Reentering the atmosphere?

Question

I understand that spacecrafts need to reenter the atmosphere at a certain angle to balance the rate of friction and the G's obtained because of a fast deceleration otherwise, it will burn in flames.

But why is the spacecraft said to be coming so fast in the first place?
Lets say that there is an object above Manhattan floating in space, and it is always keeping itself in the same place. That means to me traveling at the same speed of the earth and in the same direction, so there is no speed relative to the ground.

Then why that same object can't just come down at a slow speed and just enter the earth without complications about friction or anything? Pretty much the same as an elevator coming down vertically.

Why spacecrafts don't do the same? just place themselves right above the place where they need to land and come down at free fall speeds after a "pushdown" from zero gravity so they can enter the gravity zone?

What am i missing here?
Thanks

Answer 1

When a shuttle takes off, it has to go fast enough to escape the earth's gravity well; the speed needed is roughly 90,000 miles an hour. While it is orbiting the earth, the shuttle is still moving that fast, orbiting the earth every 90 minutes, since there is no air to slow it down. When they land, the cannot use their engines to slow down because they have expended all of their fuel on takeoff. It takes those two huge white booster rockets and all the fuel in the giant orange fuel tank (which pumps through the three big engines on the shuttle) to get up; the two little engines are only to alter the height of the orbit and the direction the shuttle is pointing, but aren't powerful enough to significantly change the speed of the shuttle. The friction of the atmosphere is the force that is used to slow down the shuttle.

When a satellite is maintaining an orbit above a single point on the earth, that is called geosynchronous orbit. Everything put into orbit has to go 90,000 miles an hour to escape Earth's gravity, and even though the satellite appears to be hovering over the earth, it is still going 90,000 miles an hour! The satellite has merely moved to a very high orbit, so far that it will be able to stay over a point on the earth, regardless of its high speed (think of a spinning wheel; a point close to the center of the wheel moves slower than a point far away from the center). This orbit is used for weather or communications satellites.

As for the idea of a "space elevator", this is the holy grail of engineering. If you could simply hoist space cargo, like satellites or space station parts, up an elevator it would save loads of energy in getting it off the surface, since you wouldn't have to strap a rocket to it. The only problem is that there is no material strong enough to make a cable that can be 50 miles long and not break; and even if there was, it would probably be more expensive than the cost of using rockets.

As for SpaceShipOne, it was not going fast enough to orbit the earth, it merely went very high, and fell back down. SpaceShipOne can only go Mach 3 (3 times the speed of sound, nowhere near fast enough to attain orbit), while the space shuttle goes Mach 25!

Hope that answers everything!

Answer 2

The point your talking about is callled "synchronous orbit"--its the distance at which an object in orbit takes exactly 24 hours to orbit the earth, exactly matching the earth's rotation--so it stays above the same point on the earth's surface.

But that point is about 22,000 miles up--100 times as high as the International Space Station. The Shuttle doesn't have the range to get that high.

And even if it did, your idea won't work--what you missed is that the spacecraft will gain an enormous amount of energy as it sinks deeper into Earth's gravity well. One way or anoter, that energy has to go somewhere--if you just let the spaceccraft fall the whole way, it will be going just as fast as the Shuuttle coming in from the orbit of the ISS--in fact, even faster. The only way to make it come ins lowly is to get rid of that energy by firing its engines to brake--and even if the Shuttle carried no payload, just all the fuel it could lift, it wouldn't be enough.

In practice, the most efficient way wwe have right now is to send rockets into low Earth orbits--because it takes an enormous amount of fuel even to do that. There's not much room left for a payload (at liftoff the Shuttle weighs over 4 million pounds--but can carry only 47000 pounds of cargo--barely more than 1%). If you go higher, you use more fuel--and that cuts down on that payload.

And the only way to even do that is to use the atmosphere for braking, instead of firing the rockets (except to guide the shuttle into the atmosphere at the right time and place) But to be in orbit the spacecraft has to be traveling about 17,5000 mph. So that's how fast its going when it re-enters the atmospher.

Higher orbits are slower--but it takes more fule to get the spacecraft higher than you save by having the slower orbit.

There is one way to make your idea work (in theory, anyway) and that's by building a "space elevator" This would be a structure that extended from the Earth's surface to synchronous orbit--not a tower, exactly; more like a superstrong and lightweight cable. that "elevator cars" could travel up and down. That is an idea that a lot of people think might be workable at some point in the future (we don't have the technology yet). If you want to find out more about that, just do a search for "space elevator" and you'll find some sites.

Answer 3

To put it succinctly, the object orbiting above a city like Manhattan is actually going faster than the city. It is this extra speed that has to be shed when reentering the atmosphere and landing. It is like a wheel spinning, where the tread is going faster than the rim. A point on the tread has to move through a greater distance in the same time as a corresponding point on the rim.

Answer 4

nicely, interior the case of a automobile like the Mercury spacecraft, they're all managed. Re-get admission to into the ambience is a tender procedure. in case you do not hit it ideal, you'll both leap off the ambience or fall apart. Shepard's 15-minute flight develop into non-orbital, meaning it under no circumstances totally were given out of the earth's environment. typically, re-get admission to % is determined through orbital altitude, and orbital speed. The automobile should be slowed down adequate to make that get admission to. i appreciate some thing about the gap software. and that is an outstanding action picture. have you ever watched "the right Stuff?" sturdy action picture, so is Apollo 13. i wish this helps!

Answer 5

In the example you give, an object stationary above New York...yeah, no problem, just come back down...no heat shield required.

The problem is re-entering from ORBIT. Because orbital velocity is anything but stationary over any point on the ground. (You can't orbit 300 miles above one point on the ground; that is physically impossible.) Theoretically, a spacecraft could fire its engines to re-enter as you say, matching velocities with the ground. Alas, that would take just as much fuel to achieve that as it took to get up there in the first place...meaning the shuttle would have to carry two external tanks and four solid rocket boosters. All of that is well beyond its payload capacity. That's why we use heatshields instead; they are simply far more practical.

You'll note that Burt Rutan's Spaceship One did not need a heatshield...since it essentially went straight up and came back down in the same area.

Answer 6

The first two answers, from AZ and cubes, are wrong. Both contain factual and conceptual errors and confusion.

Crabby's answer is correct and well stated.