Pretty sure it's Scott Manley that did an episode on the aerospike and just how hard they are to get to work right.

Needless to say, getting anything to go to space is hard.

Aerospikes are hard because you cant control the external pressure. At altitude A you have X atmospheric pressure, but at altitude B, Y pressure, that pressure is what keeps the exhaust against the surfave and exerting force, you can only design an aerospike for a certain effecient operational altitude and outside of that its just not great.

A nozzle engine doesnt have to account for this as much because the nozzle is keeping the pressure of the exhaust

	▲	dabluecaboose 22 minutes ago \| parent [-]
		Nozzle engines absolutely have to account for the external pressure. The optimal pressure as the exhaust leaves the bell should be as close as possible to ambient for full thrust. If the pressure at exhaust is higher than ambient, the exhaust pushes outward against the ambient pressure and you get huge exhaust plumes, and lost efficiency. Conversely, if the pressure at exhaust is lower, the ambient pressure pushes the exhaust inward into shock diamonds[1] and you, again, lose efficiency. Engine bells specifically yield their max efficiency at one external pressure/altitude. The reason you see shock diamonds is most often from ground-level testing (or takeoff) of engines that perform best at altitude. [1] https://en.wikipedia.org/wiki/Shock_diamond