I have been thinking lately of some of the engineering problems associated with spaceflight. the one that is currently causing me to scratch my head deals with cryogenic fuels and oxidators.
I understand that the fuel and oxidizer tanks are essentially large vaccum bottles in order to contain the cryogenic fuel. While on the surface of the earth, earths gravity will cause the liquids to collect at the bottom of the tanks, and so fuel pumps at the bottom of the tanks can transfer the liquids to the combustion chamber of the rocket motor. I understand that during acceration the same effect can happen, i.e. the liquids collecting at the rear or bottom of the tank, with respect to the direction of motion.
When a liquid fueled rocket shuts down in microgravity, the liquids should rebound and spread throughout the interior of the tank. Unless confined by a bladder of some type there should be voids that would cause the pump to fail or "lose prime". I am not aware of any substance that would remain flexable enough to be a bladder in a cryogenic environment. I have not seen any diagrams indicating such a bladder or a pressurization system for compressing such a blader in the spacecraft I have studied.
I know that this issue has been solved, at least since the saturn S-IV-B, and also with the OMS pods for the shuttle. But for me it remains something of an engineering mystery.