There's always an outlook for liquid propellant and solid propellant rocket engines along with detonation engines and so on. But I hear very little about hybrid engines, have they simply been abandoned or do they still provide significance?
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2 Answers
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Hybrid Rocket Motors are still in use, even in commercial crewed spaceflight. RocketMotorTwo, the motor powering SpaceShipTwo, the reusable crewed spaceplane built by The Spaceship Company and operated by Virgin Galactic, is a hybrid N2O/HTPB motor.
answered May 30 at 9:23
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1$\begingroup$ I really doubt hybrid will ever go "mainstream" for anything besides suborbital tourism. I know there's some light-lift LEO stuff in the works, but even there I can't see a hybrid system being more economical than existing liquid fuel engines. The only place I can see where it kind of makes sense is if you want an extremely simple maintenance cycle, which means fully recoverable, which means suborbital. $\endgroup$ Commented May 30 at 15:04
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6$\begingroup$ I fully agree. Joe Barnard from bps.space likes to jokingly respond "hybrids are trash" whenever the subject comes up. If you go through all the trouble of having valves, injectors, manifold, slosh baffles, etc. anyway, then what's the point of the solid part? Just make it a liquid bi-propellant engine or stick with a solid. The complexity of having two liquid propellants is not much bigger than having one, but one is much bigger than zero. So, if you're already making the jump, just make it all the way. $\endgroup$ Commented May 30 at 15:43
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1$\begingroup$ Rutan's thinking is explained here: space.stackexchange.com/a/39278/6944 To me it made sense for a one-off, not so much for a production vehicle. $\endgroup$ Commented May 30 at 17:16
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$\begingroup$ I remember someone saying that's why they had so much trouble scaling up SpaceShipOne to the larger version, that hybrids don't scale up well. And also why they are short of the Karman line. But I dont know how much that was a part of it. I do remember they tried a different fuel at one point then changed back. $\endgroup$ Commented May 31 at 0:20
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On currently flying orbital rockets, I'm not sure that there is any Hybrid Propulsion engine used.
But for some new companies, it's a choice that's being considered for its relative simplicity comparing to liquid engines while still getting advantages over Solid propulsion like thrust throttling. So some companies decided to develop this kind of engine like Gilmour Space, an Australian company which developed the Sirius hybrid engine and the Eris orbital rocket that should fly for the first time in just a few months.
There are other companies (mainly in Europe) developing this kind of engine, for example, HyImpulse in Germany are developing paraffin/LOX Hybrid engines for their SL1 rocket. There's also HyPrSpace, a French company mainly focused on the development of their Hybrid propulsion engine.
