Space Forum / Space Flight / August 2005

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

    Paul

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

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

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.

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.

  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.

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

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

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

In sci.space.tech khatcat@hotmail.com wrote:

Yes, but much, much slower.
When going up, the shuttle gets out of nearly all of the atmosphere in
3 minutes or so.
It's then only at about a quarter of its orbital speed, which leads to
about a sixteenth of the heating (in the same thickness of atmosphere).

When going down, it's at its full speed of 9Km/s or so, and in order to
slow down has to descend quite deep, so that the atmosphere is thick
enough to slow it.

velocity

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.

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

Re-entry must be much faster than launching.  It would make sense that it
would be - and the higher speed causes the extreme temperatures.  It's the
speed at which you travel through the atmosphere that leads to the high
temperatures.

[...]

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.

+- www.cowlark.com --+

I am hoping that someone will answer the question below as well.  I was
getting ready to post the exact same question when I discovered that
somebody else had beat me to it.  Thanks in advance :)  Cindy

First - we do need heat shielding during launch, just not as much.

Second, the energy (heat) depositted into your rocket is (about) the
energy needed to take still air and move it out of the rocket's way (as
in accelerate the air).  So, the energy from the air will be on the
order of 1/2*m*v^2.  But the m (the mass of the air) flowing through
per second (which changes the energy term to a power term) is also
dependent on veolocity (the faster you go the more air you need to move
per second), so actual heating energy flow goes with the cube of the
velocity.

There are other, more complicated ways of looking at this as well, but
this is an easy start (making the rocket have wings decreases the
heating rate by staying high and decreasing m, but ends up having to
disapate more total energy because it takes so much longer to slow
down).

On the way up, rocket controllers limit the speed during time spent in
the atmosphere (because all the heating energy is useless for getting
to orbit) - that's why almost all rockets launch straight up and then
turn over to accelerate to orbital speed.  On the way down, the rocket
is using the atmosphere to slow down.  That means it has to enter the
atmosphere at the maximum speed.

Yes, but much more slowly.  Most of the acceleration to orbital velocity
happens outside the atmosphere.  Coming back down, however, we no longer
have enough fuel to do that much deceleration outside the atmosphere --
not by a long shot.  The only way we have to slow down is by plowing
into the air (converting kinetic energy to large amounts of heat).

Best,
- Joe

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At takeoff, a lot of fuel is burned to climb (more or less) straight up.  
Only *after* altitude (above the air, more or less) is reached does the
vehicle pitch over and use it's remaining fuel to gain orbital speed.

These are not exclusive moves with nice square angles, mind you.  It's a
slow curve, with more and more horizontal drive the higher up you go.

Yes, the Shuttle would burn up on takeoff if it had more than a tiny
fraction of orbital speed near the ground.  But it doesn't get to that
speed until it's already gotten almost all of the required altitude.

Since there's no fuel left to shed all that velocity, the only way to get
rid of it is to dip down into the atmosphere...  And all of the energy
stored as speed is turned into friction heat...

Gene P.
Slidell, LA

There have, I believe, been vehicles built that needed heat-shields
when launching.  But they were missile interceptors (I think _Nike_
was one), for which it was absolutely critical to get up to the
position of the incoming missile as fast as possible; you did a very
fierce burn while right down at the bottom of the atmosphere.

Tom