The Solar Orbit Transfer Vehicle (SOTV) was an upper stage and orbital tug developed by Boeing under contract from the Defense Advanced Research Projects Agency (DARPA) in the 1990s-2000s. It was to use solar-thermal propulsion, in which a large inflatable solar concentrator dish would capture sunlight and reflect it into a heat exchanger, through which liquid hydrogen propellant would be passed. The hydrogen would be heated and boil into a gas, which would be expelled at very high exhaust velocities to produce thrust. It was hoped that this would be a more cost-effective means of delivering payloads to high orbits or interplanetary trajectories, due to both the high specific impulse of thermal propulsion and the relative mechanical simplicity of the system. SOTV would have flown on Delta III and Delta IV rockets as a third stage, but was canceled as part of a restructuring of Boeing's space division

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