Thursday, March 15, 2012

Rockets Aren't Rocks

I love the solid rocket booster videos from the shuttle launches. You probably thought you were done with me posting those 30 minute long things on here. No such luck. Today Phil Plait led me to an edited down best-of reel. Only some of the cameras have sound so they must have played sound from one over video from another to get this compilation. (Thanks to Skywalker Ranch for sound mixing.) They also made up this mph number in the upper right calculated from instrument data.

Phil was apparently as transfixed by watching these flashing numbers as me. He was watching on the ascent.
I was also interested in watching the numbers flashing past: on the upper left is elapsed time, and on the upper right is the air speed as calculated using on board instruments. Watch as the speed increases… and then the increase increases! In other words, the acceleration of the whole system increases quite a bit with time. That’s because the thrust from the rockets — the force they apply to the stack — is roughly constant, but as they burn fuel, the mass decreases. Since force = mass × acceleration (F = ma, with a hat tip to Isaac Newton!), as the mass drops, the acceleration must increase.
But I couldn't take my eyes off the numbers on the way down. I was trying to figure out where the SRB stopped going up and started coming down. Unlike the simple physics problem in high school where you throw a rock straight up, it stops, and falls back down, this never goes to v = 0. This takes college physics. I remember this from Classical Mechanics.

It stands to reason that these rocket motors are going to keep going up for a while after they detach. Of course the air speed immediately starts to decrease when the source of upward acceleration detaches, but it's not going STRAIGHT up. It's just going to make an arc where only the portion of the velocity vector perpendicular to the earth goes to zero, the vector tangent to the earth is still quite large. My gut tells me that the point where it stops continuing up and starts heading down is where the change in air speed is minimized. Around 3:46 in the video the air speed drops to around 2556 mph for a few seconds, then it begins to increases again. Still outside the atmosphere the SRB picks up speed from gravitational acceleration. Then it hits the atmosphere and very obligingly lets friction slow it down to just about what my terrible math memory remembers as terminal velocity for our atmosphere, 275 mph. It takes the whole atmosphere to get there, too, which points out that there is only just enough of that stuff up there. The thinness of our atmosphere really weirds me out if I think about it too hard, like I feel when I try to pick up trash around my driveway with cars whizzing by on the highway.

It's funny that I can remember Dr. Stanford standing at the blackboard working out that terminal velocity problem, and I can remember the answer, but I sure couldn't recreate any of those equations from memory. Just knowing I understood it once makes me appreciate this video more, though, so it was time well spent.

1 comment:

  1. Showed this video to my wife and one of my sons, who were so taken by it they collectively yawned and sucked all of the fun right out of the room. Even when I showed the the speed and time numbers flashing away!

    And you think you live alone.