“A given rocket stage operates ordinarily at constant chamber pressure. Hot gas is produced by the combustion of liquid or solid fuel, exchanging chemical energy for thermal energy. The hot gas creates some chamber pressure in the combustion chamber, which is connected to space by the throat of a rocket nozzle. If there were just an aperture in the combustion chamber, one would still have a rocket, but the gas would be emitted from that aperture in a hemisphere, rather than in a directed stream. So the momentum associated with the thermal energy would be lower than if all the gas went in the same direction. Furthermore, even if the gas were all to go in the same direction, but were to remain hot, the gas stream would have less momentum than if the gas were cooled by expansion and still kept largely uni-directional. In fact, the function of the rocket nozzle is to expand the combustion gas in such a fashion that it cools and constitutes a largely uni-directional jet. At the throat, one has hot gas with the molecules going in every direction. As the gas expands in the rocket nozzle, it repeatedly pushes on the material of the nozzle (except in the direction of the exit circle), and as the rocket nozzle diameter increases with distance from the throat, the gas expands and cools itself. So in the rocket jet that is ejected into space, the gas is quite cold, but moving extremely rapidly. The best that can be done in obtaining exhaust velocity is to convert all of the thermal energy of the fuel into kinetic energy of the exhaust (leaving nothing left over for internal thermal energy of the exhaust plume). For rocket exhaust into space (above the atmosphere), that condition is closely approached. Certainly billions of dollars have gone into wringing the last bit of performance out of rocket nozzles. Eventually one gets to a point at which the size and mass of an additional nozzle outweighs the small benefit that can be obtained by reducing the already small residual thermal energy in the exhaust plume,” Slothrop was thinking — still masturbating furiously of course, his fist a whitehot blur of pasty skin and torque, a violent fleshy coaxing, the rhythmic stretch and pull a venomous spate of self-love and self-loathing coalescing into a pitiful crescendo: pale viscous drizzlings he’d soon blot away with a sheet of Kleenex and flush without so much as a second thought…