Hacker News Reader

Comment by tstegart | Hacker News Reader

I'm surprised they landed so close together. amazing. Also, can anyone tell me why they are slowing down as they fall towards earth (before the parachutes deploy)? Shouldn't they be speeding up by 9.8m/s as they fall?

View on HN

The terminal velocity of a giant tube tumbling through the atmosphere is a lot closer to 200mph than 3,200mph.

Ahh, got it. I expected more of a speedup as they dropped the last mile or so. I guess they didn't slowdown very fast right away because there wasn't much air up in the upper atmosphere.

> I'm surprised they landed so close together.

this doesnt make sense to me. On the orbit they look like they gaining a distance between each other. Any distance on that altitude would be 150-fold back on earth. How come they landed less than 20m from each other, no idea. Anyone?

Where do you get the figure of 150? I reason that if they are nearly identical pieces, and falling near each other, wind and other atmospheric forces would shift them about equivalently. Even if they veered hundreds of miles, it seems reasonable that they would do so together.

150 was a wild guess. I think there is so many variables when they orbit in space and drop to Earth, that I don't see it that simple to land one next to another. Its like taking two peppercorns and throwing them at the same time, very close to each other from the Empire State building.

well they are similar objects, undergoing the same forces, and they were released from nearly the same spot (from which they were both traveling in the same direction) the difference is mostly from the forces due to slight differences in tumbling motion when interacting with the atmosphere. just try thinking about conducting the same situation with out an atmosphere. the objects would follow the same trajectory from the moment they were released back to the ground. I'm sure nasa has to try to calculate the specifics/landing location, you dont want SRB's falling on property.

> just try thinking about conducting the same situation with out an atmosphere.

I can't since they DID travel through the atmosphere.

you can't imagine a simple physics problem? i understand that they DID travel through an atmosphere, but the basic mechanics involved can be understood just using projectile motion equation without taking the effects of air drag.

what are you talking about? in this example they were travelling through atmosphere.

I think you can see ships in the background of the video, presumably for recovery. So maybe they know pretty close to where they land, close enough that they don't have to worry about hitting the ships. That's impressive.

The 150-fold figure might apply if they actually took divergent paths back to Earth by a degree or two, but that doesn't seem to be the case--they took very nearly parallel tracks, with a few meteres of random jostling one way or the other. I'm not sure how much effect the explosive detachment from the shuttle+main tank has though / which way those forces are directed.

Interesting reading here on the details of the booster rockets: http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/s...

Once they hit the atmosphere, their lateral motion (groundspeed) is probably greatly reduced by air resistance, while theory downward motion is maintained by gravity. So shortly after reentry, they probably mostly stop diverging.

Still, though, I feel like I would be hard-pressed to throw two objects from space and have them land within sight of each other.

I think you're forgetting to take air resistance and other factors into account? Google terminal velocity.

Edit: If you meant prior to re-entry of Earth's atmosphere, the force of gravity is changing the objects course, thereby affecting acceleration, speed, velocity, etc

I wasn't forgetting, just under estimating it. I expected the terminal velocity to be higher.

Would have been interesting to get direction as well as velocity.

Specifically, direction as well as speed. But yes, that would have self-answered my question about what the numbers mean (by showing the "sideways" component).

they should be speeding up but not until infinity of course. At some point (here around 300mph) their size with "glide" in the atmosphere causing it to slow down.

It was interesting to watch how they gained speed on the orbit while being sucked by Earth gravity, and then quickly hit the breaks when atmosphere/air got thicker... awesome! Must be amazing feeling, given you would have 3000mph-withstand material, to actually "drop" yourself from space to Earth :)

IANAP, but isn't this due to air resistance/drag?

well they were released from the same point in space, and since they are similar objects traveling nearly at the same velocity, its not that surprising. im sure nasa calculated for it, and towards the end of the video there looked like a ship on the horizon for pick up.

Reply on news.ycombinator.com