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Comment by Florin_Andrei | Hacker News Reader

> The engines are the most advanced ever made (arguably).

Maybe not arguably. They embody a technology that's been considered for a long time the Holy Grail of rocket science - the full-flow staged combustion cycle:

https://en.wikipedia.org/wiki/Staged_combustion_cycle#Full-f...

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Everyday Astronaut did a nice explanation of the Raptor and various simpler designs, and what's so great about full-flow staged combustion:

https://everydayastronaut.com/raptor-engine/

This is the internet. I say 'arguably' because someone out there wants to argue.

And look! You found a friend!

That is not strictly true.

As much as I love the Space Shuttle, I wish the US had not went all in on Hydrogen Oxygen. The RS-25 are amazing engines, but we could have had operational full flow staged engines 20 years ago if we had gone down that route. Though probably not the landing part.

Interesting that full flow LOX-LH2 engines could be easier to get, as oxygen and hydrogen can be ignited in a wide range of ratio. With LOX-kero you might have to have a two stage gas generator, when first you ignite components with a good ratio and then add a component to the already established combustion.

> With LOX-kero

Small nitpick: kerolox is much nicer sounding and what people know them as ;)

If you are going to nitpick the kerolox, why not hydrolox too? ;)

(I’m not one of the downvoters for the record)

Oh I just missed it :)

LH2 is way less dense than RP1 though, so you’d need a significantly larger fuel tank to carry the same amount of fuel.

Hydrolox (LOX-LH2) is about 30% more power than Kerolox (LOX-RP1), but is about 3.5x less density.

Source: https://en.m.wikipedia.org/wiki/Liquid_rocket_propellant#Bip...

Hydrolox - Ve: 3,816 d: 0.29

Kerolox - Ve: 2,941 d: 1.03

If you were wanting to optimize for high specific impulse, it seems LOX-LH2:Be or a fluorine based fuel is the way to go purely on the numbers (not accounting for cost or stability).

NASA should’ve kept producing the Saturn V. Over time, the production costs should’ve come down.

The shuttle was a rather mediocre trade off, in terms of LEO payload capacity.

Although granted, the shuttle landings were pretty cool.

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.

> 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?

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/

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.

> 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)

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.

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/me...

Thanks! Just started reading about rocket engines, I don't think most people realize how many moving parts the SpaceX engines have!

Okay, I'll bite, How many moving parts do the SpaceX engines have?

Assuming the fuel and oxidizer impellers are on a common shaft, that's one. There must be a throttle valve somewhere, so that's two. What am I missing? Are there reduction gears?

They use pintle injectors in the Merlins, so that does move as well (though only a short distance) and is likely what makes them so throttleable.

https://en.m.wikipedia.org/wiki/Pintle_injector

Not sure why I was downvoted. Are the moving pintle injectors separate from a throttle vavle?

I have studied a lot of pintle injector designs and it can go either way. Reguardless, a moving pintle injector typically only has two moving parts.

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