1. since conventional rocket cryogenic propcllants currently account for about 85% of lift-off mass, of which 80.90% is liquid oxygen. This will, therefore, substantially decrease liftoff mass and allow for specific impulses which are about 10 times better than those of current LH2/L02 engines (450s for U.S. Space Shuttle to projected 2000-4000s). It follows that without the high density oxidizer mass requirements, a higher payload fraction can also be obtained, Most expendable provide for a payload which is rockets
2. The aerodynamics of a spaceplane are much more complex than those of a conventional airplane or rocket. Because an air-breathing engine's thrust depends to large extent on atmospheric density, the spaceplane must accelerate to near orbital speed well within the atmosphere. At very high Mach numbers, the viscous airflow over a spaceplane's surface slows down and becomes a very thick boundary layer which affects the rest of the air flowing past the vehicle. As much as 40% of the air entering the spaceplane's intakes can consist of this low energy boundar layer which will result in loss oi? net thrust. Obviously, this effect is more crucial for a NASP type vehicle which air breathes all the way to orbit than for a HOTOL or a Sanger which will fly to Mach 5-7 and then use rocket power to achieve orbit.
3. Aerodynamic heating is another area where a considerable amount of technology effort is being expended to develop advanced materials. Again, vehicles such as the HOTOL and the Sanger, which use rocket power to achieve orbit, will not suffer maximum heating during entry into orbit, but experience it during reentry. The highest heat loads on a spaceplane will occur on the wing leading edges and the nose elements. It is expected that for the HOTOL upon reentry these areas will get as hot as 1500 - 18OO0C, while for NASP during ascent they could quite possibly reach 2700°C.
4. Spaceplane airframe and engine structural designs call for the use of light weight materials which will retain useable physical and mechanical properties to temperatures beyond those available today. Currently used spacecraft materials such as aluminum alloys and polymeric matrix composites based on carbon fibers will not he able to withstand spaceplane operational temperatures approaching and possibly exceeding 2000°C. More appropriate for use are various metallic and ceramic type materials, either in alloy or composite form. In particular, the following groupings of materials have been proposed:
5. Titanium alloys provide high specific strength and good basic corrosion resistance. They have traditionally been used for airplane structures requiring temperature tolerances from 150°C to 6OOOC. With the introduction of filament reinforcement, stiffness is improved, density is decreased, and overall temperature stability is increased to around 600'C- 800'C. The basic development of these composites has so far centered on the use of boron (B,C/R) and Sic (AVCO SCS) as filaments, with conventional Ti-6-A1-4V as thc principle alloy. Titanium alloys and their composites provide an added advantage in that they can be fashioned into very complex, thin sectioned, and stiffened structures through superplastic forming or diffusion bonding processes. Intermetallic