SP-4220 Wingless Flight: The Lifting Body Story





[155] By 1969, the lifting-body program had become a major activity at the NASA Flight Research Center, Ames, and Langley. The Air Force Flight Test Center was vigorously supporting the flight-test part of the program for the M2-F3, HL-10, and X-24A. However, I was becoming concerned that a disproportionate amount of our effort was going into supporting only one type of lifting body.

The M2-F3, HL-10, and X-24A were configurations with high volumetric efficiencies, best suited for shuttle-type missions in deploying satellites and in carrying cargo and people to and from earth orbit. All three had hypersonic lift-to-drag ratios between 1.0 and 1.4, permitting a potential cross-range capability of 700 to 1,000 miles-that is, they could range from 350 to 500 miles to either side of the orbital path during re-entry. They also had adequate lift-to-drag ratios for landing.

To me, the M2-F3, HL-10, and X-24A were the lifting-body "plow-horses," and I was becoming interested in a different kind of lifting body, a class of vehicles I considered the "racehorses." The shapes of these lifting bodies had high fineness ratios with long pointed noses and flat bottoms. The more efficient of these shapes had hypersonic lift-to-drag ratios as high as 3.0, allowing a cross range of 3,000-the ability to range 1,500 miles to either side of the orbital path. A hypersonic vehicle with a lift-to-drag ratio greater than 3.0, of course, would be considered at the top of its class in performance.

The "racehorse" class of lifting bodies could be used for special missions where flexibility was required, being able to land anywhere on earth on short notice. However, the slender shapes would not lend themselves to serving as efficient cargo containers. While these vehicles would have high aerodynamic efficiencies at hypersonic speeds, they wouldn't perform well at landing speeds and likely would need some sort of deployable wings to land.

Two of these "racehorse" shapes were the Hyper III developed by NASA Langley and the FDL-7 developed by the Air Force Wright Flight Dynamics Laboratory in Dayton, Ohio. There is some question about whether the Hyper III and the FDL-7 were true lifting-body configurations since they both had small deployable wings used for landing. Both can be called special forms of the lifting body, however, since the small wings would be stowed during most of the projected re-entry flight before landing. Another of the lifting-body features that each possessed was that, even with the wings deployed, the body still dominated the aerodynamics of the total configuration.



Graph showing cross range distances in miles plotted against hypersonic lift over drag for several vehicles returning from orbit. Notice that the <<race horse>> vehicles such as the X-24B and Hyper III have the greatest cross-range capability- around  2,500 miles.

Graph showing cross range distances in miles plotted against hypersonic lift over drag for several vehicles returning from orbit. Notice that the "race horse" vehicles such as the X-24B and Hyper III have the greatest cross-range capability- around 2,500 miles. [Click here for a larger image]


Model-Testing of Lifting-Body Spacecraft


By 1969, I was outside the mainstream of the on-going lifting-body program at the Flight Research Center, busy looking at new concepts and projects further into the future. Using the excellent radio-control equipment then becoming available to model-airplane hobbyists, I teamed up with Dick Eldredge to conduct several experiments in flying models of experimental spacecraft. We worked with what was called the "de-coupled mode" in which the basic re-entry vehicle is flown down to a certain point and then converted to a landing configuration by deploying either a gliding parachute or wings of some sort.

[157] Eldredge had been the first research engineer to join my M2-F1 lifting-body team seven years earlier, and I still thought of him as my "little buddy." Although we had remained in contact with each other throughout the lifting-body buildup program, since the early days of the M2-F1 we had not had the time to brainstorm together about new ideas. This situation began to change after I got out of management with the lifting-body program in 1965 and, by 1969, I was free to think about new ideas again.

Over the years I have often compared the relationship between me and Dick Eldredge with that of the Wright Brothers. I thought of Eldredge as being a sort of brother off whom I could bounce ideas and from whom I could get constructive feedback, much as the Wright Brothers did between themselves during the first part of their career. Even the progressive changes in our careers bore some resemblance to those experienced by Orville and Wilbur Wright. At first, the Wright Brothers treated aeronautics as a hobby and had fun. All innovation begun early in their career stopped, however, once they became businessmen and project managers. They had no time for experimentation or research once they entered competition with Glen Curtiss and others and became involved with legally protecting their wing-warping and other patents. By that time, aviation was no longer fun for the Wright Brothers. It had become serious business.

I have noticed that the same changes often occur within the careers of many innovative individuals who are motivated by fun as well as the satisfaction they receive from creating something that has never existed before. When these people enter the business world, however, they often become unhappy, their productivity diminishing. I believe I made the right decision when I took Paul Bikle's advice in 1965 and got out of management with the lifting-body program. When I returned to engineering, I essentially returned to the realm of innovation.

As I learned from my own experience over the years, NASA Headquarters operates in such a way that priority and attention tend more easily to be given to large and costly projects. Experiments or projects by two people or a small group generally do not fit into the scheme of things at NASA Headquarters. In fact, until a project has been supervised by NASA Headquarters, pondered for some time there, and then officially blessed, it usually is not considered important by headquarters people.

Nevertheless, the small projects that result from brainstorming at the NASA centers are often exciting for those who originate them and literally love the work they do. I don't think, on the other hand, that most managers at NASA Headquarters trust those who have too much fun while working. In fact, these managers coined the term "hobby-shop projects" for referring disparagingly to projects originating outside of the mainstream and control of the master plan.

Dick Eldredge and I, however, were intrigued with the idea of doing the first flight-testing of a sleek "racehorse" configuration with a pointed nose, a design we believed would give superior performance at hypersonic speeds. As we continued our radio-controlled model flying of lifting-body spacecraft, we tested models of both the "racehorse" Hyper III and the "plow-horse" M2-F2, using a Rogallo Limp Wing gliding parachute for recovery.

[158] We also designed and built a special twin-engined, 14-foot model mother-ship for carrying the lifting-body models to altitude and launching them, much as was being done with the B-52 for the full-scale lifting bodies. We envisioned future space missions where there might be a need to use the vehicle's hypersonic lateral cross-range capability to reach a meadow in Alaska, for example, and land the vehicle there softly and slowly by means of a gliding parachute for some covert military mission. Our imaginations also came up with a mission that used the hypersonic lateral range of the vehicle to take an injured astronaut back to earth, landing in a field near the hospital best able to provide the care needed.

One of us would fly the mothership by radio-control with the lifting-body model attached to its belly with a hook activated by remote control. The other would take charge of the experimental lifting body, flying it after air-launch on its own aerodynamics, then controlling it through steering control lines to a landing on its gliding parachute.

We found that the Hyper III's extremely low lift-to-drag ratio of 2.5 made it impractical to land without either a gliding parachute or deployable wings. We experimented with three types of deployable wings for the Hyper III. The first was a pair of switchblade wings that pivoted out of slots in the lower part of the body. The second was a one-piece wing that pivoted in the center and was stowed in the upper portion of the body, the right half of the wing exiting from a forward slot on one side and the left half exiting from a rearward slot on the other side. With this second type of wing, after rotating 90 degrees, the final configuration for landing was a straight wing mounted high on the body.

The third type of wing we tried was the Princeton Sailwing that had been tested in the NASA Langley full-scale wind-tunnel on a conventional glider fuselage. The Sailwing involved two D-shaped spars stowed in two slots in the body and deployed like a switchblade wing, with trailing edge cables pulling taunt from a tip rib and stretching upper and lower fabric membranes from the spar to the cable. The fabric surfaces would then curve upward, like sails on a boat, forming a cambered airfoil and producing positive-lift airflow over the wing.


Hyper III and Parawing


Our second type of deployable wing-the one-piece pivoted design-proved to be the best of the three for actual flight. NASA Langley conducted wind-tunnel tests on the Hyper III without the wing up to Mach 4.6, followed by tests with the wing deployed at subsonic speeds. I put together a plan for building a full-scale vehicle at low wing-loading similar to the M2-F1. However, I proposed to fly it without a pilot onboard. The idea of flying unpiloted vehicles at the Flight Research Center was unpopular, especially with the pilots. Paul Bikle would approve the plan only if I would build the vehicle so that a cockpit could be installed for a pilot to fly it after the initial tests were completed. Later, an X-15 type of canopy would be added slightly forward of the wing to balance the piloted version.

[159] In spite of the success of the on-going rocket-powered lifting-body program, NASA Headquarters still was not tolerant of programs as small as that of the original lifting body, the M2-F1. For this reason, I was very interested in developing a flight-test approach with the pilot doing the early hazardous flight tests in a simulator-type cockpit on the ground. This approach would put us in a better position later for getting approval for the more expensive piloted flight tests.

I managed to convince Paul Bikle that this approach had merit and we ought to give it a try. However, the idea went over like a lead balloon with the pilots. In the end, I had to turn once again to Milt Thompson for help. Even though Thompson had retired from flying, he was intrigued with the idea and offered to fly the Hyper III from a ground-based cockpit.

By this stage in 1969, I had two projects developing at the same time. The gliding parachute tests that Dick Eldredge and I had been doing with spacecraft models had attracted the interest of the NASA Johnson Space Center. I discussed our use of the limp Rogallo parachute in recovering spacecraft models with Max Faget, Johnson's director of engineering who had played a major role in designing crewed spacecraft starting with Project Mercury.1 Not yet accepting horizontal landing as appropriate for the next space mission, Faget at the time was still backing gliding parachute concepts such as the "Big G," a twelve-astronaut version of the Gemini space capsule with one astronaut steering the capsule to flare and landing at a ground site.

While talking with Faget, I offered to develop a one-pilot test vehicle that could be launched from a helicopter and used to test a pilot's ability to fly the vehicle while looking through the viewing ports typical of spacecraft. I suggested we fly the vehicle at first by radio-control with just a dummy onboard until it was determined to be safe to fly. Faget just happened to have a borrowed Navy SH-3 helicopter that was being used to practice fishing Apollo astronauts out of the water. Enthusiastic about my idea, Faget offered to let us have the helicopter for a month, plus enough money to buy large-sized Rogallo Parawings for the project.

Paul Bikle approved our Parawing Project, as it was called, and assigned NASA pilot Hugh Jackson to it. Although he was the new kid on the block among the other NASA pilots, Jackson was considered the resident expert in parachuting, having parachuted four or five times. At best, Jackson was lukewarm about participating in the Parawing Project. He likely accepted the assignment because he wasn't yet allowed to fly the NASA research aircraft.

Dick Eldredge designed the vehicle for the Parawing Project. It was built in the shops at the NASA Flight Research Center. Since we were experienced scroungers and recyclers by this time, we used surplus energy struts from the Apollo couches in the vehicle to soften the load on the pilot in hard landings. The M2-F2 launch adapter not being used with the B-52, we used its pneumatic hook-release system to launch [160] the vehicle from the side of the SH-3 helicopter. For the test configuration, we used a generic lifting-body ogive shape with Gemini viewing ports. We attached landing skids with energy straps to an internal aluminum structure containing the pilot's Apollo couch. A general-aviation auto-pilot servo was used to pull down on the parachute control lines. The pilot used a small electric side-stick to control the servo.

The plan was that before putting a pilot onboard, we would launch the lifting-body with the dummy in the pilot's seat off the side of the helicopter, deploy the parachute, then steer the vehicle to the ground by radio-control, using model-airplane servos to move the pilot's control stick. We even tied the dummy's hands in its lap so it wouldn't interfere with the control stick. Measured accelerations in the dummy and on the airframe were transmitted to the ground to record shock loads as the parachute opened...


Hyper III with single-piece, pivot wing installed. Flexible Princeton sailwing is on the ground to be installed for future tests (never performed),  and one of the fabricators of the Hyper III, Daniel C. Garrabant, is standing next to it.

Hyper III with single-piece, pivot wing installed. Flexible Princeton sailwing is on the ground to be installed for future tests (never performed), and one of the fabricators of the Hyper III, Daniel C. Garrabant, is standing next to it. (NASA photo E69 20464)


...and the vehicle made ground contact. By moving the pilot's stick directly with the radio-controlled servos, we qualified the entire control system downstream of the pilot's control stick.

[161] Dick Eldredge stayed with the Parawing Project until the system had been qualified for piloted flight following 30 successful radio-controlled flights. Hugh Jackson was getting ready to make his first flight in the vehicle when the NASA Johnson Space Center decided that the next piloted space program would not make use of a gliding parachute system but would use a horizontal-landing spacecraft instead. I think Jackson was relieved when he heard this news that made his flight unnecessary. A few months later, he left the pilots office at the NASA Flight Research Center.


Hyper III Team


Meanwhile, Dick Fischer had himself assigned as the operations engineer on the Hyper III. Fischer had other aircraft obligations, but his bosses agreed to the assignment after I had accepted the decision of management at the Flight Research Center that the Hyper III program would be conducted on a low-priority basis. A long-time friend of mine and a model-airplane flying buddy, Dick Fischer was also an excellent designer of home-built aircraft who restored antique aircraft in his spare time.

Together, Fischer and I recruited Bill "Pete" Peterson, a control-system engineer on the X-15 program, to help design the control system for the Hyper III. Peterson had worked earlier for Honeywell in Minneapolis, designing the adaptive control system for the X-15. As a Honeywell employee, he had come initially to the Flight Research Center during the X-15 flight tests to help NASA with operating the X-15's control system. He was then hired by the Flight Research Center to continue working with the control system on the X-15 and other aircraft. Peterson managed to find time to help us with the Hyper III, despite the fact that he was involved with four other aircraft at the Flight Research Center at the time.

On the Hyper III, I managed to use volunteers in the same way I had originally with the M2-F1, thanks to the influence of Paul Bikle. As in the days of the M2-F1, we found that NASA supervisors were tolerant when engineers such as Pete Peterson wanted to work on volunteer projects like the M2-F1 or Hyper III and could do so while still meeting their obligations on assigned projects.

Dick Fischer designed the structure of the Hyper III, and the vehicle was built in the NASA shops. When finished, it was 35 feet long and 20 feet wide at the tail surfaces. The fuselage was basically a Dacron-covered steel-tube frame, the nose was made out of molded fiberglass, and the four tail surfaces were constructed of aluminum sheet-metal. The aluminum wing was built from the wing kit for an HP-11 sailplane.

Frank McDonald cut and fitted the steel-tube body, and Howard Curtis did the welding. NASA aircraft craftsman Daniel "Danny" Garrabrant- a highly skilled builder of model aircraft and of home-built wooden and aluminum full-size sailplanes-assembled the wing for the Hyper III. LaVern Kelly assembled the vehicle's sheet-metal tail surfaces.

Many of the people who worked on the M2-F1 worked as well on the Hyper III, including aircraft inspector Ed Browne and painter Billy Shuler. We worked closely [162] with the NASA fabrication shops to get the Hyper III structure completed without interfering with the shops' work on other, prioritized projects. With the X-15 program winding down, I managed to recruit even more talented volunteers to work part-time on the Hyper III, including crew chief Herman Dorr and mechanics Willard Dives, Bill Mersereau, and Herb Scott.

Our skills in scrounging and recycling came in handy in building the control system for the Hyper III, which was composed of an uplink from a Kraft model-airplane radio-control system. The control surface on each of the two elevons was driven by a surplus miniature hydraulic system from the Air Force's PRIME lifting-body program. The hydraulic system was a battery-driven pump that had run two actuators for the elevons on the PRIME vehicle.

Peterson cleverly designed the system to operate from either of two Kraft receivers, depending on the strength of the radio signal at the top or bottom of the Hyper III, one receiver mounted on the top and the other mounted on the bottom of the vehicle. If either receiver malfunctioned or picked up a bad signal, an electronic circuit switched to the other receiver. Signals from the operating receiver controlled the two elevon surfaces driven by hydraulic actuators. A talented hydraulics engineer, Keith Anderson modified the PRIME hydraulic actuator system for the Hyper III.

In case we lost control during the flight tests, we mounted an emergency parachute-recovery system in the base of the vehicle. It consisted of a drogue chute that fired aft, extracting a cluster of three paratrooper-type chutes that would lower the vehicle onto its landing gear. The Northrop support contract still in effect, I managed to get the help of Northrop's Dave Gold for a few weeks. A top parachute designer, Gold had done most of the detailed design of the parachute system used on the Apollo spacecraft. Gold and John Rifenberry from the NASA pilots' life-support shop worked steadily for two weeks at the sewing machines in Rifenberry's shop while completing the vehicle's parachute-recovery system. The Flight Research Center's expert on pyrotechnics, Chester Bergner assumed the responsibility for the drogue firing system.

We tested the emergency parachute-recovery system by putting the Hyper III on a flatbed truck and firing the drogue extraction system while we were racing across the dry lakebed, but a weak link kept the three main parachutes from jerking the Hyper III off the truck. We then tested the clustered main chute by attaching it to a weight that equaled that of the Hyper III and dropping it from a helicopter. We were very fortunate that the emergency parachute-recovery system never needed to be used.

With the help of Don Yount as instrumentation engineer and Chuck Bailey and Jim Duffield as instrumentation technicians, a 12-channel FM/FM down-link telemetering system recorded data and drove instruments in the ground cockpit. Assembled by Tom McAlister, the ground cockpit was made out of plywood and looked somewhat like a Roman chariot when it was hauled out to the landing site on a two-wheeled trailer. The instruments in the ground cockpit were identical to those in our fixed-base simulator. In the center of the display, an artificial-horizon ball indicated roll, pitch, heading, and sideslip. Other instruments in the ground cockpit showed air speed, altitude, angle of attack, and control-surface positions.


[163] First Flight of the Hyper III


Bruce Peterson piloted the borrowed Navy SH-3 helicopter that towed the Hyper III aloft for its first flight on 12 December 1969. A Marine Corps pilot before joining NASA, Peterson continued to fly helicopters and jet fighters in the Marine reserves on a restricted basis following the M2-F2 crash that cost him his vision in one eye. After the crash, Peterson also flew support aircraft during various NASA flight-research missions, although he was not allowed to fly the actual research aircraft. The first flight of the Hyper III was the last lifting-body mission in which Bruce Peterson and Milt Thompson would directly participate.

After liftoff, with the Hyper III attached to the helicopter at the end of a 400-foot cable, Peterson had a difficult time getting the Hyper III to track straight on the end of the tow-line. Afterwards, we realized that we should have installed a small drag chute on the Hyper III that could have been jettisoned after launch. As Peterson struggled to get the vehicle to track straight, Milt Thompson sat in the ground cockpit located beside the planned landing site on the lakebed, relaxed and smoking a cigarette.

After starting and stopping forward flight several times during the climb, Peterson eventually got the Hyper III to stabilize in a forward climb. When Peterson radioed that he was ready to launch, Thompson flipped his cigarette onto the lakebed and hunched over the controls, intently ready to fly the Hyper III. Peterson towed the Hyper III to 10,000 feet above the dry lakebed, where the Hyper III was released from the tow-line by an electric cargo hook. For this first flight, the Hyper III was flown with the wing fixed in deployed position, the configuration that would be flown in a final low-speed approach and landing after re-entry from space.

Peterson dropped the Hyper III in forward flight on a downwind path with a northerly heading, Thompson controlling the Hyper III from the ground cockpit. Thompson flew the vehicle in a glide three miles north, guided it into a 180-degree turn to the left, and then began steering it the three miles to the planned landing site. During the straight portions of the flight, Thompson had performed research doublet and oscillation maneuvers so we could extract aerodynamic data following the flight.

Since Thompson was flying strictly by instrument flight rules in the ground cockpit with his head down, I asked Gary Layton in the control room at the Flight Research Center to watch a radar plotboard and guide Thompson by radio to landing position. Layton had often helped lifting-body pilots in this way in the past as they steered to landing sites on the lakebed runways. Since we had no experience yet in landing unpiloted vehicles at the Flight Research Center with the use of onboard video, we had not installed a forward-looking video camera in the Hyper III. Dick Fischer was standing beside Thompson in the ground cockpit to take control of the Hyper III just before the landing flare, using the model-airplane radio-control system's box during the landing-flare maneuver to touchdown.

[164] Although the sky over Edwards Air Force Base is often clear, on this particular day in December, the sky was hazy with moisture. While the Hyper III could be seen from the ground cockpit when it was overhead, it could not be seen through the haze when it was slanted at an angle three miles away. Without visual contact with the Hyper III, Fischer had to rely on Thompson's comments to know how the vehicle was flying, Thompson steadily watching the gauges in the cockpit.

On the final approach to landing, with Thompson calling out altitudes, Fischer strained to see the Hyper III through the haze. As the Hyper III broke through the haze at about 1,000 feet, Dick said, "I see it!" Thompson replied, "You've got it!" and switched control to Fischer's model-airplane control box.

Noticing no response from the vehicle as control was transferred, Fischer deliberately input a roll to verify that he indeed had control before he executed the landing flare. Still monitoring the gauges, Thompson told Fischer that the vehicle was rolling left, and Fischer replied that, yes, he had commanded it to roll. Now certain that the vehicle was responding to his control, Fischer used the pencil-sized control sticks on his box to bring the Hyper III level and complete the flare to a soft landing. The Hyper III slid safely to a stop on its three skids, landing on the lakebed in front of Fischer and Thompson in the ground cockpit.

We were gratified by the successful first flight of the Hyper III, having gotten the flight scheduled at our last possible opportunity for using the SH-3 helicopter before it was returned the next day to the Navy. Later, as quoted in a paper that I presented at an AIAA conference, Thompson described his experience flying the Hyper III from the ground cockpit.

"During my first attempts to change the vehicle's heading," Thompson said, "the vehicle appeared to be marginally stable or even unstable in roll. Vehicle motions in response to roll-control input seemed to be erratic and much too rapid when compared to the simulation. When faced with a situation of this type in a flight or in a simulator, I have always found the best procedure is to let go of the control stick momentarily to determine whether the vehicle is inherently stable. The Hyper III motions damped immediately after the stick was released, indicating adequate levels of stability and damping. I had simply been over-controlling and exciting a pilot-induced oscillation. The over-controlling resulted from much higher roll-control effectiveness than had been predicted." 2

The lift-to-drag ratio of the Hyper III turned out to be lower than expected. Rather than 5.0 maximum, it proved to be 4. Thompson had had to stretch the glide as much as possible to bring the Hyper III close enough for Dick Fischer to be able to see it and land it. Twice, as Thompson pointed out, the flight had shown that a research pilot [165] could use actual flight experience to compensate for significant deficiencies in or departures from predicted aerodynamic characteristics.

Before the flight, Thompson had worried that the lack of motion cues, particularly during short-period motions of the vehicle, might hurt his performance in piloting the Hyper III from the ground cockpit. "This apprehension was quickly dispelled once the vehicle was launched," Thompson said. "It seemed very natural to fly the gauges, just as in the simulator, and respond to what I saw rather than what I felt."

What Thompson found surprising were his reactions during the flight. "I was really stimulated emotionally and physically, just as in actual first flights," he said. After noting that he had made the first flights in such "strange vehicles" as the Paresev and the M2-F1, he said, "Flying the Hyper III from a ground cockpit was just as dramatic."

In explaining how the experience differed from flight simulators, Thompson said, "I have flown many different simulators with and without motion and visual cues, including centrifuge and airborne simulators. Although some provided a lot of realism, none stimulated me emotionally. I always knew I could hit the reset button, or in the airborne simulators, turn the vehicle back to the conventional testbed aircraft characteristics. There was no question with the Hyper III. I, and only I, had to fly it down to the landing location."

According to Milt Thompson, his experience in flying the Hyper III "tends to confirm the theory that responsibility rather than fear for personal safety is the real driver of physiological response." 3


NASA Headquarters Says "No" to Hyper III Piloted Flights


Our single flight of the Hyper III produced good aerodynamic data and demonstrated that the vehicle was safe to fly. By early 1970, I had located in Arizona the ideal aircraft for launching the Hyper III in a piloted flight program, an Air Force Albatross SA-116B seaplane with low flight time that had never been in the water, had no corrosion, and was in excellent condition. The aircraft was available to NASA as Air Force surplus. The Albatross had sufficient structure, control authority, and performance capability for carrying the Hyper III aloft under its wing at the 2,000-pound drop-tank location for air launch at 15,000 to 20,000 feet.

Paul Bikle asked NASA Headquarters to substitute the Albatross for the C-47 currently in use at the Flight Research Center as a utility aircraft. Trading the C-47 for the Albatross on a one-to-one basis would involve no additional cost to aircraft operation at the Flight Research Center. We could also make better use of the Albatross than the C-47, for only the Albatross could serve a dual purpose, being used as a utility aircraft when it wasn't being used in air launches.



Retired M2-F1, Hyper III, and remote control models on display.

Retired M2-F1, Hyper III, and remote control models on display. (NASA photo EC70 2450)


Bikle's request was turned down. By 1970, NASA Headquarters was caught up in the throes of internal politics, flexing its muscles as it cut authority within the various NASA centers for planning their own research. Without a launch vehicle such as the Albatross, the Hyper III would never achieve piloted flight. In this way, the Hyper III fell victim to political currents within NASA Headquarters.

The Hyper III program had three strikes against it. First, it was too low-cost to get the attention and support of NASA Headquarters. Second, it had been flight-tested as an unpiloted vehicle first, taking away some of the luster it would otherwise have had if first flown as a piloted vehicle. Third, it was a variable-geometry configuration, making it less competitive in weight and complexity than the simpler lifting-body configurations.

Paul Bikle was very angry when NASA Headquarters rejected his request for the Albatross. He saw the Albatross as a tool for the Flight Research Center and, as the director of the Center, he felt he should be able to select his own tools, especially when a tool was not going to cost NASA extra money. At the time, I think he was also seeing the writing on the wall, sensing that he was becoming a dinosaur, a relic of not only earlier aviation history but also NASA's previous decades of vigor, glory, and innovation. It was only about a year later-on 31 May 1971-that Paul Bikle retired from NASA.




Schematic showing the X-24A conversion to the X-24B. This was a cost-saving approach to use the same systems for both configurations.

Schematic showing the X-24A conversion to the X-24B. This was a cost-saving approach to use the same systems for both configurations. [Click here for a larger image]



A Racehorse of Another Color: the X-24B


While we were still involved with the Hyper III, Alfred Draper and others at the Air Force Flight Dynamics Laboratory in Ohio had come up with an idea for recycling the X-24A by wrapping a new shape around it. They found that the new configuration could achieve hypersonic lift-to-drag ratios near 2.5, putting it into the same "racehorse" category of lifting body as the Hyper III which, before its wing was deployed for landing, had a hypersonic lift-to-drag ratio near 3.0. The other lifting bodies-the M-2, HL-10, and X-24A- had hypersonic lift-to-drag ratios between 1.2 and 1.4.

A distinct advantage over the Hyper III was that the new X-24A wrap-around-shape designated the FDL-8 could achieve a landing lift-to-drag ratio of 4.0 without variable geometry. The more slender shape of the Hyper III gave it the higher hypersonic lift-to-drag ratio of the two lifting-body shapes. However, the Hyper III had a landing lift-to-drag ratio near 2.0, making it necessary to use a deployable wing to bring the vehicle's subsonic lift-to-drag ratio up to near 4.0 for landing.

Al Draper and his colleagues at the Air Force Flight Dynamics Laboratory believed that flat-bottomed pointed shapes like the FDL-8 would prove to be useful not only for sustained hypersonic-cruise aircraft using air-breathing propulsion but also for unpowered boost-glide orbital re-entry vehicles capable of landing at virtually any convenient airfield. Furthermore, the long flat under-surface of the FDL-8 would make an ideal compression ramp for the inlet of a future supersonic combustion ramjet engine operating at speeds up to Mach 8.

[168] At Edwards, NASA director Paul Bikle and Bob Hoey, manager of the Air Force's lifting-body program, endorsed the idea. Always attuned to thrift, Bikle was in favor of ideas that saved government money by getting the most research out of each dollar spent, the same reason why he had readily endorsed my ideas for saving money by recycling rocket engines and sharing launch aircraft with other programs.

At this point, a critical stumbling block appeared. Major General Paul T. Cooper, chief of research and technology development for the Air Force, rejected the idea of using the X-24A as a basis for the test shape that would later be designated the X-24B. Clearly opposed to the entire flight-test concept, he asked that the proposal be reviewed by a joint panel of the Air Force Scientific Advisory Board and the National Academy of Sciences. Al Draper and Bob Hoey briefed the panel on the concept. The panel concluded that the Air Force could not afford to do without the project. Thus securely endorsed, the plan advanced rapidly.

By the end of August 1970, the directors of both the NASA Flight Research Center and the Air Force Flight Test Center at Edwards had agreed that such a program was worthwhile. However, Air Force Systems Command delayed approving the program until suitable arrangements had been made for joint funding by NASA and the Air Force. Paul Bikle asked John McTigue to work with Fred DeMerritte at NASA Headquarters to come up with the money needed to get the program started. Thanks to the teamwork of McTigue and DeMerritte, NASA transferred $550,000 on 11 March 1971 to the Air Force to initiate acquisition of the aircraft. The Air Force pledged a similar amount. Finally, on 21 April 1971, the director of laboratories for Air Force Systems Command okayed the program. On 4 June 1971, the X-24A completed its last flight.

On 1 January 1972, the Air Force awarded the modification contract to the Martin Marietta Corporation. A month later, on 4 February, Grant L. Hansen, the Air Force's assistant secretary of research and development, and John S. Forster, Jr., the director of defense research and engineering, signed a memorandum of understanding between the Air Force and NASA on conducting the X-24B program as a joint Air Force/NASA lifting-body venture. The memorandum was also signed by George M. Carr, NASA's deputy administrator, and Roy P. Jackson, NASA's associate administrator for advanced research and technology. The memorandum marks the official beginning of the X-24B program. Modifying the X-24A into the X-24B meant that the new research aircraft would cost only $1.1 million. The same vehicle, built from scratch, might have cost $5 million.

At the Air Force's Arnold Engineering Development Center, hypersonic wind-tunnel tests on a model of the X-24B indicated that the proposed shape performed well at those speeds. As usual, the big question was what would happen to performance when the vehicle decelerated to much lower velocities. Many, including Fred DeMerritte, expected surprises as the vehicle passed through the transonic range.



X-24B as delivered to Edwards. Notice that the original X-24A is completely disguised inside of the X-24B shape.

X-24B as delivered to Edwards. Notice that the original X-24A is completely disguised inside of the X-24B shape. (NASA photo E73 25283)


X-24B Shell Arrives at Edwards


On 24 October 1972, the X-24B shell built around the structure of the X-24A arrived at Edwards Air Force Base, delivered by Martin Marietta's Denver plant. Systems for the X-24B were delivered separately. The structure had grown an additional 10 feet in span and 14.5 feet in length. It weighed 13,800 pounds at launch, the X-24A having weighed approximately 12,000 pounds. The X-24B had a 78-degree double-delta planform for good center-of-gravity control, a boat-tail for favorable subsonic lift-to-drag ratio, a flat bottom, and a sloping three-degree nose ramp for hypersonic trim. The sides of the forebody aft of the canopy were sloped 60 degrees relative to the Y-plane (lateral, or left-to-right, axis).

The aerodynamic design features of the X-24B were quite distinct from those of the X-24A. Like the earlier lifting bodies, however, the X-24B also used several off-the-shelf components. Portions of its landing gear, control system, and ejection system came from the Northrop T-38, the Lockheed F-104, the Martin B-57, the Grumman F11F, the Convair F-106, and the North American X-15. It had an LR-11 rocket engine and Bell Aerosystem landing rockets.

Although the basic systems in the X-24B were the same as those in the X-24A, several upgrades and additions were made in the propulsion system, control system, and nose landing gear. The LR-11 rocket engine was modified, the vacuum thrust increased from 8,480 to 9,800 pounds by increasing chamber pressure and adding [170] nozzle extensions. The engine started at a lower thrust level with thrust increased after the engine was stabilized.

Two outboard ailerons were added to the eight control surfaces that had been on the X-24A. The HL-10 also had ten control surfaces (the subject of the standing joke that HL-10 stood for "Hinge Line Ten"). The two new control surfaces on the X-24B were used only for roll control with a plus or minus five-degree pitch bias feature. Unlike the X-24A, the X-24B's split upper and lower body flaps were not used for roll control. The pitch control and shuttlecock biasing of the X-24B, however, were the same as on the X-24A. The triply-redundant rate-damping system used in the X-24A was retained in all three axes on the X-24B with variable gain control by the pilot. Most of the other control system features of the X-24B, including the hydraulic power supply and rudder biasing linked to the body flap biasing for transonic stability, were the same as on the X-24A. The biasing on the X-24B could also be used by the pilot for speed brake control.

Basically the same automatic aileron-rudder interconnect system was used in both the X-24B and the X-24A, although the system in the X-24B had more flexibility in operation. The amount of interconnect was automatically programmed as a function of angle of attack. As in the X-24A, the pilot could select two interconnect angle-of-attack schedules, a high-gain one for transonic-supersonic conditions and a lower-gain one for control at subsonic speeds. The pilot could also use a manual interconnect mode as backup to the automatic scheduling or for special test maneuvers.

The X-24B retained the T-38 main landing gear that had been used in the X-24A. However, unlike the X-24A, the X-24B used a modified Grumman F-11F-1F nose gear. The combination resulted in an unusual arrangement of landing gear, similar to but not as extreme as that on the X-15. The main gear on the X-24B was significantly aft of the landing center of gravity, and the three-point attitude was nose low. The landing gear was a quick-acting (approximately 1.5 seconds) pneumatic system. The main gear deployed forward, the nose gear aft, minimizing not only the movement in the center of gravity but also the change in longitudinal trim. From the cockpit, the pilot could actuate the landing gear only to the down position.

While the cockpit controls and instruments were basically the same in both the X-24A and X-24B, the X-24B alone had an F-104 stick-shaker. The shaker actuated at 16 degrees angle of attack to warn the pilot that the vehicle was approaching an area of reduced pitch stability. Later in the X-24B flight-test program, to provide additional sideslip monitoring for the pilot, an audio sideslip warning system was added.


X-24B Team: Preparing for Flight Tests


Following the end of the X-24A flight-test program on 4 June 1971, the X-24A crew, led by operations engineer Norm DeMar, was disbanded for 16 months while Martin Marietta was contracted and the X-24A was transformed into the X-24B. During this time between the disbanding of the X-24A crew and the formation of the X-24B crew, DeMar lost his X-24A crew chief, Jim Hankins, to the new Digital-Fly-By-[171] Wire (DFBW) program that, using an F-8 fighter as a test-bed, would create the world's first truly digital fly-by-wire aircraft. In the F-8 fighter used in the DFBW program was a reprogrammed version of the computer used earlier to control the Apollo Lunar Landing Vehicle, another example of the sort of cost-savings practiced at the Flight Research Center by recycling equipment from earlier projects into new ones.

Many of the X-24B crew recruited by Norm DeMar had experience with rocket-powered aircraft, having been on the crews of either the X-24A or the X-15. Charley Russell, a crew chief on the X-15, became crew chief for the X-24B. Three of the X-24A mechanics-Mel Cox, John Gordon, and Ray Kellogg-were assigned as well to the X-24B crew. Other X-24B crew members included inspector Bill Bastow, instrumentation inspector Dick Blair, and aircraft inspector Gaston Moore.

DeMar and the X-24B crew managed to install systems in the X-24B and prepare for systems tests by February 1973, just three and a half months after Martin Marietta had delivered the X-24B as an empty shell. Rather than full-scale wind-tunnel tests, a very detailed set of eleven types of ground and captive-flight tests was scheduled by the two X-24B program managers, NASA's Jack Kolf and the Air Force's Johnny Armstrong, to be done during the six months between February and August 1973 before the first glide flight.

During structural resonance tests on the X-24B's control system, we found an unacceptable resonance in the ailerons. It was a purely mechanical resonance, sustained solely by the actuator and its linkage. To eliminate it, we added a mechanical damper to the actuator's servo valve.

We ran ground vibration tests on the horizontal and vertical tail surfaces to verify flutter clearance margins. Since the results were significantly different from the predicted mathematical model used by Martin Marietta, we reran the flutter analysis using the experimentally determined model data, finding flutter margins to be adequate.

To establish the relationships between applied loads and strain gauge responses, we did structural loads calibration tests on all ten movable control surfaces as well as on the left fin and strake. For use later in interpreting flight results, we also measured the outputs of strain gauges and derived the appropriate load equations.

As had been done on the earlier lifting bodies, the X-24B was hung at different angles to determine the vehicle's center of gravity, then crosschecked by weighing the vehicle while it was balanced on each wheel and tipped at various angles. We used the "rocking table" technique to determine pitch and roll inertias. The vehicle was also hung on a cable and oscillated, using springs attached at both ends of the vehicle, to determine yaw inertia and the product of inertia, the coupling between roll and yaw.

On the X-24B, we expected very high landing-gear loads during X-15-like "slap-down" landings due to its long nose, forward center-of-gravity relative to the location of the main gear, and its increased weight-1,800 pounds more than the X-24A. To provide additional tire capability, we had selected 12-ply T-38 tires for the X-24B, [172] rather than the 10-ply tires used on the X-24A. During dynamic load tests on the tires at Wright Patterson Air Force Base, however, the tread repeatedly separated from the tire casing at the anticipated loading. Later tests showed that shaving the tread from the tire through the first ply resulted in satisfactory tire performance. As a result, we decided that a new set of shaved tires would be used for each flight of the X-24B.

During drag-load tests on the main gear, we found that the down-load lock released when predicted drag loads were applied, which could result in gear collapse during landing. The crew reworked the locking device so that it would maintain a securely locked position.

We did "slap-down" tests on the nose gear to verify the strength of the new backup structure as well as the energy-absorbing capability of the nose gear and new metering pins in the X-24B. For these drop tests, we elevated the nose of the vehicle with the main tires restrained and then released the vehicle from increasing heights. To produce appropriate nose-gear drag loads, we rotated the nose tires with a spinup device prior to each release. During these tests, the structure and nose-gear performance proved to be satisfactory.

Flutter while the X-24B hung in pre-launch position under the wing of the B-52 could cause structural failure on the B-52. Therefore, vibration tests were conducted on the B-52 with the X-24B hanging in launch position that assured us that no flutter would occur in flight from the B-52's wing, the lifting-body adapter, or the X-15 pylon.

We ran a series of taxi tests with incremental increases in speed to test for nose-gear shimmy, which we felt was possible due to the X-24B's nose-gear steering that made it distinctly different from the other lifting bodies. The other lifting bodies had had nonsteerable dual nose wheels that avoided all possible shimmy problems. Our primary concern with the X-24B's nose-gear steering was that the nose gear or backup structure might fail if severe shimmy occurred in the nose gear at touchdown on the first flight, given the dynamic load added to the already high landing loads that we expected.

Eight taxi runs were made at speeds from 40 to 150 knots, using the main LR-11 rocket engines as well as the 500-pound hydrogen-peroxide rockets intended to help the pilot during the landing flare. The 150-knot run across the lakebed runway was made using approximately 4,000 pounds of thrust from two LR-11 chambers. Even at 150 knots, the nose-gear steering and ground handling characteristics of the X-24B were found to be satisfactory, with no shimmy in the nose gear. However, lateral center-of-gravity was offset two inches during the test run, the liquid-oxygen tank on the left side outweighing by 1,000 pounds the alcohol fuel tank on the right, making the X-24B pull to the left. The pilot was able to compensate for the offset with intermediate right braking.

We made a final taxi test to 80 knots on the take-off runway with the X-24B hanging under the B-52. Both accelerometer measurements and comments from the pilot verified that the ride was smooth and no problems could be predicted.

During the captive-flight test of the X-24B, we had to exercise much greater care than we had in captive-flight tests of the other lifting bodies, for the pilot of the X-24B [173] could not eject while the vehicle was mated to the B-52. To obtain acceptable loads on the forward hook of the X-15 pylon, we located the X-24B adapter further aft under the pylon than we had with the other lifting bodies, a design compromise based on the proven safe operation of the X-24A.

If there had been a problem during the captive flight, X-24B pilot John Manke would have had to launch before he could have ejected safely. The B-52 was flown as slowly as possible during the climb to 30,000 feet, where structural resonance tests were conducted at speeds higher than possible on the ground. Since the X-24B was within gliding distance of the dry lakebed during these tests, Manke could have landed the vehicle if it had broken loose or had had to be launched, but no problems occurred during the single captive flight.


Flight Tests of the X-24B


On 1 August 1973, John Manke piloted the X-24B on its first glide flight, launching from the B-52 at 40,000 feet, coasting earthward at 460 miles per hour, performing a series of maneuvers to establish handling qualities, and executing a practice landing flare approach before making a 200-mile-per-hour landing on the lakebed. On the flight, the same flight-test maneuvers and evaluations were done that had been done on flights of the earlier lifting bodies. During the series of glide flights that followed, Manke and Major Michael V. Love, the Air Force X-24B project pilot, checked the vehicle's performance in a variety of configurations.

On 15 November 1973, John Manke piloted the X-24B in its first powered flight. Typical flight time in the X-24B was seven minutes, a minute longer than in the other lifting bodies. As pilots had done before flights in the earlier lifting bodies, Manke and Love completed pre-flight practices of numerous simulated approaches in the T-38 and F-104 aircraft. By the end of the X-24B project, lifting-body pilots had flown more than 8,000 such simulated approaches in support of the entire lifting-body program.

On 24 October 1974, during the sixteenth flight of the X-24B, Love reached the aircraft's fastest flight speed, Mach 1.76-or 1,164 miles per hour. On 22 May 1975, Manke made the X-24B's highest approach and landing, coming down to the lakebed from 97,000 feet-more than 18 miles above the earth's surface.

Love and Manke were pleasantly surprised by the handling qualities of the X-24B at all speed ranges, both with and without engaging the control dampers in the stability augmentation system. Even in turbulence, the X-24B flew surprisingly well. In subsonic handling qualities, the X-24B earned the very high rating of 2.5 on the Cooper-Harper pilot rating scale. In short, the X-24B was considered a fine aircraft.

Manke and Love said the handling characteristics of the X-24B compared favorably with those of the fighter aircraft, the T-38 and F-104. The X-24B's handling and riding qualities in turbulence during the final approach were superior to those of the earlier lifting bodies. The high dihedral effect of the other lifting bodies had created disconcerting roll upsets for pilots due to sideslips in turbulence. With its low values in effective dihedral, the X-24B rode turbulence with more of a side-to-side motion...



X-24B simulating future  Shuttle landings. The F-104 chase plane is behind and to the (pilot's) right of the X-24B.

X-24B simulating future Shuttle landings. The F-104 chase plane is behind and to the (pilot's) right of the X-24B. (NASA photo EC75 4914)


....that the pilots found more acceptable. The pilots also found the vehicle's dampers-off handling qualities in the landing pattern to be excellent, commenting that they couldn't believe the dampers were off. 4

Despite the fact that the X-24B was 1,800 pounds heavier than the X-24A, it had achieved a top speed of Mach 1.76 due to the lower configuration drag of the X-24B and a 14 percent increase in thrust from the uprated LR-11 engines. Although the X-24A had reached Mach 1.6, it very likely could have achieved Mach 1.7 had its test-flight program not been cut short to build the X-24B.


X-24B Simulations of Future Shuttle Landings


By the time that the Space Shuttle was well into the design phase, space mission planners wanted to know if such unpowered re-entry shapes with low lift-to-drag ratios could land successfully on asphalt or concrete runways. Convinced that the X-24B could successfully execute such an approach and landing, John Manke had recommended even earlier that the X-24B make a series of landings on Runway 04/22, the main 15,000-foot concrete runway at Edwards. For John Manke, Mike [175] Love, and other pilots, such a demonstration seemed important for developing the confidence needed to proceed with similar landings of the Space Shuttle.

In January 1974, the X-24B research subcommittee had approved Manke's proposal. Afterwards, Manke and Love began a three-week flight program, flying the F-104 and T-38 in landing approaches approximating those of the X-24B. Manke alone made over 100 of these approaches.

The payoff came on 5 August 1975. Manke launched in the X-24B from the B-52 mothership, climbed to 60,000 feet, began his descent, and-seven minutes after launch-touched down in the X-24B precisely at the planned target landing spot, 5,000 feet down Runway 04/22. Afterwards, Manke said, "We now know that concrete runway landings are operationally feasible and that touchdown accuracies of ± [plus or minus] 500 feet can be expected."5 Assisting landing accuracy, Manke commented, were the distance markers and geographic features along the concrete runway, not characteristic at the time of the lakebed runways. Two weeks after Manke's first runway landing, Love duplicated the feat in the X-24B.

These precise touchdowns demonstrated to the Shuttle program that a configuration with a comparatively low lift-to-drag ratio could land accurately without power, thereby convincing Shuttle authorities that they could dispense with the airbreathing jet engines originally planned for the Orbiters. The resultant reduction in weight added significantly to the Shuttle's payload.

Of all the vehicles flight-tested during the twelve years of the lifting-body program, the X-24B had the highest landing lift-to-drag ratio, 4.5. Next highest was the X-24A at 4.0, then the HL-10 at 3.6. Lowest among the lifting bodies was the M2-F3 with a landing lift-to-drag ratio of 3.1. Because of its relatively high lift-to-drag ratio plus good control characteristics, the X-24B was considered by the pilots to be very comfortable to land without power. The lifting-body pilots also considered the M2-F3 acceptable in landing characteristics, although the M2-F3 required more of the pilot's attention in landing, due to having less time from the flare to setting the wheels down on the runway.

By the end of the X-24B program, we had gained widespread experience with the unpowered landing characteristics of lifting-body configurations over a range of landing aerodynamic performance. In its maximum "dirty" configuration-with flaps, deployed landing gear, speed brakes, and low levels of thrust-the F-104 had been used to train pilots in landing approaches for both the X-15 and lifting-body programs, beginning in 1959 with NASA pilots Neil Armstrong (of Apollo fame) and Joe Walker. During the course of these F-104 flights, the aircraft would be landed in the worst lift-to-drag configuration-with flaps, gear, and speed brakes extended in idle power-that approached a maximum lift-to-drag ratio of 2.5. Later tests conducted by Bob Hoey and the Air Force pilots concluded that landing without aids, a vehicle with a [176] maximum lift-to-drag ratio of 2.5 bordered on the totally unacceptable- that is, a landing where the risk of crashing is highest. These test results in the F-104 served as a benchmark for the Flight Research Center while evaluating the different flight-tested lifting-body configurations for future space operations.

Landing performance and safety were critical as well in terms of the ablative heat shields used for re-entry vehicles before the development of such new heat-protection materials as the lightweight silicon tiles. We tailored the concept of the lifting body as a re-entry vehicle to the use of the ablative heat shields, the technology current at the time. As a result, landing performance and safety were linked to how the roughness resulting from the burned and melted ablative heat shields would affect the aerodynamic drag of the lifting bodies.

We had excellent data from flight tests at hypersonic speeds made during the X-15 program to use in predicting the magnitude of this effect for the lifting bodies, available in Lawrence C. Montoya's Drag Characteristics Obtained from Several Configurations of the Modified X-15-2 Airplane up to Mach 6.7.6 The report compares the drag characteristics of a clean-surfaced X-15 with an X-15 flown with an ablative coating. We also had the results of a similar test done on the X-24A during the full-scale wind-tunnel testing of the vehicle at NASA Ames. Although the X-24A was later flight-tested at the Flight Research Center only with a clean metal skin, the wind-tunnel testing of the X-24A with a coating simulating the ablative roughness typical after the heat of re-entry showed a significant reduction for the vehicle in landing lift-to-drag ratio. This reduction, in turn, would reduce significantly the time a pilot would have for making corrections in control during an actual landing of a lifting-body re-entry vehicle.

When we used the ablative roughness data from the X-15 and the X-24A tests to calculate the aerodynamics of lifting bodies with ablative roughness, we found that some lifting-body configurations previously found to be acceptable for flight would become unacceptable as re-entry vehicles with ablative roughness. The ablative roughness after the heat of re-entry would cause the drag of lifting bodies to increase between 15 and 30 percent, lowering the lift-to-drag performance. As a result, for example, the 3.1 lift-to-drag ratio of the M2-F3 would be lowered to less than 2.5, making the M2-F3 unacceptable as a re-entry vehicle unless considerable care were taken to use the correct heat-protection materials in certain places, such as carbon-carbon rather than ablative material on the leading edges. Likewise, the HL-10's lift-to-drag ratio of 3.4 would drop to a ratio that would make it barely acceptable in re-entry. With ablative roughness added, the only lifting bodies that would retain adequate lift-to-drag ratios would be the X-24A and X-24B.

When Bill Dana made the last powered flight of the X-24B on 23 September 1975, the lifting-body program drew to a close. After Dana's flight, six pilot familiarization [177] glide flights were made in the X-24B by Air Force Captain Francis R. Scobee and NASA's Einar Enevoldson and Tom McMurtry. On 26 November 1975, piloted by McMurtry, the X-24B completed its 36th and final flight. Through the spring of 1976, before being sent to the Air Force Museum, the X-24B remained in residence at Edwards Air Force Base, resplendent in its blue and white paint scheme.



1 See Henry C. Dethloff, Suddenly Tomorrow Came . . . A History of the Johnson Space Center (Washington, D.C.: NASA SP-4307, 1993), esp. pp. 62-65.

2 R. Dale Reed, "RPRVs: The First and Future Flights," Astronautics & Aeronautics 12 (April 1974): 31-32.

3 Quotations from the preceding four paragraphs all in Reed, "RPRVs," p. 32.

4 See John A. Manke and M. V. Love, "X-24B Flight Test Program," Society of Experimental Test Pilots, Technical Review 13 #4 (Sept. 1975): 129-154.

5 Ibid., p. 140.

6 Lawrence C. Montoya, Drag Characteristics Obtained from Several Configurations of the Modified X-15-2 Airplane up to Mach 6.7 (Washington, D.C.: NASA TM X-2056, 1970).