Is it feasible to redesign the Space Launch System to use a pair of reusable Falcon 9 first stages instead of the five-segment solid boosters now being developed? The reason would be to lower launch costs while still retaining a Saturn V class capability.

But could those creatures get married rationally? For example, would having to fuel the boosters with kerosene while the main stage needs hydrogen mess up the launch pad infrastructure? And SLS is vertically integrated while Falcon is horizontally integrated. And do the attachment points of the Falcon Heavy common cores and the SLS+boosters match each other? And whatnot. Is there any obvious deal breaker?

Some key figures through Wikipedia:


Solid 16,000 kN each

Falcon 7,600 kN each

Specific impulse

Solid 269 seconds

Falcon 282 seconds

Burn time

Solid 124 seconds

Falcon 162 seconds

(Some blogger loosely through out this idea because of the recent 2016 November NASA RFI about making SLS cheaper. But I can't find that source now to be credited or blamed.)

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    $\begingroup$ Look at this video m.youtube.com/watch?v=d2200YGSeKM it is fun. $\endgroup$ – Mark777 Dec 21 '16 at 13:28
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    $\begingroup$ @Mark777 Great illustration! I saw a (dry "serious" non-illustrated) presentation of a concept launching astronauts from Mars' surface in a Soyuz class spacecraft to a small docking spacecraft in low Mars orbit. And from there to a medium Mars orbit fuel depot. And from there to the going-home large transfer spacecraft. At first sight it looks crazy. But if each step is safe enough, it simply saves original launch mass from the gravity well. (Maybe that too could be Kerbally simulated?) $\endgroup$ – LocalFluff Dec 21 '16 at 21:40

The SRBs for the Shuttle and SLS are used to get the stack moving and off the pad, so that the more efficient LOX/LH2 engines can do the hard work.

In the case of the Space Shuttle, an amazing amount of thrust is needed. I like using lbs of thrust because it is more inspiring. The SRBs produced 2.8 million lbs of thrust. That is just awe inspiring huge.

While a Falcon 9 Full Thrust is working its way up to 1.7 million lbs thrust, that is not even close to what an SRB puts out.

The first stage (half stage, whatever you want to call strap on boosters) needs high thrust, performance is less important, and is needed to literally get the booster off the pad. A F9 first stage just won't provide the thrust needed.

  • $\begingroup$ Maybe with 3 or 4 falcons... or with larger falcon. $\endgroup$ – peterh May 26 '18 at 0:43

Pad infrastructure seems simple enough: just add plumbing for kerosene.

You'd go from 2 to 5 boosters (with more thrust, so higher loads), attachment points won't match and you'd need to qualify the new combination of stages.

  • $\begingroup$ So one would need five (reusable) F9 first stages to replace the two solid boosters? Because of the thrust I suppose. But how capable would an SLS with just to F9-boosters still be? (Or, what would an SLS without any boosters be capable of?) $\endgroup$ – LocalFluff Nov 23 '16 at 14:48
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    $\begingroup$ Yes, that's going by takeoff thrust alone. I don't have time to do the numbers for the other options just now, but as a first approximation you need thrust to be greater than takeoff weight. $\endgroup$ – Hobbes Nov 23 '16 at 17:06

Your question "But could those creatures get married rationally?" is a good one. Rationally - sort of. The answers here that refer to thrust forget the solid rocket boosters have to lift themselves. If you run the numbers (try the "Silverbird" tool on the web for ROM purposes) you will get the 70t payload to LEO for an approximate description of the SLS (enter the SRB's and the Core Stage specs). Replace the SRBs with the Falcon 9 (first stage only) and you get about 10% less.

The dilemma is the SRB and the External Tank derived core stage as a concept. The SRB's support the core stage. That's where the difference in payload occurs. Once I start to change out the SRB's for Falcon 9 1st stages I have to modify the core stage. The natural decision is to increase commonality. Guess what you get? The Falcon heavy at 63.8t to LEO.

The difference in payload becomes more acute with the upper stages - the SLS core stage has a larger diameter, the Falcon 9 less. It's in that final push that the SLS, if it has a saving grace, can add a larger diameter upper stage and get to 105t payload to LEO. Now the Falcon 9 MAYBE can try this, but that gets tricky, and in either case you are quickly creating custom Falcon 9's and upper stages which mean less opportunity for amortizing effort and equipment back at the plant.

So your question, could Falcon 9's replace SRB's? Not really, unless assuming lots of other "replacements", like the core stage too! That just takes you on a quick path right back to the Falcon Heavy.


In addition to the engineering answers already provided there's an even bigger one:

The only real mission for the SLS is to be pork. Using existing Falcon boosters rather than paying somebody to make new ones does not meet this essential mission requirement.

  • $\begingroup$ There's a lot to criticize about the SLS, but it is not solely pork. And I'm fairly certain NASA wouldn't want Falcon 9 as boosters even if they got to make their dream rocket. $\endgroup$ – Rob Rose Feb 18 '18 at 2:53
  • $\begingroup$ @RobRose The only other possible mission that wouldn't be a lot cheaper by other means is boosting something too heavy for any other launcher. It's pork. $\endgroup$ – Loren Pechtel Feb 18 '18 at 3:51
  • $\begingroup$ Launching things that are too heavy for any other launcher is exactly what's made for. Falcon Heavy can't do much BEO and it has a too small faring anyway. Space exploration has never and will never be a cheap endeavor. Politicians have definitely hindered the SLS, by not giving adequate funding and by adding useless pork. But we've still only spent $11B on it, which is pocket change for the US Government. $\endgroup$ – Rob Rose Feb 18 '18 at 19:02
  • $\begingroup$ @RobRose But how many payloads too heavy for the Falcon 9 are there? And will the SLS fly before the BFR is ready? $\endgroup$ – Loren Pechtel Feb 19 '18 at 3:38
  • $\begingroup$ SLS will probably fly end of 2019 or early 2020. Musk says BFR will fly next year, but he also said FH would fly in 2013 in 2011. So I doubt it will ever fly before SpaceX burns through all its cash. And pretty much anything you want going beyond earth orbit is too heavy for FH or F9. $\endgroup$ – Rob Rose Feb 19 '18 at 6:15

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