The Fourth Industry Conference

NASA held the fourth and last conference on the progress of the X-15 program at the FRC on 7

October 1965. This conference was considerably smaller than the previous ones, with only 13 papers written by 25 authors. The FRC employed 18 of the authors, while four came from other NASA centers, one from the AFFTC, and the remaining two from other Air Force organizations. Approximately 500 persons attended the event. At this point, the program had conducted approximately 150 flights over 6 years.[227]

By the time of the conference, the X-15 had essentially met or exceeded all of its revised performance specifications. The future would bring no additional altitude marks, and additional speed of less than a Mach number. For the most part, the government was using the X-15 as an experiment carrier, although X-15A-2 continued some additional aero-thermo-dynamic research. Jim Love noted that 10 pilots had used the three X-15s to accumulate almost 1 hour of flight above 200,000 feet and almost 4 hours at speeds in excess of Mach 4.[228]

The follow-on experiments were taking on unanticipated importance. Love observed, "The use of the airplane as an experimental test bed is one of the most significant extensions in the research capability of the X-15 airplanes. They have been utilized to carry various experimental packages to required environments, obtaining measurements with these packages, and then returning the experiment and results to the experimenters… several experiments were installed on each aircraft for better flight utilization. For this reason, on the X-15-1 airplane, specially constructed [wing] tip pods and tail-cone box have been installed… to accommodate the experiments… Three experiments have been completed, five are in progress, and three more are planned for next year."[229]

Love noted that "the X-15 program has never settled down to a routine operation because of the continued increase in complexity and the nature of experiments and research performed by each aircraft. This attribute is probably characteristic of research programs." The lack of routine, however, undoubtedly increased the cost of the program and placed a heavy burden on personnel to maintain safety.*230

Cold Wall

The X-15 offered investigators a unique opportunity to measure heat transfer and skin friction under quasi-steady flight conditions at high Mach numbers and low wall-to-recovery temperature ratios. This allowed them to make a direct comparison between measured flight data and calculated values. A considerable amount of heat-transfer data and some skin-friction data were obtained during the flight program, and these data indicated that the level and rate of change of turbulent skin friction and heat transfer were lower than predicted by the most widely used theories, such as those of Van Driest and Eckert. However, comparisons of the X-15 data and the theory were inconclusive due to uncertainties about the boundary layer conditions because of

non-uniform flow and conduction losses. To evaluate the problem, researchers wanted to use a highly instrumented panel in a location with known flow characteristics. They also wanted the panel to be shielded from aerodynamic heating until the airplane was in a steady-state cruise condition.[209]

Researchers selected the X-15-3 with the sharp-leading-edge modification on the dorsal rudder to carry the experiment. The test panel was located just behind the right-side leading-edge boundary-layer trips 15.1 inches below the top of the rudder, and was constructed of 0.0605- inch-thick Inconel X. Researchers installed a removable panel on the left side of the rudder to provide access to the instrumentation used for the test panel. To obtain the desired wall-to – recovery temperature ratios and ensure an isothermal test surface when the airplane reached the desired speed and altitude, it was necessary to insulate the test panel during the initial phase of the flight. Explosive charges jettisoned the insulating cover from the test panel in approximately 50 milliseconds, resulting in an instantaneous heating of the test panel (the so-called "cold wall" effect). Researchers instrumented the test panel with thermocouples, static-pressure orifices, and a skin-friction gage with the data recorded on tape by a PCM data acquisition system at a rate of 50 samples per second. A Millikan camera operating at 400 frames per second was in the upper bug-eye camera bay to record the events. The measurements obtained were in general agreement with previous X-15 data.12101

Researchers used the same general location for another test panel, but without the cold wall. This panel, which was flush with the normal surface of the rudder, had a microphone and static – pressure orifice mounted flush, and an "L"-shaped total pressure probe sticking out and forward. The microphone was located 28.8 inches from the original rudder leading edge (not the sharp extension) and 20.3 inches from the top of the rudder. The data was recorded onboard the airplane and evaluated after the flight. The intent of the experiment was to determine when the boundary layer transitioned to turbulent flow. The highest noise levels occurred during reentry as the Reynolds numbers reached their peak value. The data gathered provided a qualitative indication of the end of the transition that agreed reasonably well with wind-tunnel data. Interestingly, researchers also recorded some data while the NB-52 carried the X-15, and described the noise level as "very high" due to aerodynamic interference with the carrier aircraft. This confirmed predictions made before the first glide flight.12111

Flying Again

Bob Rushworth’s last flight (2-45-81), on 1 July 1966, was also the first flight with full external tanks. As Johnny Armstrong later observed, "with 20-20 hindsight, flight 45 was destined for failure." On X-15A-2, the propellants in the external tanks were pressure-fed to the internal tanks, and the engine received propellants from the internal tanks in the normal fashion. The fixed-base simulator had shown that the X-15 would quickly become uncontrollable if the propellant from one external tank transferred while that from the other tank did not, because the moment about the roll axis would be too large for the rolling tail to counter. If this situation developed, the pilot would jettison the tanks, shut down the engine, and make an emergency landing.-1272

The problem was that, for this first flight with full tanks, there was no direct method to determine whether the tanks were feeding correctly. Instrumentation was being developed to provide propellant transfer sensors (paddle switches), but it was not available for this flight. Instead, a pressure transducer across an orifice in the helium pressurization line provided the only information. Researchers had verified that the pressure transducer worked as expected during a planned captive-carry flight (2-C-80) with propellants in the external tanks.-282

During the flight to the launch lake, while still safely connected to the NB-52, Rushworth verified that the pressure transducer was working. Rushworth jettisoned a small amount of propellant from the internal tanks, and NASA-1 watched the helium pressure come up as the external propellants flowed into the airplane (NASA-1 had to do it since nobody had thought to provide the pilot with any indicators). However, 18 seconds after the X-15 dropped away from the NB-52, Jack McKay (NASA-1) called to Rushworth: "We see no flow on ammonia, Bob." Rushworth responded, "Roger, understand. What else to do?" McKay: "Shutdown. Tanks off, Bob." Rushworth got busy: "OK, tanks are away… I’m going into Mud." Any emergency landing is stressful, but this one ended well. Bruce Peterson in Chase-2 reported, "Airplane has landed, everything OK, real good shape."282

Jettisoning the tanks with the "full" button was supposed to initiate only the nose cartridges and not fire the separation rockets. However, in this case, apparently because of faulty circuitry, the separation rockets did fire. Fortunately, the separation occurred without the tanks recontacting the airplane. Engineers obtained a great deal of data on the tank separation because an FM telemetry system in the liquid-oxygen tank transmitted data on accelerations and rotational rates during separation. Post-flight inspection of the ejector bearing points on the aircraft indicated that the ammonia tank briefly hung on the aircraft, marring the ejector rack slightly. The drogue chutes deployed immediately after separation and the dump valve in the tank allowed the propellants to flow out. The main chute deployment was satisfactory; however, the mechanism designed to cut the main chute risers failed and high surface winds dragged the tanks across the desert. Nevertheless, the Air Force recovered both tanks in repairable condition.-1282-

Bob Rushworth left the program after this flight, going on to a distinguished career that included a tour as the AFFTC commander some years later. Rushworth had flown 34 flights, more than any other pilot and more than double the statistical average. He had flown the X-15 for almost 6 years and had made most of the heating flights. These flights were perhaps the hardest to get right, and Rushworth did so most of the time.[283]

Major Michael J. Adams, making his first flight (1-69-116) on 6 October 1966, replaced Rushworth in the flight lineup. He started his career with a bang, literally. X-15-1 launched over Hidden Hills on a scheduled low-altitude (70,000 feet) and low-speed (Mach 4) pilot – familiarization flight. The bang came when the XLR99 shut itself down 90 seconds into the planned 129-second burn after the forward bulkhead of the ammonia tank failed. Fortunately, the airplane did not explode and Adams successfully landed at Cuddeback without major incident. Perhaps Adams was just having a bad day. After he returned to Edwards, he jumped in a T-38 for a scheduled proficiency flight. Shortly after takeoff, one of the J85 engines in the T-38 quit; fortunately, the Talon has two engines. Adams made his second emergency landing of the day, this time on the concrete runway at Edwards.-1284

Flying Again

external tank transferred but the other one did not – the moment about the roll axis was too large for the rolling tail to counter. If this situation developed, the pilot would jettison the tanks, shut down the engine, and make an emergency landing. Unfortunately, this exact scenario played out on 1 July 1966 on the first flight with full tanks. Thankfully, Bob Rushworth managed to jettison the tanks and make an uneventful emergency landing at Mud Lake. (NASA)

Jack McKay seemed to have more than his share of problems, and holds the record for the most landings at uprange lakes (three). His last emergency landing was made during his last flight (1­68-113), on 8 September 1966. The flight plan showed this Smith Ranch launch going to 243,000 feet and Mach 5.42 before landing on Rogers Dry Lake. However, as McKay began his climb he noticed the fuel-line pressure was low. Mike Adams as NASA-1 recommended throttling back to 50% to see if the fuel pressure would catch up; it did not. McKay shut down the engine and began jettisoning propellants to land at Smith Lake. The landing was uneventful and NASA trucked the airplane back to Edwards.-285

The program had experienced a few flights where the pilot overshot the planned altitude for various reasons, but Bill Dana added one for the record books on 1 November 1966. On flight 3­56-83, Dana got the XLR99 lit on the first try and pulled into a 39-degree climb, or so he thought, heading for 267,000 feet. In reality, the climb angle was 42 degrees. Interestingly, Pete Knight in the NASA-1 control room did not notice the error either, and as the engine burned out he reported, "We got a burnout, Bill, 82 seconds, it looks good. Track and profile are looking very good." As Dana climbed through 230,000 feet, NASA-1 finally noticed and said, "[W]e got you going a little high on profile. Outside of that, it looks good." The flight eventually reached 306,900 feet-39,900 feet higher than planned.286

As Dana went ballistic over the top, he asked Knight if "Jack McKay [was] sending in congratulations." The reference was to flight 3-49-73 on 28 September 1965, when McKay had overshot his altitude by 35,600 feet. Dana had been NASA-1 on that flight and had needled McKay ever since. Dana’s fun, however, did not stop with the overshoot. As he reached to shut down the engine, Dana apparently bumped the checklists clipped to his kneepad with his arm. Dana later recalled, "At shutdown my checklist exploded. I don’t know how it came out of that alligator clamp, but anyway I had 27 pages of checklist floating around the cockpit with me, and it was a great deal like trying to read Shakespeare sitting under a maple tree in October during a high wind. I only saw one instrument at a time for the remainder of the ballistic portion… these will be in the camera film which I think we can probably sell to Walt Disney for a great deal." After an otherwise uneventful landing, Dana could not find the post-landing checklist, "Thank you, Pete," he joked. "Since my page 16 is somewhere down on the bottom of the floor, maybe you could go over the checklist with me?"287


As was usual for the high desert during the winter, the rains had begun in late November 1966, and during early 1967 most of the lakebeds were wet, precluding flight operations. This gave North American and the FRC time to perform maintenance and modifications on the airplanes. For instance, X-15-1 was having its ammonia tank repaired and the third skid added, X-15A-2 was having instrumentation modified, and X-15-3 was having an advanced PCM telemetry system installed. By February the lakebeds at Three Sisters, Silver, Hidden Hills, and Grapevine were dry, and Rogers and Cuddeback were expected to be within two weeks. Unfortunately, snow and ice still covered Mud, Delamar, Smith Ranch, and Edwards Creek Valley. It would be late March before all the necessary lakes were dry enough to support flight operations.-1288!

The program was also making plans to add new pilots, allowing some of the existing pilots to rotate to other assignments. For instance, John A. Manke, a NASA test pilot, went through ground training and conducted a single engine run. Unfortunately, Mike Adams’s accident would eliminate any chance that Manke would ever fly the X-15.[289]

Pete Knight would eventually set the fastest flight of the program, but before that event he had at least one narrow escape while flying X-15-1. As he related in the pilot’s report after flight 1-73-


The launch and the flight was beautiful, up to a certain point. We had gotten on theta and I heard the 80,000-foot call. I checked that at about 3,100 fps. Things were looking real good and I was really enjoying the flight. All of a sudden, the engine went "blurp" and quit. There could not have been two seconds between the engine quit and everything else happening because it all went in order. The engine shut down. All three SAS lights came on. Both generator lights came on and then there was another light came on, and I think it was the fuel low line light. I am not sure. Then after all the lights got on, they all went out.

Everything quit. By this time, I was still heading up and the airplane was getting pretty sloppy. As far as I am concerned both APUs quit.

Once the X-15 began its reentry after an essentially uncontrolled exit, Knight managed to get one of the APUs started. Unfortunately, the generator would not engage, which meant Knight had hydraulics but no electrical power. He elected to land at Mud Lake.

Once I thought I was level enough I started a left turn back to Mud. Made a 6-g turn all the way around… Once I was sure I could make the east shore of Mud Lake with sufficient altitude I used some speed brakes to get it down to about 25,000 [feet altitude] and then varied the pattern to make the left turn into the runway landing to the west. On the final, all this time the trim was still at 5 degrees for the theta that we had. I was getting pretty tired of that side stick so I began to use both hands. One on the center stick and one on the side stick taking the pressure off the stick with the left hand and flying it with the right. Made the pattern and the airplane is a little squirrelly without the dampers but really not that bad. … I settled in and got it right down to the runway and it was a nice landing as far as the main skids were concerned, but the nose gear came down really hard.

After I got it on the ground I slid out to a stop. I started to open the canopy. I could not open the canopy. I tried twice and could not move that handle, so I sat there and rested for a while, I reached up and grabbed it again. Finally, it eased off and the canopy came open. Then I started to get out of the airplane and I could not get this connection off over here. I got the hat [helmet] off, to cool off a little bit, and tried it again. Then I was beginning to take the glove off to get a hand down in there also. I never did get that done. I tried it again and it would not come so I said the hell with it, and I’ll pull the emergency release. I pulled the emergency release and that headrest blew off and it went into the canopy and slammed back down and hit me in the head. I got out of the airplane and by that time, the C-130 was there. Got into the 130 and came home.

It was one of the few times an X-15 pilot extracted himself from the airplane without the assistance of ground crews. Normally a crew was present at each of the primary emergency lakes, but Mud was not primary for this flight and no equipment or personnel were stationed there.

Based on energy management, Knight probably should have landed at Grapevine. At the time, there was no energy-management display in the X-15, so NASA-1 made those decisions based on information in the control room. However, since the airplane had no power, and hence no


radio, decisions made by NASA-1 were not much help.

It is likely that the personnel on the ground were more worried than Knight was, because when the APUs failed they took all electrical power, including that to the radar transponder and radio. At the time, the radars were not skin tracking the X-15, so the ground lost track of the airplane. It was almost 8 minutes later when Bill Dana, flying Chase-2, caught sight of the X-15 just as it crossed the east edge of Mud Lake.[292]

The problem was most likely the result of electrical arcing in the Western Test Range launch monitoring experiment. Unlike most experiments, this one connected directly to the primary electrical bus. The arcing overloaded the associated APU, which subsequently stalled and performed an automatic safety shutdown. This transferred the entire load to the other APU, which also stalled because the load was still present. The APUs had been problematic since the beginning of the program, but toward the end they were generally reliable enough for the 30 minutes or so that they had to function. Each one was usually completely torn down and tested after each flight. In this case, something went wrong. After this flight, NASA moved the WTR and MIT experiments to the secondary electrical bus, which dropped out if a single generator shut down; this would preclude a complete power loss to the airplane.*293

Paul Bikle commented that Knight’s recovery of the airplane was one of the most impressive events of the program. The flight planners had spent many hours devising recovery methods after various malfunctions; all were highly dependent upon the accuracy of the simulator for reproducing the worst-case, bare-airframe aerodynamics. NASA constantly updated the simulator with the results from flights and wind-tunnel tests to keep it as accurate as possible. The flight by Knight was the only complete reentry flown without any dampers. As AFFTC flight planner Bob Hoey remembers, "[W]e would have given a month’s pay to be able to compare Pete’s entry with those predicted on the sim, but all instrumentation ceased when he lost both APUs, and so there was no data! Jack Kolf told Pete that we were planning to install a hand crank in the cockpit hooked to the oscillograph so he could get us some data next time this happened." Fortunately, it never happened again.*294

Becker’s Lament

Despite the variety of artists’ concepts and popular press articles on an orbital X-15, in the end the new National Aeronautics and Space Administration (NASA) would decide to endorse a concept that had been initiated by the Air Force and use a small ballistic capsule for the first U. S. manned space program, renamed Mercury. Nevertheless, a small minority within NASA, mainly at Langley, continued to argue that lifting-reentry vehicles would be far superior to the non-lifting capsules. In fact, at the last NACA Conference on High-Speed Aerodynamics in March 1958, John Becker presented a concept for a manned 3,060-pound winged orbital satellite. According to Becker, this paper, which dissented from the consensus within the NACA favoring a ballistic capsule, created more industry reaction-"almost all of it favorable’-than any other he had ever written, including the initial X-15 study.-13"

What ruled out acceptance of his proposal, even more than the sheer momentum behind the capsules, was the fact that the 1,000 pounds of extra weight (compared to the capsule design presented by Max Faget) was beyond the capability of the Atlas ICBM. If the Titan had been further along, Becker’s concept would have worked, but the simple fact was that Atlas was the only game in town. If it had all happened a year or two later, when the Titan became available, Becker believes that "the first U. S. manned satellite might well have been a [one-man] landable winged vehicle." The decision to adopt the capsule concept made the X-15 a dead end, at least temporarily. It would be a decade later when the aerospace community again decided that a winged lifting-reentry vehicle was feasible; the result would be the space shuttle.

There was one other orbital X-15 proposal. At the end of 1959, Harrison Storms presented a version of the X-15B launched using a Saturn I first stage and an "ICBM-type" second stage. According to Storms, "We figure the X-15, carrying two pilots…could be put into orbit hundreds of miles above the earth. Or with a scientific or military payload of thousands of pounds…into a lower orbit." Storms estimated that it would take three to four years of development and presented the idea to both the Air Force and NASA, but neither organization was interested. NASA was too busy with Mercury, and the Air Force was occupied with Dyna-Soar and fighting off Robert McNamara.-1140-

The Research Program

Because the research program was the rationale for the X-15’s existence, flights to obtain basic aero-thermo data began as soon as North American and the government were sure the airplane was relatively safe for its intended purpose. Nevertheless, almost from the beginning, the airplanes carried a few minor experiments that had little to do with its basic aero-thermo research objectives; the B-70 emission coating and a radiation detector were early examples. Still, the first couple of years of the flight program were primarily dedicated to expanding the flight envelope and obtaining the basic data needed by aerodynamicists to validate the wind-tunnel predictions and theoretical models used to build the X-15.

As this goal was increasingly satisfied, more X-15 flights carried unrelated experiments, such as

tests of ablative materials and star trackers for the Apollo program. Usually these experiments required little support from the X-15 itself, other than some power and recording capacity. Later in the program, flights began to be conducted for the sole purpose of supporting the "follow-on" experiments, although even these usually gathered aero-thermo or stability and control data to support continued evaluation. In reality, the X-15 as an experiment ended sometime in 1963 (except for the advanced X-15A-2); after that, the airplane was mostly a carrier for other experiments.


Paul F. Bikle was born on 5 June 1916 in Wilkensburg, Pennsylvania, and graduated from the University of Detroit with a bachelor of science degree in aeronautical engineering in 1939. His career with the Army Air Forces began in 1940 when he became an aeronautical engineer at Wright Field, and in 1944 he became chief of the aerodynamics branch of the Flight Test Division. While working closely with other government agencies in establishing the first flying qualities specifications for aircraft, he wrote AAF Technical Report 50693 ("Flight Test Methods"), which was used as a standard manual for conducting flight tests for more than five years. During World War II he was involved in more than 30 test projects and flew over 1,200 hours as an engineering observer.

In 1947, Bikle became chief of the performance engineering branch and directed tests of the XB – 43, XC-99, and F-86A. When the flight-test mission was transferred to the newly formed Air Force Flight Test Center (AFFTC) at Edwards, Bikle came to the desert and advanced to assistant chief of the flight-test engineering laboratory in 1951. From there, he advanced to the position of AFFTC technical director. He replaced Walt Williams as director of the NASA Flight Research Center (FRC) in September 1959. Like Williams, Bikle had little use for unnecessary paperwork, and often remarked that he would stay with NASA as long as the paperwork level remained below what he had experienced in the Air Force. He was also an avid soaring enthusiast and established two world soaring records during a flight near Lancaster on 25 February 1961 that still stands as of 2006. In July 1962, Bikle received the NASA Medal for Outstanding Leadership for directing the "successful X-15 flight operations and research activities," and he received the 1963 FAI Lilienthal Medal. Bikle retired from NASA in May 1971 and died on 20 January 1991.[6]


Finally, at 0838 hours on 8 June 1959, Scott Crossfield and X-15-1 dropped from the NB-52A at Mach 0.79 and 37,550 feet. Just prior to launch, the SAS pitch damper failed, but Crossfield elected to proceed with the flight and switched the pitch channel to standby. At launch, the X-15 separated cleanly and Crossfield rolled to the right with a bank angle of about 30 degrees. Usually the obedient test pilot, on this flight Crossfield allowed himself to deviate slightly from the flight plan and perform one unauthorized aileron roll. However, not all was well. On the final approach to landing, the X-15 began a series of increasingly wild pitching motions. Crossfield: "[T]he nose of the X-15 pitched up sharply. It was a maneuver that had not been predicted by the simulator… I was frankly caught off guard. Quickly I applied corrective elevator control. The nose came down sharply. But instead of leveling out, it tucked down. I applied reverse control. The nose came up but much too far. Now the nose was rising and falling like the bow of a skiff in a heavy sea… I could not subdue the motions." The X-15 was porpoising wildly, sinking toward the desert at 175 knots.*63*

The airplane touched down safely at 150 knots and slid 3,900 feet while turning slightly to the right. After he landed, Crossfield said he believed that the airplane exhibited a classic case of static instability. Harrison Storms, on the other hand, was sure that the cure was a simple adjustment. In the end, Storms was right. As he would on all of his flights, Crossfield had used the side-stick controller during the flare instead of the center stick, and this subsequently proved to be the contributing cause of the oscillations. The side-stick controller used small hydraulic boost actuators to assist the pilot since it would have been impossible (or at least impractical) to move the side stick through the same range of motion required for the center stick. However, the engineers had decided to restrict the authority of these hydraulic cylinders somewhat, based on a best guess of the range of movement required. The guess had been wrong, and because of this a cable in the control system was stretching and retracting unexpectedly. What appeared to be pilot-induced oscillations during landing actually reflected the mechanics of the control system. The fix was to provide more authority to the hydraulic cylinder by changing an orifice—a simple adjustment.*643

Although the impact at landing was not particularly hard, later inspection revealed that bell cranks in both main landing skids had bent. Unfortunately, North American had not instrumented the main skids on this flight, so no specific impact data were gathered. However, the engineers generally believed that the shock struts had bottomed and remained bottomed because of higher – than-predicted landing loads. Excessive rebound loads caused by a foaming of the oil in the nose gear strut compounded the issue, although it took several more landings to realize this. As a precaution against the main skid problem occurring again, the metering characteristics of the shock struts were changed, and engineers conducted additional lakebed drop tests at even higher loads with the landing-gear test trailer used to qualify the landing-gear design. The landing gear would continue to be a concern throughout the flight program. All other airplane systems operated satisfactorily on this flight, clearing the way for the first powered flight using X-15-2. The following day North American moved X-15-1 into the hangar to hook up its XLR11s and propellant system and make other changes in preparation for its first powered flight.*65*


Scott Crossfield climbs out of X-15-1 after the first captive-carry flight. The X-15 landing gear had been deployed during the flight to demonstrate it would work after being cold-soaked at altitude. A member of the ground crew installs protective covers on the nose-mounted air data boom. (AFFTC History Office)

While the NB-52 was carrying X-15-1 as expensive wing cargo, engineers were testing the XLR11s at the Rocket Engine Test Facility using X-15-2. Despite the successful 22 May test, things were not going particularly well. Perhaps the engines had been out of service for too long between programs, or maybe too much knowledge had been lost during the coming and goings of the various engineers and technicians over the years, but the initial runs were hardly trouble-free. Various valves and regulators in the propellant system also proved to be surprisingly troublesome.

Moreover, sometimes things just went to hell. After one engine run, the ground crew began purging the hydrogen-peroxide lines of all residual liquid by connecting a hose from a ground nitrogen supply to a fitting on X-15-2. On this day, it was a new hose. Despite the careful procedures and great caution used, the hose had a slight residue of oil. When the technician applied gas pressure to the hose, the film of oil ran into the hydrogen-peroxide lines. The only thing truly compatible with peroxide is more peroxide, not oil. The result was an immediate explosion and fire that raced through the X-15 engine compartment. As always, the Edwards fire crew was standing by and quickly extinguished the fire, but not before gutting the engine bay.

One X-15 crewman was badly burned; if he had been standing two feet closer, he likely would have been killed. It took weeks to repair the airplane.[66]

Forty-six days after the first glide flight, and after the damage from the explosion was repaired, the Nb-52A took X-15-2 for a captive-carry flight with full propellant tanks on 24 July 1959.

One of the purposes of this flight was to evaluate the liquid-oxygen top-off system between the NB-52 and X-15. It proved to be erratic. Another test was to measure the time it took to jettison the propellants at altitude. While still safely attached to the wing of the NB-52, Crossfield jettisoned the hydrogen peroxide, which took 140 seconds. He then jettisoned the liquid oxygen and alcohol simultaneously, which took 110 seconds. The times matched predictions. The APUs and pressure suit performed flawlessly. Despite the failure of the top-off system, researchers considered the flight a success. The original contract had specified that North American would turn the first airplane over to the government in August 1959. For a while it looked like the company might deliver the first X-15 on schedule, but it was not to be.[67]

During August and early September, engineers canceled several attempts to make the first powered flight before the aircraft left the ground, due to leaks in the APU propellant system and hydraulic problems. There were also several failures of propellant tank pressure regulators, and on at least one occasion, liquid oxygen streamed out of the safety vent while the NB-52 carried the X-15. No flight occurred on that day. Charlie Feltz, Bud Benner, and John Gibb, along with a variety of other North American engineers and technicians, worked to eliminate these problems, all of which were irritating but not critical-other than to the morale.[68]

At the 30th anniversary celebration, Storms described the mood at the time:*69

A typical launch attempt would start the night before, and the crews would work all night preparing the X-15 and fueling it. About 8 a. m., Scott Crossfield would be in his flight gear and, after walking around the operation, get into the cockpit and start his checkout. Scott would stay in the ready condition as the countdown continued. This, unfortunately, might be as late as 3 or 4 in the afternoon before the B-52 would be allowed to take off. By the time it had reached launch altitude and attempted to hold for the required length of time with all systems in operation, sometime during this period a regulator would fail, a valve would fail, or the bearings on one or both APUs would go out. Then back to Edwards. When Scott returned, we would be scheduled to go to a press conference and meet many tired, and by that time somewhat edgy, reporters that always wanted answers that were just not available. These were not happy meetings for any of the participants.

Shortly after about the fourth such encounter, I was gathered up by General John McCoy of Wright Field and taken over to Mr. [James Howard ‘Dutch’] Kindelberger’s office, the then chairman of the board. The general explained that the country was in a bad spot with the Sputnik success and that our false starts were not very much of a positive boost to the national position. In short, "when were we going to launch that X-15?" This one time in my life all eyes were on me. Not the most desirable position. The answer I gave was to go over the conditions that we and the NASA had set up for a launch. Also, I gave my support to this approach and pointed out that we were attempting to put a new type of flying machine in the air without the loss of either millions of dollars worth of equipment or the pilot. However, if they wanted to, I would take them to the task force that set up the launch ground rules and they could either convince them of a different approach or overrule them, if possible. The whole meeting ended up with the Air Force’s plea for increased effort on out part and hope for early success. Fortunately for all concerned, the next attempt turned out to be a winner.

At last, Scott Crossfield made the first powered flight using X-15-2 on 17 September 1959. The NB-52A released the research airplane at 0808 in the morning while flying at Mach 0.80 and 37,600 feet. X-15-2 reached Mach 2.11 and 52,341 feet during 224.3 seconds of powered flight using the two XLR11 engines. Crossfield surprised everybody, including most probably himself, by performing another aileron roll, this time all the way around. As Crossfield remembers, "Storms was tickled." On a more serious note, he observed, "With the rolling tail one would expect very clean ‘aileron’ rolls without the classical adverse yaw from ailerons, and that is the way it rolled.

No big deal at all." The government’s concerns about the rolling tail were for naught.-701

Crossfield landed on Rogers Dry Lake 9 minutes and 11 seconds after launch, despite some concerns about a crosswind on the lake. Following the landing, ground crews noticed a fire in the area around the ventral stabilizer, but quickly extinguished it. A subsequent investigation revealed that the upper XLR11 fuel pump diffuser case had cracked after engine shutdown and sprayed fuel throughout the engine compartment. Alcohol collected in the ventral stabilizer and some unknown cause ignited it during landing. Crossfield noted that "the fire had burned through a large area, melting aluminum tubing, fuel lines, valves, and other machinery." For the second time in less than six months, X-15-2 went back to Inglewood. It took about three weeks to repair the damage.-1711

Edwards was not the only place where the X-15 created interest, although it was certainly the most visible. Back at Langley Research Center, just a month after the first powered flight, approximately 20,000 visitors attended the first anniversary inspection, held on Saturday, 24 October 1959. The crowds had come at NASA’s invitation, and local newspapers had spread the word that for the first time in its 42-year history Langley would be open to the public. NASA scientists, engineers, and technicians showed the public just what the new agency was doing to launch their country into space. The attractions included full-scale mockups of the X-15, XLR99, and a dummy in an MC-2 full-pressure suit. A small group of Langley secretaries acted as the hostesses for the exhibit, while both John Becker and Paul Bikle were nearby to answer questions. The event was a success with both the public and the media.-721

Back in the high desert, the third flight (2-3-9) of X-15-2 took place on 5 November 1959 when the NB-52A dropped the X-15 at Mach 0.82 and 44,000 feet. The flight got off to a bad start; during the engine start sequence, one chamber in the lower engine exploded. Chase planes reported external damage around the engine and base plate, and the resulting fire convinced Crossfield to land on Rosamond Dry Lake. Crossfield shut down both engines, but the 13.9 seconds of powered flight had been sufficient to accelerate the X-15 to Mach 1. Unfortunately, the flight attitude necessary to descend to the lakebed made it impossible to dump most of the remaining propellants. Crossfield initiated the landing flare at about 950 feet altitude and 253 knots. The aircraft touched down near the center of the lake at approximately 161 knots and a 10.8-degree angle of attack with a descent rate of 9.5 feet per second. Crossfield noted: "The skids dug in gently. The nose slammed down hard and the airplane plowed across the desert floor, slowing much faster than usual. Then she came to a complete stop within 1,500 feet instead of the usual 5,000 feet." When the nose gear had bottomed out, the fuselage literally broke in half at station 226.8, shearing out about 70% of the bolts at the manufacturing splice. The broken fuselage dug into the lakebed, creating a very effective brake.-173


A minor explosion during Flight 2-3-9 on 5 November 1959 resulted in an emergency landing on Rosamond Dry Lake that broke the back of X-15-2. As built, the X-15 was heavier than originally intended, and it did not help that Scott Cross field was unable to jettison all of the unused propellants before the emergency landing. The airplane was repaired in time for its fourth flight on 11 February 1960. (AFFTC History Office)

A contributing factor to the hard landing was the 15,138-pound touchdown weight. During development, engineers had established a limiting rate of sink of 9 fps based on design weight of 11,500 pounds. However, the as-built airplane had increased to 13,230 pounds. In addition, Crossfield had been unable to jettison some of the propellants because of the steep descending attitude necessary to reach the landing site, which further increased the landing weight. Crossfield later stated that the damage was the result of a structural defect that probably should have broken on the first flight.-74

Yet again, X-15-2 went to the Inglewood plant for repairs, and returned to Edwards in time for its fourth flight on 11 February 1960. North American repaired the damaged fuselage and strengthened the manufacturing splice by doubling the number of fasteners and adding a doubler plate, top and bottom, at the fuselage joint. The company also modified the other two airplanes to prevent similar problems.75

Radiation Detection

The next experiment came from an unlikely source. On 3 August 1961, the Air Force Special Weapons Center at Kirtland AFB, New Mexico, delivered an ionizing radiation-detection device for use on the X-15. NASA installed the 10-pound package in the left side console of the cockpit outboard of the ejection seat. Actually, the first attempt to install the experiment failed because the space allocated in the cockpit was insufficient, but Kirtland soon repackaged it to fit.731

The experiment activated automatically when the pilot turned on the main instrumentation switch during flight. The package contained an ion chamber, two scintillators, a Geiger tube, and a self- contained multi-channel tape recorder. Different thicknesses of human-tissue-equivalent plastic encased the ion chamber and scintillators. With the Geiger tube acting as a count rate monitor, the detectors recorded radiation dose rates on the surface and at depths of 0.25 inch and 1.0 inch in the plastic between 1 millirad per hour and 100 millirads per hour.741

The first flight attempt was made on 29 September 1961, but this flight (1-A-38) aborted prior to launch due to a flight-control anomaly in the X-15. The package successfully flew on 4 October 1961 with Bob Rushworth at the controls of flight 1-23-39. The experiment subsequently flew several more times during late 1961. After each flight the taped data went to Kirtland for analysis, and the results ultimately showed that the pilots received essentially a normal background dosage of radiation (0.5 millirads per hour) during the flights. Since there seemed to be no cause for concern, the Air Force deleted the radiation detector from flights beginning in 1962.75

The program flew a different radiation experiment on X-15-2 from early 1961 until September 1963. The "Earth cosmic-ray albedo" experiment investigated the cosmic-ray environment at altitudes from 50,000 feet upward to determine the cosmic-ray environment in which future manned space vehicles would operate. The experiment consisted of placing small photographic

emulsion stacks in upper and lower structural (i. e., bug-eye) camera bays to obtain information on the cosmic-ray albedo flux and spectrum, as well as the flux and spectra of electrons and protons leaking out of the Van Allen belts. The X-15-2 carried the stacks to high altitudes on as many flights as practical and placed no restrictions on the flight path or trajectory. The NB-52 carried similar stacks during the same flights to provide lower-altitude references. Researchers at the University of Miami and the University of California at Los Angeles and Berkeley analyzed the film from the stacks.[76]

In a very similar experiment, X-15-1 and X-15-2 carried two nuclear-emulsion cosmic-radiation measurement packages from the Goddard Space Flight Center on the aft ends of their side fairings to investigate the cosmic-radiation environment at extreme altitudes. These emulsion stacks were considerably larger than the Earth cosmic-ray albedo stacks and were located external to the airplane. Several flights carried the packages to altitudes above 150,000 feet. The packages required no special maneuvers and no servicing other than installation just prior to flight, and removal after landing.-177


Bob Rushworth flew the X-15 for 68 months from 4 November 1960 until 1 July 1966, making 34 flights. These included two flights with the XLR11 and 32 flights with the XLR99. Rushworth reached Mach 6.06, a maximum speed of 4,018 mph, and an altitude of 285,000 feet. His accomplishments include the first ventral-off flight, the maximum dynamic-pressure flight, the maximum temperature flight, the maximum Mach number (6.06) in the basic X-15, the first flight of X-15A-2, and the first flight with external tanks.

Robert Aitken Rushworth was born on 9 October 1924 in Madison, Maine. He joined the Army Air Forces, flying C-46 and C-47 transports in World War II and later combat missions in Korea. In 1943 he graduated from Hebron Academy, Maine. He received bachelor of science degrees in mechanical engineering from the University of Maine in 1951 and in aeronautical engineering from the Air Force Institute of Technology in 1954. He graduated from the National War College at Fort Lesley J. McNair, Washington, D. C., in 1967.[22]

Rushworth began his flight-test career at Wright Field and transferred to Edwards in 1956. Following graduation from the Experimental Test Pilot School, Rushworth reported to the fighter operations branch at Edwards and later became operations officer in the manned spacecraft section while flying the X-15. Prior to flying the X-15, Rushworth flew the F-101, TF-102, F-104, F-105, and F-106. He received the Distinguished Flying Cross for an emergency recovery of the X-15 after premature extension of the nose gear at near Mach 5 speeds, and the Legion of Merit for overall accomplishments in the national interest of initial space flights.-1231

He graduated from the National War College in August 1967 and attended F-4 Phantom II combat crew training at George AFB. In March 1968, Rushworth went to Cam Ranh Bay Air Base in Vietnam as the assistant deputy commander for operations with the 12th TFW and flew 189 combat missions. From April 1969 to January 1971, he was program director for the AGM-65 Maverick, and in February 1971 he became commander of the 4950th Test Wing at Wright-Patterson AFB. General Rushworth served as the inspector general for the Air Force systems command from May 1973 to February 1974 and returned to the AFFTC as commander until November 1975, when he became commander of the Air Force Test and Evaluation Center at Kirtland AFB, New Mexico.

Rushworth retired from the Air Force in 1981 as vice commander of the Aeronautical Systems Division at Wright-Patterson AFB. Bob Rushworth died of a heart attack on 18 March 1993 in Camarillo, California.-1241

The X-15A-2

After Jack McKay’s emergency landing in X-15-2 on 9 November 1962, North American proposed modifying X-15-2 to an advanced configuration capable of reaching Mach 8 velocities. NASA in general and Paul Bikle in particular were not particularly enthusiastic and felt the Air Force should simply repair the aircraft to its original configuration or retire it altogether. Many researchers believed that the Mach 8-capable X-15A would be of limited value for aero-thermo research. However, NASA did not press its views, and on 13 May 1963 the Air Force directed North American to repair and modify the aircraft at a cost of $4.75 million. The advanced aircraft was intended to evaluate an air-breathing hypersonic research engine (HRE) being developed at Langley. It was designed to reach 8,000 fps at an altitude of 100,000 feet, and a dynamic pressure of 1,000 psf. Heating rates up to 210 Btu per square foot per second were expected, with peak structural temperatures approaching 2,400°F.[231

The modifications did not significantly alter the physical appearance of X-15A-2. The wingspan was still 22.36 feet, but the airplane was 29 inches longer due to a plug in the center of gravity compartment between the propellant tanks. Perhaps the most obvious change was the addition of external propellant tanks on each side of the fuselage below the wings. These allowed the airplane to carry approximately 70% more propellant, a necessary ingredient in raising performance to 8,000 fps. The tanks provided an additional 60 seconds of engine burn time, for a total of 150 seconds at 100% power. Other modifications included adding hydrogen-peroxide tanks within extended aft-side fairings to supply the turbopump for the longer engine burn times, and additional pressurization gas in a spherical helium tank just behind the vertical stabilizer.12321

The fuselage extension provided additional internal volume for experiments, and the center-of – gravity compartment access doors could accommodate optical windows looking up or down. The compartment could also accommodate a liquid-hydrogen tank, with a total capacity of 48 pounds, to fuel the ramjet mounted under the fixed portion of the ventral stabilizer, but it appears NASA never actually installed the tank. Perhaps the most difficult part of incorporating this extension was moving the B-52 pylon attach points to maintain the vertical stabilizer in the appropriate position under (through) the NB-52 wing.-12331

North American strengthened the landing gear using a strut that was 6.75 inches longer than the original. This provided 33 inches of ground clearance to the bottom of the fixed ventral stabilizer; the expected ramjet was 30 inches in diameter. The new strut provided a 1,000-pound increase in allowable landing weights, but there was some concern over the effects of the longer strut on nose-wheel and forward fuselage loads. In an attempt to provide an additional margin of safety, the nose-gear trunnion was mounted 9 inches lower (effectively lowering the nose gear by the same amount), allowing an attitude at nose-gear touchdown that was similar to that of the basic airplanes.12341

Instead of the trapezoidal windows on the original airplanes, North American installed elliptical windows that used three panes of glass to withstand the increased temperatures at Mach 8. The outer pane was 0.65-inch-thick fused silica, the middle pane was 0.375-inch alumino-silicate, and the inner pane was 0.29-inch laminated soda lime glass. This time the company mounted the outside windshield-retaining frame flush with the glass to prevent the reoccurrence of the flow heating experienced early in the program.-12331

The X-15A-2 modification also included a "skylight" hatch. Two upward-opening doors, 20 inches long by 8.5 inches wide, were installed above the instrument compartment behind the cockpit to expose cameras and other experiments. North American revised the normal research instrumentation elevator so that the upper shelf could extend upward through the open hatch if needed. The modifications also included additional data recorders: five 36-channel oscillographs, eight three-channel oscillographs, two 14-track tape recorders, one 24-cell manometer, and one cockpit camera. In addition, a new 86-channel PDM telemetry system was used to transmit data to the ground in real time.-12361

The accident had seriously damaged the outer portion of the right wing. North American found that it could adequately repair the main wing box, but the outer 41 inches were a total loss. With the government’s concurrence, the company modified the wing box to support a replaceable outer panel that allowed the testing of various materials and structures during hypersonic flight. The panel provided with the airplane (and the only one that apparently ever flew) was similar in construction and materials to the standard X-15 wing except that it was equipped with a 26.7 by 23-inch access panel that allowed access to an extensive amount of research instrumentation.-12321















The X-15A-2

The X-15A-2


After Jack McKay’s emergency landing at Mud Lake, the Air Force had North American rebuild the X-15-2 with several modifications intended to allow flights to Mach 8 to support ramjet propulsion research. The X-15A-2 was designed to reach 8,000 fps at an altitude of 100,000 feet, and a dynamic pressure of 1,000 psf. Heating rates up to 210 Btu per square foot per second were expected, with peak structural temperatures approaching 2,400 degrees Fahrenheit.

A 29-inch fuselage extension provided a larger center-of-gravity compartment to hold a liquid hydrogen fuel tank for the proposed ramjet, and external tanks carried additional propellants for the XLR99. (NASA)

NASA conducted wind-tunnel tests of the X-15A-2 during the summer and fall of 1963. The tests indicated that there was little aerodynamic difference between the modified X-15A-2 without external tanks and the basic X-15. Despite the anticipated similarities, engineers decided it was prudent to conduct an abbreviated flight series to verify that the airplane still handled satisfactorily. Stability and control maneuvers conducted during the initial flights of the X-15A-2 largely verified the wind-tunnel predictions. However, the verification took significantly longer than expected when the program encountered trouble with the modified landing-gear system.-1238

North American completed final assembly of X-15A-2 in Inglewood on 15 February 1964, and the Air Force accepted the airplane on 17 February, three weeks ahead of schedule and slightly below budget. The airplane was, however, 773 pounds overweight, a condition that was expected to reduce the maximum velocity somewhat. The design launch weight was 49,640 pounds, and propellants accounted for 32,250 of these pounds (18,750 pounds internally and 13,500 pounds in the external tanks). Subsequent modifications would add another few hundred pounds in empty weight. North American delivered the airplane to the FRC on 18 February and an "official" government acceptance ceremony took place on 24 February.-239

flight was to check out the various systems, evaluate the handling qualities of the modified airplane, and gain preliminary experience with the ultraviolet stellar photography experiment


It was not bad for a "checkout" flight. Using 77% thrust, Bob Rushworth reached Mach 4.59 and 83,300 feet. In an ironic twist, Jack McKay, who had been injured on the flight that damaged the airplane, was the NASA-1 controller. As expected, Bob Rushworth reported that the modified X- 15A-2 handled much like a basic X-15. Static longitudinal stability remained about the same, despite a 10% forward shift in the center of gravity. The already low directional stability of the unmodified airplane was somewhat lower in X-15A-2, but Rushworth did not think it posed a significant threat to safety. The longitudinal trim characteristics of the modified airplane were essentially unchanged up through Mach 3 at angles of attack up to 15 degrees. The modified airplane had a trim capability reduced by approximately 3-5 degrees at higher angles of attack and Mach numbers.*241

Things got more exciting on the second flight (2-33-56). Shortly after a maximum Mach number of 5.23 was obtained, the nose gear unexpectedly extended as the airplane decelerated through Mach 4.2. After the flight Rushworth wrote, "Everything was going along fine and just about the time I was ready to drop it over [lower the nose] I got a loud bang and… the resulting conditions that I had gave me quite a little bit of concern because the airplane began to oscillate wildly and I couldn’t seem to catch up with it. I put the dampers back on and stuffed the nose down to about 5 degrees angle of attack and it seemed to be normal then except I had a sideslip and I was then required to use left roll to hold the airplane level. A couple of seconds later I realized that this sound that I had heard was very much similar to the nose gear coming out in the landing pattern so that was the only thing I could think of. I announced that and then a few seconds later I began to get smoke in the cockpit, quite a little bit more than I had ever seen before. This partially confirmed that the nose gear, at least the door was open. I wasn’t sure that the gear was out but it was; there was enough of an explosion there to make me think that the gear was out."[242]

For the time being, the chase planes were of no help in confirming the problem since they were some 10 miles below the X-15. Jack McKay as NASA-1 could not help much either, since no emergency procedures existed for this particular failure. McKay did advise Rushworth that it would probably be best if the X-15 remained at high altitude until it had slowed considerably, thereby easing the aero-thermo loads on the extended nose gear. At one point NASA-1 advised Rushworth to use the brakes to slow down a bit, but Rushworth had other ideas: "No, I don’t want to get brakes out, I want to get the damn thing home." Fifty miles away from Edwards, the X-15 was still traveling at Mach 2.5 and McKay advised Rushworth, "Let’s go max L/D, Bob. You’re looking OK now. Your heading is good. You’re on profile. Looks like you’ve got plenty of energy."*241

The chase planes finally spotted the X-15 as it was descending 20 miles northeast of Edwards. Despite the degraded control and increased drag resulting from the extended nose gear, Bob Rushworth was doing fine. Joe Engle in Chase-3 verified that the nose gear appeared to be structurally sound and in the locked-down position. As for the tires, Engle reported, "OK, Bob, your tires look pretty scorched; I imagine they will probably go on landing." There was a worry, however, that the oleo strut had also been damaged by the heat and dynamic pressure; if it failed on landing, the X-15 could break in half or worse. There seemed to be little choice. Engle was right-the tires disintegrated shortly after the nose gear came down, but Rushworth managed to stop the airplane without serious difficulty.*244*

An investigation revealed that aerodynamic heating was the cause of the failure. The expansion of the fuselage was greater than the amount of slack built into the landing gear release cable. This caused an effective pull on the release cable that released the uplock hook. An outward bowing of the nose gear door imposed an additional load on the uplock hook. The load from both of these sources caused the uplock hook to bend, allowing the gear to extend. Engineers duplicated this failure in the High-Temperature Loads Calibration Laboratory by simulating the fuselage expansion and applying heat to the nose-gear door.[245]

The same stability and control data flight plan was duplicated for the next X-15A-2 light (2-34­57) on 29 September 1964. Again, shortly after reaching a maximum Mach number of 5.20, Bob Rushworth experienced a similar but less intense noise and aircraft trim change at Mach 4.5-the small nose-gear scoop door had opened. In his post-flight report Rushworth noted, "Yes, I sensed it was the little door, because the magnitude of the bang when it came open wasn’t as large as the other experience." During the normal gear-extension sequence, air loads on the small door pulled the nose gear door open to assist in the extension of the nose gear. Although not as serious a failure as that on the previous flight, it again precluded obtaining dampers-off stability data. NASA redesigned the nose-gear door to provide positive retention of the scoop door regardless of the thermal stresses. Engineers also modified the other two airplanes since the basic failure mode was common.[246]

To check out the modifications to the nose gear, the program decided on a low-speed flight (2­35-60) to a maximum Mach number of 4.66. The flight planners decided to give Bob Rushworth a break after the two previous adventures, so Jack McKay made this flight, which went off without a problem. The nose gear performed normally.-12471

Rushworth was in the cockpit again for the next fight (2-36-63) of X-15A-2 on 17 February 1965. In a run of bad luck that is hard to fathom, this time the right main skid extended at Mach 4.3 and 85,000 feet. In his post-flight report Rushworth wrote, "Jack [McKay, NASA-1] was talking away and things were going along real nice and I couldn’t seem to get a word in there to tell him that I had a little problem. It took several seconds to get the airplane righted and dampers back on, very much similar to the nose gear coming out. Once I got it righted, I realized that I had a tremendous sideslip, I guess 4 degrees, and it took a lot of rudder deflection to get sideslip to zero. This persisted all the way down until I got subsonic. Once I had gone subsonic the airplane handled reasonably well." Again, the chase pilot was able to verify that the gear appeared structurally sound, and Rushworth managed to make a normal landing. When Rushworth finally got out of the airplane, he turned around and kicked it-enough was enough. Post-flight inspection revealed that the uplock hook had bent, allowing the gear to deploy. Again, aerodynamic heating was determined to be the source of the high load on the uplock hook.[248]

NASA flew five more X-15A-2 flights (2-38-66 through 2-42-74) before the envelope expansion program was begun. These flights were primarily conducted to study stability and control, but they also included landing-gear performance tests. Each flight carried the ultraviolet stellar photography experiment but obtained little useable data because of problems in maintaining the precise attitudes required for the experiment. Fortunately, the landing gear seemed to behave throughout these flights, but it had caused this portion of the flight program to take longer than expected.[2491

The engineers had always had some concerns about operating the X-15A-2 with the 23.5-foot – long, 37.75-inch-diameter external tanks. These attached to the airplane structure within the side fairings at fuselage stations 200 and 411. Propellant and gas interconnects ran through a tank pylon that was located between stations 317 and 397 and was covered by a set of retractable doors after the tanks were jettisoned. The left tank contained about 793 gallons of liquid oxygen in one compartment and three helium bottles with a total capacity of 8.4 cubic feet. The right tank contained about 1,080 gallons of anhydrous ammonia in a single compartment. The empty left tank weighed 1,150 pounds and the empty right tank weighed only 648 pounds; when they were full of propellants, they weighed 8,920 pounds and 6,850 pounds, respectively. Note that the left tank was over 2,000 pounds heavier than the right tank when they were full. To minimize weight and cost, the government had opted not to insulate the liquid-oxygen tank. As a result, the evaporation rate was high enough that the engineers considered the NB-52 top-off supply to be marginal. If a flight encountered a long hold time prior to launch, it might prove necessary to abort the mission and return to Edwards due to excessive liquid-oxygen boil – off.[250]

The use of external tanks on the X-15A-2 was unique in that the pilot had to jettison the tanks from the aircraft. The structural limitations of the aluminum tanks and the degraded handling qualities dictated that the maximum allowable Mach number with the external tanks was 2.6, so the pilot had to jettison the tanks before reaching that speed. In addition, a landing was not possible with the tanks installed because of the increased drag and a lack of ground clearance. Hence, the program expended considerable effort to ensure the tanks would jettison when commanded.-125^

Each tank was forcibly ejected from the airplane during flight through the use of fore and aft gas – cartridge ejectors and a forward solid-propellant sustainer rocket that imparted pitching and rolling moments to the tank after it had been ejected. For a normal empty tank jettison, both sets of gas cartridges fired and the nose rocket ignited. In the case of an emergency jettison when the tanks were full, only the nose gas cartridge fired.

The tanks were relatively expensive, so they were equipped with a recovery system that included a drogue and a main parachute. The drogue chute deployed from its nose compartment immediately after separation, and the main descent parachute deployed when a barometric sensor detected the tanks passing through 8,000 feet. Although the engineers expected some impact damage, they believed it was possible to refurbish the tanks at a reasonable cost.-252!

Wind-tunnel tests indicated that satisfactory separation characteristics existed when the dynamic pressure was less than 400 psf and the angle of attack was less than 10 degrees; acceptable separation probably existed for dynamic pressures up to 600 psf. At higher angles of attack and dynamic pressures, researchers expected the tanks to roll excessively and to pitch up within close proximity to the airplane. Tank separation characteristics with partly expended propellants were unknown and were a potential problem since there were no slosh baffles or compartments for center-of-gravity control. The researchers expected the full-tank ejection characteristics to be satisfactory for any reasonable flight conditions that might occur within 15 seconds of launch.-253!

Prior to the first flight using external tanks, the Air Force conducted two dummy tank jettison tests with X-15A-2 located over a 10-foot-deep pit in the ground beside the ramp. Technicians constructed a pair of beams with similar mass and inertia properties to simulate empty tanks. Preloaded cables attached to the beams applied simulated aerodynamic drag and side loads. The first test used a single set of ejector cartridges at simulated air loads of 400 psf, 5 degrees angle of attack, and 3 degrees of sideslip. The second test used two sets of ejector cartridges at a simulated dynamic pressure of 600 psf. Both tests were successful, and high-speed motion pictures showed good separation characteristics. During the tests, the X-15 APU supplied hydraulic and electrical power, and engineers engaged the SAS to observe its reaction to the separation, which was satisfactory.-2541


Подпись: RIGHT HAND NH3 (6850 lb FULL)

The X-15A-2LEFT HAND LOX (3920 fb FULL)

The external tanks allowed carriage of approximately 70 percent more propellant, a necessary ingredient in raising performance to 8,000 fps. The tanks provided an additional 60 seconds of engine burn time, for a total of 150 seconds at 100-percent power. The tanks were 23.5 feet long and 37.75 inches in diameter. The left tank contained about 793 gallons of liquid oxygen in one compartment and three helium bottles with a total capacity of 8.4 cubic feet. The right tank contained about 1,080 gallons of anhydrous ammonia in a single compartment. The empty left tank weighed 1,150 pounds and the empty right tank weighed only 648 pounds; when full, they weighed 8,920 pounds and 6,850 pounds, respectively. (NASA)

Despite this, during a review leading up to the first flight, engineers expressed concern over the possible separation of partially filled tanks during an emergency. The wind-tunnel tests and the tank separation system only covered full and empty scenarios. What would happen if the pilot had to abort the flight during the first 60 seconds of powered flight while the engine was siphoning propellants from the external tanks? The initial response was that the tanks, as designed, would not withstand the loads imposed during a separation with a partial load. Engineers at the FRC and AFFTC considered installing a rapid propellant-dump system, installing a set of baffles in the tanks, or even providing a system that would allow the pilot to refill the tanks using internal propellants. All seemed too complicated given the time and money available to the program.[255]

After a great deal of consultation among the engineers, flight planners, and pilots, management decided to continue, at least for the time being. The risk was considered reasonable because the XLR99 had never encountered a premature shutdown from 100% thrust-all failures had occurred either during ignition or while throttling. If a failure happened during ignition, the tanks would be full and would not present a problem, and the plan called for no throttling during the high-speed runs. Nevertheless, engineers decided to add a third jettison button in the cockpit. This one, intended for use with partial tanks, would fire the forward gas cartridge and ignite the separation rocket-sort of a middle ground between the other scenarios.-1256

Although there were physical differences between the basic X-15 and the X-15A-2 without external tanks, their aerodynamic qualities were similar. With external tanks on the airplane,
however, some rather dramatic differences existed, with the general trend toward unfavorable characteristics. The offset center of gravity caused by the external tanks further complicated the overall control task. At launch with full tanks, the vertical center of gravity was approximately 9 inches below the aircraft waterline, moving upward as the engine consumed propellants. The pilot had to use additional nose-up stabilizer trim to counteract the nose-down pitch at engine ignition caused by this offset below the thrust vector. The heavier liquid-oxygen tank on the left side displaced the center of gravity 2 inches to that side, causing a left rolling moment that the pilot also had to counteract.-257!

The nominal flight profile for the speed missions was to maintain the airplane at a 12-degree angle of attack until it reached a pitch attitude of 34 degrees. The pilot held this climb attitude until the external propellant was depleted. Tank ejection occurred at approximately Mach 2.1 and

67.0 feet, and the pilot maintained an angle of attack of 2 degrees until the airplane reached

100.0 feet. The airplane then accelerated to maximum velocity.258

As was the case with the basic airplane, the simulator predicted poor handling qualities at high angles of attack, due primarily to the large negative dihedral effect caused by the presence of the ventral rudder. For a yaw damper failure with the speed brakes out, a divergent sideslip oscillation persisted above 6 degrees angle of attack. Although the pilot could damp this divergence, it required almost continuous attention and left little time for other tasks. The simulator showed that turning off the roll damper would eliminate the divergent yaw oscillation, but then the pilot would have to fly the airplane with less lateral directional stability. From the simulator studies it was determined that, because of the relatively low-altitude profiles required, the airplane could be safely flown after a roll and/or yaw damper failure if an angle of attack of less than 8 degrees was maintained. The program accepted this restriction for the initial envelope expansion flights. However, for the projected ramjet tests, which required flights at high dynamic pressures, a divergence of this type could occur too rapidly for the pilot to take corrective action. Hence, NASA decided to provide a redundant yaw damper, similar to the ASAS used for the roll axis. The FRC began initial design work, but the flight program ended before the system was completed.-1259!

The final ground-based external tank test took place on the Rocket Engine Test Facility where X – 15A-2 had completed a full-duration engine run with the external tanks installed on the aircraft. Engineers had already corrected deficiencies uncovered during several earlier tests.268

The expected performance of X-15A-2 represented a significant improvement over the demonstrated 6,019 fps of the basic aircraft. With external tanks and the ventral rudder, the estimated velocity was between 7,600 and 7,700 fps at 120,000 feet. Replacing the ventral with an assumed ramjet configuration would decrease that to about 7,200 fps at 118,000 feet, a result of increased weight and drag for the ramjet configuration. This performance was, however, appreciably less than the design goal of 8,000 fps at 100,000 feet. As a result, Reaction Motors was investigating the development of a new injector and nozzle to provide additional thrust in an attempt to bring the performance back up to 8,000 fps. Again, the end would come before the company completed the work.261

The length of time the airplane could remain at high velocity and dynamic pressure determined the amount of useful data about the ramjet that could be obtained. Researchers expected that X – 15A-2 could stay above 7,000 fps for 50 seconds and above 6,000 fps for 110 seconds per flight. For ramjet tests that required steady conditions (that is, at a relatively constant velocity and dynamic pressure), the pilot would throttle the XLR99 to minimum and extend the speed brakes so that low acceleration existed. The expected stabilized test time for this configuration was approximately 14 seconds at 7,000 fps and 40 seconds at 6,000 fps.268

The first flight (2-43-75) with empty external tanks was on 3 November 1965, the only flight launched from Cuddeback, about 60 miles north of Edwards. Bob Rushworth jettisoned the tanks at Mach 2.25 as the airplane passed through 70,300 feet, and took the airplane to Mach 2.31 and 70,600 feet before landing at Rogers Dry Lake after a flight of only 5 minutes and 1 second (the shortest non-emergency powered flight of the program). Post-flight analysis indicated that the handling qualities were essentially as predicted by the simulator. Rushworth, who for a change was flying without deploying part of the landing gear, commented that he thought the "roll stability was significantly less than I had expected," but the "longitudinal control wasn’t quite as bad" as he had anticipated.-126^

Two ground-based mobile trackers, each with 150-inch lenses on 35-mm Mitchell cameras running at 72 and 48 frames per second, provided photographic coverage of the tank separation. In addition, six Askania tracking cameras recorded the tank recovery system. Because the events took place at quite a distance, the resulting image size was small and researchers could only make a qualitative analysis of the event. The tanks separated cleanly from the aircraft; however, it appeared that the tanks did not rotate nose down as much as expected. They exhibited a tumbling action during flight with the drogue chutes attached, and tended to trim at an angle of attack of about -110 degrees. The drogue chutes occasionally collapsed during flight, so the engineers lengthened the drogue chute riser for future flights. Impact with the desert destroyed the liquid-oxygen tank after the nose cone containing the main descent chute did not separate properly. The Air Force recovered the ammonia tank in repairable condition. The tumbling action of the tanks increased the total drag and the tanks fell short of their predicted impact points-the ammonia tank landed 2.3 miles short and 0.6 mile to the left, while the liquid-oxygen tank landed 2.7 miles short and 1.6 miles to the left. This was still well inside the bounds of the Edwards impact range and did not represent a problem.-264

Joe Engle ended up being the only X-15 pilot who would get to fly the next lifting-reentry vehicle, the Space Shuttle. He also has the distinction of being the only person to fly back from orbit, on the second Space Shuttle flight (STS-2). Milt Thompson said that "Joe Engle seemed to have a charmed relationship with the X-15" because for the most part all of Engle’s flights went according to plan. However, not everybody would agree with that assessment of his 15th flight.

On 10 August 1965, Engle took X-15-3 to 271,000 feet-his second flight above 50 miles.

Mission rules stated that the X-15 pilot should fly an alternate low-altitude mission if the yaw damper channel on the MH-96 failed during the first 32 seconds of flight. This was because it was unlikely the airplane could make a successful reentry with a failed yaw damper. On this flight (3-46-70), the yaw channel failed 0.6 seconds after the X-15 dropped off the pylon. Engle reset the damper and did not feel obligated to fly the alternate profile since the damper successfully reset. It was a temporary reprieve, however. The damper failed again 19 seconds later; the reset was successful for at least 10 seconds until it failed again. The damper failed three times in the first 32 seconds of flight. Remarkably, Engle successfully flew the mission, although he missed some of the profile for various reasons, including a preoccupation with resetting the failing yaw damper.264

At the end of 1965, NASA could see that the end of the X-15 program was in sight. Researchers had long since completed the originally envisioned basic flight research, and the aircraft were now primarily experiment carriers, although X-15A-2 was still extending the flight envelope somewhat. However, plans to use X-15A-2 as a hypersonic ramjet test bed began to unravel when, on 6 August 1965, Secretary of Defense Robert S. McNamara disapproved the funding necessary for the effort.266

Under the best-case scenario, the FRC anticipated that the flight program using the basic X-15s would begin winding down at the end of 1967 when X-15-3 began receiving its delta-wing modifications. By the end of 1969, X-15-1 would be retired, leaving only X-15A-2 and the newly redelivered delta-wing X-15-3 in service. X-15A-2 would finish its ramjet tests in mid-1970, transferring all flight activity to the delta wing.-267

Paul Bikle had long believed that any extended operation of the X-15 program beyond its original objectives was unwise and hard to justify in view of the high cost and risk involved. As early as 1961, he had suggested the end of 1964 as a desirable termination date. As time went on, Bikle felt that continued extensions of the program were becoming increasingly hard to justify, and he personally had strong doubts that either the delta wing or the HRE would ever reach flight status on an X-15. In spite of these personal misgivings, Bikle continued to support the program in his public statements.268


January 1966 was much like December 1965 in the high desert-wet. Between 12 November and 1 December 1965 more than 3 inches of rain had fallen, and 2 more inches fell during December. NASA described Rogers, Three Sisters, Silver, and Hidden Hills as "wet," while Mud Lake was only "damp." Over 95% of Cuddeback was under water and there was visible snow at Delamar. A lack of landing sites effectively grounded the X-15 program.268

This gave the program time to do maintenance on the airplanes and incorporate various modifications. For instance, engineers installed the Honeywell IFDS, finally, on X-15-3 along with a new Lear Siegler-developed vertical-scale instrument panel. All of the instrumentation wiring on this airplane was removed and replaced with new four-conductor shielded Teflon wire. It received the modifications necessary to carry the wing-tip experiment pods, and the third skid and stick – kicker needed for higher landing weights were installed.278

The pilots were not greeting the new X-15-3 instrument panel with overwhelming enthusiasm. Paul Bikle opined that "there has been some evidence of reluctance to accept the vertical-scale, fixed-index [tape] instruments." Bikle noted that previously "no objective evaluation of the suitability of the panel for the X-15 mission had been made." To correct this, engineers installed a duplicate of the panel in the fixed-base simulator and conducted runs using "measurable flight control and pilot performance parameters in a comparison of the Lear panel with the traditional panel."278

Of all the performance measures taken, only two showed consistent and significant differences. These were the absolute error in velocity at power reduction and the burnout altitude; in both cases, the statistical results favored the Lear panel. An examination of the altitude and velocity indicators on both panels showed that the differences were the result of high-scale resolution on the Lear instruments, which was almost twice that of the traditional panel instruments. The pilots were still not altogether happy with the new panel, but they no longer mistrusted it.222

The X-15A-2

For most of the flight program, the X-15 used an instrument panel that contained conventional instrumentation. In 1965, Lear Siegler developed a new panel for X-15-3 that used vertical-scale instruments that were supposed to provide enhanced situational awareness for the pilot. Similar instruments were being incorporated into the latest generation of Air Force fighters about the same time. At first, the new instrument panel was not met with overwhelming enthusiasm from the X-15 pilots, but eventually they came to accept the new instruments, although having two very different cockpit configurations complicated the simulators and training regiments. (NASA)

During this down period, X-15A-2 received a new Maurer camera to replace the Hycon unit in the center-of-gravity compartment. This was not as simple as it sounded and took almost eight weeks of work. X-15-1 received a modification that allowed ground personnel to easily remove or replace the wing-tip pods as needed to support various experiments. NASA could now swap the pods between X-15-1 and X-15-3, and was manufacturing a second set of pods.[273]

As January passed with no relief from the wet lakebeds (another half inch of rain fell at Edwards, with more snow on the upper areas of the High Range), NASA performed more modifications on the airplanes. Because the increase in stiffness of the main skids and the addition of the third skid transmitted higher loads through the structure to the nose gear, engineers decided to reinforce the skin on X-15-1 between fuselage stations 91 and 106. The Air Force sent the NB-52B to Tinker AFB for major maintenance, leaving her older sister to support the flight program, assuming the lakebeds ever dried out. The carrier aircraft returned on 8 April and the AFFTC spent the next five weeks modifying it to carry the heavier X-15A-2.[274]

The X-15A-2

The X-15A-2 configuration was tested in a variety of wind tunnels, including one at the NASA Jet Propulsion Laboratory in Pasadena, California. The JPL tests centered around determining the effects of shock-wave impingement on the proposed ramjet and finding an alternate vertical – stabilizer configuration to provide enhanced stability at Mach 8 while carrying a ramjet under the ventral stabilizer. (NASA)

NASA also used the time to complete various analyses, including a complete simulation of reentry profiles at the increased weights currently flown by the airplanes. The ground rules were that reentries would be limited to 1,600 psf using an angle of attack of 20 degrees. To avoid exceeding the structural limitations of the airplanes, NASA decided to restrict X-15-1 to altitudes under 265,000 feet and X-15A-2 to less than 250,000 feet. Mostly because it was equipped with the MH-96, NASA allowed X-15-3 to operate up to 360,000 feet. These restrictions were not really a problem since the program had already reached the maximum altitude it was planning on, although the first two airplanes would bump into these limits on several future flights.-1275

The simulations showed that, as currently configured, X-15A-2 should be able to reach a maximum velocity of 7,500 fps without the ramjet and 7,100 fps with the ramjet, both at 120,000 feet. These velocities assumed a launch weight of 51,650 pounds with the use of external tanks and an XLR99 burn time of 152 seconds. Based on these simulations, flight planners decided to conduct the X-15A-2 envelope-expansion program with the ventral on, primarily because it most closely resembled the planned ramjet configuration. However, the program was short of ventral rudders, and it was uncertain whether economic constraints would allow each flight to use one.-1275

More importantly, Langley and the Jet Propulsion Laboratory conducted wind-tunnel tests to investigate shock-wave systems affecting the proposed ramjet installation on X-15A-2. Researchers worried that shock waves impinging on the ramjet could affect inlet and engine performance, structures, and structural heating. The tests provided data for angles of attack between -5 and +20 degrees at Mach numbers between 2.3 and 4.63. A review of the data

showed that a shock wave emanating from the forward tip of the landing-gear skid would impinge on the ramjet inlet at all Mach numbers, and did not significantly vary with the angle of attack. These tests also showed that there was a complex shock impingement around the ventral stabilizer in general. Apparently, these data went unnoticed.-1272

By the beginning of April, the weather had improved considerably. The Air Force was in the process of repairing and re-marking Rogers, Grapevine, and Mud Lake. Cuddeback was dry but still too soft to re-mark. All of the other lakes were drying rapidly and were ready to use by the end of the month.-1278-