For best efficiency, rocket nozzles should expand exhaust to match air pressure. Higher altitude means lower air pressure, so the optimal nozzle size increases with altitude.
Aerospike nozzles have been considered as a way to get more expansion at lower pressure. However, they're more complicated, more expensive, and heavier.
What could be done instead is: have a standard nozzle designed for low pressure, and then a "reverse nozzle" on the end of that which brings the exhaust back up to a higher pressure. This reverse nozzle would be ablative and get discarded during flight once air pressure is low enough. Because you want the reverse nozzle to be discarded when it's producing drag instead of thrust, it could simply fall off the end of the rocket when it's no longer needed.
back to index