3 ms·

A staged combustion engine is intrinsically a lot more efficient than an engine where you dump some of your fuel to power the pumps, like in a tapoff or gas-gen

A staged combustion engine is intrinsically a lot more efficient than an engine where you dump some of your fuel to power the pumps, like in a tapoff or gas-generator engine. Compared to a oxygen-rich full flow you can reach a higher pressure while having better safety margins which will allow you to re-use the engine. The higher your chamber pressure the closer the rocket's thrust at sea level is to its maximum thrust in vacuum. Also higher pressure tends to correlate with higher thrust to weight ratios. Also, having both inputs already be gasses mean they mix better. This will have a tiny benefit in them burning more completely. But more importantly they'll burn better across a wider range of thrusts, which could be important for throttling down the engine for landing.

SECProto 8y ago
> Also, having both inputs already be gasses mean they mix better. This will have a tiny benefit in them burning more completely. But more importantly they'll burn better across a wider range of thrusts, which could be important for throttling down the engine for landing. Also, and importantly for the design of the engine, it's much much easier* to computationally model the mixing of two gases, to ensure complete mixing and no combustion instability (especially when the reaction components are limited to ch4, o2, and partial- and complete-reaction products like c02, c0, h2o, oh-, ...). * by much easier, i mean less impossible
- Symmetry 8y ago
  Have you seen SpaceX's talk at some NVidia conference about their homegrown Computational Fluid Dynamics simulator? It was really interesting. https://www.nextplatform.com/2015/03/27/rockets-shake-and-rattle-so-spacex-rolls-homegrown-cfd/ https://www.nextplatform.com/2015/03/27/rockets-shake-and-ra...
  - SECProto 8y ago
    I hadnt seen that exact article, but i recall reading (or listening to a lecture) that mentions how one advantage of methane is the possibility to use CFD in design, while that was still outside the realm of possibility with RP-1 due to complexity of reactants (differing phases and many compounds)
ummonk 8y ago
Liquid-liquid mixes extremely well. The hard part is droplet atomization, not mixing. I’m not knowledgeable about gas-gas injectors but I find it hard to believe that you’d be able to easily make them mix well.

SECProto 8y ago

> Also, having both inputs already be gasses mean they mix better. This will have a tiny benefit in them burning more completely. But more importantly they'll burn better across a wider range of thrusts, which could be important for throttling down the engine for landing. Also, and importantly for the design of the engine, it's much much easier* to computationally model the mixing of two gases, to ensure complete mixing and no combustion instability (especially when the reaction components are limited to ch4, o2, and partial- and complete-reaction products like c02, c0, h2o, oh-, ...). * by much easier, i mean less impossible

Symmetry 8y ago
Have you seen SpaceX's talk at some NVidia conference about their homegrown Computational Fluid Dynamics simulator? It was really interesting. https://www.nextplatform.com/2015/03/27/rockets-shake-and-rattle-so-spacex-rolls-homegrown-cfd/ https://www.nextplatform.com/2015/03/27/rockets-shake-and-ra...
- SECProto 8y ago
  I hadnt seen that exact article, but i recall reading (or listening to a lecture) that mentions how one advantage of methane is the possibility to use CFD in design, while that was still outside the realm of possibility with RP-1 due to complexity of reactants (differing phases and many compounds)

ummonk 8y ago

Liquid-liquid mixes extremely well. The hard part is droplet atomization, not mixing. I’m not knowledgeable about gas-gas injectors but I find it hard to believe that you’d be able to easily make them mix well.