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Secret Flight Test History - An Alternate History of the X-24C

Dynoman

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This is just a little thought exercise in a 'what-if' scenario for a hypersonic flight research program.

The US efforts to develop a hypersonic aircraft design in the 1950's (Aerospaceplace, X-15), 1960's (X-20, Lifting Bodies), and 1970's (X-24C, HYFAC) culminated, for a time, in the USAF and NASA X-24C project, which would have been a follow-on to the X-24A/B, HL-10, M2-F1/2/3, etc.

With what we know of the X-24C program this alternative history examines the questions of:
1. Which project pilots (USAF, NASA, and Contractor) would have been the likely candidates to have flown the aircraft during the projected timeframe?
2. Which manufacturer had the lead in receiving the contract (i.e. McAir or Lockheed)?
3. Which engine combination was likely the one to be used, or both in a program designed for growth?
4. Where there any additional configurations that might have been best to consider in the program (drop tanks, different carrier aircraft, fin arrangement, etc.)?
5. Which launching station would have been used and would Groom Lake have been considered as a recovery site?
6. Which experiments would have likely flown?
7. Where could the program have gone after the X-24C (e.g. X-24D)?
 

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Dynoman

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I certainly think that Milt Thompson of NASA would have been NASA's project pilot on the X-24C, due to his flight experience with the NASA lifting body programs. John Manke would have been another likely NASA candidate. Jerry Gentry would have been a good USAF candidate.
McAir also had extensive hypersonic and spacecraft development talent and experience.
LR-99 had extensive experience with the X-15 and could have been used in the X-24C's initial flight test program. The proposed use of the LR-105 would have required additional engines to sustain cruise flight above M6.
The launch locations could have mimicked the X-15's launch sites since the maximum altitude and speed profiles were within X-15 ranges.
 

Dynoman

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The McDonnell Douglas X-24C Flight Test Plan called for one test article, developed and flown over a 25 month period, with a 3 month flight test schedule.

The X-24C was officially cancelled in September of 1977, however, if it was instead authorized and continued development the vehicle could have been flown by the second quarter of 1980.

Two Primary Flight Test Experiments were planned: The Integral Liquid Hydrogen Tank Experiment (12 Flts) and the Active Cooling Flight Experiment (9 Flts).

Additional experiments to consider are an integral LH2 tank experiment, an integrated scramjet experiment, and an actively cooled insulation structure.
 

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Dynoman

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Lockheed's X-24C vehicle was a refined design. Its baseline design changed between its Phase II and Phase III studies.

Mission Profile for the Lockheed X-24C (Lockheed Phase III Study):

"A typical profile for the scramjet cruise mission of 40 seconds at Mach 6.6. Other mission profiles considered in the vehicle design included; 40 seconds cruise on sustainer rockets, (without scramjets) at Mach 6.6 and zoom to Mach 7.8 with no cruise time.

Each mission starts with a subsonic launch from the B-52 launch vehicle, at 13.7 kM MSL, where boost rocket engine is ignited to accelerate. At this point, the X-24C vehicle is heavy with large loads applied to the structure due to the positive 2.5 g limit maneuver and rocket axial thrust. Vehicle structure remains relatively cool during this initial maneuver. At the end of the acceleration phase a 0.0 g push over is initiated to align the vehicle for the desired cruise condition. At start of the cruise phase, the take-off mass has been reduced approximately 50 percent. The structure is increasing in temperature towards a peak shortly after the end of the cruise segment. To stay within the test range constraint, the vehicle performs a high drag 3.0 g pullup combined with a banking maneuver at time of cruise power burnout. Temperatures are combined with flight loads during deceleration. During vehicle descent, the structure begins to cool with the vehicle mass remaining at approximately 12.9 Mg.

The mission for the cruise on sustainer rockets follows the identical flight profile noted above for scramjet cruise.Vehicle structural differences are reflected in the skin panel changes, on removal of scramjet modules, required to accommodate the thermal gradient difference to the vehicle shell resulting from scramjet removal.

The dash to Mach mission follows the same profile as for the sramjet mission, except the acceleration phase is extended to Mach 7.8. Level flight is obtained at the end of the acceleration phase followed by the deceleration maneuver identical to the one used in the scramjet mission. Heating peaks occur shortly after the start of deceleration, like in the scramjet mission, but are 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."
 

Dynoman

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Using the X-15, X-24A, X-24B, HL-10, and M2-F3 first flight profiles and flight test events as a baseline for the X-24C a generic first flight scenario can be hypothesized considering that the subsonic handling and control characteristics of the vehicle were being designed to compare with the X-24B. John Manke, NASA Test Pilot, who flew almost all of the lifting bodies was consulted regarding the approach and landing of the X-24C.
 

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Dynoman

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The X-24C would have been transported to Edwards AFB where the vehicle would have been ground tested and prepared for flight tests. "Final checkout of the liquid hydrogen system would include fill, pressurization, leak and dump tests of the system using liquid hydrogen and facilities available at the flight test site. The experiment would then be installed aboard the X-24C aircraft and a final leak check would be conducted. The aircraft would then be ready for further ground crew familiarization and flight test" (McAir NASA Flight Experiments X-24C Final Briefing, January 1975).

McDonnell Douglas planned 12 flights at a rate of 3 per month for the Integral LH2 Tank Experiment (9 months with 3 flights per month for the Active Structural Cooling Experiment). The whole flight test program would be composed of one and a half months of flight test instrumentation and experiment installation and vehicle checkout, 4 months of flight tests, and two and one half months of flight test data analysis and evaluation.

McDonnell Douglas conceived a Flight Test schedule for the X-24C as below:
 

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Archibald

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Very cool, I like it. Maybe you should consider posting it here
https://www.alternatehistory.com/forum/forums/alternate-history-discussion-after-1900.16/
 

Dynoman

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Archibald, that looks like a very interesting website. I'll have to spend some more time on it.

I was trying to see how far I could develop the 'what if' concept, basing the project on some historical facts and a lot of suspected operations. Delving into the X-24C documentation its interesting to see how this project could have unfolded. It was an ambitious attempt at a hypersonic design that would have explored interesting technologies, such as scramjets, thermal protection materials, and structures, to name a few.

I'm currently looking at a comparison between all of the lifting body projects and the X-15, as well as other documentation, such as on TAV and HYFAC, on the stages and events of their flight test programs to see where there is commonality and which tests are relevant to the X-24C. Again, a neat mental exercise.
 

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Looking at NASA test pilots with Lifting Body aircraft experience and who would have been engaged in flight testing in the late 1970's early 1980's the list would be relatively short.

As mentioned before, Milt Thompson was very formative in the development of the Lifting Body concept and would have been an asset to the X-24C's design and test. However, his flying career as a NASA test pilot ended in 1967, when he took a more administrative duty at Dryden (Armstrong FRC).

John Manke, who had the most extensive experience with all of the Lifting Bodies would have been around for the early development of the X-24C (Manke becomes Chief of Flight Operations in 1981).

Thomas McMurtry was the last pilot to fly the X-24B. He could have been instrumental in the early development of the X-24C as his work in aerodynamic and stability testing of the F-8 Supercritical Wing (prior to X-24C timeframe) and involved in the Shuttle ALT program as a SCA B747 pilot. He could have easily been considered as an X-24C project pilot at the time.

Bill Dana was another candidate for the X-24C. He had extensive high speed flight research experience, having flown the X-15 and the Lifting Bodies.

NASA Photo: USAF Test Pilots Gentry and Hoag on left, NASA Test Pilots Manke and Dana on right. Bottom Picture NASA's McMurtry.
 

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