SpaceX's positive 'spin'
Posted: Wednesday, March 28, 2007 2:20 PM by Alan Boyle
The millionaire behind the maverick SpaceX rocket venture, Elon Musk, says the verdict on last week's partly successful Falcon 1 rocket launch is "looking increasingly positive" now that his team is getting a close look at the data. Before its second-stage engine cut off prematurely, the Falcon flew to a height of 180 miles (289 kilometers) - a performance not quite good enough to reach orbit, but good enough for Musk to declare the end of the rocket's test phase and the beginning of its operational phase.
The assessment came in a post-flight data review published Tuesday on SpaceX's Web site. Musk said the engine cutoff was traced to overly vigorous sloshing of liquid oxygen in a propellant tank - a bad spin that was exacerbated when the first stage bumped into the second stage's engine nozzle during separation.
The slosh cut off the flow of propellant to the second-stage engine - and that triggered a shutdown of the rocket, just 90 seconds prematurely, Musk said. "For those that aren't engineers, imagine holding a bowl of soup and moving it from side to side with small movements, until the entire soup mass is shifting dramatically," he wrote.
If you depended on sucking a continuous flow of broth through a straw that was stuck into that bowl, you'd run into trouble - and that's sort of what happened to the Falcon 1.
Musk said his team was already working on a fix:
"We definitely intend to have both the diagnosis and cure vetted by third party experts; however, we believe that the slosh issue can be dealt with in short order by adding baffles to our 2nd stage LOX tank and adjusting the control logic. Either approach separately would do the trick (eg. the Atlas-Centaur tank has no baffles), but we want to ensure that this problem never shows up again. The Merlin [first-stage engine] shutdown transient can be addressed by initiating shutdown at a much lower thrust level, albeit at some risk to engine reusability. Provided we have a good set of slosh baffles, even another nozzle impact at stage separation would not pose a significant flight risk, although obviously we will work hard to avoid that."
Last week's $7 million test mission, paid for by the Pentagon's Defense Advanced Research Projects Agency, carried an experimental payload consisting of equipment that might be used to monitor future launches. Musk said the next Falcon 1 mission would be for real, with a U.S. Navy satellite called TacSat 1 due to be placed in orbit. The launch window for that mission opens in September. The mission after that, scheduled for November, would put the Malaysian Razaksat telecommunications satellite into orbit.
Some observers have wondered whether SpaceX was rushing things, considering that Falcon 1's first launch ended with the rocket dropping into the sea and the second launch didn't reach orbit. Musk addressed those wonderings head-on:
"There seems to be a lot of confusion in the media about what constitutes a success. The critical distinction is that a test flight has many gradations of success, whereas an operational satellite mission does not. Although we did our best at SpaceX to be clear about last week's launch, including naming it DemoFlight 2 and explicitly not carrying a satellite, a surprising number of people still evaluated the test launch as though it were an operational mission.
"This is neither fair nor reasonable. Test flights are used to gather data before flying a 'real' satellite and the degree of success is a function of how much data is gathered. The problem with our first launch is that, although it taught us a lot about the first stage, ground support equipment and launch pad, we learned very little about the second stage, apart from the avionics bay. However, that first launch was still a partial success, because of what we learned and, as shown by flight two, that knowledge was put to good use: there were no flight critical issues with the first stage on flight two.
"The reason that flight two can legitimately be called a near complete success as a test flight is that we have excellent data throughout the whole orbit insertion profile, including well past second-stage shutdown, and met all of the primary objectives established beforehand by our customer (DARPA/AF). This allows us to wrap up the test phase of the Falcon 1 program and transition to the operational phase, beginning with the TacSat mission at the end of summer. Let me be clear here and now that anything less than orbit for that flight or any Falcon 1 mission with an operational satellite will unequivocally be considered a failure.
"This is not 'spin' or some clever marketing trick, nor is this distinction an invention of SpaceX - it has existed for decades. The U.S. Air Force made the same distinction a few years ago with the demonstration flight of the Delta 4 Heavy, which also carried no primary satellite. Although the Delta 4 Heavy fell materially short of its target velocity and released its secondary satellites into an abnormally low altitude, causing re-entry in less than one orbit, it was still correctly regarded by Boeing and the Air Force as a successful test launch, because sufficient data was obtained to transition to an operational phase.
"It is perhaps worth drawing an analogy with more commonplace consumer products. Before software is released, it is beta tested in non-critical applications, where bugs are worked out, before being released for critical applications, although some companies have been a little loose with this rule. :) Cars go through a safety and durability testing phase before being released for production. Rockets may involve rocket science, but are no different in this regard."
Will further analysis bear out the preliminary conclusions about this month's flight? Will Musk's company "make it out of beta" on SpaceX's current timetable? Stay tuned...
