Not sure if I didn't explain well enough or you just don't understand, but, anyway, the point I was trying to make is that the aerospike operates at lower efficiency at every altitude than a bell nozzle tailor-made for each altitude would. Over a range of widely varying altitudes, though, the aerospike nozzle, is, cumulatively much more efficient than a bell nozzle that only has one exact design altitude at which it is neither over- or under-expanded. Neither a bell nozzle nor an aerospike exhibit "optimal" expansion in any real-world scenario.