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To me, it seems like their business case is very dependent on rapid reuse. With this in mind, using a hybrid engine that would need to be replaced every flight seems very counter intuitive vs a liquid one which seems like it would be much easier to maintain. What factors (liquid storage, specific impulse, design time, etc...) caused them to make this decision?

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The book Burt Rutan's Race to Space by Dan Linehan describes why the predecessor of Spaceship Two chose a hybrid engine.

  1. Rutan ruled out solid motors because they cannot easily be shut off (a safety concern).
  2. He ruled out liquid motors because of cost and complexity.

Rutan settled on a hybrid engine as the best compromise between safety and cost. Also

Spaceship One could get away with having a less efficient or a less effective rocket engine.

When Spaceship Two was being designed and built, this was also done by Scaled Composites, and presumably the same logic held. The follow-on vehicle, while incorporating many design changes, was essentially a scaled-up Spaceship One.

Since then, the engine has been redesigned quite a bit and Scaled Composites is no longer building the vehicle. It's a fair question whether the same choice would be made today given hindsight. A vehicle designed to win a prize and then be retired has significantly different requirements than a passenger vehicle.

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    $\begingroup$ Yes, in my original question I assumed they could buy engines, but that is probably easier said than done. Even if SpaceX sold engines (which they don't now I believe) a single Merlin engine would give around 10G of acceleration which would likely kill everyone on board. The BE3 would give about half of that, but that is still not comfortable. Throttled down, the Isp would probably be low. I guess most people don't make rocket engines weak enough to be comfortable for a small passenger plane like this. If AR made one, I'm sure pricing would be an issue. $\endgroup$ – David Oct 10 '19 at 18:28
  • $\begingroup$ I did a bit of searching: The RL-10 engine by AR would be a good fit. Lower thrust than their current motor though, but at least the G forces wouldn't kill the passengers. The deal breaker: The engine cost 25$ Million. Amazing Isp though. $\endgroup$ – David Oct 10 '19 at 18:38
  • $\begingroup$ @David that was my first thought at well just based on thrust. It's a great engine. But...it's a cryo H2/O2 engine. The cost of the engine is just the tip of the iceberg, you need all the ground equipment to handle & store the cryogens safely. $\endgroup$ – Organic Marble Oct 10 '19 at 18:40
  • $\begingroup$ Yeah, I could see that adding cost outside the engine. Just needing a supply of NO2 is a big help, but it would also be nice to crack 100km and not have BO rubbing it in their face as they undoubtedly will until for the next decade or until they upgrade their engine. $\endgroup$ – David Oct 11 '19 at 4:23

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