“[A rocket engine] is a heat and pressure machine whose end goal is to convert…heat and pressure into workable thrust. The more that gets converted the better. This conversion is usually done by a large bell nozzle…
The further down the nozzle you go, the lower the pressure and temperature of the exhaust gets and the more it’s exchanged for higher and higher exhaust velocities. So in general, you want this nozzle to be as big as possible in order for it to convert as much of that energy as possible.
Only one problem. When the exhaust pressure at the end of the nozzle gets below the pressure of the outside ambient air surrounding, the ambient air actually starts to squeeze in on the exhaust gas. Lower the pressure too much and the ambient air will squeeze in on the exhaust so much that it will actually start to peel the exhaust off the nozzle walls and form random shock waves and spikes that will tear apart the engine. So what if you turned an engine inside out and made it so the ambient air pressure is actually pushing the exhaust IN against the nozzle instead of squeezing the exhaust away from the nozzle.”—Tim Dodd, “Are Aerospike Engines Better than Traditional Rocket Engines?” Everyday Astronaut. October 18, 2019.
I’d never heard of aerospike engines before. This seems like a good introduction.