Another Ramjet

Despite the HRE debacle, Marquardt did not give up easily. Although NASA had ruled out a ducted rocket in 1963, Marquardt managed to generate enough interest in the concept to get a study contract from the FRC in early 1964, and the AARPS came back as a separate study. The designers of the AARPS contemplated the use of advanced air-breathing propulsion cycles, such as ducted rockets and ejector ramjets. NASA awarded Marquardt a small contract to define a research and development plan for AARPS and to determine the feasibility and usefulness of flight-testing the system on X-15A-2.2821

On 3 January 1967, the company proposed a different ramjet installation for X-15A-2, actually providing power to allow a "cruise capability of approximately Mach 5." The company proposed installing an "ejector ramjet on X-15A-2 in the area now occupied by the rocket engine." The ramjet would use jet fuel and liquid oxygen as propellants, although hydrogen peroxide was listed as an alternate oxidizer.-12831

The gross weight of the airplane would increase 2,571 pounds (from 35,735 to 38,306 pounds). The amount of liquid oxygen would remain constant at 10,533 pounds, but 9,400 pounds of jet fuel would replace the normal 8,199 pounds of anhydrous ammonia. The propulsion system weight would increase from 910 pounds (XLR99) to 2,280 pounds (1,380 pounds for the engine and 900 pounds for the inlet). On the airplane itself, the liquid-oxygen tank would remain unchanged, but the proposal modified the existing ammonia tank to allow room for the inlet ducting. Unfortunately, Marquardt did not specify how it would accomplish this, given that the ammonia tank was a full-monocoque structural member of the fuselage.-2841

XLR99 (but well above the 16,000 Ibf provided by the interim XLR11s). Marquardt estimated that the acceleration to Mach 5 would take 4.21 minutes at 1.04 g (much slower than the 90 seconds or so it normally took to get to Mach 6), covering approximately 150 miles in the process. Once at Mach 5 the ramjet would provide 14.8 minutes of steady-state cruise, covering 840 miles. This

1,0- mile flight would have necessitated a major extension to the High Range, and might well have exceeded the heat-sink capability of the Inconel structure, even with an ablative coating.[285]

Marquardt also suggested that the engine could be adapted to the delta-wing airplane, and that in the future a modified engine could provide additional cruise performance. In either case, the engine featured a large rectangular inlet located under the fuselage that started slightly ahead of the wing. The inlet duct swept upward into the fuselage just ahead of the ventral stabilizer, explaining the required modifications to the fuel tank. The inlet faired into the ventral, and the ramjet engine was located where the normal XLR99 had been.[286]

It appears that NASA did not take any action based on the study results.