Slide 41 of 125
Boeing investigated HTHL SSTO rocketplanes that would either utilize in-flight propellant
transfer or a launch sled to reduce the mass of the landing gear, which then would not have to
support the weight of a fully fueled vehicle. Sled-launched HTHL SSTO (128t/1216t) appeared to
be the more attractive option since they weigh less than an in-flight fueled vehicle (150t/1257t)
, which also would require the development of a very large subsonic tanker aircraft capable of
transferrring 76t/minute of propellant. The HTHL design would have required a relatively large
wing and efficient tankage but the propulsion mass and re-entry wing loading would also be less
than for the VTHL. The HTHL SSTO launch mode (sled assist, in-flight propellant transfer or
rolling takeoff gear) also reduces the T/W requirement and vehicle mass, but VTHL nevertheless
appears to be most attractive as long as limitation to 1-2 launch sites is not detrimental.
The gross liftoff mass of this sled-launched 66-meter long vehicle would have been 1180t and the
dry mass 117t. The structure consists of a relatively advanced brazed honeycomb, internally
braced hot structure which serves the multipurpose role of tankage, load-carrying structure and
thermal protection system. The metallic TPS is a relatively poor insulator which nonetheless
appears feasible for a HTHL SSTO. An advanced technology sled-launched all-rocket HTHL SSTO
would cost $5B and $1.6M/flight and have a mass of (99t/1140t) according to Robert Salkeld. The
propellant costs for the advanced HTHL SSTO would still be higher ($0.3 million vs. $0.140M for
the VTHL) since it uses all hydrogen fuel. VHTL SSTOs, on the other hand, would clearly
benefit from tripropellant propulsion.
A larger 3438-tonne version was proposed for heavy-lift missions; it could have carried a
payload of 113.4t. The total length was 102m, the wingspan was 70m and the vehicle weighed
376.4t empty. Boeing claimed HTHL SSTO would be practical e.g. if tripropellant (LH2,LOX,
hydrocarbon) engines were used)
"Looking Beyond the Space Shuttle" -- Dooling, SPACEFLIGHT 1976/p.68 & p.367
"Advanced Launch Vehicle Systems and Technology" -- Bell, SPACEFLIGHT 1978/p.135
"Advanced Technology and Future Earth-Orbit Transportation Systems" -- Henry & Eldred, Space Manufacturing Facilities II (Proceedings of the Third Princeton/AIAA Conference, May 9-12 1977) p.43
"Space Transportation Systems 1980-2000" -- Salkeld,Patterson & Grey, AIAA Aerospace Assessment 1978/Vol.1
