The scramjet engine was a key to the NASP multi-cycle engine. In a scramjet the onrushing hypersonic air is compressed and passed into a combustion chamber in which hydrogen is injected and burned by the hot, compressed air. The exhaust is expelled through a nozzle creating thrust. The efficient functioning of the engine depends on the aerodynamics of the airframe, the underside of which serves as the air inlet and the exhaust nozzle. Design integration of the airframe and engine are thus critical to the success of the design.
Other enabling technologies include the development of materials that would maintain structural integrity at very high temperatures, sometimes in excess of 1,800°F. The enormous heat loads associated with hypersonic flight have required the development of active cooling systems and advanced heat-resistant materials.
Using atmospheric oxygen instead of tanked LO2 for the majority of the mission, the NASP airbreathing engines were projected to have a specific impulse that is approximately 3 to 4 times that of a LO2/LH2 rocket engine. This enables a single-stage-to-orbit (SSTO) vehicle with propellant mass fraction of approximately 76 percent to achieve orbital speeds. So the principal technical challenge was to achieve the high performance of the airbreathing engines, while limiting the impact of the inert mass of these engines and the additional mass of the thermal protection system required to protect the vehicle during its ascent to orbit.
Despite significant progress in structural and propulsion technology, the program had substantial hurdles to overcome. DOD wanted it to carry a crew of two, plus a small payload. The demands of being a man-rated vehicle operating as a SSTO vehicle made the X-30 more expensive, larger, and heavier than is required for a demonstrator vehicle.
As a SSTO vehicle with a turnaround time of 24 hours or less, proponents initially viewed the X-30 as leading the way to faster, cheaper access to low Earth orbit. What became obvious was that the claims made for NASP as a space launch vehicle were similar to the initial claims made for the space shuttle in the early 1970s. The assertions that NASP would have airplane-like operating characteristics were assumptions, not conclusions based on detailed analysis.
NASP never achieved flight status and finally fell to budget cuts in 1993. But it also was cancelled because of its severe technological overstretch. Although the X-30 program never came near to building significant hardware, NASP development work contributed in an important way to the advancement of propulsion technology and high-temperature materials as well as materials capable of tolerating repeated exposure to extremely low temperatures (the cryogenic fuel tanks). By 1990, NASP researchers had realized significant progress in titanium aluminides, titanium aluminide metal matrix composites, and coated carbon-carbon composites. Moreover, government and contractor laboratories had fabricated and tested large titanium aluminide panels under approximate vehicle operating conditions, and NASP contractors had fabricated and tested titanium aluminide composite pieces.
When NASP was cancelled, the government admitted to making a $1.7 billion investment in the National Aerospace Plane, but parts of the research and development were classified, and the official costs may have been higher.2
Lessons learned from the NASP program include the following:
• SSTO vehicle technologies using airbreathing engines are beyond the existing state of the art;
• Aerothermodynamics of sustained flight at high hypersonic speed within the atmosphere creates significant challenges with regard to materials and thermal management; and
• Propulsion technology should be independently matured prior to initiation of large-scale vehicle development programs.
The X-33 VentureStar was intended to be a one-third-scale prototype of a Reusable Launch Vehicle (RLV), designed to significantly lower the costs of launching payloads. VentureStar was a very ambitious effort by NASA and Lockheed Martin to develop a fully reusable, vertical launch and horizontal landing, manned, (SSTO vehicle. To achieve SSTO capability, VentureStar needed to incorporate high thrust and high-specific-impulse propulsion
2 R. Launius, After Columbia: The Space Shuttle Program and the crisis in space access, Astropolitics 2:277-322, 2004.