Comments by "TheThirdMan" (@thethirdman225) on "Bloodhound SSC - 1000 MPH Hybrid Rocket System - Explained" video.
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The Jag engine was chosen because it was available but in the greater scheme of things, the choice of fuel pump was not that difficult compared with the other challenges they had to overcome. There is absolutely no reason why the fuel pump could not have been a gas turbine. A Pratt & Whitney PT-6 or similar would have been an excellent choice (very available, powerful and reliable) and it would have been considerably smaller and lighter than the Jag engine. The trouble would be the size of the reduction gearbox needed to get the RPM down to a manageable size. Extracting heat would have been harder too. On the upside, it could have shared the fuel type with the EJ-200, which would make things simpler and it would almost certainly be lighter overall.
But what are we talking about are? The fuel pump itself uses an extremely clever priming system which is only hinted at in this video and the hybrid rocket is actually throttleable, which is amazing by itself. That's not so hard with liquid fuel and/or hypergolic fuels but it's very difficult with a solid fuel component. All that is worthy of a video on its own. And really, the Jag engine is only 540 hp of non-motive power against 135,000 hp which will actually power it. It's chicken feed but it gives some idea of how much power this thing produces.
For the record, the Saturn F-1 engines from the Apollo Moon rockets used a 55,000 hp turbo pump, which was itself a gas turbine.
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