Running a vacuum-optimised engine in atmosphere means the exhaust is overexpanded, which causes severe flow seperation and chaotic flow. Even small amounts of

Running a vacuum-optimised engine in atmosphere means the exhaust is overexpanded, which causes severe flow seperation and chaotic flow. Even small amounts of overexpansion are harmful to the nozzle; overexpansion to vacuum-type ratios is likely to cause an Rapid Unscheduled Disassembly.