Space elevators are still pretty far out there. And if we ever do end up building one, we're going to need pretty damn good rockets to do that. To reach maximu

Space elevators are still pretty far out there. And if we ever do end up building one, we're going to need pretty damn good rockets to do that. To reach maximum theoretical performance, a rocket engine nozzle needs to expand the exhaust to be of equal pressure as the ambient air. Being too underexpanded can actually destroy the engine, and even well before that being over- or underexpanded saps efficiency, so you can improve performance by specializing for the pressure you target. But of course, as a rocket ascends, pressure falls. This means that the expansion ratios of traditional engines are compromises over the pressure range they are expected to operate in. Aerospike engines use a neat hack to make a "virtual nozzle", where the pressure of outside air is used to push on the exhaust stream. This makes it slightly less efficient than a traditional de Laval nozzle that is specialized for the exact pressure, but it can maintain that not-perfect but high level of efficiency for the whole ascent, from atmospheric to vacuum. When everyone was trying to build single stage to orbit vehicles, aerospikes sounded very promising as they would allow a single engine to be used from the launch pad to vacuum with reasonable efficiency. However, now that first stages are routinely returning to the launch site and landing on their own, SSTOs are not nearly as attractive, and with a two-stage architecture, you want to give the second stage a proper vacuum engine, and then the first stage won't lose that much if it's optimized for near-sealevel conditions. I'm not sure aerospikes make that much sense anymore.