Today, solid rockets dominate most military applications because they take up relatively little space, can be launched on a moment’s notice, and offer a high thrust-to-weight ratio. Meanwhile, liquid bipropellant rockets enjoy the lion’s share of orbital launch vehicle and spacecraft propulsion system use because they are more fuel efficient, offer a higher specific impulse, and can be throttled and restarted when needed.

Hybrid rocket engines, which combine a solid fuel with a liquid oxidizer, offer the best of both worlds, including immediate launch and the ability to be throttled and restarted. More important for commercial space transport use, they offer significantly greater safety than either solid or liquid rocket engines. But hybrid motors have been studied for many years and have seen little use for several reasons, including high motor-to-motor performance variability, high development costs, poor performance efficiencies, and inability to scale to high-volume production.

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