Copyright 2022 Robert Clark
The Boeing Exploration Upper Stage(EUS) is only about 1/10th the size of the SLS core stage. Why couldn't this upper stage be built in the same fashion as the core?
The SLS core like the shuttle External Tank before it is built by welding together barrel sections:
Then the upper stage could have been formed in the same fashion simply by using fewer/shorter barrells. Indeed, cost could have cut even further by constructing the upper stage at the same time as the core.
Being only ~1/10th the size of the core it likely would have fit in the same building during the construction:
Instead of this simple approach, Boeing chose a more complicated construction that resulted in higher development costs:
In fairness to Boeing it should be said that they most likely wanted to save on dry mass on the stage. Unlike for lower stages like the core, extra mass on the upper stage results in direct reduction in that mass from the payload.
However, to accomplish the same payload with a less mass efficient stage they could have just increased the stage propellant size. For instance the Delta IV first stage has a 200 ton propellant load at a 26 dry mass. You would not need the full thrust and high mass of the sea level engine the RS-68 used on the stage for this upper stage use. Smaller thrust and weight engines such as 4 RL-10's or a single J-2X would suffice.
The mass efficiency for the Delta IV stage without the 6 ton RS-68 engine is also pretty good at a 200 ton propellant load for the 20 ton structural mass sans engine. Note the Delta IV first stage achieves this good mass efficiency without using advanced mass saving methods such as common bulkhead, balloon tanks, or specialty high strength stainless steels as used on the Centaur upper stages.
BOEING EUS COSTS.
The total development and production cost for the Boeing EUS will probably be $10 billion:
More far-out facts: The EUS will probably cost $10 billion and (it's being built by Boeing, recall) won't be completed for at a minimum of five years. Do you think a fully expendable $2.5 billion super heavy lift rocket will have any relevance at that time?
https://twitter.com/SciGuySpace/status/1446478856840433669?s=20&t=LILNIi_t_RepDRJMMTX49Q
And the per stage cost likely ~$880 million each:
The story also uses a simple cost estimator model to put a per-unit price on Boeing's Exploration Upper Stage. The result? $880 million. That's just the upper stage, and doesn't include the core stage, SRBs, integration, ground systems, etc. etc.
Note the entire SLS development was in the range of $20 billion, with an expected per lauch cost of $2 billion. Then this EUS stage at ~1/10th the size of the SLS core and without the extra expense of the SLS side boosters or current interim SLS upper stage would amount to half the cost of the entire current SLS.
ENGINE COSTS.
While the RS-68 engine(with a vacuum optimized nozzle extension) would be over powered for the upper stage use, it would be significantly cheaper than using 4 RL-10's for example. The RL-10's are estimated to cost $25 million for a total of $100 million for 4. The RS-68 in contrast is estimated to only cost $10 to $20 million. For the J-2X engine, being an engine only developed but not yet used, there are no estimates on its cost, but it's very likely to cost much less than the $100 million for the 4 RL-10's. Another possibility might be the Vulcain engine used on the Ariane 5. It's estimated cost is only $10 million.
As an estimate on how much the stage aside from the engines might cost if built in the same fashion as the SLS core and shuttle external tank, look at the costs for the shuttle ET built by Lockheed. They were given a contract for 18 External Tanks for a total $3 billion, about $166 million per tank. Note too an SLS upper stage at ~200 ton propellant load would be less than 1/3rd the size of the External Tank. Given this much lower cost Lockheed should be asked how much would be a stage built in the same fashion as the shuttle ET at ~ 200 ton propellant load.
Robert Clark
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