COLORADO SPRINGS — SpaceX is thought to be focusing on static friction in an engine throttle valve as the prime suspect for the loss of the Falcon 9 first stage during the third attempt at recovering the booster.

The Falcon 9 was seconds away from what would have been the first successful landing of a used booster stage on SpaceX’s Autonomous Spaceport Drone Ship (ASDS) when the vehicle toppled over and was destroyed. The landing attempt occurred following the launch on April 14 of SpaceX’s sixth cargo resupply mission to the International Space Station from Space Launch Complex 40 at Cape Canaveral Air Force Station, Florida.

Video of the stage descending to the landing ship showed the vehicle approaching quickly but decelerating. However, closer to the platform the Falcon 9 showed an excessive horizontal velocity component that prompted the single engine used for landing to gimbal to correct the flight path angle. Exhaust from the Merlin engine could be seen raising clouds of water from around the platform as the stage maneuvered close to the edge of the landing zone. The control system then commanded vectoring of the engine nozzle to an angle that effectively over-compensated for the previous flight path angle correction. By this time the vehicle was too low to make further corrections and landed at too great a tilt and speed to safely land.

SpaceX founder and chief technology officer Elon Musk tweeted that “excess lateral velocity caused it [the booster] to tip over post landing.” In a later tweet that was subsequently withdrawn, Musk then indicated that “the issue was stiction in the biprop throttle valve, resulting in control system phase lag.” In this statement, Musk was referring to “stiction” — or static friction — in the valve controlling the throttling of the engine. The friction appears to have momentarily slowed the response of the engine, causing the control system to command more of an extreme reaction from the propulsion system than was required. As a result, the control system entered a form of hysteresis, a condition in which the control response lags behind changes in the effect causing it.

Despite the failure of the latest attempt, SpaceX will be encouraged by the landing accuracy of the Falcon 9 and the bigger-picture success of its guidance, navigation and control (GNC) system in bringing the booster back to the drone ship.  The GNC also worked as designed during the prior landing attempt in January, which ended in the destruction of the vehicle following a hard touchdown on the edge of the platform.

Other control system modifications also appear to have functioned according to plan. These include aerodynamic mesh fins that were added in place of gaseous nitrogen control thrusters that were tested to control the rotation that occurred on the first test flight in September 2013.  Prototype versions of the steerable fins were tested for the first time in May 2014 on a flight of the Falcon 9 Reusable (F9R) experimental vehicle during a test that reached an altitude of 1,000 meters.  SpaceX is developing a new F9R after the first one was destroyed in August last year during a test at the company’s rocket development facility in McGregor, Texas.