An Engineering Analysis of the Falcon 9 First Stage Landing Failure

schwit1 writes: AviationWeek has posted an analysis of SpaceX's latest attempt to land its Falcon 9 rocket on an ocean barge. Quoting: "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 In a later tweet that was subsequently withdrawn, Musk then indicated that "the issue was stiction in the biprop throttle valve, resulting in control system phase lag."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." In related news, SpaceX is hoping to attempt its next landing on solid ground.

Comments

Video from the barge

[Bruce Perens]

A video from the barge itself is here. Everything goes wrong in the last second of landing, with over-correction putting it down on one leg, and then the leg crumples.

Re:Video from the barge

[DerekLyons]

Kind of makes me wonder if using the barge as such a small target is contributing to the hard landings, simply because it's such a tiny target relative to the area that the rocket has to come down on

Since the first attempt hard landed because it ran out of attitude control gas, and the second hard landed because of a control valve problem... how would a larger target have helped? In case of the first attempt, you've still got to control your attitude regardless of the size of the field. In the second, the size of the field is irrelevant if you can't properly control the vehicle in the first place.

Seriously, don't be misled by the frantic activity in the final seconds of the most recent attempt. That burst of activity was the vehicle attempting to null it's horizontal velocity and then trim it's attitude before landing - something it has to do regardless of the size of the field.

The basic flaw in the landing sequence isn't the size of the target, it's the design of the vehicle. Its minimum T/W ratio is well over unity at landing, meaning it can't hover, can't ease itself down, and you have to take great care to not end up with positive vertical velocity. The only way it can land (with any reasonable sized target) is to approach at high speed, then at the last second try to null horizontal velocity without excessively reducing vertical velocity (I.E. bouncing), followed by a return to vertical and touchdown.

You could avoid this by having a circle of paved ground a quarter to half a mile in diameter - but that's not cheap to build or maintain given the need to resist a rocket's exhaust. Long term, given that the tests are essentially free*, it's cheaper and easier to figure out how to land precisely on a smaller target.

* The first stage is bought and paid for by the launch customer - and so long as the added equipment for landing poses no undue risk during ascent, they don't care what happens to it after separation.

Re:Video from the barge

[Bruce Perens]

If there's one thing they should work on, it's not thrusters but having the capability to throttle to hover. That would potentially change the entire low approach. It is complicated by the fact that engine performance goes nonlinear in the low range.

Re: Video from the barge

[Anonymous Coward]

Their ultimate goal is to land this on another planet, then take back off. That is why they are working so hard on unassisted landing.

Re:You Can See

[Anonymous Coward]

That would require strengthening the body to withstand forces from other directions. More weight. This is a kludge and the wrong way to solve the problem.