Ask A Scientist© Environmental Earth Science Archive

name         Andy
status       student
grade        6-8
location     OR

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