Military Aviation

Faster than the X-15: the never told story of the Lockheed X 24C L-301, the Mach 8 hypersonic test aircraft that never was

The Lockheed L-301, also called the X-24C, though this designation was never officially assigned, was an experimental hypersonic aircraft project.

As the North American Aviation X-15 program wound down in the mid-1960s, NASA and the USAF considered follow-on hypersonic test aircraft. As explained by James C. Goodall in his book 75 years of the Lockheed Martin Skunk Works, the USAF had significant classified work underway, while NASA Langley undertook two study programs: HYFAC (Hypersonic Research Facility) for a Mach 12 aircraft, and HSRA (High Speed Research Aircraft) for a Mach 8 aircraft. The Air Force revealed that it had intentions to build a Mach 3–5 test vehicle, and an incremental growth vehicle which would gradually be taken from Mach 4.5 to Mach 9. By July 1974 NASA and the Air Force selected the FDL-8 lifting body configuration. Two versions were proposed: one with cheek air intakes and air-breathing engines, and one with the XLR-99 rocket engine of the X-15. By September 1977 (officially) budget overruns were apparent and NASA agreed to cancel further X-24C work. But given the stories of similar USAF test aircraft in the 1980s, perhaps the project merely went deep black.

The Lockheed L-301, also called the X-24C, though this designation was never officially assigned, was an experimental hypersonic aircraft project. NASA and the Air Force organization, the National Hypersonic Flight Research Facility, with the Lockheed Skunk Works as the prime contractor, developed it. Beginning in January 1977, the program was tentatively scheduled to operate two vehicles for eight years and to conduct 100 flights per vehicle.

Originally intended to carry the same XLR-99 engine used by the X-15, the primary engine was changed to the LR-105, which was the sustainer engine used on the Atlas launcher. This rocket engine, burning RP-1 and liquid oxygen (LOX), was intended to accelerate the X-24C to hypersonic speeds in order to ignite the hydrogen-fueled, air-breathing ram/scramjet (supersonic-combustion ramjet) mounted in the belly of the airframe with which it would attain cruise speeds of at least Mach 6 and peak velocities of Mach 8+ at altitudes of 90,000ft or more. As such, this vehicle was plainly not intended to reach orbit.

Each mission was to start with a subsonic launch from the B-52 launch vehicle, at 45,000ft above Mean Sea Level (MSL), where its boost rocket engine was ignited to accelerate. At this point, the X-24C vehicle was heavy with large loads applied to the structure due to the positive 2.5g limit maneuver and rocket axial thrust. Vehicle structure would have remained relatively cool during this initial maneuver. At the end of the acceleration phase a 0g pushover would have been initiated to align the vehicle for the desired cruise condition. At the start of the cruise phase, the takeoff mass had been reduced approximately 50 percent. The structure was increasing in temperature towards a peak shortly after the end of the cruise segment. To stay within the test range constraint, the X-24C would perform a high drag 3g pullup combined with a banking maneuver at the time of cruise power burnout. During these maneuvers, airframe temperatures, combined with flight loads during deceleration, would cause the structure to cool.

The mission for the cruise on sustainer rockets exactly followed the flight profile noted above for scramjet cruise. The dash to Mach mission followed the same profile as for the scramjet mission, except the acceleration phase was extended to Mach 7.8. Level flight was obtained at the end of the acceleration phase followed by the deceleration maneuver identical to the one used in the scramjet mission. Heating peaks occurred shortly after the start of deceleration, as in the scramjet mission, but were not as high as in the Mach 6.6 mission due to the shorter mission elapsed time involved. Consideration of this mission, however, was made due to the difference in the vehicle shell temperature gradients in the non-scramjet configuration.

They were originally scheduled to fly 200 flights over eight years, reaching a maximum speed of Mach 8 and being able to cruise at over Mach 6 for 40 seconds.

75 years of the Lockheed Martin Skunk Works is published by Osprey Publishing and is available to order here.

X-24C configuration images circa January 1977.

Photo credit: unknown via and Harry G. Combs / NASA

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

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