Overheating when going through atmosphere

velocity

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

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

Going from mach 1 to mach 25 is much more gradual into a thinning and
eventuallly zero atmosphere. Coming back is hitting the rapidly increasing
atmosphere at mach 25 trying to slow back down to mach 1 in a hurry. If they
had a way to slow down to mach 1 in space, then they could come down slower
and cooler.

Nog wrote:
Going from mach 1 to mach 25 is much more gradual into a thinning and
eventuallly zero atmosphere. Coming back is hitting the rapidly increasing
atmosphere at mach 25 trying to slow back down to mach 1 in a hurry. If they
had a way to slow down to mach 1 in space, then they could come down slower
and cooler.

Hence, the adjustable wing design of SpaceShipOne, which allows it come
in like one of those badminton birdies, whatever they're called.

wrote:
Hence, the adjustable wing design of SpaceShipOne, which allows it come
in like one of those badminton birdies, whatever they're called.

SpaceShipOne was great for a mach 4 re-entry, but it would need a
massive redesign for a mach 25 (orbital speed) re-entry.

Mike Miller

" writes:

wrote:
Hence, the adjustable wing design of SpaceShipOne, which allows it come
in like one of those badminton birdies, whatever they're called.

SpaceShipOne was great for a mach 4 re-entry, but it would need a
massive redesign for a mach 25 (orbital speed) re-entry.

The basic idea is still sound. Have lots of surface area when you're
high up and fast in thin atmosphere and shrink down later.


Jochem

--
"A designer knows he has arrived at perfection not when there is no
longer anything to add, but when there is no longer anything to take away."
- Antoine de Saint-Exupery

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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JazzMan ) writes:
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,

Correct.

they keep the throttle down until they've cleared that.

Wrong. At the moments of " Max Q ", which is maximum aerodynamic
pressure on the vehicle ( Due to thickness of air times velocity;
later on, the shuttle is flying 3-10 times faster, but the air is
10-50 times thinner ), the SSMEs throtle down to about 65% thrust,
but the SRBs keep burning flat out.

After well under a minute, the SSMEs throttle back up to full rated
power.

Once above the atmosphere the shuttle
goes mostly horizontal and accelerates to orbital velocity.

Not quite. The trajectory is more slanted than that, for most of that
time.

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.

If you are in any physical object at 160 km up, and your burn has
eliminated your forward velocity, the acceleration DOWN will swiftly
give you much of that speed back, just in a different direction.

Not only will you need the tiles, you'll need double thickness, as
the shuttle's re-entry course is designed to shed a lot of speed
*before* you hit denser air. Free falling straight *down*, you
get no such benefit.

Andre

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

Andre Lieven wrote:

JazzMan ) writes:

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.

If you are in any physical object at 160 km up, and your burn has
eliminated your forward velocity, the acceleration DOWN will swiftly
give you much of that speed back, just in a different direction.

Not only will you need the tiles, you'll need double thickness, as
the shuttle's re-entry course is designed to shed a lot of speed
*before* you hit denser air. Free falling straight *down*, you
get no such benefit.

From only 160km you'll gain over 17k MPH accellerating
straight downwards at 32ft/sec^2? At what altitude will
atmospheric density start giving you an effective terminal
velocity?

JazzMan
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Please reply to jsavage"at"airmail.net.
Curse those darned bulk e-mailers!
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Don't need heat shielding when launching since speeds and atmosphere
are different on launch compared to re-entry, with speed 0-1000 km at
high density ground level, with ship going straight up 10 km to get out
of 99% of atmosphere and only then turning more to speed up
perpendicular to earth surface to orbit. If you wanted to orbit at 0
km, you would need heat shielding. Actually, I may be missing some
points, like atmosphere helpfully carrying away heat, so others also
please comment. Hmmm, maybe due to landing craft intentionally using
broad shape (flat cone) to maximize friction to slow, rather than
narrow cylinder rocket slicing through air using rockets NOT FRICTION
for speed change. Also, maybe some ships like shuttle use path on
landing that avoids extreme temps but requires tiles that can take
medium baking but are reusable unlike other shields. Meteors I hear
sometimes hit earth and are cold to the touch, so it is a question of
timing.......

AWR

AWR

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?

When vehicles launch, they tend to do most of their acceleration above the
atmosphere, so they're never travelling through the atmosphere at orbital
velocities. Partly this is to avoid this very problem.

That said, on the way up vehicles do travel very fast through the air by
aviation standards and they do have to cope with this, but the heating
involved is pretty much on a par with any other high-speed jet.

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