Tuesday, January 10, 2006

Launching in 50 MPH winds

I was a little curious about the idea of launching in 50 MPH winds. Could the vehicle be blown into the tower?

The Falcon gets off the pad with something like 1.3 Gs of acceleration, so to clear a 21 meter tower will take about 2 seconds.

Plain horizontal translation is not enough to worry about. I'm not sure what the horizontal drag coefficient of Falcon is, but it might be around 0.5. Assuming that, a horizontal 50 MPH wind would accelerate the vehicle laterally at about 0.3 m/s. By the time it gets off the pad, that's a little over half a meter.

One wonders how much the vehicle sways under wind. If the nose was deflected a meter and a half towards the tower when the holddown clamps release, that would add a meter of translation by the time the rocket clears the tower. I think the tower is more like 2 meters away, and I find it pretty hard to imagine pushing the launch button on a vehicle whose nose is whipping around anything like the complete distance to the tower.

So, no problem, but I thought the numbers were interesting so I'm posting them.

P.S. I think the SpaceX LOX boiloff solution of insulated panels velcroed onto the rocket, that get ripped off at launch, is cheap and cute. But this isn't going to change their procedural problem that they need to unload a bit of kerosene if they need to sit on the pad for any length of time. It was an error during the unloading of kerosene due to a launch delay that caused the tank damage.

And I'll bet those panels are going to get blown to bits when the rocket blast hits them. Not that it matters a lot, but they are going to get polystyrene bits all over their lovely tropical isle. If they ever want to clean that up for a VIP visit it's going to take a lot of time, and where are they going to find low-wage workers out in the middle of nowhere?

And finally, I think SpaceX gets my vote as best nerd entertainment of the decade. At some point, some producer is going to realize that real-time updates on spectacular engineering projects are better than "reality" TV programs, and then I'm going to have to start watching TV again. I wonder how much the buzz they're getting is worth, in term of popular support when they need to negotiate with the government?


  1. umm,
    from astronautix.com:
    the mass of falcon 1 is 27,200 kg and
    the thrust is 318.00 kN, so if we substract gravity, we get little acceleration:
    (318000-9.81*27200)/27200 = 1.8 [m/s^2]

    I have to go now...

  2. meant to say that, based on info from astronautix.com, the initial acceleration of the rocket is 1.7 m/s or 0.2 gees. (G=m*g is a force, so yes, the force felt is 1.2G but nobody says that you're "accelerating at 1 G" when you're sitting on a chair.). :)

    But yeah, 2.4 seconds to clear the tower with that.

    But I don't know about the vehicle dynamics when there is a gimballed force at the bottom and a wind pushing, total sum of that wind force is probably at above the center of gravity.

    Didn't the shuttle experience some unexpected behaviour at first liftoff too?

    One would think that if the computer is programmed to keep the rocket at a certain attitude, it will make it "slide" horizontally with the wind but keep it upright. In this case the total force moving the rocket downwind is greater than just the wind, because the engine is gimballed that way too.

    But if the intention is to keep as close as possible to a fixed 3d flight path, then the engine must first be gimballed so that the rocket is leaning against the wind, and then gimballed back straight.

    This moves the base of the rocket quite a lot in the downwind direction. Such a case probably asks for control engineering and simulation to give an answer about how much the rocket base will move.

  3. The only TV I've watched in the last year has been Dancing with the Stars, but that's because I'm a dance geek. But I saw an ad for a potentially-interesting show: American Inventor. It looks like "American Idol" with, well, inventors instead of singers. I'm going to catch the first episode in a few weeks to see if it's any good.