X-24C Credit - USAF

Two X-24C NHFRF (National Hypersonic Flight Research Facility) aircraft were to be built under a $ 200 million budget. They would fly 200 flights over ten years, reaching a maximum speed of Mach 8 and being able to cruise at over Mach 6 for 40 seconds. They were cancelled in 1977 after budget overruns.

As the X-15 program wound down in the mid-1960's, NASA and the USAF considered follow-on hypersonic test aircraft. 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 it had intentions to build a Mach 3 to 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 1980's, perhaps the project merely went deep black.

• 1974 July 1 - FDL-8 lifting body configuration selected for the X-24C. -
  NASA and the Air Force selected the FDL-8 lifting body configuration for the X-24C. Two versions of the hypersonic aerospacecraft were proposed: one with cheek air intakes and air-breathing engines, and one with the XLR-99 rocket engine of the X-15. Two X-24C were to be built under a $ 200 million budet. They would fly 200 flights over ten years, reaching a maximum speed of Mach 8 and being able to cruise at over Mach 6 for 40 seconds.

• 1977 September 1 - X-24C cancelled -
  Budget overruns were apparent and NASA agreed to cancel further X-24C work. But given the stories of similar USAF test aircraft in the 1980's, perhaps the project merely went deep black.