Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

Milt Thompson flew the X-15 for 22 months from 29 October 1963 until 25 August 1965, making 14 flights with the XLR99 engine. Thompson reached Mach 5.48, a maximum speed of 3,723 mph, and an altitude of 214,100 feet.

Milton Orville Thompson was born on 4 May 1926 in Crookston, Minnesota. Thompson began flying with the Navy and served in China and Japan during World War II. Following six years of active duty, Thompson entered the University of Washington and graduated with a bachelor of science degree in engineering in 1953. After graduation Thompson became a flight-test engineer

for the Boeing Aircraft Company, testing, among other things, the B-52.

Thompson joined the HSFS on 19 March 1956 and became a research pilot in January 1958. At the time, there were only five pilots at the station: Joe Walker, Stan Butchart, Jack McKay, Neil Armstrong, and Thompson. In 1962, Thompson became the only civilian pilot on the X-20 Dyna – Soar, but Robert McNamara canceled that program just over a year later. On 16 August 1963, Thompson became the first person to fly a lifting body, the lightweight M2-F1. He flew it 47 times and made the first five flights of the all-metal M2-F2. Thompson concluded his active flying career in 1967 and became chief of research projects two years later. In 1975, he became chief engineer and retained the position until his death on 6 August 1993. Thompson also served on NASA’s Space Transportation System Technology Steering Committee during the 1970s. In this role he was successful in leading the effort to design the Space Shuttle orbiters for power-off landings rather than increase weight with air-breathing engines. His committee work earned him the NASA Distinguished Service Medal.

Thompson was a member of the Society of Experimental Test Pilots, and he received the organization’s Iven C. Kincheloe Trophy as the outstanding experimental test pilot of 1966 for his research flights in the M2 lifting bodies. He also received the 1967 Octave Chanute award from the AIAA for his lifting-body research. In 1990, the National Aeronautics Association selected Thompson as a recipient of its Elder Statesman of Aviation award (this award has been presented each year since 1955 to individuals who made contributions "of significant value over a period of years" in the field of aeronautics). Milt Thompson died on 6 August 1993.Г26

Thompson wrote about his experiences with the X-15 in At the Edge of Space: The X-15 Flight Program (Washington, D. C.: Smithsonian Institution Press, 1992). Anybody who is interested in an inside look at the program should pick up a copy; it is a fascinating read.

The Air Force initiated Project 7969, the manned ballistic rocket research system, in February 1956 with a stated goal of orbiting and recovering a manned space capsule. By the end of 1957, a joint Air Force-NACA team had evaluated at least 10 serious proposals during a conference held at Wright Field on 29-31 January 1958. Avco, Convair, Goodyear, Lockheed, Martin, and McDonnell proposed spherical reentry vehicles or blunt capsules, while Bell, North American, Republic, and Northrop all proposed winged vehicles.234

The North American proposal included a "stripped" X-15 with an empty weight of 9,900 pounds. Cape Canaveral would launch the vehicle on a two-stage booster that allowed a single orbit with an apogee of 400,000 feet and a perigee of 250,000 feet. The launch vehicle consisted of four Navaho boosters. Three were clustered together in the first stage and one acted as the second stage. The XLR99 in the X-15 was the third stage. The X-15 would be equipped with beryllium oxide leading edges and a Rene 41 alloy shingle heat shield, plus a thicker Inconel X hot structure. Due to the low perigee and aerodynamics of the X-15, no retrorocket was required for reentry. The pilot would eject and descend by parachute just before ditching the X-15 in the Gulf of Mexico, with the aircraft being lost. North American expected that it could conduct the first manned orbital flight 30 months after a go-ahead, at a cost of $120 million.-1135

Given the early state of development of the X-15, there was almost no real engineering associated with this proposal. Nevertheless, it was further along than many of the others since researchers already knew that the basic X-15 shape was stable in most flight regimes, and both the airframe and XLR99 were at least under active development.

After the launch of Sputnik 1, Project 7969 was reoriented into the Man In Space Soonest (MISS) project to ensure that a U. S. Air Force pilot would be the first human in outer space. On 27 February 1958, General Curtiss E. LeMay, the Air Force vice chief of staff, was briefed on three alternatives that included the X-15 derivative, speeding up the Dyna-Soar program, and building a simple non-lifting ballistic capsule that could be boosted into low orbit by an existing ICBM – derived booster. LeMay apparently expressed no preference, and although it was a long and complicated process, the result was that a ballistic capsule appeared to offer the best hope of immediate success. This idea formed the basis for Project Mercury after NASA was formed on 1 October 1958 and the first American manned space effort was transferred to the civilian agency.-135

X-15B

Nevertheless, engineers at North American continued to refine their Project 7969 concept. A few days after the Soviet Union orbited Sputnik 1 on 4 October 1957, North American packaged everything into a neat report and Harrison Storms took the idea to Washington. This version used two Navaho boosters clustered together as the 830,000-lbf first stage, a single Navaho booster as the 415,000-lbf second stage, and an X-15B powered by a 75,000-lbf Rocketdyne XLR105 Atlas sustainer engine as the third stage. Unlike the 7969 proposal, this one had a great deal more engineering in it, although it was still very preliminary since North American had not conducted wind-tunnel tests or detailed calculations on heating or aero loads.-137

The X-15B was larger than the basic X-15 and was capable of carrying two pilots. The Inconel X skin was made thicker to withstand the increased reentry heating, and the vehicle had larger propellant tanks to feed the Atlas sustainer engine that replaced the XLR99. However, the shape and many of the internal systems were identical to those of the basic X-15 then under construction. Engineers had already demonstrated the supersonic and subsonic stability of the X – 15 during numerous wind-tunnel tests, and keeping the same shape eliminated the need to repeat many of them.

The flight plan was simple. Eighty seconds after launch from Cape Canaveral, the first stage would drop away and the second stage would fire. At an altitude of about 400,000 feet, the second stage would burn out and the X-15B would continue using its own power. The vehicle would eventually get up to 18,000 mph, enough for three orbits. The pilot would fire the XLR105 at a point that would allow the X-15B to land at Edwards using the reentry profiles already developed for the basic X-15. It was a grand plan, and years ahead of its time. Unfortunately, when Storms got back from Washington he reported that "there were exactly 421" other people who had competing proposals. Eventually the X-15B just quietly faded from sight.138

In the excitement caused by the Soviet launch of Sputnik, North American proposed a heavily modified X-15B as an early orbital vehicle. Although the aerodynamics of the X-15 were well – understood by this time, the X-15B did not have nearly the maneuverability of the Air Force Dyna-Soar while returning from orbit, and in fact, many X-15B proposals had the pilot ejecting over water instead of attempting to land. (North American Aviation)

November 1961 saw the first industry conference held in three years (NASA had held previous conferences in 1956 and 1958). The classified conference at the FRC featured 24 papers from 56 authors, including 4 X-15 pilots, and was attended by 442 people. Of the authors, 5 came from North American, 37 from various NASA centers, 13 from the Air Force, and 1 from the Navy. The attendees represented virtually every major aerospace contractor in the country, all of the NASA centers, several universities, the various military services, and the British Embassy.-154

At the time the papers for the conference were prepared, the program had made 45 flights during the 29 months since the initial X-15 flight. The first of these was a glide flight, and of the subsequent powered flights, 29 had used the XLR11 engines and 15 used the XLR99. A maximum altitude of 217,000 feet (flight 2-20-36) and a velocity of 6,005 feet per second (flight 2-21-37) had been achieved.155

Researchers had already accomplished quite a bit of analysis on aerodynamic heating, one of the primary research objectives of the X-15. Several theoretical models had been developed to predict heating rates, but little experimental data were available to validate them since it was uncertain whether wind tunnels were capable of realistically simulating the conditions. The X-15 provided the first real-world experience at high Mach numbers in a well-instrumented, recoverable vehicle. Data from the X-15 showed that none of the models were completely accurate, although all showed some correlation at different Mach numbers. The data showed that the wind tunnels were reasonably accurate.156

A particular area of interest to researchers was how the boundary layer transitioned at different Mach numbers and angles of attack. Researchers used two methods to detect laminar and turbulent areas on the airplane in flight. The first was to use thermocouple data reduced to heat – transfer coefficients, which showed a much higher level of heat transfer in a turbulent boundary layer than in a laminar one. The second method was to use temperature-sensitive "DetectoTemp" paint applied over large areas of the airplane. In general, NASA applied the paint to the left side of the airplane, and the thermocouples were on the right side.157

The first use of the paint was on 4 August 1960 for flight 1-9-17, which was the XLR11 maximum speed attempt. The results were promising inasmuch as the paint established a semipermanent pattern of contrasting colors at different temperature levels. The pattern retained on the wing and vertical stabilizer after the flight clearly indicated all of the heat-sink locations and areas of high heating. For instance, the internal spars and ribs stood out as heat sinks, while areas such as the expansion joints on the wing leading edge stood out in the color pattern as concentrated heating areas. Researchers decided that they could use the paint to collect qualitative temperature data, particularly in small areas that were not equipped with thermocouples.-1581

One of the notable discoveries made using the paint was that patterns indicated high – temperature, wedge-shaped areas originating at the wing leading-edge expansion joints and extending for a considerable distance rearward. The 0.080-inch-wide expansion joints appeared to result in a turbulent flow during the entire flight, producing 1,000°F temperatures in an 8-inch wedged-shaped area behind them. The measured heat-transfer data on the other wing supported this view, offering "a classic example of the interaction among aerodynamic flow, thermodynamic properties of air, and elastic characteristics of structure." Although the rates were well within the limits of the airframe, engineers installed small 0.008-inch-thick Inconel X shields over the expansion joints in an attempt to minimize the interference. Flights with these covers showed that the turbulent wedges still existed, although they were smaller, and researchers theorized that they would be present for shorter periods on each flight.159

The conclusion drawn from this was that the "boundary layer transition, which may be produced by such discontinuities in the surface of a high-speed vehicle, would be extremely difficult to predict. As yet, for the X-15, there has not been established parametric correlation which would allow the prediction of the transition location on the wing a priori. Under these circumstances, it would seem that conservative estimates of transition should still be required."169

To show how the preflight estimates and flight data correlated, the authors presented data for one thermocouple on the lower surface of the right wing about 1.4 feet from the leading edge at mid-semispan. For the high-speed flight profile, the measured data indicated an all-turbulent flow with a high skin-heating rate and high maximum temperature. The calculated skin temperature agreed quite well during the high heating period, but slightly overestimated the measured value near its peak and during a period of cooling just afterwards. A close look at the trajectory during this period of disagreement showed a high angle of attack, and researchers believed the differences were due to their inability to properly predict the local flow conditions.

For a high-altitude mission, however, this point of the wing appeared to experience laminar flow, at least at times. An all-turbulent flow prediction resulted in a higher temperature than was actually measured during the exit phase of the trajectory, greater cooling during the ballistic portion, and an overestimate of the maximum temperature during reentry. The assumption of laminar flow during the latter part of the exit phase resulted in better agreement between the measured and calculated data. Researchers noted, however, that one of the turbulent wedges originating on a wing leading-edge expansion joint might affect the thermocouple in question. Researchers did not understand exactly what might cause the location to go laminar, but theorized that either the turbulent wedge vanished or its lateral spread was delayed.161

The wing leading-edge expansion slots produced problems in addition to the wedge-shaped boundary layer issue. On one flight the area directly behind the expansion slots buckled. One reason for this was that the fastener spacing directly behind the slot was wider than on other sections of the leading edge, providing less support for the area. It was also determined that the original segmentation of the leading-edge heat sink did not adequately relieve the thermal compression loads. The skins at the expansion slots acted as a splice plate for the solid heat-sink bar, and as a result buckled in compression. Engineers made several changes to solve this problem. The shield installed over each expansion slot to help the boundary layer problem minimized the local hot spot, but engineers also added a fastener near each slot and three additional expansion slots (with shields) in the outboard segments of the leading edge. This presented some concern since North American had designed the original expansion slots with shear ties to prevent relative displacement of the leading edge, and it was not cost-effective to provide shear ties for the new slots because the entire wing structure would have required modification. A structural analysis showed that sufficient shear stiffness was present in the leading edge to meet the design requirements without shear ties, but engineers expected some relative displacement at the three new slots. Actual flight tests showed that this displacement averaged about 0.125 inch. Overall, the modifications prevented any serious leading-edge buckling, although minor distortions continued throughout the flight program.-162

The X-15 program was one of the first to employ temperature-sensitive paint that established a semipermanent pattern of contrasting colors at different temperature levels. The paint clearly showed the different heating loads absorbed by the hot-structure airframe. In general, NASA applied the paint to the left side of the airplane; the thermocouples were on the right side. (NASA)

The conclusion drawn from the available data was that "when the boundary layer is known to be either laminar or turbulent, the skin temperatures can be predicted with reasonable accuracy." The problem was to figure out what the boundary layer would do under different flight conditions.163

The effect of temperature is not linear, and at Mach 6 the heating load on the X-15 was eight times that experienced at Mach 3. Unsurprisingly, the front and lower surfaces of the aircraft experienced the highest heating rates. During the conference, researchers discussed several intriguing aspects of the temperature problems. One was surprising, given that the program had always worried about high temperatures: "The first temperature problem occurred on the side­fairing panels along the LOX tank before the X-15 was first flown. Pronounced elastic buckles appeared in the panels as a result of contraction when the tank was filled for the first time."

Adding a 0.125-inch expansion joint to the tunnel fairing near the wing leading edge relieved the buckling.1164!

However, after a Mach 4.43 flight (2-13-26) on 7 March 1961, several permanent 0.25-inch buckles formed in the outer sheet of the fairing between the corrugations near the edge of a panel. Since the panel only carried air loads (not structural loads), the buckles did not seriously affect structural integrity. During the flight, the panels that buckled had experienced temperatures between 490°F (near the wing leading edge) to 590°F (near the front of the fairing). On this particular flight, the pilot shut down the engine prior to propellant depletion, leaving about 20% of the liquid oxygen in its tank. The maximum temperatures occurred after shutdown, and it was theorized that the cold tank (-260°F), together with the high outer-skin temperatures, resulted in large thermal gradients that caused the buckles. These gradients were higher than had been calculated for the original design, since the estimates had assumed propellant depletion on all flights. Based on this experience, engineers added four expansion joints in the fairing ahead of the wing that allowed a total expansion of slightly over 1 inch. This modification appeared to prevent any further buckling.!1661

Researchers expected the surface irregularities produced by the buckles to cause local hot spots during high-speed flights. To investigate this, NASA covered the buckled areas with temperature – sensitive paint for flight 2-15-29. The results from the Mach 4.62 flight showed that the maximum temperature in the buckle area was essentially the same as in the surrounding areas with no evidence of local hot spots. The researchers went back to their slide rules to come up with revised theories.-11661

Other heating problems experienced during the early flight program included hot airflow into the interior of the airplane, which caused unexpected high temperatures around the speed brake actuators, and loss of instrumentation wires in the wing roots and tail surfaces. In a separate incident, cabin pressure forced the front edge of the canopy upward, allowing hot air to flow against and damage the seal. NASA resolved the canopy problem by attaching a shingle-type strip to the fuselage just ahead of the canopy joint to prevent airflow under the edge of the canopy. A similar problem developed in the nose landing-gear compartment: a small gap at the aft end of the nose-gear door was large enough to allow the airstream to enter the compartment and strike the bulkhead between the nose-gear compartment and the cockpit. This stream caused a local hot spot that melted some aluminum tubing used by the pressure-measuring system on flight 2­17-33. During the Mach 5.27 flight, the bulkhead heated to 550°F, high enough to scorch the paint and generate some smoke inside the cabin. It was a potentially catastrophic problem, but fortunately no significant damage resulted. In response, engineers added an Inconel compression seal to the aft end of the nose-gear door and installed a baffle plate across the bulkhead.11671

On 23 August 2005, 40 years of aerospace controversy ended. For years, many aviation historians and enthusiasts had questioned why the Air Force pilots who flew the X-15 to altitudes above 50 miles received astronaut ratings, while the NASA pilots who accomplished the same feat in the same airplanes did not. The answer came on a small stage at the DFRC when Navy Captain Kent V. Romminger, chief of the NASA Astronaut Office at the Johnson Space Center, presented certificates proclaiming three NASA test pilots as astronauts. NASA administrator Shawn O’Keefe authorized the recognition, and Romminger, associate administrator for the Space Operations Mission Directorate William F. Readdy, and DFRC director Kevin L. Peterson signed the certificates. The purposefully small ceremony was a private moment for a very special group of men and their families.-*402*

In August 2005, NASA finally recognized three NASA pilots who had flown over 50 miles altitude but had not received astronaut wings like their military counterparts. Three former X-15 pilots joined in the ceremony. From left, Robert M White, William H. Dana (proudly wearing the new wings on his flight jacket), Neil A. Armstrong, and Joe H. Engle. The families of Jack McKay and Joe Walker were present to accept their astronaut wings from Kent Romminger, the Chief of the Astronaut Office. (NASA)

In the late 1960s, these three men-William H. Dana, John B. McKay, and Joseph A. Walker-had piloted the X-15 to altitudes in excess of 50 miles. Although five of their colleagues had received Air Force astronaut ratings for similar accomplishments, NASA had never recognized the three civilian pilots. Now, 40 years after the fact, the agency did. Only Bill Dana was still alive to receive his certificate, and to have his wife, Judy, place the blue and gold name tag with the astronaut wings emblem on his flight jacket. However, almost the entire McKay and Walker families were on hand to receive the tribute. Joe Walker, who always had a smile on his face, was the first human being to fly into space twice; now his son has a set of astronaut wings to proudly display. On hand to honor their colleagues were three former X-15 pilots: Joe H. Engle, who after his X-15 flights became the only person to fly the space shuttle back from orbit under manual control; Robert M. White, perhaps the least known of the test pilots (odd considering he was the first person to fly to Mach 4, Mach 5, and Mach 6, as well as the first person to fly to 200,000 feet and then to 300,000 feet; Bob still holds the world absolute-altitude record at 314,750 feet); and Neil A. Armstrong, who needed little introduction. Unfortunately, Scott Crossfield could not attend due to previous commitments. There was not a dry eye in the house.J403

The ASD sponsored experiment #3 to measure the exhaust characteristics from a liquid-oxygen – ammonia rocket engine (the XLR99). It used the same basic equipment as experiment #2, without the stabilized platform. The first flight (3-41-64) was made on 23 April 1965 with Joe Engle at the controls. By the end of 1965, the high-resolution Barnes ultraviolet scanning spectrometer and solar-blind radiometer that had proved so troublesome on experiment #2 had successfully obtained good data. As a follow-up, researchers installed a Millikan dual-channel radiometer in X-15-3 during the weather down period at the beginning of 1966, and installed a vacuum ultraviolet spectrometer later in the year. The Millikan radiometer flew on flight 3-55-82 but froze due to a failed heater, and there is no record of it flying again. Similarly, no record exists of the spectrometer ever being flown.-194!

Experiment #4: Langley Horizon Definition a 12,000-foot mountain using a simple photometer and several interference filters. The data indicated that the "stable phenomena" hypothesis appeared to be correct, but emphasized the need for observations made from outside the Earth’s atmosphere using equipment that was more sophisticated. Researchers flew variations of the experiment on sounding rockets and the X-

15.1951

Researchers installed a radiometer in the tail-cone box of X-15-3 along with a 16-mm motion – picture camera pointing out the rear. The camera provided wide-angle coverage to check for clouds or haze during the data-gathering period. The radiometer included a motor-driven scan mirror that provided a 30-degree field of view, and reflected energy into a parabolic mirror that focused the energy on the detector. The radiation passed through an optical bandpass filter to select the appropriate spectral band. The angle of the scan mirror and the output of the detector were recorded on an FM-FM magnetic tape recorder.1961

The experiment first flew on 2 May 1963 (flight 3-16-26) and made five additional flights during 1963. Three of these six flights provided meaningful data for the MIT-Apollo horizon photometer experiment (#17). Another successful flight (3-30-50) on 8 July 1964 investigated the near infrared in the 0.8-2.8-micron region. After the flight, the experiment returned to Langley for modifications, and was intended to fly at least three more times. In the end, only two additional flights were flown during 1965 (3-42-65 and 3-44-67), since the more sophisticated MIT experiment had already begun flying aboard X-15-1.

Langley was generally happy with the X-15 as an experiment platform: "Not only is the design of the experiment simplified because there are few restrictions due to size and weight limitations, but also the availability of standard X-15 attitude and position data are an important advantage… the radiometer is reusable… and good weather data is available." This was in contrast to sounding rockets that provided comparatively short flights, had minimal onboard instrumentation, and, of course, were not generally recoverable.-1971

This experiment provided the first infrared data gathered on the Earth’s limb from above 30 miles. From these data, researchers modeled the horizon profile to an accuracy of 4 kilometers for use in attitude-referencing systems carried aboard early orbiting spacecraft.1981

John Becker is widely regarded as the father of the X-15, having served as the leader of the Langley researchers who defined the general concept of a hypersonic research airplane.

John Vernon Becker was born in 1913 in Albany, New York. He earned a bachelor of science degree in mechanical engineering (aero option) in 1935 and a master of science degree in aero engineering in 1936, both from New York State University. He joined the NACA Langley Memorial Aeronautical Laboratory as a junior aeronautical engineer in 1936.-^

Becker served as head of the 16-foot high-speed wind-tunnel branch from 1943 until 1947 and chief of the Compressibility Research Division from 1947 through 1957. During this time Becker contributed to the design and understanding of the Bell X-1, Bell X-2, Douglas X-3, the Century Series fighters, and the XB-70, in addition to his work on the X-15. In 1958 Becker became the division chief of the Aero-Physics Research Division, a position he held until his retirement in 1974. During that time he contributed to the X-20 Dyna-Soar and various lifting-reentry vehicles that led to the Space Shuttle, Mercury, Gemini, and Apollo, as well as Project FIRE, Sprint, the hypersonic cruise vehicle, the hypersonic research engine, and others. After he retired from NASA, Becker was a consultant for the General Applied Sciences Laboratory, Burns and Roe, and the NASA Office of Aeronautics and Space Technology.

Becker authored over 50 research papers in addition to numerous technical journal articles. In 1955, New York University cited Becker as one of its 100 outstanding graduates from the College of Engineering. He received the Sylvanus Reed Award from the American Institute of Aeronautics and Astronautics (AIAA) for 1960, and received other AIAA awards in 1961, 1968, and 1973. Becker delivered the 3rd Eugen Sanger Memorial Lecture at the Deutsch Geaellschaft Fur Luftfahrforschung, Bonn, Germany, in December 1968.-15

Joe Walker flew the X-15 for 41 months, from 25 March 1960 until 22 August 1963, making 25 flights. These included five flights with the XLR11 and 20 with the XLR99. Walker reached Mach 5.92, a maximum speed of 4,104 mph, and an altitude of 354,200 feet. His accomplishments include the first government flight, the maximum speed (4,104 mph) flight of a basic X-15, and the maximum altitude (354,200 feet) flight.

Joseph Albert Walker was born on 20 February 1921 in Washington, Pennsylvania, and graduated from Washington & Jefferson College in 1942 with a bachelor of arts degree in physics. During World War II he flew P-38 fighters for the Army Air Forces in North Africa, earning the Distinguished Flying Cross and the Air Medal with seven oak leaf clusters.

He joined the NACA in March 1945 at the Aircraft Engine Research Laboratory (now the Lewis Research Center), where he was involved in icing research and spent many hours flying into the worse weather the Great Lakes region could dish out. He transferred to the HSFS in 1951 and became chief pilot in 1955. He served as project pilot on the D-558-1, D-558-2, X-1, X-3, X-4, X-5, and X-15, and flew the F-100, F-101, F-102, F-104, and B-47. He was the first man to pilot the Lunar Landing Research Vehicle (LLRV) used to develop piloting and operational techniques for lunar landings.

Prior to joining the X-15 program, Walker did some pioneering work on the concept of reaction controls, flying an JF-104A to peak altitudes of 90,000 feet. The indicated airspeed going over the top of this maneuver was less than 30 knots, providing an ideal environment for evaluating

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the reaction control system.

Walker was a joint recipient of the 1961 Robert J. Collier Trophy presented by President John F. Kennedy at the White House in July 1962. Walker also received the 1961 Harmon International Trophy for Aviators, the 1961 Iven C. Kincheloe Award, and the 1961 Octave Chanute Award. He received an honorary doctor of aeronautical sciences degree from his alma mater in June 1962, and in 1963 the National Pilots Association named him Pilot of the Year for 1963. He was a charter member of the Society of Experimental Test Pilots, and one of the first to be designated a fellow. Tragically, Walker died on 8 June 1966 in a mid-air collision between his F-104 and the second XB-70A.[28] On 23 August 2005, the Walker family received a set of astronaut wings honoring Joe Walker’s flights above 50 miles altitude in the X-15.

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

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

1966 FLIGHT PERIOD

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-

126:[290]

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

[2911

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

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-

By late 1961, most of the people involved in the flight program expected it to end in December 1964. This would allow an orderly investigation of the remaining aero-thermo environment, an evaluation of the MH-96 adaptive control system, and a few follow-on experiments. This was in general agreement with the original 1959 Air Force plan, although it consisted of only 100 flights instead of the anticipated 300 flights.11681

A quick look at the labor required to support the X-15 shows that it was not a small program. The following table counts only government employees, not contractors, in "equivalent" man years, meaning that there may have been more people actually supporting the program than shown, but they were doing so on a less than full-time basis. In general, the Air Force figures consisted of

about 55% civil servants and 45% military personnel. The Air Force paid the civilians an average of $8,370 per annum at the ASD and $7,850 at the AFFTC. The FY65 numbers reflect the period between June 1964 and December 1964 (the government fiscal years at the time ran from 1 July to 30 June).[169]

The next table shows the projected propellant and gas requirements at the same point in the program:[170]

required for the operation of the NB-52s, other support and chase aircraft, propellant analysis and servicing, instrumentation, data processing and acquisition, photo lab, biomedical support, engineering, and test operations.

X-15 FLIGHT RESEARCH PROGRAM

ЧЬЪЕДКОт ДІКНІ. ІЖЕ. ____ COMMIT TEL

JASA ІГШУ :U£AF

SFERFIY —1

The X-15 program was a joint venture between the Air Force, Navy, and NASA, although the Navy generally played the role of a silent partner. The Air Force developed and paid for the airplanes and operated much of the support infrastructure at Edwards AFB, while NASA flew the airplanes (often with military pilots) and performed the maintenance. This 1961 organizational chart delineates the various interrelationships. In all, it worked well. (NASA)

Additional funds were budgeted for travel. Again, these are only Air Force funds; the equivalent NASA funding could not be ascertained.

20 Dyna-Soar. At the time, Honeywell had tested the system on a McDonnell F-101 Voodoo, but many researchers wanted to get some high-performance experience with the system prior to committing it to space flight on the X-20. When an XLR99 ground test almost destroyed X-15-3, the Air Force seized the opportunity to include a prototype MH-96 when North American rebuilt the airplane.171

The MH-96 was the first command augmentation system with an adaptive gain feature that provided invariant aircraft response throughout the flight envelope. The MH-96 used a rate command control mode whereby a given control-stick deflection would produce a specific rate response for the airplane. For example, a 1-inch pitch stick deflection would result in a 5- degrees-per-second pitch rate, regardless of how far the control surfaces had to deflect to produce that rate. This meant that the response would be the same regardless of airplane speed or flight condition.1721

In a conventional aircraft of the period, the pitch rate response would vary with airspeed. In an airplane with a large speed envelope, such as the X-15, a 1-inch stick deflection with a conventional system could result in such disparate responses as almost none at low speed to an extremely violent one at high speed. The MH-96 was an attempt to cure this. However, nothing comes free. With an invariant response, the pilot lost the "feel" for the airplane; for example, the controls did not become sloppy as it approached a stall. The system automatically compensated for everything right up to the point that the airplane stopped flying. The same problem would confront the first fly-by-wire systems.-11731

The rate-command system eliminated the need to modify the trim settings because of configuration changes, such as deploying the landing gear or flaps. The system also masked any shifts in the center of gravity. In many respects, these were good things because they eliminated mundane tasks that otherwise needed to be accomplished by a pilot who already had his hands full of rocket plane. On the other hand, they eliminated many of the normal cues the pilots used to confirm that certain things had happened, such as the trim change after the landing gear deployed. It took an open mind-and some experience-to get comfortable flying the MH-96.-11741

The MH-96 was potentially superior to the basic flight-control system installed in the other two airplanes, for a couple of reasons. The first was that it was more redundant than the SAS (even after the ASAS was installed), which eliminated many of the concerns of flying to high altitudes. Also, the MH-96 blended the ballistic controls and the aerodynamic controls together beginning at 90,000 feet. The pilot moved the same stick regardless of altitude and the MH-96 decided which controls were appropriate to command the airplane. The MH-96 also offered a few autopilot modes (such as roll hold, pitch-attitude hold, and angle-of-attack hold) that significantly reduced the pilot’s workload during the exit phase.-1751

The system minimized any extraneous aircraft motions by providing much higher damper gains. The pilots appreciated this feature particularly during altitude flights, and X-15-3 was designated the primary airplane for altitude flights. Neil Armstrong had been heavily involved in the development and evaluation of the MH-96 and made the first four evaluation flights with the system.

North American moved the rebuilt X-15-3 from Inglewood to Edwards on 15 June 1961, and finally delivered the airplane, along with the XLR99 and MH-96, to the government on 30 September. After various ground tests were completed, Neil Armstrong attempted to take the airplane for its first flight on 19 December 1961, but a problem with the XLR99 resulted in an abort. The flight (3-1-2) successfully launched the next day, with additional flights on 17 January

and 5 April 1962. As it turned out, the MH-96 worked remarkably well, but Armstrong and others realized the system would require a considerable period of evaluation before researchers could thoroughly understand it. The MH-96 provided good service to the X-15 program until a fateful day in 1967.-176