why shuttle doesn't have an atmospheric exit heating problem, like it does in re-entry?

Question

Thank you for all your answers. I understand the energy transformation analogy. If problem is heat created during slowdown, why doensnt the shuttle begins slow down much earlier, slower. Cant it be done outside the earth atmosphere?

Answer 1

When leaving the atmosphere, there is no "heat drag" like there is when entering the atmosphere. It's only upon entering the gravitational "pull" of a planet that a spacecraft will experience the resistance which is the cause of the heat energy.

Answer 2

On launch the shuttle is in the atmosphere for a relatively short time, and is using the energy of its rockets.

Think of the launch and reentry as energy management exercises, because that is exactly what they are.

On launch the rockets make the energy needed to achieve the escape velocity--about 18,000 miles an hour.

On landing the energy is dissipated in the form of heat from reentry friction in the atmosphere. That heat is generated by the atmospheric action that brakes the shuttle to a safe landing speed. It has to be roughly equal to the energy needed to launch the shuttle, and that's a lot of heat.

Heat is energy in another form. Speed is kinetic energy. Fuel is potential energy.

There really is no safer or more efficient way to slow the shuttle down before atmospheric re-entry. The reason the shuttle has wings is so it can fly to a landing. That means it must maintain a certain amount of speed to fly safely.

All manned spacecraft so far have variations on this challenge. Most use some sort of heat shield which functions much as the wings and belly of the shuttle do on reentry. The shuttle is actually a bit safer because it can spread the heat out over a larger area.

If you slowed the vehicle down before reentry you would be entering the atmosphere at a rather steep angle and that could make for a very hard landing.

Future American spacecraft will probably use the more traditional heat shields unless some method can be devised to develop lift as the vehicle decelerates and reenters the atmosphere.

Remember, the shuttle must slow down, continue to develop lift, and must dissipate heat all at the same time.

Final note (I hope). Most reentry vehicles, including Mercury, Gemini, Apollo, Soyuz etc., have a heat shield that dissipates the heat as the capsule comes through the atmosphere.

As it descends into the atmosphere and slows to something less than its escape velocity (a lot less, actually), parachutes slow the vehicle and eventually larger chutes deploy and bring it to a safe landing, either on water (American capsules) or land (Russian capsules). This would allow a somewhat steeper descent into the atmosphere, but not too steep as speed has to be carefully controlled.

Our next generation of spacecraft will use similar technology, so the descent may be quicker and cooler in future spaceflights.


Hope this helps.

Answer 3

The reason the shuttle gets so hot is from friction in the atmosphere from moving so fast (17000 to 18000 mph at first). At launch, it leaves the atmosphere before getting anywhere near this speed.

Theoretically a spacecraft could slow down outside the atmosphere and return to Earth in a way that would not include a great deal of friction, thereby reducing the risk involved but there is a distinct problem with this. I'm sure you've noticed that in order to reach that high speed of 18,000mph, the shuttle carries a huge fuel tank and two big rocket boosters. If it wanted to slow down enough to get back to Earth without a hot reentry, it would have to bring up that same fuel with it so it could use it to slow down. It would also have to bring more fuel to compensate for the extra weight of bringing all that extra fuel, and more fuel to compensate for that fuels weight etc etc.

Essentially, we use the friction of the atmosphere to slow down because it's free energy that we don't have to bring fuel up into orbit to get. It's more dangerous, certainly, but it's really the only thing that makes orbital spaceflight possible.

Answer 4

There is a myth here I would like to try to dispell. The heat generated during re-entry is NOT due to friction. As the shuttle (or any other spacecraft) hits the atmospere the air beneath it is compressed. Compression causes heat. That is where all the heat comes from.

"When air is processed by a shock wave, it is superheated by compression and chemically dissociates through many different reactions (contrary to myth, friction is not the main cause of shock-layer heating). "

Answer 5

because of the angle of attack. going straight up it hits much less atmosphere at a slower speed that when coming in fast and at a shallow angle.

Answer 6

b/c when going up, it gains most of the speed outside atmosphere.

when going down, it looses all that speed in the atmosphere.

Answer 7

Great question, I had no idea that's why I had a look here to see what answers had been given.

Thanks to Bill P (above) for your answer, what you said makes a lot of sense.

Answer 8

First off, you probably overlooked the fact that you don't know how hot it is when you do it. Second, I am pretty sure the acceleration upward is far less than gravity acceleration.