Incorporation of airbreathing can provide many propulsion options; however, vehicle design choices are not completely arbitrary as requirements and propulsion performance define practical solution space, as discussed in Chapter 3. A priori decisions such as horizontal versus vertical takeoff can doom success before starting on an otherwise solvable problem. From the governing equations, the two keys appear to be offloading some of the carried oxidizer, and designing for sustained operations over a long operational life with maintenance, not continuous overhaul and re-building. As illustrated in Figures 3.27, 3.28 and 3.29, the design space solvable with current industrial capabilities and materials is readily identifiable. New discoveries and industrial capabilities are always important, but, as was clearly demonstrated in the 1960s, neither discovery of new technologies nor the identification of technology availability dates (TADs) are necessary to fabricate an operational space flight system with more capability than the current hardware. Even a cursory review of the North American X-15, or Lockheed and Kelly Johnson's SR-71 would show that the presence of bureaucratic roadblocks such as TADs would have meant neither aircraft would have been built or flown. It was curiosity, resourcefulness, skill and knowledge that enabled the North American and Lockheed teams to succeed. Governmental planning had little to do with their success. The teams adapted what was available and created what was not, only if and when necessary. The latter is the late Theodore von Karman's definition of an engineer [Vandenkerckhove, 1986], contained as a personal note to Jean, one of von Karman's last graduate students: "scientists discover what is; engineers create that which never was''.
There is an excellent documented example of what just written above in a book published by the Society of Automotive Engineers (SAE) entitled Advanced Engine Development at Pratt & Whitney by Dick Mulready. The subtitle is "The Inside Story of Eight Special Projects 1946-1971.'' In Chapter 6, "Boost glide and the XLR-129—Mach 20 at 200,000 feet'' two McDonnell Aircraft Company persons are named, Robert (Bob) Belt and Harold Altus (sic). The spelling should be Altis. The former was known to lead the ''belt driven machine.'' Figure 4.1 comes from Figure 6.7 in that book and compares the development testing of the XLR-129 turbopump to its design value of 6705 psia, with that of the NASA 350K turbopump that became later the main SSME component. In the last paragraph of the chapter the sentence is: ''The liquid oxygen turbopump was the next component in line. However, before it was funded, NASA had started the Space Shuttle campaign, and the Air Force gave the XLR-129 program to NASA, granting free use of the existing hardware to Pratt & Whitney. NASA promptly canceled the liquid oxygen turbopump because it would be unfair to our competitors to fund it. I bet there were times when NASA wished it had continued the program.'' And with it disappeared a rocket engine with a run record of 42 simulated flights (in the test chamber) without any overhaul or repair.
Applying this viewpoint, a cross-section of propulsion system options based on available, demonstrated hardware and materials are discussed with both pros and cons. Airbreathing propulsion can be beneficial over at least a part of the flight trajectory. Historically, there are three broad categories of airbreathing propulsion:
(a) A combination of individual engines operating separately (sometimes in parallel, sometimes sequentially) that can include a rocket engine [The Aerospace Corporation, 1985].
Months after start of tests
Figure 4.1. Comparison of XLR-29 qualification (circa 1965) with that of the Space Shuttle main engine (SSME) (circa 1972).
(b) An individual engine (usually a rocket engine) operating in conjunction with an engine that can operate in more than one cycle mode [Tanatsugu et al., 1987, 1999; Nouse et. al., 1988; Balepin et al., 1996], or a combined cycle engine.
(c) A single combined cycle engine that operates in all of the required cycle modes, over the entire flight trajectory [Maita et al., 1990; Yugov et. al., 1989; Kanda et al. 2005].
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