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To the best of my knowledge, all solid rocket boosters so far use a propellant that is a solid mixture of both the fuel and the oxidiser parts of the reaction. However, given that SRBs are generally used in-atmosphere during the earliest parts of a launch (hence the "booster" appellation), one could potentially get a lot more oomph out of a given-sized SRB by only packing the fuel, and grabbing the oxidiser from the ambient atmosphere as you go. Since the SRB is moving at hypersonic speeds near the end of its burn, it'd pretty much be restricted to a ramjet-style intake, necessitating that the propellant immediately surrounding the central bore still contain oxidiser in order to start getting the SRB up to speed; once it's going fast enough, though, it wouldn't need any extra internal oxidiser, allowing the booster to either be made smaller and lighter while still delivering the same thrust for the same amount of time, or to have a longer and/or more powerful burn than a conventional SRB of the same size.

Here's a (very simple) sketch of what I have in mind:

enter image description here

So why don't airbreathing SRBs exist?

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Solid fuel ramjets exist Anatomy of a Solid-Fuel Ramjet (SFRJ) diagram

The problem for a launch system is you would need a lot of oxidiser anyway — ramjets only start to produce thrust from Mach 1.5–2. They also start to lose thrust above Mach 4–5. Add onto this the huge amount of intake air that would be needed for a system with a high thrust/weight ratio and the complexity of dealing with the varying thrust levels and it doesn't seem as attractive an idea as it did at first.

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    $\begingroup$ I guess ramjets might lose thrust above Mach 4-5 but then you'd make a scramjet right? $\endgroup$
    – user541686
    Commented Jun 10, 2018 at 10:34
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    $\begingroup$ No one has yet made a practical scramjet. $\endgroup$
    – Organic Marble
    Commented Jun 10, 2018 at 14:02
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You'd need a complex intake: it has to be closed during ignition, with a large closing force (otherwise you'd have half the thrust going out the intake), then it has to move out and keep changing its position as speed increases (to keep the shockwaves coming off the inlet cone in the correct position), then after about (handwaving) 30 seconds it has to close again as inlet pressure becomes too low to keep the thrust flowing in the right direction.

It's a lot of added complexity to save 20 seconds worth of oxidiser.

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  • $\begingroup$ I think that the exhaust would go out the top anyway as soon as your intake opened, regardless of the speed of the craft. Also, since you are going to close the intake while the solid rocket is still burning, then that means that the fuel in the solid rocket has to contain oxidizer anyway. $\endgroup$
    – Itsme2003
    Commented Mar 26, 2020 at 6:00

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