> the test really needed to be performed inside of a vaccuum If I'm not mistaken they did, and the thrust was almost the same than the one in air. No perfect

> the test really needed to be performed inside of a vaccuum If I'm not mistaken they did, and the thrust was almost the same than the one in air. No perfect vacuum here, but still. "Vacuum conditions are provided by two roughing pumps and two high-speed turbopumps, and all vacuum tests are performed at or below 8×10^−6 torr" "The vacuum test data collected show a consistent performance of 1.2±0.1 mN/kW, which is very close to the average impulsive performance measured in air (also 1.2 mN/kW)"