Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

The development of an escape system had been the subject of debate since the beginning of the X-15 program. North American’s decision to use a combination of an ejection seat and a full – pressure suit was a compromise based largely on the ejection seat being lighter than the other alternatives. It was also heavily lobbied for by Scott Crossfield.

The Aero Medical Laboratory had recommended an escape capsule, as prescribed by existing Air Force regulations, as early as 8 February 1955. However, the laboratory admitted that an escape capsule would require a long development period and would probably be unacceptably heavy. The laboratory’s alternative was an ejection seat with limb restraints used together with a full – pressure suit. Meetings held during October and November 1955 resulted in a direction to North American to develop an ejection seat that would incorporate head and limb restraints. The Air Force also told North American to document the rationale for adopting such a system.!1281

Privately, Scott Crossfield had already decided he did not like capsule designs. Part of this came from experience with the Douglas D-558-2 program. According to Crossfield, "We had a capsule nose on the Skyrocket but knew from the wind-tunnel data that if you separated the nose from the fuselage, the g-force would be so great it could kill you. I made up my mind I would never use the Skyrocket capsule. I would ride the ship down and bail out." Later events with a similar system on the X-2 would prove this fear correct.!1291

The North American analysis of potential accidents that could cause the pilot to abandon the X – 15 produced some surprising results. Despite the high-altitude and high-speed nature of the mission profiles, North American determined that 98% of potential accidents were likely to occur at dynamic pressures below 1,500 psi, Mach numbers below 4.0, and altitudes less than 120,000 feet. Using these as criteria, North American investigated four potential escape systems: fuselage – type capsules, cockpit capsules, encapsulated seats, and open ejection seats. The comparison included such factors as cockpit mobility, escape potential, mechanical reliability, post-separation performance, and airframe compatibility. This effort took some 7,000 man-hours to complete.

The results showed that an open ejection seat imposed the fewest performance penalties on the aircraft and took the least time to develop. The estimates from North American showed that a satisfactory escape capsule would add 9,000 pounds to the 31,000-pound airplane. Just as importantly, North American—and Scott Crossfield, who would be making the first flights in the airplane-believed the ejection seat offered a better alternative in the event of an emergency, mainly due to its relative mechanical simplicity.-1130!

Despite the report, the Air Force was not completely convinced. During a meeting at Wright Field on 2-3 May 1956, the laboratory again emphasized the perceived limitations of ejection seats. Primarily due to the efforts of Scott Crossfield, the Air Force finally agreed that "the X-15 was probably its own best capsule." The meeting also resulted in another action for North American, once again, to document its rationale for selecting the stable-seat and full-pressure suit combination.-131!

North American held the first formal cockpit inspection in July 1956 at its facility in Inglewood. This inspection featured a fully equipped cockpit mockup, complete with instruments, control sticks, and an ejection seat. The seat was a custom design that featured a new type of pilot restraint harness and small stabilizers to "weather-vane" it into the wind blast and prevent fatal tumbling or oscillation. A solid rocket motor provided about 3,000 lbf to ensure that the seat would clear the X-15. Despite Air Force policy to the contrary, nobody raised any objections about the seat during the inspection. By default, it became part of the official design.-132!

By November 1956, North American had tested a 0.10-scale isolated pilot-seat model of its design in the Naval Supersonic Laboratory wind tunnel at the Massachusetts Institute of Technology (MIT). Although the seat seemed to stabilize randomly in different orientations, the results were generally encouraging. In itself, this did not represent a serious problem, although all participants wanted to understand the dynamics involved. North American conducted additional tests in the Southern California Co-Operative Wind Tunnel in Pasadena to develop the final stabilization system configuration and determine the influence of the forward fuselage without the cockpit canopy.-133!

The debate over the X-15 ejection seat intensified on 27 September 1956 when Captain Milburn G. Apt was killed in the X-2. However, the accident also weakened the case for an escape capsule. The X-2 used a semi-encapsulated system whereby the entire nose of the aircraft, including the cockpit, was blown free of the main fuselage in an emergency. Unfortunately, Bell engineers had expected the pilot to be able to unbuckle his seat straps and manually bail out of the capsule after it separated, something Apt was unable to do. It demonstrated that an encapsulated system was not necessarily the best solution, but then neither was an ejection seat. Almost by definition, piloting X-planes was—and would remain—a dangerous occupation.134

During early 1958, researchers began testing the X-15 ejection seat on the rocket sleds at Edwards, with the preliminary runs concluding on 22 April. The series got off to a good start, with the first test seat ejected at 230 knots and the parachute successfully opening at 120 feet, lowering the anthropomorphic dummy gently to the ground. The dummy was equipped with telemetry that relayed data from rate gyros, accelerometers, and pressure transducers. The second test, this one at 620 knots and a dynamic pressure of 1,130 psf, also went well. The third test, under similar conditions, was again satisfactory. However, during the fourth run the shock­wave generator catapult exploded at Mach 1.26 and 2,192 psf. The accident damaged the seat, suit, and anthropomorphic dummy beyond repair. Engineers fired another seat during a static test on 24 April, but the post-ejection operation failed because of a mechanical problem in the initiation hardware. During the second static test on 14 May 1958, the parachute and parachute lines became tangled with the seat. In all, the test series provided mixed results. North American made several minor modifications in preparation for a second series of tests scheduled for June.-1135!

The high cost of the rocket-sled runs, coupled with the damaged seat hardware, was quickly exceeding the budget for the escape-system tests. Because of this, the X-15 Project Office decided to conduct only two tests, at 125 psf and 1,500 psf. Despite the earlier difficulties, Air Force and North American engineers believed these two tests could adequately demonstrate seat reliability.-1136!

The Air Force conducted the test at 125 psf on 4 June 1958, and the results appeared to be satisfactory. Three successful tests took place during June, but the fourth test, on 3 July, revealed serious stability problems. North American discontinued further tests until it could determine a cause for the failures. A detailed analysis revealed that the seat would need several major modifications.-137!

The Air Force conducted the first test of the revised North American seat on 21 November 1958, but several of the sled rockets failed to ignite and reduced the desired 1,500-psf pressure to about 800 psf. Two tests during December also suffered from the failure of sled rockets. The only test conducted during January failed when the right-hand boom and fin failed to deploy. The leg restraints also failed during the test, but North American believed an instability caused by the boom malfunction caused this. The parachute failed to open until just before the test dummy hit the ground, causing significant damage to the dummy.138!

The ejection seat for the X-15 was a remarkable engineering achievement, and was the most sophisticated ejection seat yet developed at the time of the first X-15 flight. Still, it was much simpler than an encapsulated ejection system would have been. (U. S. Air Force)

The schedule was getting tight since the X-15 was nearly ready to begin captive-carry flights. On 12 January, the Aircraft Laboratory verbally approved the seat for the initial captive and glide flights between Mach 0.377 and Mach 0.720 at dynamic pressures between 195 and 715 psf. The X-15 Project Office considered this satisfactory given the inability of the NB-52 to go much faster.^

As developed by North American, the ejection seat contained provisions to restrain the pilot’s arms and legs to keep them from flailing in the airstream after leaving the aircraft, and also booms and canards to stabilize the seat during separation. After the seat left the aircraft, the pilot unbuckled and jumped from the seat, coming down on his own parachute. (North American Aviation)

Because of the unsuccessful January test, North American carefully rechecked and strengthened the booms and pressure-tested the seat’s gas system. The Air Force conducted the final sled-test on 3 March 1959 at Mach 1.15 and 1,600 psf—conditions somewhat in excess of requirements. Despite the failure of the leg manacles, the test was the most successful to date. North American proposed additional tests and a parachute program in April 1959, but the X-15 Project Office was happy with the results of the tests already run and declined. The X-15 finally had an ejection seat.^1401

The pilot used a backpack-type parachute after he separated from the seat. However, because of the design of the pressure suit, seat, and cockpit, neither the Air Force nor North American considered the standard quarter-deployment bag and 28-foot-diameter C-9 parachute acceptable. Instead, North American produced a special 24-foot-diameter chute and "skirt bag" specifically for the X-15. The company extensively tested this combination on a whirltower to verify the design of the skirt bag, the optimum pilot-parachute bridle length, and the effect of having the seat headrest permanently attached to the pilot chute. The tests in early 1958 included opening speeds up to 300 knots, and subsequent free-fall tests with an anthropomorphic dummy released from a Fairchild C-119 Flying Boxcar over the National Parachute Range in El Centro, California. During the initial tests, the C-119 released the dummy in a head-down attitude at 125

knots and 1,200 feet. These tests were unsuccessful because the pilot chute deployed into a low – pressure zone in the wake of the dummy and was not capable of pulling the main chute from the pack. North American extended the bridle length to 70 inches, allowing the pilot chute to escape the low-pressure area, and subsequent tests were successful.-141

Initially North American used the 24-foot diameter chute because it was the largest they could easily accommodate in the backpack and the engineers thought it would open more quickly, allowing safe ejection at lower altitudes. However, several flight surgeons had concerns that it would allow too high a descent rate for the pilot, and urged the certification of a larger parachute for use on the X-15. During October 1960, North American tested a repackaged 28-foot – diameter parachute at the National Parachute Range. These tests were successful and indicated no significant difference in opening time between the smaller and larger chutes. It became policy that each pilot could select whichever size parachute he wished to use. Most continued to use the 24- foot chute because the reduced thickness of the backpack made it more comfortable to sit on in the cockpit.142

In June 1965, NASA authorized North American to purchase five new 28-foot parachutes to replace the 24-foot units that had reached their 7-year service limit. The new chutes had a disconnect device that allowed the pilots to release one-half of the shroud lines during descent. They were less comfortable because they were thicker than the original parachutes, but as personnel at Edwards discarded the smaller units, they became standard.142

Despite the confidence Scott Crossfield and the North American engineers had in the ejection seat, apparently it was not universal. Pete Knight once commented, "They tell me that the seat is good for Mach 4 and 120,000 feet. I take it with a grain of salt, but I think the safest place to be is inside the airplane until we get to a more reasonable environment…. If you had to, as a last resort certainty you would take the chance, but I think most of the pilots have felt that we…would stay with [the airplane] as long as possible." At least everybody agreed that the cockpit was a safe place. Crossfield demonstrated that when the X-15-3 exploded on the ground while he was testing the XLR99 engine.-144-

The X-15 ejection seat, like all other seats of the era, was tested on the rocket sled track at Edwards AFB, California. The sled test results were mixed, with many failures of both the sled and the seat for various reasons, but ultimately the Air Force, NASA, and North American were satisfied that the seat would work as advertised. (U. S. Air Force)

Of the 11 XLR99 flight engines that were produced during 1958-1960 to support the flight program, one (s/n 105) was destroyed in the 1959 ground accident and another (111) was destroyed in the 1967 crash of the X-15-3. During September 1975, researchers at Edwards conducted an inventory of existing engines and engine spares in anticipation that the engine might possibly be used in a future flight program. Seven flight-rated and one ground-test engine remained at Edwards, but the Air Force had already scrapped the others or given them to museums. Although the engineers thought most piece-parts were available from various sources, three high-cost spares (thrust chamber/injector assemblies, turbopump cases, and igniters) were in short supply.-1114

because of cracks in the tubing or injector spud. Six pump cases ($12,000 each) had been replaced during the X-15 flight program, mainly due to corrosion, and there were eight cases available for future use. Only 10 igniters ($4,000) were available, but the flight program had used 17, mainly due to detonation at shutdown-a condition that Reaction Motors had largely corrected.-1115!

In addition to the possibility of using existing engines in another program, several proposals had been made for augmented or improved versions of the XLR99 to support various projects. The first serious effort was to support the hypersonic research engine (HRE) experiment on the X – 15A-2. On 30 October 1963, Douglas E. Wall, the project manager for airborne hypersonic research at the Aeronautical System Branch at the FRC, wrote to James E. Love, the NASA X-15 program manager, advising him that the X-15A-2 would likely fall far short of the performance requirements for the HRE program.-116!

The region of interest for supersonic combustion testing was from 7,000 to 8,000 fps at dynamic pressures between 1,000 and 2,000 psf. Although Wall cautioned that he could not ascertain the extent of the performance shortfall until after preliminary flight tests, at the time it looked like the X-15A-2 would fall approximately 1,000 fps short. At a meeting held at Wright-Patterson on 25 September 1963, researchers recommended that the X-15 Project Office fund an upgrade to the XLR99, and the AFFTC and FRC representatives proposed three different modifications. The first was the use of an extended nozzle to increase performance at the mid-altitudes (^100,000 feet) for the expected ramjet experiments. The other modifications included a modified injector assembly and the use of a hydrazine fuel additive. Researchers expected that these modifications would take between 12 and 14 months to develop and implement. The X-15 Project Office agreed to look into the matter; however, there appears to be no record indicating that any action was taken.117!

Nevertheless, Reaction Motors did conduct several studies during 1964-1965 on possible improvements to the XLR99. At least one of these investigated the use of axisymmetric and two­dimensional nozzles, and another studied possible improvements to the thrust chamber. Reaction Motors engineers also kept up with the published reports from other rocket-engine manufacturers to see if any of their developments might be applicable to the XLR99.118!

The FRC already had some experience with increasing rocket-engine performance by using nozzle extensions on the Douglas D-558-2. These extensions were small, radiation-cooled members that permitted the rocket exhaust gases to attain higher exit velocities by expanding within the nozzle to ambient pressures. Because of their small size, the extensions had no serious aerodynamic effect or structural design implications. It appeared to researchers at the FRC that a lightweight, radiation-cooled nozzle extension could provide a desirable performance increase for the X-15A-2. The researchers admitted, however, that it would be more difficult to design such a nozzle for the XLR99 than for the XLR11 because of the former’s larger size and more severe operating environment. The size issue loomed largest because there was a possibility of adverse aerodynamic interference with the afterbody flow.119

In order to evaluate this potential, researchers ran a series of wind-tunnel tests that used several different nozzle extension designs. The tests were quite extensive and included various speed brake and horizontal stabilizer positions, ventral stabilizer shapes, and ramjet installations. Tests were conducted over free-stream Mach numbers from 2.3 to 8.0 using the Unitary Plan Tunnel at Langley (Mach numbers up to 4.63) and the von Karman Gas Dynamics Facility Tunnel B at the Air Force Arnold Engineering Development Center (AEDC) at Mach numbers 6.04 and 8.01. To withstand the high Mach numbers, researchers modified the 1/15-scale model to withstand temperatures of 900°F for up to 30 minutes.-1120!

The tests included nozzle extensions of various exit diameters and lengths representing expansion ratios of 22.1:1 to 33.6:1, along with various aerodynamic shrouds to reduce interference effects. In all, researchers investigated nine candidate nozzles, and the tests indicated that none of the nozzle extensions had any appreciable affect on overall drag or static margin, although the 22.2:1 nozzle was most suitable. The use of this nozzle increased the burnout velocity by 400 fps with no other changes to the airplane or engine.-121!

During January 1966, researchers at Langley ran more tests on the 1/15-scale model of the X – 15A-2 in the 4 by 4-foot unitary tunnel. These obtained data on various XLR99 nozzle extensions, including ones with area ratios of 11.2:1, 28.8:1, and 33.6:1 at Mach numbers up to 4.63. The X015 models used in the wind tunnels included various other modifications, including a redesigned aft fuselage boat-tail meant to smooth over the larger engine nozzle. All of the nozzle extensions actually improved the base drag coefficients over the basic configuration, and all exhibited less drag than the boat-tail configurations. Despite the seemingly minor cost of the nozzle modifications, neither the Air Force nor NASA took any action to produce any hardware or perform actual engine or flight tests.122!

In early 1967, Reaction Motors began another investigation of an improved nozzle for the XLR99 designed to increase thrust at high altitudes. The Air Force issued a work order for the study as an extension of the XLR99 engineering support contract, but did not record the exact reason for the study. The new nozzle had an expansion ratio of 22.5:1 instead of the 9.8:1 used on the existing XLR99s, resulting in an increase in vacuum thrust and vacuum-specific impulse of approximately 7% at a chamber pressure of 600 psi. Two percent of that improvement was the result of using a contoured nozzle instead of the 20-degree conical nozzle used on the original 9.8:1 extension.123

During the investigations of the new nozzle, all other parts of the engine remained unchanged, so it would have been easily possible to retrofit existing engines. The new engine produced a specific impulse of 298-lbf-sec/lbm and a thrust of 63,378-lbf in a vacuum. The new engine could be operated at sea level without flow separation, although its performance was somewhat below the standard XLR99 at low altitudes. The recommended nozzle design was an overturned bell nozzle composed of tangent circular arcs with a length and end diameter roughly equivalent to the normal 20-degree conical nozzle. The nozzle was designed with an exit angle of approximately 5 degrees rather than zero. This is because the last few degrees of wall-turning only added weight, since friction losses canceled out the theoretical thrust gain. Again, no further action resulted from the study.123

Perhaps the most ambitious upgrade was the one proposed to support the delta wing X-15 concept. One of the desired missions for the delta-wing airplane was a sustained 1-g Mach 7 cruise capability, and Reaction Motors sought a way to allow the XLR99 to act as a "sustainer" engine producing 8,000-10,000 lbf for several minutes at a time. The company investigated two different possibilities to provide the sustainer capability. The first used the existing XLR99 chamber to provide the same 57,000-lbf thrust and a separate, remotely located chamber to provide additional thrust during main engine operation and sustainer thrust during cruise. This was conceptually similar to the system used on the Atlas ICBM and the ill-fated Curtiss-Wright XLR25 in the Bell X-2. The second idea was to modify the existing chamber to both provide increased thrust and allow the sustainer function, and to use the previously investigated 22.5:1 expansion ratio nozzle. This second concept was similar to what the 1963 meeting at Wright – Patterson had recommended to fix the X-15A-2 performance shortfall. Reaction Motors estimated that it would take two years to develop and test the modified engine.-1125!

Surprisingly, Reaction Motors preferred using a separate sustainer chamber since it presented less risk and required less development time. Throttling the main chamber produced between 26,000 and 62,000 lbf, and the remote chamber produced between 8,000 and 21,000 lbf. This would have provided an engine capable of infinite throttling between 8,000 and 83,000 lbf. The Air Force disagreed with the risk assessment and considered the problem of integrating a second thrust chamber and nozzle into the X-15 too great, so the delta-wing program selected the single-chamber design despite the longer development time required.-126!

The major constraint imposed in considering the maximum thrust available from modifications to the XLR99 was the number of changes that had to be made to the turbopump. Unlike some other components of the XLR99, the turbopumps had been relatively trouble-free during development and operation. However, because of this lack of problems, nobody was thoroughly familiar with the pumps and their operation. To address this, Reaction Motors brought the original turbopump engineer, Haakon Pedersen, out of retirement. Pedersen proposed relatively modest changes to the turbopump that could provide a 40% increase in pumping capacity. The solution was deceptively simple: speed up the pump. This increased speed was not expected to "generate difficulties with the seals, bearings, or critical speed" or to "affect cavitation adversely." Pedersen did caution that he based these predictions on his own intuition since Reaction Motors had never tested the turbopumps at greater than 100% power. The increased speed, however, required a new turbine because the existing one could not accommodate the 72.5% increase in hydrogen – peroxide flow.122!

There is no record that Reaction Motors ever accomplished any testing on the modified XLR99 or its components. Given that NASA terminated the delta-wing X-15 project early in its development, it is likely that Reaction Motors never modified any hardware.

During the course of the X-15 program, various drawings and artist concepts were released that showed the research airplane-particularly the proposed delta-wing version-carried by a North American XB-70 bomber. The use of this Mach 3+ capable aircraft would have greatly extended the performance envelope of the X-15. However, given the theoretical uncertainties of launching an object from the back of a larger aircraft traveling at Mach 3, it is unlikely that the Air Force or NASA ever seriously considered this concept. After the fatal crash on 30 July 1966 of a Lockheed M-21 Blackbird while launching a D-21 drone from a similar configuration, it became even more unlikely. Nevertheless, sometime during 1966 North American conducted a study (logically called "XB-70/X-15"); unfortunately, however, no copy could be found in any archive, so its contents and conclusions are unknown.-1228

The use of the Mach 3+ capable XB-70A as a carrier aircraft would have greatly extended the performance envelope of the X-15. However, given the theoretical uncertainties of launching an object from the back of a larger aircraft traveling at Mach 3, coupled with the fact that only two Valkyries were manufactured, it is unlikely that the Air Force or NASA ever seriously considered this concept. (North American Aviation)

The WADC evaluation of the NACA proposal arrived at ARDC Headquarters on 13 August. Colonel Victor R. Haugen, director of the WADC laboratories, reported that his organization believed the proposal was technically feasible. The only negative comment referred to the absence of a suitable engine. The WADC estimated that the development effort would cost $12,200,000 and take three or four years. The cost estimate included $300,000 for studies, $1,500,000 for design,

$9,500,000 for the development and manufacture of two airplanes, $650,000 for engines and other government-furnished equipment, and $250,000 for modifications to a carrier aircraft. Somewhat prophetically, one WADC official commented informally: "Remember the X-3, the X-5, [and] the X-2 overran 200%. This project won’t get started for twelve million dollars."-81

A four-and-a-half-page paper titled "NACA Views Concerning a New Research Airplane," released in late August 1954, gave a brief background of the problem and attached the Langley study as a possible solution. The paper listed two major problems: "(1) preventing the destruction of the aircraft structure by the direct or indirect effects of aerodynamic heating; and (2) achievement of stability and control at very high altitudes, at very high speeds, and during atmospheric reentry from ballistic flight paths." The paper concluded by stating that the construction of a new research airplane appeared to be feasible and needed to be undertaken at the earliest possible opportunity.-^

A meeting between the Air Force, NACA, Navy, and the Office of the Assistant Secretary of Defense for Research and Development took place on 31 August 1954. There was general agreement that research was needed on aerodynamic heating, "zero-g," and stability and control issues at Mach numbers between 2 and 7 and altitudes up to 400,000 feet. There was also agreement that a single joint project was appropriate. The group believed, however, that the selection of a particular design (referring to the Langley proposal) should not take place until mutually satisfactory requirements were approved at a meeting scheduled for October.-101

Also on 31 August, and continuing on 1 September, a meeting of the NACA Subcommittee on High-Speed Aerodynamics was held at Wallops Island. Dr. Allen E. Puckett from the Hughes Aircraft Company was the chair. John Stack from Langley gave an overview of the proposed research airplane, including a short history of events. He reiterated that the main research objectives of the new airplane were investigations into stability and control at high supersonic speeds, structural heating effects, and aeromedical aspects such as human reactions to weightlessness. He also emphasized that the performance of the new airplane must represent a substantial increment over existing research airplanes and the tactical aircraft then under development. In response to a question about whether an automatically controlled vehicle was appropriate, Stack reiterated that one of the objectives of the proposed program was to study the problems associated with humans at high speeds and altitudes. Additionally, the design of an automatically controlled vehicle would be difficult, delay the procurement, and reduce the value of the airplane as a research tool.-11 design of the airplane" and that the establishment of a design competition was the most desirable course of action. The subcommittee forwarded the recommendation to the Committee on Aerodynamics for further consideration.-1121

Major General Floyd B. Wood, the ARDC deputy commander for technical operations, forwarded an endorsement of the NACA proposal to Air Force Headquarters on 13 September 1954, recommending that the Air Force "initiate a project to design, construct, and operate a new research aircraft similar to that suggested by NACA without delay." Wood reiterated that the resulting vehicle should be a pure research airplane, not a prototype of any potential weapon system or operational vehicle. The ARDC concluded that the design and fabrication of the airplane would take about 3.5 years. In a change from how previous projects were structured, Wood suggested that the Air Force should assume "sole executive responsibility," but the research airplanes should be transferred to the NACA after a short Air Force airworthiness demonstration program.-121

During late September, John R. Clark from Chance-Vought met with Ira H. Abbot at NACA Headquarters and expressed interest in the new project. He indicated that he personally would like to see his company build the aircraft. It was ironic since Chance-Vought would elect not to submit a proposal when the time came. Many other airframe manufacturer representatives would express similar thoughts, usually with the same results. It was hard to see how anybody could make money building only two airplanes.141

The deputy director of research and development at Air Force Headquarters, Brigadier General Benjamin S. Kelsey, confirmed on 4 October 1954 that the new research airplane would be a joint USAF-Navy-NACA project with a 1-B priority in the national procurement scheme and $300,000 in FY55 funding to get started.15

At the same time, the NACA Committee on Aerodynamics met in regular session on 4 October 1954 at Ames, with Preston R. Bassett from the Sperry Gyroscope Company as chairman. The recommendation forwarded from the 31 August meeting of the Subcommittee on High-Speed Aerodynamics was the major agenda item. The following day the committee met in executive session at the HSFS to come to some final decision about the desirability of a manned hypersonic research airplane. During the meeting, various committee members, including De Elroy Beeler,

Walt Williams, and research pilot A. Scott Crossfield, reviewed historic and technical data. Williams’s support was crucial. Crossfield would later describe Williams as "the man of the 20th Century who made more U. S. advanced aeronautical and space programs succeed than all the others together. He was a very strong influence in getting the X-15 program launched in the right direction." Williams would later do the same for Project Mercury.161

The session at the HSFS stirred more emotion than the earlier meeting in Washington. First, Beeler discussed some of the more general results obtained previously with various research airplanes. Then Milton B. Ames, Jr., the committee secretary, distributed copies of the NACA "Views" document. Langley’s associate director, Floyd Thompson, reminded the committee of the major conclusion expressed by the Brown-O’Sullivan-Zimmerman study group in June 1953: that it was impossible to study certain salient aspects of hypersonic flight at altitudes between 12 and 50 miles in wind tunnels due to technical limitations of the facilities. Examples included "the distortion of the aircraft structure by the direct or indirect effects of aerodynamic heating" and "stability and control at very high altitudes at very high speeds, and during atmospheric reentry from ballistic flight paths." The study admitted that the rocket-model program at Wallops Island could investigate aircraft design and operational problems to about Mach 10, but this program of subscale models was not an "adequate substitute" for full-scale flights. Having concluded that the

Brown group was right, and that the only immediate way known to solve these problems was to use a manned aircraft, Thompson said that various NACA laboratories had then examined the feasibility of designing a hypersonic research airplane. Trying to prevent an internal fight, Thompson explained that the results from Langley contained in the document Milton Ames had just distributed were "generally similar" to those obtained in the other NACA studies (which they were not), but were more detailed than the other laboratories’ results (which they were).[17]

Williams and Crossfield followed with an outline of the performance required for a new research airplane and a discussion of the more important operational aspects of the vehicle. At that point, John Becker and Norris Dow took over with a detailed presentation of their six-month study.

Lively debate followed, with most members of the committee, including Clark Millikan and Robert Woods, strongly supporting the idea of the hypersonic research airplane.

Surprisingly, Clarence L. "Kelly" Johnson, the Lockheed representative, opposed any extension of the manned research airplane program. Johnson argued that experience with research aircraft had been "generally unsatisfactory" since the aerodynamic designs were inferior to tactical aircraft by the time research flights began. He felt that a number of research airplanes had developed "startling performances" only by using rocket engines and flying essentially "in a vacuum" (as related to operational requirements). Johnson pointed out that "when there is no drag [at high altitude], the rocket engine can propel even mediocre aerodynamic forms to high Mach numbers." These flights had mainly proved "the bravery of the test pilots," Johnson charged. The test flights generated data on stability and control at high Mach numbers, Johnson admitted, but aircraft manufacturers could not use much of this information because it was "not typical of airplanes actually designed for supersonic flight speeds." He recommended that they use an unmanned vehicle to gather the required data instead of building a new manned airplane. If aeromedical problems became "predominant," Johnson said, a manned research airplane could then be designed and built, and it should have a secondary role as a strategic reconnaissance vehicle.[18]

Clarence L "Kelly"Johnson, the legendary founder of the Lockheed Skunk Works, was the only representative on the NACA Committee on Aerodynamics to vote against proceeding with the development of the X-15. Previous X-plane experience had left Johnson jaded since the performance of the research airplanes was not significantly advanced from operational prototypes. As it turned out, the X-15 would be the exception, since no operational vehicle, except the Space Shuttle, has yet approached the velocity and altitude marks reached by the X-15. (Lockheed Martin)

into flight research in the shortest time possible." In comparing manned research airplane operations with unmanned, automatically controlled vehicles, Crowley noted that the X-1 and other research airplanes had made hundreds of successful flights despite numerous malfunctions.-1191 In spite of the difficulties—which, Crowley readily admitted, had occasionally caused the aircraft to go out of control—research pilots had successfully landed the aircraft an overwhelming percentage of the time. In each case the human pilot permitted further flights to explore the conditions experienced, and in Crowley’s opinion, automated flight did not allow the same capabilities.-1291

After some further discussion, and despite Johnson’s objections, the committee passed a resolution recommending the construction of a hypersonic research aircraft:1211

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соиштёе oil ашфшамюэ, 5 ocrcam 1954

VKCREAS, The весе в з It/ of supremacy

In the air continues to place great urgency on solving the problems of flight with man-carrying aircraft at greater speeds and extreme altitudes, rth-i

МВДЩЦЗ, Ргордіа ion systems are now capable of propelling eufih aircraft to speeds and altlt^ea that Impose entirely new and unexplored aircraft design problems, and

WHEftEAS, It now appears feasible to construct a research airplane capable of initial eirploraticn of these problems,

HE ГГ HHtiW RESOLVED, That iJie Ccamlttee on Aerodynamics sudarses the proposal of the tetuadlata Initiation of a project to design and construct a research airplane capable of aohleving speeds of the order of №oh Number 7 and altitudes of several hundred thousand feet for the exploration of the problems of stability and control of maimed aircraft and aerodynamic heating In the severe form associated with flight at extreme speeds and altitudes.

The "requirements" of the resolution conformed to the conclusions from Langley, but were sufficiently general to encourage fresh approaches. Appended to the specification under the heading of "Suggested Means of Meeting the General Requirements" was a section outlining the key results of the Becker study.1221

Kelly Johnson was the only member to vote nay. Sixteen days after the meeting, Johnson sent a "Minority Opinion of Extremely High Altitude Research Airplane" to Milton Ames with a request that it be appended to the majority report, which it was.1231

On 6 October 1954, Air Force Headquarters issued Technical Program Requirement 1-1 to initiate a new manned research airplane program "generally in accordance with the NACA Secret report, subject: ‘NACA Views Concerning a New Research Aircraft’ dated August 1954." The entire project was classified Confidential. The ARDC followed this on 26 October with Technical Requirement 54 (which, surprisingly, was unclassified).1241

In the meantime, Hartley Soule and Clotaire Wood held two meetings in Washington on 13 October. The first was with Abraham Hyatt at the Navy Bureau of Aeronautics (BuAer) to obtain the Navy’s recommendations regarding the specifications. The only significant request was that provisions should exist to fly an "observer" in place of the normal research instrumentation package. This was the first (and nearly the only) official request from the Navy regarding the new airplane, excepting the engine. In the second meeting, Soule discussed the specifications with Colonel R. M. Wray and Colonel Walter P. Maiersperger at the Pentagon, and neither had any significant comments or suggestions.

With an endorsement in hand, on 18 October Hugh Dryden conferred with Air Force (colonels Wray and Maiersperger) and Navy (Admiral Robert S. Hatcher from BuAer and Captain W. C.

Fortune from the ONR) representatives on how best to move toward procurement. The parties agreed that detailed technical specifications for the proposed aircraft, with a section outlining the Becker study, should be presented to the Department of Defense Air Technical Advisory Panel by the end of the year. The Navy reiterated its desire that the airplane carry two crew members, since the observer could concentrate on the physiological aspects of the flights and relieve the pilot of that burden. The NACA representatives were not convinced that the weight and cost of an observer could be justified, and proposed that the competing contractors decide what was best.

All agreed this was appropriate. Again, the Air Force requested little in the way of changes.-1251

Hartley Soule met with representatives of the various WADC laboratories on 22 October to discuss the tentative specifications for the airplane. Perhaps the major decision was to have BuAer and the Power Plant Laboratory jointly prepare a separate specification for the engine. The complete specification (airplane and engine) was to be ready by 17 November. In effect, this broke the procurement into two separate but related competitions: one for the airframe and one for the engine.

During this meeting, John B. Trenholm from the WADC Fighter Aircraft Division suggested building at least three airplanes, proposing for the first time more than the two aircraft contained in the WADC cost estimate. There was also a discussion concerning the construction of a dedicated structural test article. It seemed like a good idea, but nobody could figure out how to test it under meaningful temperature conditions, so the group deferred the matter.

Also on 22 October, Brigadier General Benjamin Kelsey and Dr. Albert Lombard from Air Force Headquarters, plus admirals Lloyd Harrison and Robert Hatcher from BuAer, visited Hugh Dryden and Gus Crowley at NACA Headquarters to discuss a proposed Memorandum of Understanding (MoU) for conducting the new research airplane program. Only minor changes to a draft prepared by Dryden were suggested.-261 The military representatives told Dryden that a method of funding the project had not been determined, but the Air Force and Navy would arrive at a mutually acceptable agreement for financing the design and development phases. During the 1940s and 1950s it was normal for the military services to fund the development and construction of aircraft (such as the X-1 and D-558, among others) for the NACA to use in its flight research programs. The aircraft resulting from this MoU would be the fastest, highest-flying, and by far the most expensive of these joint projects.

The MoU provided that technical direction of the research project would be the responsibility of the NACA, acting "with the advice and assistance of a Research Airplane Committee" composed of one representative each from the Air Force, Navy, and the NACA. The New Developments Office of the Fighter Aircraft Division at Wright Field would manage the development phase of the project. The NACA would conduct the flight research, and the Navy was essentially left paying part of the bills with little active roll in the project, although it would later supply biomedical expertise and a

single pilot. The NACA and the Research Airplane Committee would disseminate the research results to the military services and aircraft industry as appropriate based on various security considerations. The concluding statement on the MoU was, "Accomplishment of this project is a matter of national urgency."[27]

The final MoU was originated by Trevor Gardner, Air Force Special Assistant for Research and Development, in early November 1954 and forwarded for the signatures of James H. Smith, Jr., Assistant Secretary of the Navy for Air, and Hugh L. Dryden, director of the NACA, respectively. Dryden signed the MoU on 23 December 1954 and returned executed copies to the Air Force and Navy.[28]

John Becker, Norris Dow, and Hartley Soule made a formal presentation to the Department of Defense Air Technical Advisory Panel on 14 December 1954. The panel approved the program, with the anticipated $12.2 million cost coming from Department of Defense contingency funds as well as Air Force and Navy research and development funds.-129

After the Christmas holidays, on 30 December, the Air Force sent invitation-to-bid letters to Bell, Boeing, Chance-Vought, Convair, Douglas, Grumman, Lockheed, Martin, McDonnell, North American, Northrop, and Republic. Interested companies were asked to attend the bidders’ conference on 18 January 1955 after notifying the procurement officer no later than 10 January. An abstract of the NACA Langley study was attached with a notice that it was "representative of possible solutions" but not a requirement to be satisfied.-129

Also accompanying the invitation-to-bid letters was a simple chart that showed the expected flight trajectory for the new research airplane. It was expected that each flight would provide about 130 seconds of good research data after engine burnout. This performance was almost exactly duplicated by the X-15 over the course of the flight program. (NASA)

This was undoubtedly the largest invitation-to-bid list yet for an X-plane, but many contractors were uncertain about its prospects. Since it was not a production contract, the potential profits were limited. Given the significant technical challenges, the possibility of failure was high. Of course, the state-of-the-art experience and public-relations benefits were potentially invaluable. It was a difficult choice even before Wall Street and stock prices became paramount. Ultimately, Grumman, Lockheed, and Martin expressed little interest and did not attend the bidders’ conference, leaving nine possible competitors. At the bidders’ conference, representatives from the remaining contractors met with Air Force and NACA personnel to discuss the competition and

the basic design requirements. The list of participants read like a Who’s Who of the aviation world. Robert Woods and Walter Dornberger from Bell attended. Boeing sent George Martin, the designer of the B-47. Ed Heinemann from Douglas was there. Northrop sent William Ballhaus.-131

During the bidders’ conference the Air Force announced that each company could submit one prime and one alternate proposal that might offer an unconventional but potentially superior solution. The Air Force also informed the prospective contractors that an engineering study only would be required for a modified aircraft in which an observer replaced the research instrumentation, per the stated Navy preference. A significant requirement was that the aircraft had to be capable of attaining a velocity of 6,600 fps and altitudes of 250,000 feet. Other clarifications included that the design would need to allocate 800 pounds, 40 cubic feet, and 2.25 kilowatts of power for research instrumentation. A requirement that would come back to haunt the procurement was that flight tests had to begin within 30 months of contract award.

Another major piece of government-furnished equipment was the all-attitude inertial system, called a "stable platform" at the time. Early on, researchers realized the performance of the research airplane required a new method to determine altitude, speed, and attitude information. The original Langley study, as well as each of the contractor proposals, had suggested the use of a stable platform. Unfortunately, such as system was not readily available.

A meeting held at Wright Field on 14-15 November 1955 implied that the WADC would furnish the stable platform. Arthur Vogeley, the NACA representative, assumed that the Air Force had already developed a suitable device since his report stated that a newly developed Bendix platform weighed only 28 pounds and occupied less than a cubic foot of volume. Others within the NACA and North American were not as certain. During a meeting with North American personnel, Walt Williams specifically asked who was responsible for the stable platform, and no answer was immediately forthcoming.-1145!

Researchers apparently did not discuss the requirements for a stable platform until 24 May 1956 during a meeting at Langley. In attendance were representatives from Eclipse-Pioneer (a division of Bendix), the NACA, North American, and the WADC. This group discussed the platform mentioned at the November 1955 meeting, and Eclipse-Pioneer acknowledged that it was only a conceptual design and not a forthcoming product. Nevertheless, the meeting attendees thought

that development of a suitable platform would take only 24 months. Since the platform provided research data in addition to flight data, the NACA agreed to charge 40 pounds of the estimated 65-pound weight against research instrumentation. There was no mention as to why the original 28-pound estimate had grown to 65 pounds.-146

Despite its early participation, Eclipse-Pioneer did not exhibit any further interest, so the Flight Control Laboratory asked the Sperry Gyroscope Company if it was interested. By August 1956, Sperry had prepared a preliminary proposal, and on 4 October the X-15 Project Office held a technical briefing for Sperry at Wright Field.147

On 26 December 1956, the Flight Control Laboratory began the process to procure eight inertial flight data systems (six "Type A" units for the X-15 and two "Type B" units for ground research). The laboratory recommended awarding the $1,030,000 contract to the Sperry Gyroscope Company.-1148!

For unexplained reasons, the Air Materiel Command did not take immediate action and did not release a formal request for proposal to Sperry until 6 February 1957. Two weeks later Sperry replied, and the Flight Control Laboratory approved the technical aspects of the proposal on 28 March. In the meantime, however, a controversy had developed over contracting details. The negotiations reached a deadlock on 11 April 1957 and the Air Materiel Command informed the X – 15 Project Office that it intended to find another contractor. The Flight Control Laboratory and X – 15 Project Office argued that Sperry was the only company that stood a chance of meeting the X – 15 flight schedule, but procurements were the domain of the Air Materiel Command and the warnings fell on deaf ears.-1149!

It was evident that the issue was rapidly exhausting the patience of all concerned. On 22 April 1957, the director of development at the WADC, Brigadier General Victor R. Haugen, informed the Air Materiel Command that Sperry was the only company capable of developing the stable platform within the schedule constraints of the X-15 program. Having a general officer intervene was apparently the answer, and a cost-plus-fixed-fee contract signed on 5 June 1957 provided $1,213,518.06 with an $85,000 fee.156

Because of the contracting delays, the expected December 1958 delivery of the initial Sperry unit would not support the first flight of the X-15. This was not a significant problem since the initial X-15 flights would be low and slow enough to use a standard NACA flight test boom to provide the data ultimately supplied by the stable platform and ball nose. In fact, the NACA would likely have used the flight test boom even if the other instruments had been available, since it provided a known, calibrated source for acquiring initial air data. Most experimental aircraft use similar booms during early testing.151

More disturbing, however, was that it quickly became apparent that the weight of the stable platform had been seriously underestimated. In May 1958, Sperry undertook a weight-reduction program that, unfortunately, was particularly unsuccessful. By August, Sperry was reporting that the weight was approximately twice the original specification.157

It was just the beginning of serious trouble. By June 1958, the estimated cost was up to $2,741,375 with a $105,000 fee. Less than a year later the cost reached $3,234,188.87 with an $119,888 fee, mostly due to efforts to reduce the weight of the stable platform.157

specification, the system weighed 185.25 pounds. An alternate shock mount that did not meet the requirements but was probably acceptable brought the weight down to 165.25 pounds. Interestingly, Sperry admitted it had known about the weight problem for some time, but did not explain why it had not brought the issue to the government’s attention at an earlier date.-1154

Sperry defended its actions by listing the changes it had made to eliminate excess weight. These included substituting aluminum for stainless steel in some locations, reducing the thickness of various covers, and reducing component weight wherever practical. The need to include power supplies not anticipated in the original proposal also increased the weight of the system. Finally, Sperry also concluded that the stable platform was lighter and more accurate than any competing system. Apparently, Sperry’s justification was satisfactory since the X-15 Project Office accepted that the system was going to remain overweight and took no further action on the subject.[155]

As finally delivered, the stable platform was an Earth-slaved, Schuler-tuned system aligned in azimuth to a guidance vector coincident with X-15 centerline. The unit provided attitude, velocity, and altitude to the pilot with reference to these coordinate systems. There were three major components to the stable platform: the stabilizer, computer, and displays. Together they weighed approximately 165 pounds, occupied about 3 cubic feet of volume, and required a peak electrical load of 600 W. The stabilizer used three self-balancing accelerometers and three single-degree – of-freedom gyroscopes. A four-gimbal system provided complete attitude freedom in all axes.

An analog computer computed velocity and position data, and applied the necessary acceleration corrections. The computer was shock-mounted and shaped to conform to the contours of the X – 15 instrumentation compartment. Gaseous nitrogen from the X-15 cooled the stabilizer and computer to counteract the internal heat generated by the units, and the extreme external temperatures. The system was "designed to operate over a limited portion of the Earth’s surface." Specifically, it could accept a launch point anywhere within a 275-mile-wide corridor extending 620 miles uprange and 205 miles downrange from Edwards AFB.[156]

Sperry shipped the first stabilizer and computer to Edwards in late January 1959, and the Air Force intended to use the NB-52 carrier aircraft as a test vehicle. This was delayed for unknown reasons, so the Air Force made a KC-97 that was already being used for similar purposes by the Convair B-58 program available to the X-15 project. The first flights in the KC-97 took place in late April, but were of limited value given the low speed of the piston-powered Stratocruiser. In June 1959, North American successfully installed the Sperry system in X-15-3 prior to its delivery to Edwards. By the end of May 1960, there were four complete stable platforms at Edwards: one in X-15-1, one in X-15-3, one spare, and one undergoing repair.-1157

As delivered, the stable platforms could provide the following data:[158]

However, Sperry had made several compromises during the development of the X-15 stable platform, either to meet schedule or reduce weight. The designers knew that 300 seconds after launch (i. e., as the airplane decelerated to land) the pressure instruments would be adequate for vehicle altitude and velocity data, and that a system capable of operating from carrier aircraft takeoff to X-15 landing would be too heavy and bulky for the X-15. The final design had a very limited operating duration. The pilot aligned the system just before the X-15 separated from the NB-52, and the stable platform provided just 300 seconds of velocity and altitude data, along with 20 minutes of attitude data. This limited operating duration provided some relief for the weight problem.-115^

As it turned out, the lighter shock mount developed by Sperry was not adequate for the X-15. It performed fine during the XLR11 flights, but vibration tests in October 1960 prior to the beginning of XLR99 tests showed that the mount would not withstand more than 1.5 g at 110 cycles. North American redesigned the mount, since by this time saving weight had become a non-issue for the most part; having a reliable airplane was worth more than the few miles per hour the weight cost.

Over the course of the flight program, the stable platform was the subject of several other changes that greatly improved its reliability. Many of these were the result of suggestions from John Hursh at the MIT Instrumentation Laboratory and Dr. Allen Smith from Ames, both of whom spent a great deal of time at the Flight Research Center during late 1960 working on the problems. As an example of these changes, NASA changed all critical germanium transistor amplifiers to silicon during November 1960. NASA also made changes to operating procedures as well as to hardware. Initially, a gyroscope failure required that the entire stable platform be returned to Sperry for repair, taking the unit off flight status for three to six weeks. In response, the FRC developed an in-house repair capability that significantly shortened turnaround times. Even better, during late 1960 NASA substituted a higher-quality gyroscope manufactured by Minneapolis-Honeywell, which resulted in fewer failures.-1160

The X-15 was one of the first aircraft to require what is today called an inertial measurement unit, or stable-platform. Gyroscopes of the era were large, heavy, and consumed a considerable amount of power. This model shows the three interlocked rings required to determine position in three dimensions. (NASA)

In retrospect, the performance specifications established in 1956 were well beyond the state of the art with respect to available gyros, accelerometers, transistors, and circuit techniques.

However, the system as originally built was able to perform at levels that, although marginal or subpar compared to the original specification, still allowed the X-15 to realize its full performance capabilities. Compared to modern laser-ring-gyro and GPS-augmented systems, the X-15 stable platform was woefully inaccurate, but it routinely bettered its 70-fps error specification for velocity. Initially its altitude-measuring ability was somewhat substandard, averaging about 2,200 feet (rms) uncertainty. The requirement was 2,000 feet, but the system eventually improved and met its specification. Reliability was initially poor, but by mid-1961 the overall reliability was approaching the high 90th percentile, with the altimeter function proving to be the most unreliable. Unfortunately, this improved reliability proved to be short-lived.-1161

The initial operational experience with the stable platform showed that it had a large error potential that grew as time passed from the initial alignment due to drift and integration noise. The unit integrated velocities to provide distance (X, Y, and Z) and specifically altitude, which had even more error buildup with time. Early flight tests showed that the displayed velocities were marginal even after the 90-second engine burn, and that the altitude was undependable for determining peak altitude or reentry setup. Because of this, the flight planners and pilots began to consider two other sources for controlling the energy imparted to the airplane: 1) engine burn time, as measured by a stopwatch in the NASA-1 control room, and 2) radar-measured velocity,

[1621

as displayed in the control room.

For the first government flight (2-13-26) with the XLR99 engine, the flight planners decided to use radar velocity as the primary indication with a radio call to Bob White at the desired engine shutdown condition. After the successful flight, researchers calculated that the airplane had exceeded the intended speed by about half a Mach number. Further analysis showed that the radar velocity display in the control room incorporated considerable smoothing of the data to provide a readable output. This introduced a lag of 4 seconds between the actual speed and the displayed speed, thus accounting for the overshoot. For the next few flights, NASA-1 started a stopwatch in the control room at the indication of chamber pressure on the telemetry, and radioed the pilot when it was time to shut down the engine.-163

Using a stopwatch to measure powered flight time proved to be the simplest and yet most accurate method of controlling energy, so a stopwatch was installed in the cockpit of all three airplanes. A signal from the main propellant valves started and stopped the stopwatch so that it displayed the total burn time even after shutdown. The pilot could then assess whether he had more or less energy than planned, and evaluate his energy condition and best emergency lake in the event of a premature shutdown. Although the reliability of the stable platform increased considerably during the course of the program and was eventually operating within its design specifications, the pilots continued to use the stopwatch (with a backup stopwatch in the control room) for most flights. It was cheap and easy, and almost never failed.164

By 1963 an increasing number of stable platform failures began to occur—some because of design deficiencies, others simply due to component deterioration. This led to NASA placing a new set of restrictions on X-15 flights, keeping them below 160,000 feet. Progress by Sperry to resolve the issues was slow, so an analysis was undertaken at the FRC to determine what in-house efforts could be made to bolster system performance and improve reliability.163

Beginning in late 1963, the FRC began redesigning critical components to improve both accuracy and reliability. Eventually, NASA engineers redesigned some 60% of the subassemblies in the stable platform. Overall, the volume used by the accelerometers, accelerometer electronics, and power supplies was reduced over 50%, and an accompanying reduction in power and cooling requirements was also realized. Although some of the improvements resulted from correcting deficiencies in the original design, most were achieved because the state of the art had improved considerably in the four years since work had begun. NASA completed the initial redesign efforts on the accelerometer loops and power supplies during the summer of 1964, and the first flight of the new components was in X-15-2 on 14 August 1964 (2-33-56). Technicians subsequently installed the revised components in X-15-3 also. This system allowed NASA to cancel the 160,000-foot altitude restriction on the airplanes.

Although the initial performance of the revised components was a little erratic, the increase in accuracy was substantial. For instance, 400 seconds into the flight the original system would have a +8,000-foot error in altitude; the revised system generally had a -1,000-foot error. (In both cases the specification required a less than -5,000-foot error; nothing on the positive side was satisfactory.) Eventually the engineers tuned the erratic performance out of the system. By May 1966, components designed at the FRC had essentially replaced the entire Sperry stable platform, and the system was redesignated the "FRC-66 Analog Inertial System."166

and a set of pilot displays. This system was even smaller and required less power and cooling than the redesigned FRC-66 analog system. In addition, the X-20 IFDS could automatically erect itself and perform an alignment cycle on the ground while the NB-52 was taxiing, and completely eliminated the need for information from the N-1 compass and APN-81 Doppler radar on the NB – 52. This made it somewhat easier to pilot the carrier aircraft as the X-15 approached the launch position; the APN-81 took 90 seconds to stabilize after even a gentle turn, requiring the NB-52 pilot to think well ahead of the drop time. To improve accuracy, however, the IFDS altitude loop was synched to the NB-52 pressure altimeter until 1 minute before launch.-1167

The inertial measurement unit was a gyrostabilized, four-gimbaled platform that maintained local vertical orientation throughout the flight. The inner platform contained three pendulous accelerometers that formed an orthogonal triad. The coupler electronics unit contained the power supplies and interface equipment, and a dual-function digital computer performed all computations. NASA first checked out the digital system in X-15-1 on 15 October 1964 (1-50­79), with satisfactory results.1168

The overall performance of the IFDS during its first 16 flight attempts was excellent, with only two failures. However, problems with the IFDS caused two attempted launches in a row (1-A-105 and 1-A-106) to abort during June 1966. After the first abort, technicians replaced a relay and fixed a loose wire, but the second flight attempt a week later ended the same way. Engineers from Autonetics (a division of North American), Honeywell, the FRC, and Wright Field began investigating the problem. The failures were determined to be the result of yet more wiring problems, all easily corrected.-168

JH5 RESEARCH SYSTEM

TYPICAL MISSION

KtmiuiUtei

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Because of limitations in both the gyroscopes and onboard computers, the X-15 stable platform could only function for a limited amount of time in a 275-mile-wide corridor extending 620 miles uprange and 205 miles downrange from Edwards AFB. Later modifications to the system were more reliable and versatile, and at the end of the program, two of the X-15s were using digital inertial flight data systems developed for the Air Force Dyna-Soar program. (NASA)

At the same time, the installation in X-15-3 was not going as well as it had in X-15-1. On 6 January 1965, representatives from Honeywell met with FRC personnel to discuss problems with the installation. There were four primary concerns: cooling and thermal conditions, space availability, cabling, and the interface to the MH-96 adaptive control system. This latter issue was surprising since the X-20 also used a version of the MH-96. Also discussed was the relative accuracy expected from the new system versus data from the ball nose. It was pointed out by the

Honeywell representative that at low velocities there would be a significant difference between the IFDS-computed angle of sideslip and that sensed by the ball nose, but at high velocities the difference should be small.[170]

By April 1965 the FRC had made little progress installing the system in X-15-3, and only X-15-1 was flying with the Honeywell inertial system. Fortunately, by this time the modified Sperry systems were proving to be reliable, and no substantial problems had been experienced by X-15- 2 or X-15-3 since December 1964. Engineers finally installed the Honeywell IFDS in X-15-3 during a weather-induced down period at the end of 1965.-1171

Although the Honeywell IFDS was considered an improvement over the modified Sperry stable platform, the FRC decided that the FRC-66 system was preferred for the Mach 8 flights in the modified X-15A-2, so that airplane never received an IFDS. By the end of 1965, engineers had modified one of the Sperry computers to have Mach 8 scaling coefficients in preparation for the X-15A-2 envelope-expansion program.-1172

The improvements did not stop there. Eventually the FRC modified X-15-3 to include an Ames – developed guidance system that was applicable to future aerospace vehicles. This system coupled the IFDS inertial system, MH-96 adaptive control system, and ball nose to an Alert digital computer to investigate boost guidance command techniques. The navigation functions continued to be performed by the inertial system while the Alert computer handled the research objectives, including providing new displays to the pilot. This program allowed the pilot to fly a velocity – altitude window during boost, a bounded corridor during hypersonic cruise, and a precise corridor during reentry. It was an advanced system, and one that Space Shuttle only duplicated in its waning years.-1173

Although the XLR99 proved to be a remarkably capable research engine given its relatively short development period and limited operational experience, proposals were made from time to time to replace it. Usually these revolved around the idea of using a derivative of the Aerojet LR91 engine. In October 1966, Aerojet-General submitted an unsolicited proposal to North American that detailed the use of the LR91-AJ-7 engine in the X-15. Aerojet probably intended the proposal to support the concept of using an LR91 in the delta-wing modification.12^

The LR91 powered the Titan II ICBM, the Titan II Gemini Launch Vehicle, and the Titan III family of space launch vehicles. Aerojet had delivered over 180 engines at the time of the proposal, and had run more than 1,400 engine tests. The engine was man-rated for the Gemini application and the Titan IIIM developed for the Manned Orbiting Laboratory (MOL). The LR91-AJ-7 developed 100,000 lbf at 250,000 feet using nitrogen tetroxide and Aerozine-50 propellants.12^

Aerojet believed that the engine offered several advantages for the X-15. The storable propellants provided a higher bulk density, allowing additional specific impulse to be stored in the same volume, although Aerojet suggested limiting the X-15 to 92 seconds of powered flight. The propellants also eliminated the liquid-oxygen top-off system in the NB-52s since they had a very low boil-off rate and would not have to be replenished in flight. An autogenous pressurization system provided tank pressurization gases from the engine in proportion to propellant consumption, eliminating the need for separate pressurization gases and their mechanical systems (regulators, valves, etc.).130-

Aerojet pointed out that since the engine was in large-scale (for a rocket engine) and continuous production, costs would be lower, and a continuous-improvement program was in place that could benefit the X-15 program. The major changes to the LR91 configuration for the X-15 included modifying it to operate in a horizontal attitude and strengthening the engine to allow it to be reusable. These changes (especially the one to allow horizontal operation) were not as straightforward as they might seem, and a simple description of them took several pages. The modifications to make the engine reusable also took several pages to describe. Nevertheless, Aerojet believed it could provide an engine quickly-beginning by July 1967 allowed the first X-15 flight in March 1969.131

The government did not take any action on this proposal or others made along similar lines. Although working with liquid oxygen and anhydrous ammonia presented some issues for the ground crews, it was decidedly simpler than dealing with the hypergolic propellants in the LR91. Moreover, nobody readily believed that the engine would be as reliable and reusable as the XLR99 without a major development effort, something the X-15 program could not afford. Although an additional 40,000 pounds of thrust would have more than restored the performance lost due to the continual weight gains on the X-15, in the final analysis it just was not worth the time and money. Maybe it would have been worth it for the delta wing; but then, perhaps not.

In addition to the NB-52s there were numerous chase and support aircraft, mostly provided by the Air Force. The number of chase aircraft differed depending on what the flight profile looked like. The program generally used three chase aircraft on the early low-speed X-15 flights, four on most research flights, and five for the very long-range flights. Of course, all things were variable and additional chase aircraft were not uncommon, particularly during the middle years of the program.

Chase-1 was the prelaunch chase, and was usually a North American F-100F Super Sabre during the early years and a Northrop T-38A Talon later, although NASA used a Douglas F5D Skyray on a couple of occasions. Al White frequently flew this chase during the North American flights, but an Air Force pilot generally flew the airplane once the government took over. Chase-1 took off with the NB-52 and flew formation during the climb-out and cruise to the launch lake. The chase pilot visually verified various parts of the X-15 checklist, such as control surface movements, propellant jettison, ballistic system checks, APU start, and engine priming. The use of the F-100 presented some problems at the beginning of the program because the aircraft could not maintain a low enough speed to fly formation with the NB-52 during a right-hand turn; however, the T-38 proved to be more satisfactory.

Chase-2 was the launch chase and provided assistance for the X-15 pilot in the event of an emergency landing at the launch lake. Chase-2 was usually a Lockheed F-104 Starfighter flown by either another X-15 pilot or a NASA test pilot. The F-100 and T-38 could not produce enough drag to fly the steep final approach used by the X-15, which largely dictated the use of the Starfighter for this role. Conversely, the F-104 could not cruise at 45,000 feet due to its high wing loading, which made it unsuitable as Chase-1. Chase-2 normally stayed below 35,000 feet until 3 minutes before launch, and then went into afterburner and climbed to 45,000 feet just before the X-15 dropped. The pilot trailed the NB-52 during launch and then tried to keep up with the X-15 as it left the launch lake area. It was a futile gesture, but it proved useful on the few occasions in which the X-15 engine failed soon after ignition.

Chase-3 covered landings at the intermediate lakebeds and was usually an F-104 flown by either another X-15 pilot or an Air Force test pilot. Unlike Chases 1 and 2, which took off with the NB – 52, Chase-3 waited until 30 minutes before X-15 launch to take off so that it would have enough fuel to loiter for a while. On flight profiles that had multiple intermediate lakes, Chase-3 would orbit between them. In the event the X-15 had to make an emergency landing, the F-104 would attempt to join up to provide support for the X-15 pilot during final approach and touchdown.

For flights out of Smith Ranch there were two intermediate chases, usually called 3 and 4 (the Edwards chase became Chase-5 in these cases).

The Lockheed F-104 Starfighter was used as a chase airplane and to practice landing maneuvers. In addition to the F-104Ns owned by NASA, various F-104s from the Air Force Flight Test Center were used as needed. (NASA)

Chase-4 covered the Edwards landing area, usually with an Air Force pilot. Again, only an F-104 could keep up with the X-15 in the landing pattern. This chase took off at the same time as Chase-3 and orbited 30-40 miles uprange along the flight path. The pilot began accelerating on cue from NASA-1 in an attempt to intercept the X-15 at the maximum possible speed and altitude as the X-15 descended into the Edwards area. Usually the chase pilot took his cues from the vapor trail left as the X-15 pilot jettisoned his residual propellants, since the research airplane was too small and too dark to acquire visually until the chase pilot was right on top of it. Chase-4 would make a visual inspection of the X-15 as it descended and provide airspeed and altitude callouts to the X-15 pilot during the final approach, in addition to verifying that the ventral had successfully jettisoned and the landing gear extended.-1129

Ferrying men and supplies to the contingency landing sites and High Range stations kept the NASA Douglas R4D (C-47/DC-3) Skytrain busy. In addition, the Air Force used Lockheed C-130 Hercules to move fire trucks and other heavy equipment. The C-130s also carried rescue teams during flight operations to ensure help would arrive swiftly in the event of a major accident.

(NASA)

At times there were other chase aircraft, with a photo-chase or a "rover" being the most frequent. The photo-chase filmed the X-15, although Chase-1 was frequently a two-seater and carried a photographer in the back seat as well. Rover was usually another X-15 pilot who just felt like tagging along. All of the X-15 pilots flew chase aircraft, as did many AFFTC test pilots, and students and instructors from the test-pilot schools at Edwards. The chase pilots (particularly other X-15 pilots) tended to use first names for themselves and the X-15 pilot during radio chatter; alternately, they simply used "chase" (without a number) since there was seldom more than one chase aircraft in the vicinity.

A number of other aircraft provided various support functions. In particular, the program used the NASA Gooney Bird (R4D/DC-3) to ferry men and supplies to the uprange stations and to inspect the lakebeds as necessary. The Air Force used several Lockheed C-130 Hercules turboprops to transport fire engines and other material to the lakebeds and High Range stations for each flight. These aircraft often made several trips per day carrying men and equipment. During the actual flight one of them orbited midway down the flight corridor, usually with a flight surgeon and response team in case the X-15 had to make an emergency landing. The program took safety very seriously.

Piasecki H-21 Shawnee helicopters were also shuttled to the primary emergency landing lake in case of an emergency, and additional H-21s were located at Edwards. These provided a quick means of moving emergency personnel to an accident scene, surveying the runways, and evacuating the X-15 pilot if necessary. The H-21 pilots also knew how to disperse fumes from a damaged X-15 by hovering near the crashed airplane, and they used this technique on at least one occasion, probably saving the life of the X-15 pilot.