ROCKET PROPELLED LIFTING BODIES

In preparation for the development of a reusable space glider or space shuttle, in the early 1960s NASA started to investigate so-called ‘lifting body’ aircraft. As the name implies, such planes have fuselages shaped to provide all or most of the necessary lift, dispensing with the need for large wings. For a glider or spaceplane returning from orbit such a configuration was thought to be ideal: wings need to be relatively thin, making it hard for them to handle the brutal aerodynamic forces of hypersonic flight and the extreme heat of re-entry, whereas a lifting body can be very robust and have a large volume relative to the surface that is heated up. Keeping a lifting body cool and in one piece is therefore theoretically simpler than preventing protruding wings from melting or being ripped off at hypersonic speeds. A lifting body basically combines the robustness and structural simplicity of a ballistic capsule with the maneuverability, flight range and landing accuracy of an aircraft. The questions to be answered were whether a lifting body was sufficiently controllable at all speeds and could be landed safely.

In 1962 the NASA Flight Research Center at Edwards (now called Dryden Flight Research Center) approved a program to build a very simple, unpowered lifting body prototype for low-speed flight tests on a shoe-string budget of $30,000; equivalent to about $220,000 in 2011. It had to be so lightweight that it could be towed into the air behind a car for the initial take-off and low-speed flights. In this mode it would also be used to train pilots, before progressing to towing to higher altitudes with Dryden’s C-47 transport aircraft and release for gliding trials. The resulting contraption, which looked like a horizontal cone cut in half, was designated the M2-F1 with the ‘M’ referring to ‘Manned’ and ‘F’ indicating it concerned a ‘Flight’ version. However, its unusual shape quickly earned it the nickname ‘Flying Bathtub’.

The M2-Fl’s structure consisted of a tubular steel frame made by Dryden, which was then covered over with a plywood shell by the Briegleg Glider Company, a local glider manufacturer. The fixed undercarriage was taken from a Cessna sports plane. The Uttle aircraft had a maximum take-off weight of about 570 kg (1,250 pounds).

For towing the M2-F1 over the hard, flat surface of Rogers Dry Lake a powerful and fast, but not very expensive car was required. Members of the flight test team bought a Pontiac Catalina convertible with the largest engine available, which was then fitted with a special gearbox and racing slicks by a renowned hot-rod shop near Long Beach. With these modifications the Pontiac could tow the M2-F1 into the air

The M2-F1 in tow behind a C-47 [NASA],

in 30 seconds at a speed of 180 km per hour (110 miles per hour). The first car-tow test run in April 1963 did not go all that well because the M2-F1 started to bounce uncontrollably on its two main wheels the moment that NASA research (and X-15) pilot Milt Thompson raised the nose off the ground. The problem was quickly found to be in the rudder control and was fixed. About 400 successful car-tow tests were made, all with Thompson at the controls. They produced enough flight data about the aircraft to proceed with flights behind the C-47 starting in August of that same year. For this the M2-F1 was equipped with a simple ejection seat as well as a small solid propellant rocket motor in the rear base. If required, this “instant L/D rocket” could provide a thrust of 300 Newton for 10 seconds; just enough to keep the plane in the air for a little bit longer if the pilot were to find himself descending too rapidly just prior to touchdown. The tow plane typically released the M2-F1 at an altitude of 3.7 km (12,000 feet) for a 2 minute glide down to Rogers Dry Lake at a speed of 180 to 190 km per hour (110 to 120 miles per hour). Apart from Thompson, who flew most of the glide tests, several other pilots took the controls of this strange aircraft, among them the famous test pilot Chuck Yeager.

A total of 77 aircraft-tow flights were performed, the success of which convinced NASA and the Air Force of the feasibility of the lifting body concept for horizontal landings of atmospheric entry vehicles. The solid rocket motor was only used once, on the last flight when USAF pilot Captain Jerauld Gentry accidentally rolled the relatively unstable M2-F1 onto its back just after take-off. Flying inverted behind the C-47 a mere 100 meters (300 feet) above the lakebed he released the tow line, finished the barrel roll into level flight, fired the rocket and made a perfect landing. The roll instability was caused by a lack of wingspan.

The success of the M2-F1 encouraged NASA to put some real money into lifting body research and gave rise to both the M2-F2 by NASA Ames Research Center and the HL-10 by NASA Langley Research Center (with ‘HL’ for ‘Horizontal Landing’ and TO’ referring to the design number). Unlike the lightweight M2-F1 glider these new aircraft were all-metal and were fitted with an XLR11 rocket engine (previously used in the X-l series, the D-558-2 and early X-15 missions) for testing their lifting body shapes at high speeds and high altitudes. As with the rocket propelled X-planes the XLR11 was used for a short powered flight phase, after which the plane would glide to a landing. In emergencies the engine could be reignited just before landing, eliminating the need for the solid propellant rocket of the M2-F1.

To be able to attain high speeds and long flight times the rocket propelled lifting bodies were carried to about 14 km (45,000 feet) under the wing of a B-52 and then released at a speed of about 720 km per hour (450 miles per hour). In fact they used the same carrier aircraft as the X-15, whose flight program was running concurrently. Special adapters were designed to enable the lifting bodies to use the wing pylon that was developed to carry the X-15. Both the M2-F2 and the HL-10 were equipped with pressurized cockpits and ejection seats.

The HL-10 and M2-F2 [NASA],

The M2-F2 was built by Northrop and was similar in shape to the M2-F1. It made its first unpowered free flight on 12 July 1966 with Milt Thompson in the cockpit. Another 14 successful glide flights followed and revealed that, just like the M2-F1, the aircraft had a stability problem which often caused it to violently oscillate in roll during the ascent. The last planned unpowered flight in May 1967 ended in disaster when, just prior to landing, Bruce Peterson suffered from a pilot – induced oscillation problem. The situation was similar to that experienced by Scott Crossfield during his first X-15 flight, except that rather than a pitch oscillation the M2-F2’s problem was roll control. With the craft rolling from side to side and also having to avoid a rescue helicopter that was in his way, Peterson fired the XLR11 rocket engine to prolong the landing approach but nevertheless smashed onto the lake bed before the landing gear was fully down and locked. The aircraft skidded

The M2-F3 with test pilot John Manke [NASA].

across the ground in a cloud of dust, rolled over six times and came to rest upside down. Peterson was severely injured; he later recovered but had permanently lost the vision in his right eye. Some of the film footage of the crash was used by the 1970s’ TV series The Six Million Dollar Man. It took three years to rebuild and improve the M2-F2, which was redesignated the M2-F3. The most important modification was the addition of a third, central vertical fin to improve low-speed roll control. NASA pilot Bill Dana made the M2-F3’s first glide flight on 2 June 1970 and found it to possess much better lateral stability and control characteristics than the M2-F2. After only three glide flights the aircraft was taken on its first powered flight on 25

November. The M2-F3 subsequently made a total of 24 flights under rocket thrust, during which it attained a top speed of Mach 1.6 and, on its last flight in December 1972, a maximum altitude of 21.8 km (71,500 feet). This unique vehicle is now hanging in the Smithsonian Air and Space Museum in central Washington D. C.

Northrop also built the HL-10, which made its first glide flight in December 1966 with Bruce Peterson at the controls. Peterson found the plane to be very unstable and only managed to maintain control by keeping the speed up: once again actual flight testing had proven its worth as proof-of-concept of aerodynamics and control theory. The HL-10 was grounded while NASA engineers studied data recorded during the flight, as well as from additional wind tunnel tests. It was found that so – called flow separation at the outboard fins was the culprit: because the air was not flowing over them properly they were ineffective in providing lateral stability. The leading edges of the outboard fins were modified and the ensuing glide tests established that this fix worked. After 11 unpowered flights the first powered flight was made on 23 October 1968 by Jerauld Gentry, the pilot who made the last (and unusually exciting) flight of the M2-F1. During this first powered flight the XLR11 engine malfunctioned shortly after launch, forcing him to jettison the remaining propellant and make an emergency landing on a conveniently located dry lake bed. On 13 November NASA pilot John Manke made the first successful powered flight. The HL-10 was flown a total of 37 times, logging (on different flights) the highest speed and altitude in the entire lifting body program: a top speed of Mach 1.86 and a maximum altitude of 27,440 meters (90,030 feet). Although it had its share of teething problems, pilots eventually found the HL-10 to be more stable and easier to fly than the M2-F3 and in this respect also better than the later X-24A (see below). On its two last flights the XLR11 engine was replaced with three small hydrogen – peroxide engines that provided a continuous low thrust of 4,000 Newton in total during the final approach for landing, reducing the glide angle from 18 down to 6 degrees. However, it was found that this provided few benefits over a completely unpowered glide landing and led to the conclusion that adding low-thrust landing engines to aircraft with relatively low lift over drag ratios didn’t make much sense. It added complexity and weight, offered little assistance to the pilot, and the weak thrust did not enable him to abort a landing and fly around for another try. This HL-10 result later helped engineers to decide not to put any landing engines on the Space Shuttle Orbiter.

In his book Wingless Flight: The Lifting Body Story, HL-10 engineer Dale Reed describes a plan that he proposed for sending this vehicle into orbit. It would require to be fitted with reaction control thrusters for attitude control in space and an ablative heat shield for re-entry. It would be launched unmanned on a Saturn Y moonrocket along with a manned Apollo capsule; the HL-10 would basically take the place of the lunar module in the adapter of the upper stage. Once in orbit, one of the astronauts would make a spacewalk from the Apollo capsule and enter the cockpit of the lifting body. On the first of two such missions the pilot would make in­orbit checks of the vehicle and return to his spacecraft, whereupon the HL-10 would return to Earth automatically. On the second mission an astronaut would actually pilot the vehicle all the way back to Edwards. This very adventurous plan was never

The Northrop HL-10 with flyby of the B-52 carrier plane [NASA],

implemented, although Wernher von Braun was apparently enthusiastic about using his Saturn Y for these missions.

The HL-10 can be found guarding the entrance of NASA Dryden Flight Research Center, mounted on a pedestal as if coming in for a landing. The Six Million Dollar Man also used footage of the HL-10.

The M2-F2 and HL-10 were followed by the joint USAF/NASA X-24A. It was built by the Martin Aircraft Company and looked somewhat like a fat version of the M2-F3 with a curved back, three vertical fins and an XLR11 engine. Jerauld Gentry piloted this “potato with three fins” on its first unpowered flight on 17 April 1969, as well as on its first powered flight on 19 March the next year (it thus flew before the wrecked M2-F2 reappeared as the M2-F3). During its flight test phase this aircraft was flown 28 times, achieving a top speed of Mach 1.6 and a maximum altitude of

21.8 km (71,400 feet).

The Martin Marietta Corporation (which the Martin Aircraft Company became) went on to strip the X-24A down and rebuild it as the X-24B, which looked rather different. Whereas the X-24A was round and fat the X-24B had a triangular ‘double delta’ shape with a flat bottom and pointed nose, and was affectionately called the

The Martin Aircraft X-24A [NASA].

‘Flat Iron’. The double-delta planform meant it had delta wings which incorporated a bend (as on the Space Shuttle). This shape, derived from a study by the Air Force Flight Dynamics Laboratory of possible future re-entry vehicles, resulted in a more stable aircraft with a much better lift over drag ratio owing to its greater area of lift­generating surface. The reuse of much of the X-24A’s equipment and airframe saved the Air Force a lot of money in comparison to what it would have cost to build a new vehicle from scratch.

The first to take the X-24B up (or rather, down) for a glide flight was NASA pilot John Manke on 1 August 1973, and he was also at the controls on the first powered flight on 15 November. During its total of 36 flights the X-24B managed to reach a speed of Mach 1.76 and a maximum altitude of 22.6 km (74,100 feet). The X-24B also made two landings on the main concrete runway of Edwards to demonstrate that accurate runway landings were possible for a lifting body glider with a low lift over drag ratio (it had nose-wheel steering, unlike the X-15 and the other lifting bodies).

In 1975 Bill Dana made the final flight of the X-24B, drawing to a close not only the lifting body test program but also rocket aircraft flying at Edwards in general. For the occasion the team prepared a sign depicting the X-l and the X-24B and the text “End of an era; Sept. 23, 1975; Last rocket flight.” It truly was the end of one of the most interesting periods in aviation history, with rocket planes pushing technology, flight speeds and altitudes to levels that had been mere dreams when the X-l broke the speed of sound almost three decades earlier.

There were various proposals for an X-24C, including one by the Lockheed

The Martin Aircraft X-24B with test pilot Tom McMurtry [NASA].

Skunk Works for an aircraft that would use scramjets (able to function at higher speeds than a ramjet) to reach Mach 8, but in the post-Vietnam era the mihtary had Uttle money to spare and NASA was developing the Space Shuttle. The lifting body program thus ended with the last flight of the X-24B, after which the aircraft found a home in the National Museum of the US Air Force at Wright-Patterson Air Force Base. Although the original X-24A no longer exists, a very similar vehicle was put next to the X-24B in the museum to represent it. It is not a replica, but actually a conversion of a never-flown, jet-powered version of the X-24A lifting body called the SV-5J.

The lifting body program taught NASA invaluable lessons for the Space Shuttle, which it started to develop in the early 1970s. Although it was decided that the Space Shuttle Orbiter would have a relatively conventional fuselage with wings, rather than a lifting body shape, its low lift to drag ratio would produce a similarly steep gliding descent for landing. The lifting body flights showed that accurate and safe landings could be made with such a vehicle, without the need for a means of propulsion. The earher planned jet engines for the landing were discarded, simplifying the design and lowering the vehicle’s weight. The Orbiter went on to routinely land on the runway at Kennedy Space Center, and its pilots never found themselves wishing they had the jet engines available.

In the 1990s the shape of the X-24A returned in the form of the X-38 technology demonstrator that was expected to lead to a Crew Return Vehicle to enable astronauts aboard the International Space Station to return to Earth in an emergency. The X-38 made several unmanned ghde test flights after release from a B – 52 but the program was canceled in 2002. Currently the SpaceDev company is developing the somewhat similar Dream Chaser mini-shuttle partly funded by NASA under the Commercial Orbital Transportation Services program intended to encourage private companies to develop space transportation vehicles for servicing the International Space Station. If introduced, the Dream Chaser lifting-body vehicle will be launched atop an Atlas Y rocket to take crew and cargo into low orbit.