Overheating when going through atmosphere

I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

BigKhat

wrote:

I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

Yes, but not nearly as fast. A launcher does most of its acceleration
after it has risen above the atmosphere.

Paul

wrote in message
ups.com...
I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

BigKhat


Not as fast. Less energy == less friction == less surface heating.

wrote:

I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

BigKhat

Most of the accelleration that occurs with the shuttle is after it's
left the thickest part of the atmosphere, they keep the throttle
down until they've cleared that. Once above the atmosphere the shuttle
goes mostly horizontal and accelerates to orbital velocity. When
re-entering the shuttle has full speed as it hits atmosphere and
it has to scrub off speed using the atmosphere. If you had unlimited
fuel you could fire the engines and mostly stop the shuttle in orbit,
then let it drop pretty much straight down with no atmospheric friction
heating at all.

JazzMan
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Most launch vehicles first go pretty much straight up to get past the
atmosphere. They use compact, high thrust, but not necessarily
efficent, propellents like kerosene or solid fuel to lift the stack
above most of the air, while minimizing the cross-section. It is only
when they are free of the bulk of the atmosphere that they tilt over to
get the horizontal velocity required for orbit. There they can use much
more efficent, but slower burning and bulkier fuels like hydrogen.

For example, the shuttle primarily uses solids for the first couple of
minutes, which have high thrust, a narrow cross-section but dont last
long. Once it drops the SRBs, it is essentially moving horizontally,
burning hydrogen for six more minutes to reach orbital velocity.

All that energy, of course, has to be expended within the atmosphere
during entry.

) writes:
I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

Heres why. When you launch from the ground, you start moving through
the thickest air you are going to encounter, but you are moving through
it very *very* slowly, relative to the speeds when coming back in.

When the shuttle, or any re-entering spacecraft coming in from orbit,
starts to hit air, granted very thin air very high up, the spacecraft
is flying at about 17,400 MPH, or about *five miles per second*.

Wheras, on the way up, you don't hit such speeds until you're far more
out of the athmosphere. And, on the way up, as you accelerate, the
air is thinning out even faster, while on the way down, you need that
air drag to slow you down, so you don't stick your pointy nose through
it, but you point the thickest and widest part of your spacecraft
frontwise so that you can catch more of the air, so that it's drag can
slow you down.

And, all that air drag does slow you down, from orbital speeds of
five miles per second, to the 180 MPH speed of when the shuttle's tires
contact the runway. Thats a lot of slowing down, and the air does it
all for you, but in the process, it makes a LOT of heat as a result.

Does that help ?

Andre

--
" I'm a man... But, I can change... If I have to... I guess. "
The Man Prayer, Red Green.

wrote:

I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

BigKhat


Yes. But there's a difference between going up, when speed is
increasing, as air density is decreasing, and coming down, where air
density is INcreasing, and you're tryibg to use that atmosphere to
decelerate (rather than rockets, which would be impractical in this
case), and that kinetic energy can only be converted to heat.

This is one of the problems with the idea of flying hypersonic
airbreathers to orbit. By definition, it has to stay in the meaningful
atmosphere to keep picking up oxidizer at increasing speed. As close to
orbital speed as your materials will allow you, with a small rocket kick
the rest of the way to orbit.

Pure rockets can take the fastest path into less dense air. Not just
for termodynamic reasons, but because rocket engines are more efficent
in decreasing atmospheric density.

--

You know what to remove, to reply....

wrote:
I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

1) Launchers often DO need heat shielding during launch. The nose of
the shuttle's external tank is darkened and scorched a bit by launch.

Notice how the nose of the ET is darkened here compared to the sides:
http://www.orbit6.com/et/img/etank2.jpg

But in this picture, the nose is the same color as the rest of the
tank:
http://www.orbit6.com/et/img/91etksc2.jpg

2) Yes, it's the same atmosphere, but the rocket is moving much slower
during launch than re-entry. During launch, rockets are generally
moving at only a couple thousand miles per hour before they're above
most of the atmosphere. During re-entry, they hit the atmosphere at
17500mph.

Mike Miller

writes:

I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

Apart from the smug reply that the heat shield will probably already
be in place on launch... consider this:

Heating comes roughly from velocity times drag, with drag coming
roughly from density of the air, which decreases rapidly with
altitude. In this sense, it is not the same atmosphere, as the
upper layers (100km height and more) are almost a vacuum
compared to sea level.

On launch, energy from chemicals is used to accellerate from 0 to
mach 23 or something - orbital speed. You want to use the least
amount of chemicals to get at the required speed. Any heating
cum drag will WASTE chemicals, by putting their energy into
heating the air, instead of increasing the speed of your vehicle.
Thus, the engineers are told to avoid drag cum heating as good
as possible. Solution: go up almost vertically, at not-so-high
speed, until most of the air is below you. Then start to really
accellerate.

On reentry, it is the other way round. The vehicle must be
decellerated from mach 23 to 0 (or you won't survive landing...),
so all that energy must be shed. Now, it's a nice thing to have
this atmosphere cum drag - you don't need any more chemicals
to burn for decelleration! So, in this case, the engineers are
told to search for drag cum heating (within the limits of the
heat shielding, of course), because this drag is exactly the
thing you use as a brake.

I hope this layman's understanding was both correct and understandable.

best regards
Patrick

On or about 28 Jul 2005 11:32:29 -0700, made the sensational claim that:
I think I understand why objects heat up when going through the
atmosphere and therefore why re-entry vehicles need heat shielding. My
question is why we don't need heat shielding when launching. Aren't we
going through the same atmosphere?

We do, that's part of the ET foam's job.
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