This could be done by leading tapped-off exhaust gases through heat-exchangers in the tanks (being careful that the exhaust gases don't freeze and block the pipe). I'm imagining this as an alternative to carrying separate pressurization gases in pressure-fed engines.
I will clarify a bit. I'm aware both of expander cycles, and of leading heated propellants back into the tank. This is similar, but I'm interested in the specific case of leading exhaust gases through fuel tanks. I couldn't find any previous description of this, so that's why I'm asking.
Being more specific, there are at least two options:
- In a first variant, some amount of exhaust gas is tapped from the combustion chamber and driven through heat exchangers in the propellant tanks. The cooled gas could be injected along the nozzle walls to protect them (ala F-1), or otherwise expelled.
- In a second variant, one could have the combustion chamber at the top of the rocket, and lead the exhaust gas through the propellant tanks, before expelling it through a diverging nozzle at the bottom.
I expect that there is there some reason for a considerable loss of energy here compared to other (turbomachinery-free) engine cycles, but I am too inexperienced to realize where. Hopefully someone thinks this is interesting and can offer an educational take on it.
EDIT 2: Tightened the wording in the heading.