I have a question regarding the design of rocket nozzles. I am not an engineer nor a physicist but I am involved in amateur experimental rocketry and I was curious regarding grossly overexpanded nozzles. According to Wikipedia a grossly overexpanded rocket nozzle (very bottom of the above image) burns more efficiently but the exhaust jet is not stable. I would assume that is because in the case of a grossly overexpanded nozzle, the jet flows more freely but maintaining pressurization within the motor casing and thus creating an even burn rate becomes a problem. I might be fundamentally wrong on this assumption, but, assuming the above is correct, theoretically couldn't you make a nozzle that converges on the throat, diverges into gross overexpansion, and then releases into a sort of second nozzle. I guess I'm thinking like a two-stage nozzle that will capture the efficiency of gross overexpansion, but fix the problem of engine pressurization and unstable exhaust jet. Please feel free to correct any and all misconceptions above.