In theory, aerospike engines are pretty efficient at nearly any altitude. It has to do with the pressure and temperature variations as the rocket travels through the atmosphere. (There are pockets of isothermal regions, or regions where the temperature is the same in the atmopshere, and there are pockets where the temperature varies; therefore the Mach number varies). The atmospheric pressure and temperature affect the flame of the aerospike and act like a "variable" nozzle.
Aerospikes aren't as popular because,
1) they're heavy. more deadweight mass on a launch vehicle is a big no no. By deadweight I mean everything on the launch vehicle that doesn't contain the a) payload b) propellant. Because once you burn out (or run out of propellant), usually that's when you stage (or separate) your payload from the launch vehicle. At that point, the launch vehicle is useless; deadweight.
2) they require a massive amount of cooling: you're basically heating up a piece of metal to launch temperatures during the launch sequence. You have to make sure your "spike" doensn't melt.
3) There is a bunch of research and development into other cheaper alternatives, and we already have cheaper and efficient alternatives. This basically pushed aerospikes to the "novelty" category. I'm sure if there was a big technological "push" for aerospikes I'm sure we could get something that's more efficient but that research and time will cost more than to just use the rockets we have now.
In theory, aerospike engines are pretty efficient at nearly any altitude. It has to do with the pressure and temperature variations as the rocket travels through the atmosphere. (There are pockets of isothermal regions, or regions where the temperature is the same in the atmopshere, and there are pockets where the temperature varies; therefore the Mach number varies). The atmospheric pressure and temperature affect the flame of the aerospike and act like a "variable" nozzle.
Aerospikes aren't as popular because,
1) they're heavy. more deadweight mass on a launch vehicle is a big no no. By deadweight I mean everything on the launch vehicle that doesn't contain the a) payload b) propellant. Because once you burn out (or run out of propellant), usually that's when you stage (or separate) your payload from the launch vehicle. At that point, the launch vehicle is useless; deadweight.
2) they require a massive amount of cooling: you're basically heating up a piece of metal to launch temperatures during the launch sequence. You have to make sure your "spike" doensn't melt.
3) There is a bunch of research and development into other cheaper alternatives, and we already have cheaper and efficient alternatives. This basically pushed aerospikes to the "novelty" category. I'm sure if there was a big technological "push" for aerospikes I'm sure we could get something that's more efficient but that research and time will cost more than to just use the rockets we have now.
ADDENDUM; here's a video from Rocketdyne explaining aerospike engines: https://www.youtube.com/watch?v=EWf4iOMSPNc