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Physics, Astronomy & Cosmology / Re: Why Does Re-Entry Have To Be Fast ?
« on: 05/08/2005 17:32:20 »quote:
Originally posted by neilep
...thanks guys, of course !!..it's obvious now...and I was missing it !!
.....But maybe in the future when we've sorted all that braking-against-fuel-effficiency stuff out, will,we then be able to come to a halt and just drop in.......thanks again.Men are the same as women.... just inside out !!
Right you are, neil. Remember the Startrek episode, where they ran out of fuel in the shuttle craft, and were going to make a re-entry without fuel? The shuttle was burning up, but Sulu found them in time and Scotty transported them safely to the Enterprise. Sadly, we couldn't do the same for the crew of the Columbia.
To hang some ball-park numbers on the re-entry: Typical orbital speed is 25,000 fps (7,620 m/s) at an altutude of around 800,000 feet (241,000 m). The fuel of the SRBs and the external tank was expended to reach that orbit. Now a change of only 200 fps (61 m/s) will lower the perigee of the orbit to about 400,000 feet (121,000 m). Orbital speed is actually slightly higher at this point, and the apogee remains where it was. However, the shuttle encounters atmosphere at the new perigee, and begins to slow. Re-entry ensues.
The re-entry burn is nominally performed by OMS, but if that system is inoperative, the forward or sternward RCS can each perform the re-entry burn. Triple redundancy insures the crew would never be stranded in orbit.
However, I still don't understand why there is no backup heatshield for the carbon-carbon wing leading edge. It would have saved Columbia, and the three remaining shuttles as well.