The SLS is a very opinionated subject among bloggers. I'm looking for what substance there might be behind the following general claim (without specific source). Is it truthful and if so, what is the official plan concerning it:

"The SLS will reuse Space Shuttle Main Engines (SSME), but they are currently not manufactured and there are only 16 of them in storage. Since each SLS launch consumes 4 of them, the SLS only has a lifetime of 16/4=4 launches altogether."

  • Is it rational to invest in re-starting the manufacture of an engine which was basically developed in the 1970's?

  • How expensive and how long time would it take to develop a brand new custom made engine for the SLS first stage?

  • Are there ready replacement alternatives to the SSME engine available, and would their implementation compromise the general SLS architecture which presumably is optimized for the SSME?

  • $\begingroup$ Russell Borogove's answer is correct (he provided an excerpt from the relevant Wikpedia page). I would add that the original development of the RS-25 (SSME) was in the 1970s, but improvements were made on later versions into the 1990s. Also, there are very few LH2/LOX engines that size around, the SSME is a proven, reliable design, and the expendable variant(s) planned for SLS when the 16 existing engines are gone is (are) simpler and cheaper to make than the reusable ones on the shuttle. $\endgroup$
    – Kirkaiya
    Dec 1, 2014 at 19:33

2 Answers 2


Some changes appear to be under consideration for future production of SLS RS-25s:

From Wikipedia's SSME page, about the SLS plan:

Once the remaining RS-25Ds are used up, they are to be replaced with a cheaper, expendable version, currently designated the RS-25E ('E' for expendable). This engine may be based on one or both of two single-use variants which were studied in 2005, the RS-25E (referred to as the 'Minimal Change Expendable SSME') and the even more simplified RS-25F (referred to as the 'Low Cost Manufacture Expendable SSME'), both of which were under consideration in 2011.


Engines at the size and performance of an SSME are very difficult to develop.

The SSME has one of the highest ISP's ever attained in a production engine. (Not some crazy Fluorine involving demo). It is anything but a simple, or even 'old' engine. Its re-manufacture would likely benefit from modern techniques, like the people examining starting F-1 engine rebuilds.

It has been estimated (I have no source, only what is usually quoted) to take 6-10 years to develop a large engine.

The SSME is not that high in thrust, compared to the F-1 (660 Klbs SSME vs 1.5-1.8 MLbs thrust) but is much higher than something like a Merlin 1D (165 Klbs thrust).

The closest comparable (LOX/LH2) engine is the RS-68 used by the Delta 4 booster but was rejected because the heat load under the first stage, surrounded by the SRBs was unsurvivable by the RS-68. (Whereas the SSME was always designed to be 'near' the SRBs during launch. I say 'near' since of course, on the Shuttle they were offset being on the back of the orbiter, not directly between as they would be on SLS)

Is it rational to restart SSME production? Maybe. It is a well understood, mature engine, that might benefit from moderm manufacturing technologies. It is a very advanced engine with excellent performance. It has been pointed out in the comments that some of the newest engines of that remaining set of 16 were manufactured as late as 2008.

I have this graphic of which SSME was used on which STS flight.

SSME history of use on STS missions

How expensive would a new engine be? Very. It would require performance similar to the SSME, else the size of tanks or # of engines would cascade rippling changes throughout the entire SLS stack probably killing it.

The SSME was quite expensive and something comparable to it would be similarly hard to develop.

Any ready replacements? Nothing really. The RS-68 is the closest and already rejected. Would a Russian engine be accepted? Not sure there is an appropriatly sized LOX/LH2 Russian engine on the shelf. The Russians have been great at BIG Kerosene engines. (RD-170, RD-180, RD-193) But LH2 mechanics at the large scale have usually caused them issues.

The Ariane 5 Vulcain main engine might be an interesting idea, as it currently runs beneath the A5 main stage with an SRB on either side of it, but I would guess the sizing is wrong.

Having said all that, it seems like more reason to consider SLS less of a great option.

  • $\begingroup$ I take it that you conclude that resuming the SSME production seems to be the best option, given of course that the SLS shall have a long future. I suppose it was manufactured until only a few years ago, so tooling and competence might still be around and ready to get going again. $\endgroup$
    – LocalFluff
    Dec 1, 2014 at 18:20
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    $\begingroup$ @LocalFluff I conclude SLS is not the optimal solution. However, restarting SSME seems like a likely path forward, if SLS is to actually happen. Should be possible to simplify SSME to only use once to reduce costs. But I have a hard time believing current NASA can deliver it. $\endgroup$
    – geoffc
    Dec 1, 2014 at 18:35
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    $\begingroup$ You might add that SSME has been continually upgraded since its introduction, with Block II going into service in 2001, so it's a bit of a stretch to describe it as "an engine which was basically developed in the 1970s". $\endgroup$ Dec 1, 2014 at 18:50
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    $\begingroup$ If half the thrust of the SRB is radiating toward the base of the core, you are not going to space today. ;) $\endgroup$ Dec 2, 2014 at 17:01
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    $\begingroup$ I did find this article: nasaspaceflight.com/2013/06/… which mentions the heat issue in the Ares V, and mentions that RS-25 is regeneratively cooled while RS-68 is ablative, so, yeah, that would make a big difference in heat capacity. Looks like they were thinking about arranging RS-68s in an odd pattern to get them as far from the SRBs as possible! $\endgroup$ Dec 2, 2014 at 17:15

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