Comment by blahedo | Hacker News Reader

Comment by blahedo | Hacker News Reader

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Can someone explain what the nominal "velocity" counter means? I thought it made sense going up (although I wasn't sure why its rate of change---which would be the acceleration---seemed to increase a bit), but then I was totally baffled after the separation. It goes down from 2900 to 2500, then back up to 2920, then it plummets to around 300, then gently lowers down to the 50 or so that it's at for splashdown; there is no discernible change in acceleration when the chutes blow around 190.

Here's what I would have expected: numbers increasing at a relatively constant rate until separation, then a rapid decrease to zero (and since 1mph ~~ .5m/s, expected decrease at about 20mph/s), then increasing again (maybe with a negative sign) until atmosphere, then a decrease to terminal velocity, then another big decrease when the chutes blow.

So aside from the fact that there is no direction (not even a minus sign), so it's at best a speed counter, there still is something I really don't understand about that number. But... what?

(Edit: fixed dumb math error)

The velocity counter means airspeed. The reason you aren't seeing the results you expect is that the shuttle launches in a ballistic arc to enter orbit. The boosters also follow that same trajectory, but lack orbital velocity. So what you see is them losing speed until they reach the peak of that arc, then gaining speed as they are drawn back towards the earth.

This gives you an idea of the arc:

http://media.tcpalm.com/media/img/photos/2010/04/05/0406_LOC...

The shuttle isn't going straight upward at separation. It's actually going mostly eastward, so when the boosters separate they retain most of their speed, and even regain a bit as they drop through the ultra-thin upper atmosphere.

You're right about the rest -- a decrease to terminal velocity and then a big decrease as the chutes take effect.

See also: http://en.wikipedia.org/wiki/Space_shuttle#Launch

But there wasn't a big decrease when the chutes deployed. It took a while. I found that odd. I expected an immediate decrease in speed the second the chutes deployed. Maybe the counter wasn't synched with the video?

> "there wasn't a big decrease when the chutes deployed. It took a while."

They take some time to completely expand. They're actually deployed in a constricted ("reefed") configuration, and then after several seconds they're allowed to fully open.

They're also trying to slow down a large piece of metal -- the booster rockets are about 150 feet long and weigh about 200,000 pounds (empty weight) each. The chutes produce a lot of force, but they're pulling on a lot of mass too (acceleration = force/mass.)

See also: http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Boos...

The rocket engine was still running after separation and we could not see when it stopped propelling the rocket so it is difficult to really understand its motion by just using its interaction with gravity and the athmosphere as our only variables affecting its behavior.

My guess on why accelerating is increasing is that both air resistance and gravity is decreasing the higher you go (the former much more than the latter). So with constant force, the acceleration would increase.

When they separate they are no longer flying straight up, they are moving toward a more orbital trajectory.

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