CONVERGINGTHROATDIVERGINGCHAMBEREXIT
The throat insert: the narrowest point, 18.9 mm across, where the flow reaches the speed of sound and chokesFive pressure tappings and their meters: pressure, Mach number and temperature at each station (ideal-gas model)Supply, fast valve and thrust stand: a carbon-fibre bottle at 5,500 psi, an electric valve, a carriage pushing a load cellHow a cold-gas thruster makes thrust, with no combustion. Air stored at 5,500 psi (379 bar, the figure Tesla's reveal screen showed) waits behind an electric valve that opens in about 2 ms. The settling chamber fills in about 7 ms. The gas speeds up as the bore narrows and reaches exactly the speed of sound at the throat: Mach 1.00 at 200 bar (0.528 of the chamber) and -29 °C. Past the throat the widening bore makes supersonic gas go faster still, so its pressure and temperature fall: at the exit it leaves at Mach 2.94, 611 m/s, 11.3 bar and -166 °C. That cold is why the demo made a white cloud: moisture in the surrounding air freezes out where the jet mixes. The exit is still above the 1 bar outside, so the jet keeps expanding in a train of shock cells. This nozzle (throat 18.9 mm, exit 37.9 mm) is sized for one quarter of an estimated 66 kN, so it pushes about 16.6 kN (est.), using 25.2 kg of air a second (Isp 67 s); four of them would empty 250 kg of air in about 2.5 s. All numbers come from an ideal-gas model, so treat them as estimates: at 379 bar real air is about 16% less dense than ideal (compressibility about 1.19), and as the jet expands past the exit it crosses the temperature where air itself starts to condense (81.6 K at 1 atm). Motion is slowed: the opening and closing take milliseconds, and the shock cells are drawn closer together than the roughly 0.25 m they would span.