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Comment by belval | Hacker News Reader

> NASA recently said that it is working with the primary contractor of the SLS rocket's main engines, Aerojet, to reduce the cost of each engine by 30 percent, down to $70.5 million by the end of this decade. [...] And even at $70.5 million, these engines are very, very far from being affordable compared to the existing US commercial market for powerful rocket engines. Blue Origin manufactures an engine of comparable power and size, the BE-4, for less than $20 million. And SpaceX is seeking to push the similarly powerful Raptor rocket engine costs even lower, to less than $1 million per engine.

If their engine is between 5-100x more expensive than an equivalent engine from competiting companies. Why is it so impossible to be a little bit more agile and consider the other engines?

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Because the other engines are not made in the relevant senators' states.

Rockets are not LEGO.

You can't just swap engines without a significant redesign of the rocket and ground support equipment to match.

the RS-25 burns liquid hydrogen and liquid oxygen; the BE-4 and Raptor burn methane and liquid oxygen, but the Raptor uses supercooled densified propellants while the BE4 takes them at closer to their boiling point. Switching between BE-4 and Raptor would require nearly as much work as a switch from RS-25 to either.

The varying density, temperature, and fuel-to-oxidizer ratios mean the propellant tanks would need to change size, and that may have some tricky structural implications with respect to the side boosters of the SLS. And the ground support equipment (launch pad, tower, etc., ) would of course also need to be adjusted to match.

> Rockets are not LEGO.

For a moment there I thought that Low Earth Geostationary Orbit might be a thing.

It could be for a few minutes if your rocket has a LOT of fuel to burn

The other engines burn methane, these RS-25s burn hydrogen.

This is a good point, but to extend it a bit, changing the fuel means changing the chemistry of the whole system, changing tradeoffs and thresholds, changing capabilities, all sorts. Rockets very much need to be designed around a particular fuel, it defines so much of the rest of the rocket either directly or indirectly.

I imagine it also impacts all the other infra (eg, what reagents dou need to store at the launchpad, what do you need to store nearby, do you need cryogenic cooling, etc

It does! The take-off temperature is a very big concern, how much chilling of the rocket needs to happen etc. The ground support equipment I gather is also complex, although I think storage and transport of all those sorts of fuels around tank farms is much more of a solved problem, I suspect you could buy much of it off the shelf (not that NASA would, but SpaceX seem to be buying regular commercial equipment for theirs).

At a savings of USD 80 million or more per engine, I'd say it is well worth it to push deadlines or whatever we need to do to consider the cheaper options.

Right now it looks and smells like corruption.

I think it's a false comparison to say 80m per engine today.

When SLS was being architected 10+ years ago, I believe the idea was that the RS-25 engines would be cheaper, and there were spares around. Also there were no $1m engines, not even close. Perhaps there might have been a competitor for 1/2 the price, but would that be worth it over an engine that you already have flight ad maintenance experience with? Probably not, it's certainly not a convincing argument.

Lastly there weren't any Methane engines – both of the hottest engines in the market now (pardon the pun) are Methane based, the Raptor and BE-4, but the decision to use Hydrogen as the fuel for SLS was set in stone years ago and unrealistic to change.

Basically there weren't convincingly better options at the time the decision was made, and changing that decision now would mean basically going back to the drawing board on the entire rocket.

Methane is a lot more dense than hydrogen. You'd have to completely redesign the rocket.

And the whole reason for the hydrogen burning was to keep the space shuttle contractors jobs. Once again it's not a technical reason but a pork barrel one.

Possibility of corruption aside, these are clones of the Saturn 5 rocket engines and tech just scaled up - the whole thing is built for hydrogen which is clean burning (water is the byproduct) but you'd think SpaceX could take the design and make something for 20mil

The RS-25 is related to the J-2 used in S-II and S-IVB upper stages - a very successful engine. The STS program came up a new design based on a high-pressure combustion chamber running at 3,000 psi (21,000 kPa) for higher performance.

The first SLS flights will use available Block II RS-25D engines left over from the shuttle program, and when those run out (and if SLS is still flying) the rocket will switch over to the RS-25E, a cheaper, expendable version.

The F-1 engines used on the first stage of the Saturn V were built to burn Kerosene, and the RS-25 series has no common heritage.

  > The RS-25 is related to the J-2

How so? Other than sharing a fuel type, and thus surely things learned during J-2 development and operation influenced the RS-25 development and operation, so far as I know the two engines are completely different. Different cycles, different power packs (e.g. turbopump), wildly different packaging, different head pressure, different chamber pressure, throat, etc etc etc. I'm pretty sure that the J-2 did not cool the bell with the fuel, though I could be mistaken. Are the combustion chambers similar? What makes the two engines related, other than the fuel type and of course manufacturer?

I defer to http://www.astronautix.com/h/hg-3.html

A development of the J-2 using a high-performance high-pressure chamber engine. "Technology led to Space Shuttle Main Engines".

Edit to add: "In essence, the HG-3 concept eventually became the space shuttle main engine."

https://web.archive.org/web/20051115064042/http://history.ms...

SpaceX is on record with not wanting to use hydrogen for the "pain in the ass factor".

It boils at a lower point than oxygen, so you have to insulate the tanks from each other, it's very.. undense (sparse?) per volume so you need bigger tanks, and it's the smallest molecule that exists and makes leaks and shipping harder.

Methane is slightly less efficient but way easier logistically.

Don’t forget hydrogen embrittlement, a major risk in reusable rockets.

Yeah I bet if they could do without the LOX they'd do it too. But for the oxidiser there is no choice but to use a compressed gas, obviously.

There are some alternatives to O2, various molecules that contain oxygen which react readily. But those same properties that make them good oxidizers (namely, reactivity) also make them toxic and difficult to handle.

Liquefied, not compressed. Pressure vessels are heavy, just liquefy it by getting it really cold. It only needs to perform for 10 minutes in booster stages.

Those engines didn't exist, their development wasn't publicly announced, and their developers had little if any reputation for successful engine development when NASA locked in their design.

How quickly we forget that just a few years ago the space industry looked very different from how it does today.

Because it is rocket science after all. Changing engines means changing fuel system, weight distribution, avionics, etc.

Because those engines aren't mandated to be used by law.

Because they aren’t made by the parties who lobbied for SLS.

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