[babyrainbow]

> so much as the ability to execute a controlled landing of the first stage...

Pardon me if I am wrong here. But it seems to me that you are suggesting that most of the damage that a rocket sustains, that makes it not fit for reuse, does not happen during the launch and re-entry, but during landing and salt water?

But if that is so, why was this the case with space shuttles?

> Although the Space Shuttle Main Engines (SSME) were reusable and going to be used on the SLS rocket, NASA doesn't plan to reuse them. The refurbishing and recertification costs make reuse more expensive than manufacturing new engines.

https://www.quora.com/Havent-other-space-companies-thought-o...

[Inconel]

Apologies again for not understanding, but I’m unsure exactly what you’re asking with the question why was this the case with space shuttles.

With regards to the SSME, the SSME is an engineering marvel, but it is significantly more complex than the Merlins you’ll find on Falcon, both in initial build as well as refurb. I know a few techs here who used to work on them at Rocketdyne. Raptor will be more analogous to the SSME but there have been significant advancements in metallurgy and materials science since the introduction of the SSME, which should hopefully lead to easier reuse. Also, it should be noted that the SSME were in fact refurbed and reused when they were part of the Shuttle program.

As to your quote from Quora, I find it somewhat ambiguous. Again, the SSME was routinely refurbed and reflown as part of the Shuttle Orbiter. I’m no expert on the SLS program, although I do keep up with things space related, but I’ve yet to see any cost breakdowns of refurb vs new build in regards to SSME specifically. That quote makes it unclear to me whether NASA found that reusing the SSME as part of the SLS program is cost prohibitive, or whether making the first stage of the SLS reusable, which happens to use the SSME, is cost prohibitive. It should be noted that the SSME and RS-25 are largely the same engine, and later flights of SLS will switch to a cheaper non-reusable version of the RS-25. In any case, the first stage of the SLS is going to end up in a giant pond of salt water, along with the attached SSME/RS-25s. SLS was never designed for propulsive landing, and can not be made to do so now, so those engines are ending up in the ocean no matter what. Perhaps the poster means that NASA found the refurb costs of the SSME to be prohibitive after they have been dunked in salt water, again, since this is the only possible outcome with SLS. Obviously if the first stage of SLS landed on solid ground, or a ship at sea, the refrub cost of the SSME would be completely different.

[nickik]

I think we don't yet know what the rebuilding of the production line for the RS-25 will cost. They did promise quite a bit of cost saving because they need not be reusable anymore but as you probably know better then I setting up a completely new rocket engine production will not be cheap.

[mikeash]

What damages a rocket depends on the rocket.

For the SSMEs, it was just plain use which damaged them. The SSMEs were engineering marvels, but the ludicrous nature of the Shuttle demanded extreme performance, which meant that they ran on razor thin margins. By the time they finished a ~8 minute burn, they had taken enough of a beating to need a lot of refurbishment. SpaceX's Merlins, on the other hand, are much lower performance and built more for robustness.

Think of it like an F1 race car, which needs a lot of work after every race and a new engine several times a season, versus a daily driver which can probably go 50,000 miles without ever opening the hood. (Not that this is recommended.)

[babyrainbow]

But what specific thing does the SSME's had to do that Merlins doesn't?

In your race car example, you can differentiate it from a daily driver that the F1 car has to endure tremendous accelerations, cornering and down forces acting on it and an engine that reaches insane rpm's that puts tremendous amount of stress on all the critical engine components..

Can you differentiate between these rocket engines in that way?

[Inconel]

Sorry I'm typing from mobile so my posts may be even less informative than usual.

The main thing that the SSME has to do that's different from the Merlins is quite simply generate more thrust and a higher specific impulse. The SSME is a much higher performing engine. It achieves this performance through fuel choice and design, namely by using liquid oxygen/liquid hydrogen vs using RP-1(which is basically kerosene) and by being a staged combustion rather than gas gen cycle engine. While it has much higher performance than Merlin, that performance comes at significantly greater overall complexity, particularly in the turbomachinery and pre-burner components.

[mikeash]

The SSME also has to run from sea level all the way into space, whereas the Merlin is able to have one design optimized for sea level and a second design optimized for space.

The difference in efficiency is striking. The SSME's specific impulse (the closest equivalent to MPG in a car) is 452 in vacuum and 366 at sea level. Merlin's is 311/282 for the sea level version, and 348 for the vacuum version.

Of course, this is not a criticism of Merlin, just a comparison. By being less efficient, they're able to optimize for other stuff like cheapness and robustness.

[nickik]

I think after the landing the reentry is probably the most problematic. If you can prevent them from burning up and you can land them (and one is implied by the other) you are on a good road.

I think the Space Shuttle engines would have been capable of more direct relights. They test fired them quite a bit and the were reusable. NASA was just taking a now risk approach and did not really push the technology forward.

Boeing has just received a contract for a first stage with wings that should fly 10 times in 10 days and it essentially uses a SSME. So they seem to believe that they don't need to do that much to make it work.