Not just the Shuttle, but why do objects have to travel thousands of miles per hour to enter the Earth’s atmosphere?
Why, for example, couldn’t the Shuttle just slow itself down until it was in geosychronous orbit, then just fall (aided by parachutes as the air grew thicker, then glide once it was thick enough)?
In other words, if you’re going 0 mph relative to the earth beneath you, why do you have to “burn up” on re-entry?
Well, as they slow down, they start re-entry. The speed of the orbiter is what allows it to stay in orbit. To slow down to a ‘stop’ would not allow them to stay in orbit. And would require a tremendous amount of fuel.
So, they slow down enough to degrade their orbit, and let the atmosphere slow them down.
You may want to check out this past thread:
why 12000 mph?
They don’t have to travel thousands of miles per hour to enter the atmosphere, they have to travel that fast to stay up there in the first place.
When you want to come down, you have to get rid of that speed somehow. You could slow down using a reaction engine, but that would use nearly as much fuel as it took to get up there in the first place. Since you can’t realistically carry that much, your best alternative is to use the air friction to slow down for re-entry.
Because it would have to carry a hell of a lot of fuel into orbit to decelerate from orbital speed, which is what, 20,000mph? Roughly the same amount of fuel it currently uses at launch.
Lots of problems with that plan.
The space shuttle is never even close to being in geosynchronous orbit. That is much, much, much too high for the shuttle to reach. Geosynchronous orbit is about 22,236 miles above the earth. Geosynchronous orbit is not about being stopped in space. That is impossible. It is caused by being so high up that the time that it takes you to travel around the earth exactly matches the earth’s rotation. Once you descend, you are no longer stationary relative to earth anymore.
The shuttle can’t change orbits when it is in space. It simply in a constant free-fall around the earth.
Even if it was in geosynchrous orbit it wouldn’t have enough fuel to slow down that much relative to the earth. It is largely unpowered after it reaches space and requires the friction of the atmosphere to slow it down.
Let’s say that you got your wish and the shuttle could simply stop relative to earth almost instantly and just fall down. That would still be bad. A falling shuttle would be uncontrollable. Airplanes sometimes get themselves in just that sort of situation. It is called a flat spin and is often unrecoverable.
Joey G,
Got it. That thread pretty much covers all the bases. Thanks, all.
Because you need to be going that fast to be in orbit at the edge of the atmosphere. Being in orbit means that you are falling, but you are moving fast enough to be past the horizon by the time you get that low. To get to a geostationary orbit would require boosting the shuttle higher, not closer to the surface.
A geosynchronous orbit is one where you are far enough out from the surface to take 24 hours to travel the whole path. That is far beyond the edge of the atmosphere, and you are still moving at a good clip. A lower orbit (such as one at the edge of the atmosphere) requires a higher velocity.
The typical shuttle orbit is about 300 km up. At that elevation, the orbiter must move fast enough to go completely around the Earth in about ninety minutes. If you slow that down, (as in an attempt to maintain the low elevation but extending the period to 24 hours) gravity will pull you into a faster, lower orbit which eventually intersects with the surface of the Earth. This is to be avoided.
This is darn hard to explain without pictures. Google on “orbital velocity.”
I always wondered too, why didn’t they just use the forcefiled like in Star Trek NG?
According to Howstuffworks it can:
To quote Kryten from the TV show Red Dwarf: “A superlative suggestion sir, with just two minor flaws. One, we don’t have any defensive shields, and two, we don’t have any defensive shields. Now I realise that technically speaking that’s only one flaw but I thought it was such a big one it was worth mentioning twice.”
Here’s a Staff Report about the possibility of force fields:
http://www.straightdope.com/mailbag/mforcefield.html
And to answer the OP taken literally: The shuttle is significantly heavier than air, and has no lighter-than-air ballast available for deployment upon landing, so it doesn’t “float” very well in air.
-lv
It would be possible to reduce the speed of an orbital vehicle such as the Shuttle to allow a slower reentry, even without using atmospheric braking or a large amount of fuel …
a rotating tether can take energy out of the system, slowing the vessel down by speeding up the counterweight…
the technology doesn’t yet exist for such strong tethers yet, but according to the late Robert Forward, AMONG OTHERS,it will be…
Originally posted by Shagnasty
2) The shuttle can’t change orbits when it is in space. It simply in a constant free-fall around the earth.
According to Howstuffworks it can:
I guess if you want to get technical about what a change in orbits is then your point holds. However, the changes that they are describing are very small. It is just to get into position to do a docking etc. Also, I suppose the shuttle could start at a higher orbit and move to a lower one (although I don’t know why you would want to do that very often).
The OP was referring to large orbit changes which the shuttle cannot do. After the space shuttle Columbia crashed, reporters kept asking NASA why they didn’t check for tile damage in space and why couldn’t they just have sent the crew to the International Space Station to fix it or wait for help. The answer was that even though the shuttle often docks with the space station, it was in a lower orbit on that mission and there was no way to get it into a higher one. The shuttle cannot just move around in space. It is in a very specific freefall and it doesn’t have the fuel or the power to break very far from it.
An orbiting object’s ground speed (i.e. its relative speed over the surface of the Earth) has nothing to do with anything (except for tidal forces which are meaningless to anything other than planetoid size objects). An object in geosychronous orbit is in no way ‘floating’ any differently than any other object in orbit. It simply appears to not be moving because it’s altitude (@ 22K miles) makes it orbit the Earth once a day.
A satellite could orbit 90° to the equator (i.e. a polar orbit) but if it was 22,000 miles high it would still make one orbit a day.