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Related to my previous question regarding expending the RS-25s. Why aren't engine blocks, especially those expensive RS-25s on the SLS, made ejectable from the fuel tank assembly and parachute to be recovered? I'm assuming it is because of the complexity of cutting fuel lines, exemplified through the Atlas program.

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    $\begingroup$ It's been proposed many, many times before. ulalaunch.com/docs/default-source/evolution/… aiaa.org/docs/default-source/uploadedfiles/about-aiaa/… $\endgroup$ Jan 28, 2022 at 16:26
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    $\begingroup$ For an undamaged recovery of a complete rocket engine with the turbo pumps, parachutes alone will not suffice. For a soft landing retrorockets will be needed. For a splashdown an inflatable raft will be needed too. Placement of the parachutes, the retrorockets and the raft at the engine will not be easy. $\endgroup$
    – Uwe
    Jan 28, 2022 at 17:18
  • $\begingroup$ @Uwe What about being caught by a large cushion, or an inflatable cover to increase drag so they won't be damaged? $\endgroup$
    – WarpPrime
    Jan 28, 2022 at 18:49

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Stealing some math from here a parachute pack for a 3100kg RS-25 weighs around 600kg. SLS has four engines so 2400 kg, which is not a too major hit on the 100ish tonne payload to orbit

What that gets you is an engine landing somewhere at the Atlantic having made an uncontrolled descent.

To upgrade this to 'you get the engines back' you need to add:

  • Flotation systems, hopefully enough to keep the engines out of salt water

  • Guidance or at least sequencing and a beacon

  • A separation method that works on the main structural members the engine pushes through.

  • The area around the engine gets exposed to engine exhaust so a fair bit of heat protection for parachutes and flotation

All up this is starting to look like a 5-10 tonne penalty, on a vehicle where the justification for existence (and cost) is lifting cargoes that nothing else can.

You also need a small fleet of recovery vessels a long way out to sea to find and fish out the four engines, easy if you can call on the US navy but very expensive if doing regular commercial launches.

And evidence from SpaceX and Corona is that you will still fail at recovery, and you have a finite pool of engines to learn with.

As linked by Organic Marble in the comments engine re-use has been studied (mid air collection by helicopter) suggesting it is possible technically and financially.

The challenge with doing so today is:

  • If SLS loses lift capacity against the likes of Falcon Heavy it becomes harder to justify a higher cost per launch
  • If it takes longer to fly it risks losing to Starship and other developments
  • The Delta IV stopping production and very low number of Falcon heavy flights indicates few people WANT massive payloads lifted to orbit.
  • The engines being recovered have limited life, so all the costs MUST be recovered in those saved engines (suggested as three flights each), rather than across an entire program.

Which means that the reason for SLS to exist is Artemis, and there are few enough Artemis launches that engine reuse is not required. It is certainly possible we will see a future system designed from the ground up for this, possibly China or Russia who have first stages coming down over land and therefore benefit from controlled descent even if re-usability rates are initially low.

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  • $\begingroup$ "The engines being recovered have limited life... (suggested as three flights each)" I understand that's true of the new expendable RS-25 design that they're supposedly starting production on, but the original RS-25 engines had vastly more longevity than that, and if recovery was possible then we wouldn't need the new engines (or not more than a few) anyhow; we could keep using the old ones. What am I missing? $\endgroup$
    – CBHacking
    Jan 30, 2022 at 4:11
  • $\begingroup$ @CBHacking the current RS-25 engines are used, some certainly very used. I had thought I'd seen an average life remaining of three flights in discussions around extending shuttle operations but admit that is an interesting question that should be sourced. Fact remains there are a small number of engines with finite life which complicates re-usability math. $\endgroup$ Jan 30, 2022 at 4:56
  • $\begingroup$ I guess a cynical view is that saving a few bucks in the long run isn't worth it. SLS is so very expensive that I doubt there's much they can do to make it cost competitive. Luckily, and here's the cynicism, cost competitiveness is not a priority for certain Congressional delegations. $\endgroup$ Jan 31, 2022 at 0:57

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