Chair Force Engineer

Monday, April 20, 2009

Shaky Math

SpaceX is delaying its next Falcon 1 launch because of "dynamic interactions" between the launcher and its RazakSat payload. A lot of commenters are coming down hard on SpaceX or wondering how this issue could have been left to simmer until the very last minute. Having observed a spaceflight program dealing with serious launch vibration issues, it's pretty easy for me to see how this happened.

Every launch vehicle users' manual contains a vibration profile for the launcher across the range of frequencies at which the rocket is expected to vibrate. SpaceX has been publishing users' guides since at least 2005, three years before the vehicle made its first successful flight. Furthermore, the flight configuration is somewhat different from the original one in the first users guides, after the change from a Merlin 1 to a Merlin 1C engine on the first stage.

The most likely scenario is that RazakSat was designed to the old vibe specs that were published for Falcon 1 several years ago (after all, RazakSat wasn't designed, fabricated, and integrated overnight.) It wasn't until all the data came back from the successful September 2008 Falcon launch that the vibe problem was discovered with RazakSat. Perhaps it affected certain structural modes of RazakSat, or maybe the vibe profile was more intense across the spectrum. Either way, it's time to go back to the drawing board.

The vibe problem doesn't require any drastic solutions. By placing a series of Softride isolators between the launcher and the payload separation system, vibrations can be damped down to a survivable level. A coupled loads analysis is absolutely necessary to examine the full launcher-softride-payload stack and determine how the isolators can be tuned for the RazakSat mission. I don't know how much time CSA Engineering would need to solve the RazakSat issue, but it would seem like the quickest possible option for getting the next Falcon 1 successfully off the pad.