Spacecraft Re-entry

[treacle-tart]

When a space vehicle re-enters I understand the vehicle speed and entering the atmosphere creates friction resulting in tremendous heat. Why can't it just slow to a stop and fall? or, if its the atmosphere thats moving, why can't it move at the same speed thereby avoiding friction?

Sorry if its a stupid question!

[Space]

"slow to a stop and fall" ? If it falls, it's moving...
Do you mean "fall really slowly" ?

[treacle-tart]

Sorry space, I mean slow to a speed where gravity alone affects the vehicle. I aways thought there was a maximum speed that anything can free fall which, I assume, wouldnt be fast enough to generate enough friction to burn the vehicle up?

[Catso]

It can't just stop because a/ It's going so fast and b/ What do you propose using to slow it? It doesn't have wheels and brakes! And at X-thousand MPH, a parachute would be ripped off, even if it didn't catch fire.

They are trying to slow down and let gravity bring them down. The only way to do it is to let air resistance (IE the friction) slow them.

[Catso]

I believe initial re-entry speed is around 17,000MPH.

[Catso]

And the amount of fuel necessary for 'retro-rockets' to slow it down would be roughly equivalent to the amount of fuel it lifted off with. IE a very, very large tank.

[Loosehead]

It's a fuel issue, essentially the fuel necessary to slow down would have to be lifted up there and would probably make it impossible to get up there in the first place. So the resistance of the atmosphere is used to slow the vehicle until it starts to fall into the atmosphere.

[Birt]

Couldnt they refuel up there?
Surely BP or Shell have a station up there now. The prices they charge they could have them all over the galaxy!!!

[Clanad]

The maximum speed that anything falls referenced by treacle-tart is Terminal Velocity... somewhere near 300 mph. However this velocity restriction is only encountered when the falling body encounters the more dense air in the Stratosphere and more especially, the Troposphere. As an example, a reentering ballistic missle falls, if that's the correct verb, relatively straight down as wished by the questioner. It still experiences extremely high tempratures when entring the top of the Stratosphere, where it begins slowing due rto the drag of the air. So... a space vehicle attempting to fall straight down would encounter the same results. Many earth orbiting vehicles are in stationary orbits, but when the fall to earth they burn up like a meteor...

[alvin100]

Surely it takes next to nothing in fuel to start the slowing down process. When the Shuttle takes off it has to fight the density of the atmosphere and the massive force of gravity pulling against it, When slowing down in space they only require enough force to act against the shuttles own forward momentum. As every action has an equal and opposite reaction a few squirts on the forward thrusters starts the slowing process untill the the shuttle starts to fall back to earth (still going pretty fast)
If its going to fast or at too shallow an angle then in theory it would bounce off the atmosphere back into space.

[mibn2cweus]

In order to achieve orbit a space vehicle must acquire a forward velocity relative to the Earth's surface such that its rate of motion around the Earth is just enough to arrive at a balance between shooting off into space and falling back down to Earth due to the gravitational attraction of the Earth. The Moon has such an orbital velocity around the Earth that allows it to maintain its orbital distance from the Earth without fuel by relying only on its established momentum. Closer to the Earth, orbital velocity must be increased to compensate for the greater attraction of gravity that accompanies being closer to the source of gravity, the Earth.

Once an orbit has been established a small reduction in orbital velocity allows the downward pull of gravity to overtake the forward momentum necessary to maintain orbit and the vehicle will begin to descend. As the vehicle descends it encounters the thinner layers of the atmosphere and this acts as a break further slowing the vehicle and allowing gravity to have an even greater effect thereby lowering the vehicle into denser layers of the atmosphere, further slowing the vehicle, etc. etc . . .

While at first glance the atmosphere might seem a detriment to �safe� reentry, it is the gradual breaking effect of the air in the atmosphere that makes it possible to return to the Earth�s surface using very little fuel, however, the angle of descent and heat-shielding are critically important to the reentry process.