1. Geometry General Configuration Configurationbuildupbymeansofelementary elements: semisphere, half-cylinder, truncated
2. The constraints for maximum heat flux during the equilibrium glide section of the trajectory is provided by Glaman18in terms of the wing loading and nose radius, see equation 4 in Galman.18The TPS technology level identifies the constraining regions. When assuming the thermal protection system technology level of the Space Shuttle,35it provides a wider valid region than the solution space, thus it does not affect the current developed solution design space for the X-20. Assuming a reduced technology level, one equivalent to that of the X-20 program era, it can be seen in figure 11 that the solution space decreases significantly. The TPS technology level limits the feasible design space to a max crew size of 3 and τ = 0:23. The second constraint is recognizing the payload limit of the Titan-IIIC launch vehicle. The Titan-IIIC was selected as the primary launch vehicle for the X-20 glider in the early 1960s.9The Dyna-Soar program was canceled before the first flight of the launch vehicle. As can be seen clearly from Figure 11, the launch vehicle ability also reduces the overall solution space significantly. However, although the feasible design solution space reduces significantly after application of those constraints, both configurations, the X-20 glider and the glider with trans-stage, still lie comfortably in the feasible design space. The second phase of the X-20 program was planned with a higher performing mission profile and increased crew member capacity. Consequently, vehicles of this increased payload capability would not lie in the solution space shown with Figure 10 and 11.
3. It is important to mention again that the X-20 program was the first large scale industrial-level program for the development of an operational LRV vehicle and occurred more than two decades before the first flight of the STS. Most of the technology, knowledge, data, and even the test-facilities developed during the X-20 program found application in the follow on generations of lifting body programs, eventually leading to the development of the Space Shuttle. This is further confirmed by several noted authors, historians, and researchers9,10,35-38as almost every account of the history and development of the STS program mentions the X-20 program as a major contributor towards the Space Shuttle vehicle.