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TL;DR: Liquid-fuel rockets work by combining a liquid fuel with oxygen. To get the fuel and oxygen into the combustion chamber in huge quantities, they use a bi

TL;DR: Liquid-fuel rockets work by combining a liquid fuel with oxygen. To get the fuel and oxygen into the combustion chamber in huge quantities, they use a big fancy pump. To power this big fancy pump, all liquid-fuel rockets burn fuel in a separate combustion chamber, using the hot gas to power the pump. After the gas powers the pump, it has to go somewhere. Most rockets just dump the gas (these are called 'Gas-generator cycle' engines). This is nice and easy (ha), but it also means that the gas isn't going through the main combustion chamber, so it isn't propelling the rocket. The rockets on Starship re-use the gas that drives the pumps, pushing them into the combustion chamber and burning any unused fuel/oxygen to propel the rocket.

mschuster91 6y ago
> The rockets on Starship re-use the gas that drives the pumps, pushing them into the combustion chamber How is that possible? From a total layman's point of view, the pressure in the combustion chamber should always be greater than whatever the pump can put out, thus leading to a pump stall... what am I missing?
- maccam94 6y ago
  Actually the pump has to be able to put out as much if not more pressure than the pressure generated in the combustion chamber, otherwise it wouldn't be able to push a steady stream of fuel and oxidizer into the chamber. I recommend watching or reading https://everydayastronaut.com/raptor-engine/ https://everydayastronaut.com/raptor-engine/
- lutorm 6y ago
  As the other post says, this cannot be true, otherwise how could the engine work. You'd not get any propellant into the combustion chamber.
mabbo 6y ago
> To power this big fancy pump, all liquid-fuel rockets burn fuel in a separate combustion chamber, using the hot gas to power the pump. There's some engineers in New Zealand who might have something to say about that... https://en.m.wikipedia.org/wiki/Electron_(rocket) https://en.m.wikipedia.org/wiki/Electron_(rocket)
- marcusverus 6y ago
  Fascinating! Their rocket's trust-weight ratio (~73, according to Wiki) is WAY below the state of the art gas-generator cycle Merlin (~185), presumably because of the added weight of the motor and batteries required to operate an electric turbopump. It's always interesting to see folks look past a supposedly bad design and discover that its other benefits (in this case, cost and simplified design, apparently?) can still render it a viable option.
_Microft 6y ago
Here is a picture of a Merlin engine on a test stand. To the right of the nozzle, the black and sooty exhaust of the turbopump can be seen. https://old.reddit.com/r/engineteststands/comments/8f7c1q/merlin_engine_loxrp1_static_fire_test_by_spacex/ https://old.reddit.com/r/engineteststands/comments/8f7c1q/me...

mschuster91 6y ago

> The rockets on Starship re-use the gas that drives the pumps, pushing them into the combustion chamber How is that possible? From a total layman's point of view, the pressure in the combustion chamber should always be greater than whatever the pump can put out, thus leading to a pump stall... what am I missing?

maccam94 6y ago
Actually the pump has to be able to put out as much if not more pressure than the pressure generated in the combustion chamber, otherwise it wouldn't be able to push a steady stream of fuel and oxidizer into the chamber. I recommend watching or reading https://everydayastronaut.com/raptor-engine/ https://everydayastronaut.com/raptor-engine/
lutorm 6y ago
As the other post says, this cannot be true, otherwise how could the engine work. You'd not get any propellant into the combustion chamber.

mabbo 6y ago

> To power this big fancy pump, all liquid-fuel rockets burn fuel in a separate combustion chamber, using the hot gas to power the pump. There's some engineers in New Zealand who might have something to say about that... https://en.m.wikipedia.org/wiki/Electron_(rocket) https://en.m.wikipedia.org/wiki/Electron_(rocket)

marcusverus 6y ago
Fascinating! Their rocket's trust-weight ratio (~73, according to Wiki) is WAY below the state of the art gas-generator cycle Merlin (~185), presumably because of the added weight of the motor and batteries required to operate an electric turbopump. It's always interesting to see folks look past a supposedly bad design and discover that its other benefits (in this case, cost and simplified design, apparently?) can still render it a viable option.

_Microft 6y ago

Here is a picture of a Merlin engine on a test stand. To the right of the nozzle, the black and sooty exhaust of the turbopump can be seen. https://old.reddit.com/r/engineteststands/comments/8f7c1q/merlin_engine_loxrp1_static_fire_test_by_spacex/ https://old.reddit.com/r/engineteststands/comments/8f7c1q/me...