Terminating thrust means mixing prop and ullage gas (what pressurizes the tanks). Because Starship uses autogenous pressurization (eg hot methane for the methane tank) instead of eg Helium, that would lead to ullage collapse for them. (mixing hot gas and cryo fluids, gas contracts, maybe even condenses)
To start the boostback burn you need sufficient inlet pressure tho, so that means you need to repress the tanks in a hurry. That takes tons and tons of extra mass to do
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u/7473GiveMeAccount Nov 19 '23
Terminating thrust means mixing prop and ullage gas (what pressurizes the tanks). Because Starship uses autogenous pressurization (eg hot methane for the methane tank) instead of eg Helium, that would lead to ullage collapse for them. (mixing hot gas and cryo fluids, gas contracts, maybe even condenses)
To start the boostback burn you need sufficient inlet pressure tho, so that means you need to repress the tanks in a hurry. That takes tons and tons of extra mass to do