Except LANTR solves a very similar problem, which is the requirement for different performance parameters at the beginning and at the end of a rocket flight (hi

Except LANTR solves a very similar problem, which is the requirement for different performance parameters at the beginning and at the end of a rocket flight (high thrust and propellant density in first stage, low thrust and high Isp in the second stage) in a single stage vehicle. And I mentioned it because its way of operation is very similar to the thing describe above, i.e., injecting something extra into the nozzle, so it reminded me of it. Not because I consider it comparable to using aerospikes on Earth (I'm not convinced that's useful at all) but because this is one of the few such variable Isp options that appears to provide any useful benefits at least somewhere (even if it's not on Earth).