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What if NASA ditched the SRBs and strapped four Falcon 9s to the SLS instead? I ran the numbers.
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What if NASA ditched the SRBs and strapped four Falcon 9s to the SLS instead? I ran the numbers.


Hey all, here's a quick rundown of a terrible shower thought I had today: could the twin SRBs of the SLS be replaced with four Falcon 9s? I was inspired by this video that popped up on my YouTube.

First of all, why would NASA want to do this? Cost, mainly. The specific cost-dollar amounts for a single SRB are not publicly known, but some independent estimates put them at $200-300 million per booster, per launch. So for A SINGLE Artemis mission, the SRBs are $400-600 million, alone. But, the SRBs provide roughly 29.36 MN (6.6 million lbf) of combined thrust, which is great when your fueled launch mass is 2.61 million kg (2875 tons). The SRBs additionally have an excellent service record (outside of that one time); with failure rates estimated to be anywhere from 0.1% to 0.001%.

Contrast this with a Falcon 9 Block 5. They have about half the thrust of a single SRB, at about 7.6 MN (1.7 million lbf). With four Falcon 9s, you'd have roughly 30.4 MN, MORE than the SRBs. SpaceX currently charges $74 million for a single Falcon 9 launch, so 4 of them would be $296 million (the specific amount would fluctuate based on engineering investment, package deals, contracts negotiation, etc.). So, roughly, the booster cost to NASA per mission would be reduced by 26-51%! And if NASA wants to keep their pledged SLS launch cadence of 1 every 6 months, this would save $208-608 million per year, and over the life of the program (a planned 79 future launches) it would save $16.43-48.03 BILLION.

Obvious reasons why this will never happen:

  1. The SLS simply wasn't designed for the load paths this would introduce,
  2. This would require extensive redesigns that NASA does not have or want the budget for,
  3. Four complicated boosters instead of two relatively simple boosters introduces a lot of risk,
  4. I probably am not understanding some intricacy about the rocketry physics at play here.

But there's my write-up. I hope you enjoyed reading it!

by ah85q

View 11 Comments

11 Comments

  1. LeoLaDawg on

    Are those designed to be slaved together? I know nothing of the subject, just curious.

  3. sceadwian on

    The basic numbers look attractive. It’s the logistics that kill the idea.

    NASA doesn’t do efficient on these kinds of programs.

  4. 17eggg on

    4 Falcon 9s are much heavier than 2 SRBs. SLS may not be able to get off the ground in the configuration you’re proposing.

  6. ThoughtsandThinkers on

    Just adding, I find it ironic that none of the elements derived from the Space Shuttle are reusable with the SLS

    The beautifully designed and crafted RS-25 reusable main engines are thrown away. The SRBs aren’t recovered. The only component that is reused is the capsule. I wonder how that works out as a cost ratio between single use and reused

    I don’t mean to suggest that more of the SLS should be reused. ULAs SMART reuse concept seems impractical

    But I do mean to call out that requiring that SLS reuse Space Shuttle technology likely created constraints and didn’t take advantage of the original equipment’s key strengths. I think starting with a clean sheet design likely would have been faster and cheaper, but perhaps also more vulnerable to being canceled since it would have the protection of Congressional districts already involved in the Space Shuttle program

  8. Enorats on

    You forgot the most important point – The SLS is literally designed to cost just about as much as it possibly can specifically so money can be funneled from the government to the companies building those boosters. That’s why using Space Shuttle technology was a requirement. Not to save money, but to spend it. Specifically, to spend it at those companies so various Congressmen can go back to their districts and brag about how much good they’re doing for their voters.

  9. vector_search_blue on

    Let’s say both F9 and SRBs have a failure rate of 0.1%

    You’re nearly doubling your failure rate by going to 4 over 2.

    `1 – (0.999^4)`