The spacecraft coasts weightless inside the Elevated Evacuated Tube and exits through the airlock into rarefied atmosphere. On egress it encounters aerodynamic drag; a restartable rocket engine counters that drag and provides attitude control via thrust-vectoring. Establishing quick, reliable ignition as a requirement simplifies the overall system design.

Today’s flight-proven options already meet this need: bi-propellant hypergolic engines are well understood, start rapidly, and have excellent reliability. Higher-Isp, non-hypergolic systems may also satisfy the requirement given the mission context. The vehicle carries only a modest propellant load for this brief post-exit segment (e.g., ~1,877 kg for a ~26,063 kg spacecraft to offset climb-out drag), so tank mass is a minor driver compared with pad-launched rockets. That relaxes feed-system constraints: pressure-fed operation is plausible, or electrically driven pumps (as in Rocket Lab's Electron’s Rutherford engines) could be used with relatively small batteries, since burn durations are short. Those batteries can be recharged by solar prior to the Mars landing burn.

Additional advantages further reduce ignition risk: the first ignition occurs with tanks fully topped, so no propellant settling is required, and the total burn time is short, easing thermal and life-cycle stresses relative to long-duration, high-reuse engines. Taken together, these factors make rapid, reliable engine start a reasonable and achievable requirement for the spacecraft’s post-exit phase.