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Comment by chasd00 | Hacker News Reader

I like how small and slender Raptor 2 is compared to Raptor 1. The turbo pump looks even scarier than usual though. It's amazing that something so small is producing 100k HP (the pump turbine).

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I thought 100k HP was either too incredible or a typo. Then I looked it up:

“You def don’t want electric pumps on a rocket engine! Raptor turbopumps alone need 100,000 horsepower per engine. That’s not a typo.” —Elon Musk

x 32 engines. Amazing! For comparison, this is an internal combustion engine with 100k HP:

https://www.zmescience.com/science/biggest-most-poweful-engi...

That is interesting, so could you replace the biggest and most powerful internal combustion engine in the world that weighs 2,300 tons by [whatever powers] turbopumps from a raptor engine and get the same power output? Where is the catch? Less fuel efficient? Less torque?

Edit: What I meant to ask was: Can one use "whatever is powering the turbopumps and delivering the 100k HP" to power say a ship propeller, instead of that 2.3k ton internal combustion engine. Like, make some variant of raptor engine that delivers this power to a shaft.

It's a totally different problem.

Yes it's less efficient, you spent all this energy cooling off all the fuel and oxidizer to cryogenic temperatures before using it. The turbopumps in the raptors are also running very far off from stochiometric fuel mixtures (which is fine in the rocket because they're exhaust is used as fuel for the next stage) - which means if you're just using it for the pump your not actually burning most of the fuel. The exhaust is also really hot, and if you're not somehow using that energy it's definitely inefficient.

None of those issues matter to the rocket. Energy used on the ground to cool down propellants is "free", the colder the better because it means you can fit more propellants in the same tanks. The exhaust is just being piped into the main combustion chamber, so it will eventually be fully combusted extracting the left over chemical energy. The heat of the exhaust (after the main chamber) is the whole point, with a nozzle converting it into kinetic energy.

You've also got a bunch of safety issues. High pressures, liquid oxygen, and so on.

> You've also got a bunch of safety issues.

Yeah, plus, there’s no fixing a broken or poorly running turbo pump turbine. It either works or explodes, no inbetween.

The turbopump on its own won't do anything. The turbopump takes the 100k hp, and uses that to compress fuel and liquid oxygen. It's powered by the engine. It doesn't produce power on its own.

IIRC it's powered by a relatively small in size pre-burner - and I think it's fair to assume that he was including that pre-burner in his comparison.

Also known as gas generator, and while it's small, in Raptor two of them consume all (or practically all?) of both fuel and oxidizer in tanks.

The raptor burns highly purified methalox for a couple minutes at most. The ship engines have to work for weeks and years economically running on bunker fuel. From fundamental thermodynamics, the bigger an engine is the more efficient it is but you obviously need to compromise on that in a rocket.

Soyuz rocket takes 9 minutes to get to orbit. Raptors are multiple-use engines, their lifetimes could be hours of work, which is a lot for rocket engines. However it could be possible to increase lifetimes at least another order of magnitude - an NK-33 engine once run uninterrupted for 14 thousands seconds. Consumed a lot of kerosene.

RL-10 can avoid too big stresses, at least not heating to high temperatures, so this is another hint on possibly long working rocket engines...

You could in theory do that. The downsides are that a turbopump needs comparatively perfect fuel, requires liquid oxygen instead of air, and would be inefficient in non-rocket applications (the ‘waste’ has other uses like cooling/protecting the rocket nozzle in some rockets).

Think of it as an engineering tradeoff where you get to pick 2 things to optimize for: weight, efficiency and size. A turbopump (and preburner) optimizes for weight/size. A ships engine picks efficiency twice, making it massive and heavy.

Used everywhere, more complex and has a more limited operating parameters though. In the maritime industry it has generally been military or high speed ferries using them. Essentially, if your ship can get away with a larger more mechanically simple engine it is more economical because as long as they stay on the surface you are not paying the fuel price of also lifting the engine.

Commercial jets use the same mechanics with their turbofan engines. That is why they are growing huge, we're just encasing the propeller inside the cowling, with some other nice side-effects.

https://en.wikipedia.org/wiki/Turboshaft

https://en.wikipedia.org/wiki/Turbofan

Every time I look at jet engine diagrams I get chills thinking about what it must take to make mechanical parts that function with that much precision for that many revolutions under that much heat and pressure.

like many here, I've spent my professional life in software. You can goto walmart, buy a $500 laptop, hook it up to free wifi at a coffee shop and you have access to all the tools you need to build cutting edge software technology. That is not the case in meatspace. Going from an empty space in a factory to a jet engine ready to be installed on a plane blows me away on so many different levels.

You could potentially use a turbine engine as a generator in a hydrocarbon-electric setup. It would in theory be lighter, simpler, and require less maintenance.

For now there's already a lot of momentum with ICE and they are very polished products. But I think the potential for turbine is there given the shift towards electrification.

Some hurdles include working with materials that can tolerate very high stress, and finding ways to improve efficiency from significant heat loss.

https://www.wikiwand.com/en/Microturbine

Here's a guy with a 48,000 HP marine turbine for doing ships propellers. https://www.navyhistory.org/2016/12/commercial-use-of-marine...

And some other images https://www.google.co.uk/search?q=Rolls-Royce+MT+30&sxsrf=AP...

It's not as small and powerful as the SpaceX one but I guess it needs some extra bits. Also it runs off air rather than liquid oxygen which must reduce power a tad.

>You def don’t want electric pumps on a rocket engine

The https://en.wikipedia.org/wiki/Rocket_Lab_Electron has them and made it to orbit

Yes, for a tiny rocket and they are not using them for anything larger.

Its doable if you throw batteries out of your second stage when flying.

But no company that makes a rocket payload then maybe a few 100kg would ever use them. And for sure not launch humans or go to Mars.

Even if you assume gigantic improvements in batteries by 400% you wouldn't use if for anything major.

It has potential for in space applications as there you can charge it with sunlight.

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