second half of the twentieth century, very little research was conducted into space travel. During the 1960s, some of the first concept designs for this kind of craft began to emerge. One of the earliest SSTO concepts was the expendable One stage Orbital Space Truck (OOST) proposed by Philip Bono, an engineer for Douglas Aircraft Company. A reusable version named ROOST was also proposed. Another early SSTO concept was a reusable launch vehicle named NEXUS which was proposed by Krafft Arnold Ehricke in the early 1960s. It was one of the largest spacecraft ever conceptualized with a diameter of over 50 metres and the capability to lift up to 2000 short tons into Earth orbit, intended for missions to further out locations in the solar system such as Mars. The North American Air Augmented VTOVL from 1963 was a similarly large craft which would have used ramjets to decrease the liftoff mass of the vehicle by removing the need for large amounts of liquid oxygen while traveling through the atmosphere. From 1965, Robert Salkeld investigated various single stage to orbit winged spaceplane concepts. He proposed a vehicle which would burn hydrocarbon fuel while in the atmosphere and then switch to hydrogen fuel for increasing efficiency once in space. Further examples of Bono's early concepts (prior to the 1990s) which were never constructed include: ROMBUS (Reusable Orbital Module, Booster, and Utility Shuttle), another design from Philip Bono. This was not technically single stage since it dropped some of its initial hydrogen tanks, but it came very close. Ithacus, an adapted ROMBUS concept which was designed to carry soldiers and military equipment to other continents via a sub-orbital trajectory. Pegasus, another adapted ROMBUS concept designed to carry passengers and payloads long distances in short amounts of time via space. Douglas SASSTO, a 1967 launch vehicle concept. Hyperion, yet another Philip Bono concept which used a sled to build up speed before liftoff to save on the amount of fuel which had to be lifted into the air. Star-raker: In 1979 Rockwell International unveiled a concept for a 100 ton payload heavy-lift multicycle airbreather ramjet/cryogenic rocket engine, horizontal takeoff/horizontal landing single-stage-to-orbit spaceplane named Star-Raker, designed to launch heavy Space-based solar power satellites into a 300 nautical mile Earth orbit. Star-raker would have had 3 x LOX/LH2 rocket engines (based on the SSME) + 10 x turboramjets. Around 1985 the NASP project was intended