Extreme Punchout: The Ejection Seat of the X-15

The hypersonic speed and extreme altitude performance of the North American X-15 demanded one of the most complex ejection seats ever put into service. Earlier NASA research rocket aircraft like the Douglas D-558-2 Skyrocket and the Bell X-2 featured ejectable nose sections that the pilot would then bail out of conventionally once it had separated from the aircraft and stabilized. However, the weight and volume restrictions on the X-15 made such a system impractical and North American in conjunction with engineer/test pilot A. Scott Crossfield, North American Aviation designer Jerry Madden and the David Clark Co. (who had long made pressure suits for the military and NASA) designed an integrated system that combined the pressure suit design along with an advanced ejection seat.

The X-15 ejection seat in the aircraft
(USAF Museum)

David Clark's MC-2 pressure suit was the key to making an open-faced ejection at high Mach and high altitudes possible. It not only protected the pilot from the extreme windblast of hitting the airstream at Mach 3+, it also functioned as a pressure suit to protect the X-15 pilot at altitudes in excess of 250,000 feet. Despite the advanced nature of the pressure suit, it was understood that kinetic heating during a high-Mach ejection would probably result in mild burns to the head, knees, and toes which in effect projected into the airstream.

Because the rocket motor of the X-15 ejection seat had to be powerful to propel the pilot clear of the X-15's hypersonic shockwave, a novel means was used to transfer loads from the pilot's rear end to the seat pan. Each X-15 program pilot sat on a weather balloon filled with plastic beads and wiggled into it like a bean bag. A vacuum was applied which held the shape of the balloon. Once the pilot stood up, plaster of Paris was poured into the depression, creating an exact copy of the pilot's rear end. A block of Balsa wood was then carved to precisely fit the mould and this became the seat cushion of the X-15's ejection seat- not only was it custom fit which allowed the optimum transfer of shock loads from the pilot's body to the seat pan, it also made for a very comfortable seat!

Scott Crossfield in the MC-2 pressure suit
(NASA/Dryden Flight Research Center)

To prevent the flailing of the arms and legs during a high-Mach ejection, special articulated restraints would protect the pilot's legs and feet (also acting as a windblast deflector to minimize heat burns on the feet) as well as to the arms and hands. The articulated arms deployed gauntlets to protect the pilot's hands from aerodynamic heating as well.

Once the articulated arms deployed into place, the emergency oxygen supply took over pressurization of the suit and a heating unit activated to keep the pilot's helmet visor clear of ice. Once the canopy was blown off and the seat traveled up the rails, special wings on the sides of the seat deployed to stabilize the seat in the high-Mach airstream. In a conventional ejection seat, a drogue chute would be deployed to slow the seat down but at the X-15's speeds, such a chute would have melted instantly, so the X-15's seat deployed a pair of telescopic booms that projected aft and outward from the bottom of the seat to provide aerodynamic braking and helped the wings stabilize the seat.

Rocket sled test of the X-15 seat- note the deployed booms
(Boeing)

If ejection took place over 15,000 feet, a built-in barostat kept the pilot attached to the seat which allowed use of the seat's emergency oxygen supply. Once 15,000 feet was reached, the seat automatically released the pilot and activated his parachute. If ejection took place below 15,000 feet, a three second timer allowed the wings and drogue booms to deploy and stabilized the seat before separating from the pilot.

One little-known fact was that the X-15 cockpit was pressurized with nitrogen instead of oxygen as was used in the Mercury and Gemini spacecraft. The pilot breathed oxygen from the his suit which was only pressurized upon ejection. This meant the cockpit was fireproof, something that NASA tragically learned with the launch pad fire on Apollo 1.

The X-15s set an absolute speed record of Mach 6.7 and an absolute altitude record of 354,000 feet which qualified several of its program pilots for astronaut wings. Fortunately the ejection system was never needed and the one fatality, Michael Adams, occurred when his X-15 lost control and broke up on re-entry into the thicker levels of the atmosphere and the X-15's complex ejection system might not have saved him.

Interestingly, the David Clark MC-2 pressure suit served as the basis for the space suits for the Mercury and Gemini programs. The aluminized fabric exterior of the MC-2 suit carried over to the space programs and heavily influenced Hollywood cinematic depictions of space suits well into the 1970s.

Related reading: 

Lockheed's Contribution to the Space Shuttle Program
The Boeing 747 SCA Shuttle Carrier Aircraft
Flight Testing on the Cheap: The Groundbreaking M2F1 Lifting Body
How the Shuttle Orbiter Lost Its Jet Engines

Source: Eject!: The Complete History of U.S. Aircraft Escape Systems by Jim Tuttle. MBI Publishing, 2002, p136-143.

The X-28A Osprey, the Most Unlikely X-Plane

In 1969 homebuilt aircraft builder and boat designer George Pereira of Sacramento, California, started work on the Osprey I homebuilt flying boat. Using his technical background in powerboat design and construction, Pereira designed the Osprey I as a high-performance STOL sea plane intended for the growing homebuilt aircraft market. With folding wings, a single-seat open cockpit and able to be transported by a boat trailer, the Osprey I was made of wood, fiberglass, and styrofoam with a Continental C90 four-cylinder 90-hp piston engine mounted on a dorsal pylon driving a fixed-pitch two-bladed propeller. With one pilot and a full load of fuel, the aircraft weighed only 900 lbs and had a top speed of 135 mph. After finishing the design and construction, Pereira had to demonstrate that the Osprey I could safely be flown during an FAA mandatory 50-flight hour restricted flying period to be completed in less than six months.

Having duly met the FAA's requirements for a kitbuilt aircraft, Pereira was approached in 1971 by three Navy officials about seeing a demonstration of the Osprey and having one of their pilots checked out in the sole prototype. Pereira and later the Navy pilot operated the aircraft successfully over a three day period from the Sacramento River at which point the Naval Air Development Center informed Pereira that they were interested in the Osprey for Project Air Skimmer, a plan to deploy a simple single engine seaplane to Southeast Asia for air policing duties. The NADC's requirement called for not just an easy to build and fly aircraft but it also had to have a reasonable performance over existing kitplanes and be able to operate from the many rivers and lakes in South Vietnam's Mekong Delta region. Plans were for local manufacturer of the Air Skimmer aircraft by the South Vietnamese and that it be easy to train pilots to fly the aircraft in VFR conditions, operate from canals as little as 25 feet wide and be transportable by trailer with a maximum weight under 1,000 lbs.

Funded as the X-28A Osprey, the single prototype and its trailer were purchased by the US Navy on 27 July 1971 to be test flown from the Delaware River at the NADC's Philadelphia Naval Base. Since the test site sat under the Class B airspace of Philadelphia International Airport and the X-28A had only minimal instrumentation, it was restricted to 300 feet maximum altitude but it was felt that this wouldn't adversely affect the evaluation process. Flight tests took place between 16 September 1971 to 22 October 1971 and the X-28A was found to be easy to fly and easy to master by pilots of limited experience.

However, with the winding down of the Vietnam War, Project Air Skimmer quietly ended with no further plans for use in Southeast Asia. Pereira, however, refined the design with a two-seat enclosed cockpit and retractable landing gear as the Osprey II. First offered in kit form in 1974, nearly 600 have been built and flown. Not only will the X-plane be one of the smallest of the X-planes, it's the only homebuilt one, not to mention the only one that can still be built today by any homebuilt enthusiast of reasonable skill.

Source: The X-Planes- X-1 to X-45 by Jay Miller. Midland Publishing, 2001, p291-293.

As the Vietnam War escalated in the 1960s and air crew losses began to mount to increasingly sophisticated air defenses, thoughts began to circulate in the USAF if there was a way to provide downed air crew with a better means of descent than just the winds and parachute. What if downed aircrew could have a better choice in where to land? And better yet, what if they could fly away to friendly territory? The USAF began to discuss with aircraft manufacturers and designers if there wasn't a way to package an ultralight aircraft or glider into part of the ejection seat and cockpit that would give shot-down air crew more options on bail out. Proposals were issued to the industry in 1967 and Bensen Aircraft Corporation, a well-known builder of homebuilt autogyro aircraft, submitted an unpowered version of one of their autogyros to be used as an autogyro glider. Bensen's proposals easily won the USAF over and contracts were issued in 1968 and the program received the X-plane designation of X-25A (a powered version) and X-25B (unpowered glider version). A third version called the DDV (Discretionary Descent Vehicle) was an even lighter and simpler version of the X-25B designed for one time use with automatic blade deployment to become a rotor chute even from a supersonic ejection.

The USAF took delivery of the X-25A and X-25B on 16 February 1968 and immediately embarked on a series of flights to determine the length of a pilot training cycle necessary to master the autogyro controls. The X-25A was used to power itself to an assigned altitude and then shut down its engine and autorotate. This was to determine the glide performance of the aircraft. Twenty pilots all with fixed wing experience and no rotary wing time were then checked out in the X-25B to see how easy it was to operate. All 20 pilots were able to master the X-25B within 30 minutes. The DDV was then used with anthropomorphic test dummies to measure G-loads and glide performance with an incapacitated pilot.

The winding down of the Vietnam War ended interest in the autogyro solution and it was felt that such an aircraft could be packaged to be accommodated in most combat aircraft cockpits with the current technology of the day. The X-25A is currently on display at the National Museum of the US Air Force in Dayton, Ohio, while the X-25B is at the Museum of Aviation at Warner Robbins, Georgia. The final disposition of the DDV is unknown but the X-25 will always be known as the smallest of the X-planes if not one of the most obscure.

Source: The X-Planes- X-1 to X-45 by Jay Miller. Midland Publishing, 2001, p272-275.

As early as 1957 NACA (NASA's predecessor) was already conducting lifting body research started by Dr. Alfred Eggers of the Ames Research Laboratory. His pioneering work on lifting body applications to re-entry vehicles for spacecraft and missiles led the US Air Force to initiate START- Spacecraft Technology and Advanced Reentry Test. By 1960 Dr. Eggers' work was taken up by several aerospace firms, most notably Martin Aircraft and Northrop to a lesser extent. Within two years, Martin's engineers had come up with their own designs for a reentry vehicle based on a lifting body design. The USAF considered the Martin designs to be possible foundations for either a maneuverable ICBM warhead that could evade Soviet anti-ballistic missile defenses or for a data/film return vehicle for a spy satellite that could maneuver on reentry to a more favorable recovery point. At the time, the first of the Corona spy satellites were operational and used a ballistic reentry capsule to return the film images to Earth. The timing of the return had to coincide with a point in the Corona's orbit that the capsule would hit a predesignated recovery point. A maneuverable lifting body capsule could be ejected on short notice from less favorable orbits and be maneuvered to a recovery point.

In August of 1964 Martin got the contract for the SV-5D PRIME (Precision Recovery Including Maneuvering Entry). Interestingly, it was assigned the X-23 designation after the completion of the program. Research work by USAF historians demonstrated that all contemporary documents of the day used the SV-5D designation.

Only about six and a half feet in length, the X-23/SV-5D had the same lifting body configuration as the later manned Martin X-24 aircraft. With a lightweight and high-temperature tolerant structure of titanium and beryllium, the craft with three types of ablative silcon and carbon-based heat shielding depending upon the expected maximum temperatures on various parts of the vehicle. The centrally-located equipment bay housed the guidance system, telemetry and data units and the recovery parachute system. The equipment bay was surrounded by a cold-wicking system- two plates surrounded a fluid-filled absorbent material. As the heat built up during reentry, the fluid slowly boiled off and was vented overboard as steam.

The only propulsion on the vehicle was a gas jet thruster system for exo-atmospheric maneuvering as well as body flaps for endo-atmospheric maneuvering. The X-23/SV-5D was carried aloft on an Atlas missile launched from Vandenberg AFB in California. Once the Atlas reached apogee, the nose shrouds were jettisoned and the X-23/SV-5D was released to begin its reentry maneuvers.

The first X-23/SV-5D was launched on 12 December 1966 but only pitch maneuvers were demonstrated. However, the recovery system failed at the end of the 30-minute mission and the first vehicle was lost in the Pacific. The second test took place on 5 March 1967 and during reentry at hypersonic speeds, the X-23/SV-5D maneuvered as much as 500 miles on each side of a ballistic reentry path, proving for the first time the work of Dr. Eggers and Martin Aircraft. Unfortunately, it too was lost in the Pacific when it came loose from its flotation collar and sank. The third and last flight took place on 18 April 1967 and the vehicle performed a full series of test maneuvers and was successfully recovered in midair near Kwajalein Island by a Lockheed JC-130B Hercules. The successful recovery of the third X-23/SV-5D allowed NASA and Martin engineers to study the effects of reentry on the different heat shield materials used on the craft. Due to the success of the third test and the partial results from the failed second test, the fourth X-23/SV-5D was never flown.

At one point there was discussion of using the design of the X-23/SV-5D as the basis for an unmanned hypersonic reconnaissance vehicle but the project never went forward. The third X-23/SV-5D is now on display at the National Museum of the United States Air Force in Dayton, Ohio. The data gleaned from the X-23 PRIME project proved immensely useful to NASA and Rockwell's engineers during the design of the Space Shuttle and the USAF used the data in its work on the reentry vehicles for its ICBM force.

Source: The X-Planes- X-1 to X-45 by Jay Miller. Midland Publishing, 2001, p256-259.

The Schweizer X-26 Frigate: The Longest Running X-Plane Program

In 1968 the US Naval Test Pilots School (NTPS) at NAS Patuxent River, Maryland, was struggling how to teach its test pilot students the phenomenon of inertia roll coupling, where the inertia of the heavier fuselage can potentially overcome the stabilizing effects of the wing and tail, particularly in high speed flight. Intertia roll coupling became more of an issue in the fighter aircraft of the day which often boasted long slender fuselages and relatively short span wings. The standard naval jet trainer of the day, the North American Rockwell T-2 Buckeye, despite its benign handling characteristics as a trainer, would react too quickly and at times dangerously to be an effective teaching tool on the phenomenon. The Navy needed an aircraft that had a unusually slow roll rate with a slow speed and good recovery characteristics to that the students of the NTPS could see and experience the evolution and recovery from inertia roll coupling.

The longer the wing span, the slower the roll rate. This made a long-span wing necessary and having a slow speed and good recovery characteristics made a glider aircraft the ideal platform. In an unprecedented move to quickly acquire the right aircraft for the NTPS, the Navy purchased two stock Schweizer SGS 2-32 two-seat sailplanes that could be towed aloft by another unusual aircraft in the NTPS inventory, De Havilland Canada DHC-2 Beavers which were in use to acquaint the NTPS students with tailwheel aircraft and STOL flight.

In order to circumvent the usual lengthy and bureaucratic procedures that come with aircraft acquisition, the Navy had the X-plane designation X-26A assigned to the gliders. They later would be christened "Frigate" after the frigate sea bird. The two X-26A Frigates entered service with the NTPS in August 1968. As the Navy's pilots were inexperienced in glider operations, the NTPS instructors were trained at Schweizer's factory in Elmira, New York. The first two X-26A Frigates were lost in fatal accidents in 1971 and 1972. A third X-26A was procured and unfortunately it, too, was lost in a fatal accident in 1980. Two more X-26A Frigates were purchased to replace the losses that year and to this day they still fly with the NTPS. With two classes going through the rigorous NTPS course each year with an average class size of 30-36, the two Frigate gliders are kept busy.

At one point there was an X-26B. Lockheed and DARPA had a concurrent program that wasn't part of the X-plane program to develop a quiet sensor platform for the Vietnam War. Two Schweizer SGS 2-32 gliders were used in the Project Prize Crew and when they became redundant to the the program, they were passed on to the NTPS as a powered gliders and designated X-26B. One was kept as a spares source for the other which flew, but the X-26B had to be withdrawn from the NTPS due to maintenance problems with its unique modifications for the Project Prize Crew.

This year marks the 42nd consecutive year of X-26A flight operations at NAS Patuxent River, making the Frigate glider the longest-running X-plane program in history, beating out the second longest program, the Bell X-14 VTOL testbed, by a substantial margin.

Source: The X-Planes- X-1 to X-45 by Jay Miller. Midland Publishing, 2001, p282-283.

In 1945 there wasn't a large body of technical knowledge on supersonic flight when the design of the Bell X-1 was formulated. Many engineering and design decisions had to be made based on the best possible estimates of the aircraft's performance. The fusleage's shape was the end product of the study of the .50 caliber bullet, as it was known to be stable at supersonic velocities. Bell's engineers tried to fill in the data gaps on supersonic flight by observing objects known to fly at supersonic speeds- in this case, the .50 caliber round. Discussions with ballistics experts concluded that little was known about the aerodynamics of the round after it was fired, but since there was no question that it was stable at high speeds, its shape formed the basis for the X-1's fuselage.

Source: Bell X-1 Variants, Aerofax Datagraph 3 by Ben Guenther and Jay Miller. Aerofax, 1988, p6.

Forty-three percent of the empty weight of the Rockwell/MBB X-31 is made up of over 600 items from other aircraft. In an effort to reduce costs and development time, off-the-shelf components were used wherever possible in the design and construction of the X-31.

McDD F/A-18 Hornet: Windscreen, canopy, electrical generators, airframe-mounted accessory gearbox, leading edge actuators, cockpit instrumentation, cockpit controls

Lockheed F-16: Main undercarriage, rudder pedals, nosewheel/tire, emergency power unit, fuel pump

Lockheed F-16XL: leading edge flap drives

Lockheed C-130 HTTB: Flight control computers

Bell/Boeing V-22: Rudder acutators, trailing edge control modules

Cessna Citation III: Mainwheels and brakes

Vought A-7: Main tires

Rockwell T-2: Zero-g fuel accumulator

Northrop F-20: Emergency air-start system

Rockwell B-1B: Canard pivot and spindle

Source: World Air Power Journal, Volume 24 (Spring 1996). "Rockwell/MBB X-31" by Robert F. Dorr.

When the Boeing X-51A WaveRider begins its flight testing (provisionally scheduled for this October), it will mark several firsts in aviation- the first attempt to fly a fuel-cooled scramjet, the first attempt to fly an aerodynamically unstable, control-augmented hypersonic vehicle, and the first liquid-hydrocarbon fueled scramjet to fly (the engine a design from Pratt & Whitney Rocketdyne).

The WaveRider will demonstrate sustained flight at Mach 6.5 using JP-7 fuel instead of cryogenic fuels.

Source: Aviation Week and Space Technology, March 30, 2009. "Scramjet Reality" by Guy Norris, p32.