Both NASA and the military investigated various reusable TSTO launch vehicle concepts during the early 1960s. In November 1962, the heads of the military and civilian future space transportation efforts signed an agreement to coordinate their hypersonic research vehicle programs. The U.S. Air Force initially was very interested in airbreathing HTHL SSTO aerospaceplanes -- General Dynamics and North American received $1.5-million contracts for preliminary USAF concept studies in June 1963 and the Department of Defense had spent some $46 million on advanced airbreathing vehicle research by FY 1963 -- but quickly concluded that scramjets and other propulsion systems were not yet sufficiently lightweight and efficient for single-stage vehicles.
Airbreathing propulsion and rocket propulsion on HTHL TSTO boosters, on the other hand, would be bigger since they require large wings and huge hydrogen fuel tanks and consequently have a higher dry mass which translates to higher cost. For these reasons, NASA preferred all-rocket TSTO boosters for its "Space Transporter" class of vehicle, since the required engines already had been developed for the Saturn program. The Space Transporter studies were based on the following specifications: (1) 10 passengers + crew of 2 with 3,000kg of cargo to LEO, (2) reduced payload into polar orbit, (3) maximum acceleration of three G, (4) 95% mission reliability and 99.9% probability of passenger survival, and (5) launch rate options of four, eight and sixteen per month over an operational period of 10 years.
In general, the 1960s RLV studies were focused on mission/technology requirements rather than detailed vehicle design. NASA's main priorities for the 1970s were large space stations and manned lunar & planetary missions; the reusable "space transporters" and post-Saturn heavy-lift rockets were simply regarded as necessary adjuncts to reduce the transportation cost. NASA initially regarded horizontally launched TSTOs as safer for passenger transport than vertically launched systems, since the launch G-loads were reduced and the abort characteristics were better than for VLs. However, the US Air Force had more flexibility with respect to G limits and was willing to consider both vertically and horizontally launched Aerospaceplanes.
Martin's "Astrorocket" would have been launched vertically because the designers felt the VL mode frees design from gross liftoff weight constraints (Martin regarded about 450t as the upper limit for a HTHL TSTO). The vertical takeoff mode would provide additional mission flexibility since no rocket powered horizontal launch sled would be required. The Astrorocket would have used storable propellants on both stages so consequently the liftoff weight was high: 1134t. The payload capability to a 555km orbit was only 2.27t and the crew of three astronauts could stay in orbit for up to two weeks. Both stages carry turbojets for powered landing and self-ferry between launch sites. The liftoff thrust would be 13,350KN and stage separation would occur at an altitude of 64km while the vehicle was traveling at 9600km/h.
Crew Size: 3.
Gross mass: 151,927 kg (334,941 lb).
Payload: 2,270 kg (5,000 lb).
Height: 30.00 m (98.00 ft).
Span: 20.00 m (65.00 ft).
Thrust: 2,150.00 kN (483,330 lbf).