GENERAL DYNAMICS “TRIAMESE” 
The General Dynamics proposed an ingenious “Triamese” concept for the US Air Force “Integral Launch & Reentry Vehicle” program. This system (originally developed in 1965 for a classified USAF SAMSO study) would have utilized three virtually identical reusable booster/orbiter element vehicles rather than develop two different booster and orbiter spaceplanes. General Dynamics estimated that the Triamese only would cost $1-2 billion to develop (=$4.5-9B at 1999 economic conditions) and be operational by 1976. Two of the Triamese vehicles would supply propellants for the engines of all three units during ascent. The illustration depicts the stage separation process after all rocket fuel in the boosters has been depleted. The orbital element continues to orbit with its engines fed by its own internal propellant supply. The projected payload capability was 11,340kg into a low-inclination Earth orbit or 8,390kg into polar orbit from the Kennedy Space Center. Up to 12 passengers could have been carried in the vehicle's 5.2 * 4m cargo bay; the booster cargo compartments would have been outfitted with additional propellant tanks. General Dynamics claimed that the marginal launch cost would be as low as $0.3 million in 1969 dollars, i.e. $1.36 million at 1999 economic conditions ($120-200/kg).
The Triamese vehicles would have incorporated fold-out wings plus jet engines for atmospheric cruise & horizontal landing at a standard airport runway. Unlike the partially reusable drop-tank Starclipper or McDonnell-Douglas ILRV designs, the Triamese could be launched from inland areas in any direction without undue hazards to people or property on the ground. The operating cost would also be less, although the vehicle would weigh more and be slightly more complex to maintain and operate than a traditional two-stage-to-orbit VTHL spaceplane.
The structural design was quite simple and consisted of an expendable launch vehicle-type structural core (propellant tanks, thrust structure+engines) protected by an aerodynamic fairing to provide thermal protection. The propulsion system would have consisted of two 1,112.5-KN thrust P&W XLR-129 oxygen/hydrogen engines. The projected gross liftoff weight was 518,456kg.
“Reusable Space Ferry Considered” -- AW&ST 1969/June16/p.57
“Aircraft Design and Development Experience Related to Reusable Launch Vehicles” -- Dore, AAS Science & Technology 1969/vol.21/p. 43