Could the solid boosters of SLS be replaced by reusable Falcon 9 first stages?

Question

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:

Thrust

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.)

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.