>>13074146They aren't more efficient, they are heavier, they are bigger for a given thrust output, they are fundamentally harder to keep cool, and overall they're kinda shit.
The fuss about aerospikes comes from a misunderstanding about how they work. I blame Ksp for this, actually. Aerospikes are ALWAYS about 10% LESS efficient than an optimized bell nozzle. What this means is, when running at sea level, a sea level bell nozzle will always be more efficient (and also lighter and so forth) than an equivalent aerospike nozzle engine, and when running in vacuum a vacuum bell nozzle will always be more efficient than an aerospike. However, as rockets climb out of the atmosphere they gain efficiency (true of any nozzle type), and aerospikes are more "altitude compensating" in that they gain efficiency with reducing ambient pressure faster than sea level bell nozzles do. This means that if you are only comparing sea level bells to aerospikes, it would make sense to use aerospikes in some cases as they would be more efficient than a sea level optimized bell once the rocket reached space. What cases are these? The booster-sustainer launch vehicle architecture that America was obsessed with and continues to subscribe to to this day is pretty much the only case where you would consider an aerospike. The advantage of booster-sustainer is that you don't need to ignite big engines while in zero G between staging events. While on the ground the efficiency and thrust decrease by using an aerospike is in theory made up for by the higher efficiency once you reach vacuum compared to using a sea level optimized engine.
Aerospike nozzles were never about being better than bell nozzles, and they are in no way any better for SSTO anything. They were about improving booster-sustainer rockets. A two stage design with sea level optimized bells on the lower bit and vacuum nozzles on the upper stage offers far better performance.
