A rocket test stand exists for one reason. To find out what the simulation got wrong. Portal Space Systems fired its Flare thruster inside a vacuum chamber in Bothell, Washington, and the result reframes what liquid propellant actually means. Flare carries liquid ammonia. It never burns it. A conventional liquid rocket is a chemistry problem. Fuel and oxidiser meet in a combustion chamber, release energy through exothermic reaction, and the resulting gas accelerates through a converging-diverging nozzle. Performance is capped by the energy stored in the molecular bonds you brought with you. Solar thermal propulsion moves that ceiling. Concentrators focus sunlight onto an additively manufactured heat exchanger. Liquid ammonia flows through internal channels, absorbs the heat, expands to gas, and exits the nozzle. The energy source is the Sun. The propellant is only working fluid. What the test demonstrated: - Full-power operation of the heat exchanger thruster under vacuum - Operati...