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Name: Andy
Status: student
Grade: 6-8
Location: OR 
Country: N/A
Date: 1/17/2005



Question:
Is a heat shield needed to leave the atmosphere on a spacecraft? Why? If not, why do you need one to enter?


Replies:
A heat shield is not necessary for leaving the atmosphere, as the spacecraft is still working against Earth's gravity. (Travelling a bit slower) On the return though, most spacecraft essentially "fall" into the atmosphere. This re-entry is usually at a very great speed, causing tremendous heat build up from friction with the air, requiring a heat shield to protect the spacecraft.

Space ship one, however, used a slightly different means to re-enter the atmosphere. It is much slower re-entry speed allowed it to return to earth without a heat shield.

Ryan Belscamper


You need a heat shield whenever you are moving so fast through the air that friction between your craft and the air will raise the temperature of your spacecraft higher than it can stand. This could happen when you are leaving the atmosphere, or even when you are just flying around in it, but it almost never does. Rocket engines are not made powerful enough to get rockets into this kind of trouble. Usually a rocket engine is just barely powerful enough to raise the weight of the spacecraft up into space, and then make it go fast enough to stay in orbit. If you had a more powerful engine, you probably would have used it to carry more stuff.

Re-entry is different. When you enter the atmosphere, you are going very, very fast, because orbital speed is very fast, and because you have been travelling downhill since you left orbit. In order to land, you have to end up at the same speed as the ground. You could use a rocket to slow yourself down, but that would be very expensive. If you had the money for that, you probably would have used it to carry more stuff into orbit in the first place.

The cheapest way to do it is to use the air as a brake. But notice: all the energy the rocket engine gave you to get you up into orbit has to be given away now, or stored somehow, or transferred into heat. Friction with the air can turn it into heat, and that is the easiest way to get rid of this energy you no longer want. But it is a LOT of energy, and that means there is going to be a lot of heat.

Tim Mooney


We usually do not need as strong a heat-shield for launching into space.

The rocket is going faster than sound in the atmosphere, so it does get warm, but not nearly as hot as when it is coming down.

For going up, we choose our rocket's path to go up first, then curve over and speed up sideways, until it is at orbital velocity at the altitude we want. This avoids most of the air when at very high speeds, and it does not cost us extra fuel. In fact, it is the most economical path. Air drag matters almost as much as gravity, when launching a rocket.

We could do this on the way down, too, IF we were willing to burn as much rocket fuel coming down as we do going up. We are not willing, and for very good reasons. Using the atmosphere to slow us down instead of rocket fuel means we do not have to carry nearly as much fuel into space, and that means we can launch with far less too. (see below)

However, using the atmosphere to slow us down is what makes the ship get extremely hot.

Maybe someday we will have a ship with so much light-weight high-energy fuel, that can do a hovering descent from geosynchronous orbit. It would not need any heat shields at all, except maybe inside its rocket engine.

Regrettably, this particular solution seems unlikely. I have hope there will be others.

Jim Swenson


A heat shield is needed on reentry since the spacecraft is travelling so rapidly, around 5 miles per second. The kinetic energy of this object must be dissipated as the spacecraft slows down due to air friction. To get a feeling for the magnitude of the problem, consider a spacecraft landing from low earth orbit. In that orbit, it circles the earth (a little more than 24,000 miles) in 1.5 hours. This is 16,000 mph or 4.4 miles per second or (roughly) 8,000 meters/second (m/s).

An object with a mass of 1 kilogram (kg) travelling at this speed has a kinetic energy of 32,000,000 Joules = 3.2E7 J. If all this energy were used to heat the object, it would be enormous, sufficient to not only melt but also to boil the object! If it were made of solid aluminum (specific heat 900 J/(kg C)), its temperature would increase by some 35,000 C; if steel, 71,000 C (if the specific heat were independent of temperature)! Since aluminum melts at 660C and boils at 1800C and steel melts at 1535C and boils at 3000C, this is clearly a serious problem. The heat shield helps by ablation (melting of the shield takes heat energy), by transmitting some of the heat to the air streaming by, and by insulating the spacecraft from the worst of the heat. A defect in the shield, however, can easily lead to such a disaster as befell the Columbia shuttle.

To answer your question, there is not much problem when the shuttle is taking off since its speed when going through the atmosphere is much less. Initially the rocket thrust is hardly more than the weight of the shuttle (including fuel) and the rocket accelerates slowly. Before the speed gets large enough to cause a heating problem in the atmosphere, the shuttle is above most of the atmosphere. As the shuttle approaches its orbit, the rocket turns almost horizontal and the force of the rocket increases the rocket's speed to the speed necessary for it to stay in orbit (around 5 mi/s). Since it is then above the earth's atmosphere, it is not heated.

Best, Dick Plano...


No, a shield is not necessary for launch, just for reentry. That is because as the rocket goes up, it is out of the atmosphere by the time it is gets going really fast where heating would be a problem.

Rockets accelerate upwards from rest, and pass through most the atmosphere at a speed where heating is not a problem. When it gets beyond about 85 km height, there is not much atmosphere, and the rocket continues to accelerate until it attains orbital velocity.

That is not to say that the trip up through the atmosphere is easy. About a minute after launch, while doing a thousand miles per hour or so, rockets experience maximum dynamic pressure. The pressure causes stress to the whole structure. Some rockets throttle down to reduce stress at this time, then throttle up when above most of the atmosphere.

Actually, if one had a lot of fuel, it would be possible to use retrorockets to slow a spacecraft down for reentry and no heat shield would be needed. But it is impracticable to carry that much rocket fuel. So spacecraft simply reenter the atmosphere at full orbital velocity and use air drag to slow them down, instead of using retrorockets.

Bob Erck


When a rocket takes off it initially is not traveling very fast so there is not much friction between the spacecraft and the atmosphere. By the time its speed increases to a point were the heat from friction would become a problem it is high enough that the density of air is not great enough to cause a heat problem. On the other hand, when a spacecraft returns, it is traveling at a high rate of speed with respect to the earth and as it re-enters the atmosphere the density of the air increases causing intense frictional heating.

Vince Calder


Question is outside my scope of knowledge, however I offer a layman's opinion.

Going up, the rocket pierces the atmosphere, the shuttle is attached and shielded. The nose of the rocket takes the stress of heat and speed.

Coming down, the shuttle has to deal with the speed and temperature issues, hence the need for a shield.

The rockets additional tasks, the power to break through the atmosphere and gravitational forces, once beyond the shuttle can manage. Likewise coming down, the shuttle is at the whims of gravity etc. my guess, it acts similar to a glider, hence the need for accurate firing of power and timing to achieve the path for reentry and hitting the target.

James Przewoznik



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