Two engines, one vacuum chamber, one altitude. Pull the chamber from sea level toward vacuum and watch which one stays near ideal.
Fig 3Altitude chamberDrag across the chamber, or use the slider0 km · 1.01 bar · bell over-expanded · column 100%
Gauge needles: chamber pressure and column feedFixed bell: only right near its design altitudeGas column 118: trims with the chamberOne altitude for both engines. The pump takes the chamber from sea level (1.01 bar) toward vacuum. The bell's exit pressure is fixed, so its plume is pinched by thick air and balloons in thin air. The virtual aerospike's column runs wide at sea level and thins as the pressure falls, so its exhaust expands inward and the plume stays a column. Schematic: plume shapes are illustrative and driven by the ISA pressure ratio.