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The thought of pairing SLS solid boosters on just about anything one wants into orbit led to the thought of recovering them on barges at sea.

We all remember Robert Goddard's top mounted rocket engine from the 1920's, which lived on as the aborted launch escape system for manned space capsules.

Since solid rockets burn to exhaustion from the bottom, could a controllable liquid fueled system be placed on top for a controlled powered landing.

As technology exists far beyond parachute recovery of solid boosters at sea in the 1980s, would such a system be technologically and economically feasible?

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    $\begingroup$ interesting; the SRBs already have to be able to survive tension loading because of the parachutes, which are also heavy and complicated on their own accord. Congrats on making a good Robert-Goddard-style rocket question. $\endgroup$
    – Erin Anne
    Nov 20 at 1:01
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    $\begingroup$ Is this intended to exploit stability from top mounting? - aka the en.wikipedia.org/wiki/Rocket#Pendulum_rocket_fallacy $\endgroup$ Nov 20 at 1:39
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    $\begingroup$ "Since solid rockets burn to exhaustion from the bottom" – They don't. They burn from the inside out, hence why the 5-segment SLS boosters have the same burn time but 25% more thrust than the 4-segment STS boosters. $\endgroup$ Nov 20 at 2:44
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    $\begingroup$ @GremlinWranger: I think the idea is to put the landing engines at the top since you can't put them at the bottom because that's where the main engine nozzles are. $\endgroup$ Nov 20 at 2:45
  • $\begingroup$ @Robert Digiovani .... could you clarify if you are proposing separate liquid fueled rockets discharging through their own nozzles, or discharging through the booster nozzles? $\endgroup$
    – Woody
    Nov 20 at 5:38

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You will need substantially more thrust, since a spent SRB is around a hundred metric tons of steel tubing. The landing rockets would either have to be outboard, projecting outward from the sides, or angled outward, in which case there would be significant cosine losses from not pointing them straight back. The latter would likely be undesirable, given the already-high thrust requirements. The upper location would let you locate the propellant tanks inline, but that might not be favorable compared to essentially strapping on recovery rockets.

Additionally, you would be unable to combine aerodynamic surfaces and rocket propulsion as effectively as Falcon 9 boosters do. Another issue is the location of the landing legs compared to the landing rockets. If the rockets are forward, the legs are exposed to their exhaust. Rear-mounted landing engines can be interleaved with the landing legs.

But the biggest problem is that there's simply less to gain from reusing a SRB casing and whatever avionics/etc can be salvaged. The expensive part has been converted to rocket exhaust. The nozzle/etc can't take advantage of regenerative cooling and so are made of ablative materials that need to be replaced. The casing is a heavy steel pressure vessel, not some exotic ultralight structure. The Falcon 9 booster is fully reusable, it goes through inspections and routine maintenance before being readied for its next flight. No SRB would be similarly reusable, it would be a matter of assembling a new SRB from salvaged components.

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  • $\begingroup$ Cosine 15 degrees is 0.966. Really not too bad. 20 Super Dracos could land it. $\endgroup$ Nov 20 at 6:33
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“ As technology exists far beyond parachute recovery of solid boosters at sea in the 1980s, would such a system be technologically and economically feasible?”

Two different questions. Plenty of things are possible but we don’t do them. If you really want economy, you would… ditch the SRBs. Not the individual flight units, the very concept of the SRB , and in particular giant, segmented solid motors. Ariane did. H-II did. Even SLS planned to ditch giant solids after some number of initial flights, per NASA admin at the time Charlie Bolden.

The Senate then overruled Bolden, demanding that NASA use only Shuttle designs, by holding the NASA budget hostage. Key senators drafted the 2011 budget bill to explicitly force solids, the SSME, tank, etc. and implicitly keep taxpayer billions (Billions, with a “B”) flowing to their districts, which already had Shuttle work. In other words, pork.

A vague or ill-posed question will draw an ill-formed answer. Do you want economy? Then cut the pork. Do you want more space exploration? Then don’t recover the SRBs… zero them out.

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  • $\begingroup$ And replace them with what, 29 more Raptor 2s? Solids add a huge amount of thrust, and are simple and reliable. They have been used since the dawn of rocketry, in some cases (Nike Hercules) actually replacing liquid fueled systems. What NASA should really do is create 2 separate heavy lift systems: lower G human rated and solids to loft supplies with high G tolerance (or combinations of the 2). If one really wants a neat idea, put wings on the SRBs and glide them home. $\endgroup$ Nov 20 at 15:11
  • $\begingroup$ "And replace them with what, 29 more Raptor 2s?": It's a vastly more economical and logistically flexible and scalable approach, so...yes. $\endgroup$ Nov 20 at 16:10
  • $\begingroup$ @ChristopherJamesHuff I'd like to see the numbers, and I'm not anti-SpaceX by the way (merging the 2 heavy lift programs may be most economical). But check this out: if SRB's separate 40 miles up and 120 miles downrange ... 3:1 glide ratio! Use the "Goddards" for return-burn insurance. If the SRBs needed wings at all, they would be tiny. $\endgroup$ Nov 20 at 16:26

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