Slide 46 of 125
Martin Marietta investigated technology requirements for advanced single-stage Earth-orbital transportation systems in the 227-tonne payload class. The company discovered that winged heavy-lift VTHL SSTOs (left) will be impractical unless high thrust-to-weight tripropellant dual-expander engines + shorter storable payload shrouds can move the center of gravity forward during reentry. An "advanced VTHL SSTO RLV (300t/4800t) would weigh slightly less than a comparable VTVL SSTO (299t/5400t) and the development ($7B vs. $7.2B) and operational costs ($3.6M/flt. vs. $4.3M/flt.) would be reduced as well. The aircraft-type horizontal landing mode appears to be more attractive than VTVL for extremely high flight rates and is better suited as a carrier of passengers.
”Space Transportation-New Headings for the Future” -- Salkeld & Patterson, Astronautics & Aeronautics 1978/April/p.28
”Space Transportation Systems 1980-2000” -- Salkeld,Patterson & Grey, AIAA Aerospace Assessment 1978/Vol.1