This is perhaps a somewhat naive question about rocket exhaust. If I understand correctly, one of the challenges in designing a rocket nozzle - say, the main thrusters for the Saturn V - is that it needs to operate efficiently at sea level as well as at altitude. The difference in air pressure affects expansion of the gases, which require a different shape to the bell. Exhaust nozzles must be designed as a compromise to operate reasonably well - though not optimally - at the range of altitudes required. (This is one of the challenges that the aerospike engine is designed to overcome, by essentially having only one side of the bell and letting the atmo do the rest.) Do I have that right? What I don't understand is how the thrust coming out of a Saturn V nozzle - all 1.5 million pounds of it - cares at all about 1 atmosphere of pressure. Surely, it's essentially vacuum compared to the thrust. What am I missing?