#SpaceFact: Did you know? To ignite a #LiquidRocketStage in space, it has to be thrusting already. Otherwise, the propellants will float around in its tanks and the engine will ingest gas instead of liquid. That means you have to ignite when the lower stage is thrusting - or use a separate small rocket motor (called an #ullage motor) to thrust it before igniting.

It took the loss of a few early space rockets to learn this fact!