20 Years of SLS and Constellation – 50 Years of the Same RS25 Engine and Each One Costs $145 Million

SLS (Space Launch System) uses a 50 year old rocket engine design that now costs $145 million each (not including labor and testing) which is over 300 times more expensive than the more powerful SpaceX Raptor 3 engine. It is a flying museum piece that is using difficult liquid hydrogen which was the reason for the leak and delay on the wet dress rehearsal preparation for the second flight in 20 years for the post-Shuttle SLS-Constellation programs. $100+ billion for SLS, Orion and Constellation to get one flight so far. The thinking after the Space Shuttle had 135 flights costing over $200 billion was lets keep using the RS-25 engine because it works and we can go back to an Apollo style rocket configuration. This will be tested, reliable and save some money. Less technical risk?

The last Space Shuttle lost was Columbia during STS-107 on February 1, 2003. The orbiter disintegrated during reentry over Texas due to damage from foam debris striking the left wing during launch, killing all seven crew members. This was the second (and final) major loss in the program. The first loss was Challenger on January 28, 1986. The last Space Shuttle flight was 2011.

The 50 year old RS-25 engine design costs $145 million. Modern engines like SpaceX Raptor (less than $1M) or Blue Origin BE-4 ($8M) are cheaper due to high-rate manufacturing. They are also not flying museum pieces.

SpaceX Raptor 3.X engines have serial production is ramping up. Over 75 Raptor 3 engines have been made by early 2026, and 3-4 engines tested per day at peak. 71 RS-25 engines have been made over 50 years. Each sea-level Raptor 3 will produce ~280 tf, equivalent to approximately 2.75 million newtons (MN) of thrust. Vacuum variants are targeted at ~306 tf or ~3.00 MN. Mid-2026, Raptor 3.x variants are expected to push thrust to 300 tf (2.94 MN) with minor iterative changes, such as refined chamber designs or propellant ratios. This could debut in Flights 13+ if testing succeeds post-Flight 12. In 2027, the Raptor 4 is could reach 330+ tf or ~3.24 MN) and have further weight reductions and cost drops to less than $250,000 per engine. This would be about 600 times lower cost than RS-25. Raptor 3.X can be over 50% more powerful than RS-25 and Raptor 4 could be 60% more powerful.

One RS-25 engine costs more than the entire 45 engine SpaceX super heavy starship. The RS-25 per unit engine does not include the extra testing and other costs associated with hardware that has flown once in 20 years and has had a handful of wet dress tests and 1 flight.

The Tail Service Mast Umbilical (TSMU) design for the Space Launch System (SLS) draws significant heritage from the Space Shuttle program’s Tail Service Masts (TSMs), which date back to the 1970s–1980s Shuttle era (with conceptual roots even earlier in Apollo/Saturn V designs from the 1960s).

The recurring LH2 leaks at the TSMU interface (seen in Artemis I WDRs 2022 and Artemis II 2026) stem from cryogenic challenges (super-cold LH2 contraction/expansion on seals/quick-disconnects). These were known in Shuttle ops (minor incidents like STS-112 close calls) but have been more problematic for SLS due to higher flow rates, different interfaces, or ground support tweaks.

LH2 must be stored at extremely low temperatures (~-253°C / -423°F), making it highly cryogenic.

Modern designs prioritize methane because it offers a better balance of performance, cost, and operability—especially for reusable vehicles. Higher boiling point (~-162°C / -260°F) → much easier to store and handle cryogenically. Often only passive insulation suffices, with far less boil-off than LH2.

Methane has larger molecules so it is easier to seal.

The RS-25 engine (originally the Space Shuttle Main Engine or SSME) was developed by Rockwell International (later acquired by Boeing, then Pratt & Whitney Rocketdyne, and finally Aerojet Rocketdyne in 2013, which was acquired by L3Harris Technologies in 2023). Aerojet Rocketdyne/L3Harris is the current prime contractor, but earlier entities handled Shuttle-era work.

The engine’s involvement across programs is as follows:
Space Shuttle (development started 1972, operational 1981–2011): Primary use and reusable engines.
Constellation (2005–2010): Planned for Ares rockets. Limited R&D on RS-25 derivatives (RS-25D/E), but program canceled before production. No engines built or flown. Mostly component tests and studies.
SLS/Artemis (development started 2011, ongoing). Uses 16 refurbished Shuttle engines for first four flights (Artemis I–IV). New production for Artemis V onward.

Total 71 RS-25 Engines Produced

Shuttle: 47 built (46 flown across 135 missions. 1 unused spare).
Constellation: 0 full engines (some prototypes/components for testing).
SLS: 16 refurbished from Shuttle stock used/allocated so far. New production: 24 under contract (6 from 2015 restart + 18 from 2020 extension). First new engine (E20001) completed in 2024; assembly ongoing for Artemis V engines as of early 2026. None flown yet.
Grand Total: 47 historical + 24 new = 71 (with more possible if SLS continues).

Estimated Total RS-25 Engine Revenue Across Programs — Without Inflation Adjustments
~$10–15 billion (Shuttle ~$6–9 billion. Constellation ~$0.2–0.3 billion. SLS ~$4–5.5 billion to date). This is conservative. Actuals could be higher with inflation adjustments and unpublicized mods.