Hybrid motors suffer two major drawbacks. The first, shared with solid rocket motors, is that the casing around the fuel grain must be built to withstand the full combustion pressure and often extreme temperatures as well. However, modern composite structures handle this problem well, and when used with nitrous oxide and a solid rubber propellant (HTPB), a relatively small percentage of fuel is needed anyway, so the combustion chamber is not especially large
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Huh! TIL.
Til as well thanks to your initial comment making me think “oh yes that is not obvious I wonder what the answer is?”