• Abort capability with engine out (two engines provided best performance and lowest cost);
• Department of Defense (DOD) cross-range requirements drove the configuration and added complexity;
• Continuous additional requirements were introduced during the development process; and
• The complexity of the configuration and its resulting sensitivity and required interactivity provided design challenges, which affected the operational complexities and flight constraints.
Even if the original, totally reusable two-stage human-rated space shuttle system had been properly funded from the outset, it is doubtful that the initial low cost goals would have been met. This conclusion is further supported by what is now recognized as an unrealistically projected high launch rate.
The orbiter’s design was performance driven. It needed to achieve low Earth orbit, provide life support for its crew for several weeks, reenter Earth’s atmosphere, and glide to a safe landing. The ability to reserve some orbiter weight during its development to make its ground turnaround operations more efficient was not possible. As a result, the anticipated flight rate was never achieved because ground turnaround operations were extensive and very costly.
Maintenance issues associated with the orbiter’s reentry thermal protection system (TPS) tiles made rapid ground turnaround impossible. After every flight, each of the orbiter’s more than 27,000 tiles had to be individually inspected for damage and adhesion and manually replaced, if necessary.
The orbiter’s three main reusable engines (space shuttle main engines, or SSMEs) also required inspection after landing. Because of access and interference issues with other orbiter components, SSME’s were removed and replaced after every space shuttle orbiter mission, beginning with operational mission STS-6. Off-orbiter engine processing during the operational phase was done for several reasons. Checkout of the SSMEs required power-up of electrical and fluid subsystems, which interfered with other orbiter processing. Also, access was often needed in the orbiter’s engine compartment to other main propulsion system components, and the installed SSMEs restricted access and often resulted in serial processing work in the aft compartment. It was easier (and more efficient) to remove and replace engines than to accommodate these other needs with engines in place. Orbiter and engine processing work could be accomplished in parallel with removal of the engines, thus any unforeseen contingency processing would not impact other components.
This orbiter ground processing experience resulted in the development by NASA of operability/maintainability design considerations for next generation reusable launch vehicles.
NATIONAL AEROSPACE PLANE HISTORY
The National Aero-Space Plane (NASP) was a project jointly funded by NASA and DOD to create a single-stage-to-orbit spacecraft.1 The NASP concept evolved from the “Copper Canyon,” the Defense Advanced Research Projects Agency (DARPA) project, running from 1982 to 1985. The vehicle was planned for a crew of two and was meant to serve access-to-space missions. In 1990, McDonnell Douglas, General Dynamics, and Rockwell International formed a national team to develop a demonstrator vehicle, the X-30, to deal with the technical and budgetary issues.
There were six technologies considered critical to the success of the project; three of them were related to the propulsion system. The X-30 was intended to use an engine that could shift from a low-speed propulsion system to a scramjet system as the vehicle ascended; it would burn liquid hydrogen fuel with oxygen taken from the atmosphere. An auxiliary LO2/LH2 rocket engine was used to augment the scramjet engine at very high speeds and for propulsion needs in space.
1 B.W. Augenstein and E.D. Harris, The National Aerospace Plane (NASP): Development Issues for the Follow-On Vehicle, R-3878/1-AF, RAND Corporation, Santa Monica, Calif., 1993; Encyclopedia Astronautica, X-30, available at http://www.astronautix.com/lvs/x30.htm, accessed on October 9, 2012.