How Space X's Falcon 9 Rocket Flew After Losing an Engine

On Sunday, SpaceX launched a Dragon capsule full of supplies on a mission to the International Space Station on a Falcon 9 rocket. The Dragon successfully reached orbit and is on its way to ISS, but the flight wasn't perfect. During the rocket's ascent, one of its nine Merlin engines shut down prematurely, just one minute and 19 seconds after launch.

Even with nine engines on board, the loss of one still affects thrust and flight controls, which makes the successful flight all the more impressive. SpaceX doesn't know (or if they do, they aren't telling us just yet) what caused the engine to shut down, but it has said what likely happened. The Falcon 9's onboard computer probably detected a malfunction like a sudden loss of pressure in one of the first stage engines, which triggered an immediate shut down. With one engine gone, the flight computer recomputed a new ascent profile for an eight engine Falcon 9, instructing the remaining engines to burn 30 second longer to recover the thrust lost from the one engine's failure. So all's well that ends well. There weren't any major effects to the Dragon capsule or the cargo resupply mission as a whole.

But the failure did impact a secondary cargo carried aloft on the Falcon 9. Orbcomm, a global satellite data communications company, had a prototype of its second generation of satellites (called OG2) on the rocket that launched on Sunday and it didn't make it into its planned orbit. That's because the Falcon 9's first stage engine's failure precluded the planned second burn. The OG2 prototype satellite was delivered into a much lower orbit than was intended, leaving Orbcomm and Sierra Nevada engineers troubleshooting ways to use the satellite's own propulsion system to lift itself into its proper orbit.

Still, the loss of thrust from one failed engine didn't cripple the mission because rockets, at least the big ones that we send into space, don't rely exclusively on their engines for directional control.

Rockets have these really neat things called inertial guidance platforms, or inertial navigation systems (INS), that are made of three accelerometers mounted on a gyro-stabilized platform and a computer. Rockets, like airplanes, can rotate around three axes: pitch, yaw, and roll. Each accelerometer in the trio is pegged to a specific axis, and te stabilized platform isolates the accelerometers from the vehicle's rotational motions so they maintain the correct orientation.

The computer reads the information from the accelerometers to determine a rocket's true position and velocity, then sends that information in the form of steering commands to the control system. The engines swivel accordingly. So in the same way that the remaining engines fired longer to make up for the loss of thrust from the one engine's failure, the Falcon 9's onboard computer can also tell the remaining engines how to move to keep the rocket on its new trajectory.

SpaceX's single-engined Falcon 1 uses an inertial guidance system, and its algorithms played no small part in designing the Falcon 9's guidance system. The rocket was designed to deal with an engine loss in the first stage, and the onboard system is programmed to adjust the trajectory in real time. The whole things is designed to ensure a smooth and computationally simple trajectory without accruing serious orbital insertion inaccuracies. SpaceX was also quick to point out that the Falcon 9 is designed to shut down two of its nine engines before the others on a nominal flight. This limits the loads on acceleration to 5 gs. This means the rocket could have lost another engine and still completed its mission.

These failsafes and redundancies are part of SpaceX's quest to man rate (or prove its safety for manned flight) the Falcon 9, and are getting up one step closer to regaining manned launch abilities on American soil.