Reentry at high temperature

Someone please tell me why spacecraft are designed to reenter the earth's
atmosphere at high speed. Isn't there some way to come down slowly,
so the heat shields wouldn't be needed? Has anyone modeled the idea of
unfolding some large wings to add a lot of surface area, or using
propellers to resist falling, or parachutes? Thank you.

--

Mike Lepore in New York - email with the 5 deleted

Mike Lepore wrote:
Someone please tell me why spacecraft are designed to reenter the earth's
atmosphere at high speed. Isn't there some way to come down slowly,
so the heat shields wouldn't be needed?

They are initially traveling very fast, since they are either in orbit
or are coming from far away and have fallen into Earth's
gravity well.

Slowing without drag in the atmosphere would mean using rockets,
which would require a prohibitively large quantity of propellant.

Has anyone modeled the idea of
unfolding some large wings to add a lot of surface area, or using
propellers to resist falling, or parachutes? Thank you.

Certainly. The time required to brake during reentry is
increases as the lift/drag ratio increases, so this can be
used to prolong the reentry. The altitude also is dependent
on the ballistic coefficient (mass/area) of the vehicle,
allowing a broad, light vehicle to slow higher in the
atmosphere, spreading the heat over a larger area. But the
energy still has to be dissipated somehow.

Paul

Mike Lepore wrote:
Someone please tell me why spacecraft are designed to reenter the earth's
atmosphere at high speed.

Because they orbit the Earth at high speed and the amount of fuel
needed to slow down for a gentle re-entry is heavier than a heat
shield.

Isn't there some way to come down slowly,
so the heat shields wouldn't be needed? Has anyone modeled the idea of
unfolding some large wings to add a lot of surface area, or using
propellers to resist falling, or parachutes? Thank you.

Yes, most of those ideas have been modeled. However, they all have to
deal with the same total energy release per kilogram of mass in orbit.
Larger wings or other drag systems (like parachutes or ballutes) can
spread out the heating, but they're not a perfect answer and usually
add weight for little gain. It usually ends up being easier (or
lighter, or more proven) just to use a plain vanilla heat shield.

Different re-entry profiles can help, too. The original civilian
designs for the US space shuttle used metallic heat shields. When the
USAF signed on, it had requirements for the shuttle that included more
demanding re-entries (a lot more steering, or "cross-range", than the
civilian shuttle designs needed) and materials with higher temperature
tolerances were called for, like the fragile ceramic tiles of the
current shuttle. It'd be interesting to see a flight-proven metallic
heat shield on a shuttle.

Mike Miller

Someone please tell me why spacecraft are designed to
reenter the earth's atmosphere at high speed.

The answer is really quite simple when you think about it. Slowing
down from orbital velocity requires exactly the same change in speed as
attaining orbital velocity. It is entirely possible to slow down with
rockets instead of air resistance, but the ISP of those rockets would
have to be basically the same as is required to get into orbit.

You know the Space Shuttle, with that large tank of fuel and those two
huge boosters? All the power from those boosters and that fuel is used
to accelerate the shuttle to orbital velocity. Sure, it's possible to
slow the shuttle down a lot so that it would enter the atmosphere at a
leisurely 200kts, but doing that would require the same power as is
required to get it into orbit in the first place. So basically we're
talking about having the shuttle in orbit with a large, *full* external
tank at least. Getting the shuttle into orbit with a large, full
external tank would require three times the amount of thrust required
to put the bare shuttle into orbit.

So just imagine the shuttle sitting on the launch pad with not one but
three external tanks, and six external boosters. That's on the order
of magnitude of what would be required to get it into orbit with the
fuel to brake out of orbit. That's a larger stack than anything that
anyone has ever launched. That's much larger than the Saturn V or the
Russian Energia. It's much too large to be practical.

And of course there are other considerations, like keeping all that
fuel cooled for the duration of the mission. It's really just not a
workable idea.

Has anyone modeled the idea of unfolding some large
wings to add a lot of surface area

This is similar to the idea of a ballute.

http://en.wikipedia.org/wiki/Ballute

It's certainly helpful, but for a full reentry in less than one orbit
you still need a heat shield. Slowing down more gently in the very
high atmosphere, as you're suggesting, results in a ballistic
trajectory that brings you down into the lower atmosphere before you
can bleed off enough speed to no longer need the heat shield.

Another idea, that I don't know enough about to speak to, is to drop
down into the atmosphere and then pitch up so that you fly out of the
atmosphere like a rock skipping on a pond. You're still on a sub
orbital trajectory though, you don't fly off into space, you come back
down into the atmosphere and repeat the process.

This idea was employed by the X-20 Dyna-Soar.

"Mike Lepore" wrote in message
...

Someone please tell me why spacecraft are designed to reenter the earth's
atmosphere at high speed.

The only reason any spacecraft remain in orbit to begin with is because of
their high speed.
Imagine, if you will, that you could build a 200 mile high ladder and could
climb up to a platform at the top with a scale on it. If you were to weigh
yourself on that scale you would only weigh about 5% less than you would
on the ground. A space shuttle might just zip on by you at 17,000+ mph with
all the astronauts on board in freefall and feeling mighty weightless - but
if you
were to step off the platform you would would drop like a rock.
In order for an orbiting spacecraft to return to earth it has to do
something
with that excess 17,000+ mph. It has to lose it somewhere. Using the
atmosphere as a brake is the least expensive way to go.

Isn't there some way to come down slowly,
so the heat shields wouldn't be needed?

The amount of fuel needed to slow it down enough in space
would be more than it could carry up there in the first place.

Has anyone modeled the idea of
unfolding some large wings to add a lot of surface area, or using
propellers to resist falling, or parachutes?

You betcha.
As the saying goes:
Google is your friend.

Be fearless, look it up.

In article ,
Mike Lepore wrote:
Someone please tell me why spacecraft are designed to reenter the earth's
atmosphere at high speed. Isn't there some way to come down slowly,
so the heat shields wouldn't be needed? Has anyone modeled the idea of
unfolding some large wings to add a lot of surface area, or using
propellers to resist falling, or parachutes? Thank you.

There is a LOT of energy in an Earth orbit, and altitude makes
little difference. Propellers would be useless until one got
into thick enough air, and probably even then; the speed is
about 18,000 miles an hour, and propellers are not much use
even at 1000.

I presume shallower angles of entry have been considered, but
the air friction gets rather high before wings or parachutes
can be used; parachutes have been used for reentry after the
speed is low enough. Until the speed is low enough, just keep
the heat outside the critical part of the vehicle, unless you
have your antigravity device available.

Mike Lepore in New York - email with the 5 deleted




--
This address is for information only. I do not claim that these views
are those of the Statistics Department or of Purdue University.
Herman Rubin, Department of Statistics, Purdue University
Phone: (765)494-6054 FAX: (765)494-0558

In article ,
Mike Lepore wrote:
Someone please tell me why spacecraft are designed to reenter the earth's
atmosphere at high speed. Isn't there some way to come down slowly,
so the heat shields wouldn't be needed?

Not literally, no. The spacecraft *is* arriving at the outer edges of the
atmosphere at high speed; the only way to change that is with lots and
lots of rocket fuel, totally ridiculously impractical amounts (unless you
are talking about advanced nuclear rockets).

Has anyone modeled the idea of
unfolding some large wings to add a lot of surface area...

There has been a little bit of exploration of the idea of very large
surface areas. It doesn't solve all the problems; you do still decelerate
relatively rapidly. But the deceleration happens higher up, in thinner
air, and the heating is spread over a larger surface area, so the
materials problems are easier. It's an interesting idea, but there hasn't
been funding for full-scale testing, which is what's really required.

(In fact, there are at least half a dozen unorthodox reentry concepts
which *might* work, and look promising, but will become credible to
mainstream designers only after a full-scale demonstration. This is the
sort of technology R&D that NASA should be doing, and isn't.)

The idea of doing a very gradual reentry, spreading the heating out over
a long *time*, is appealing in principle, but nobody knows a way to make
it work. There is just no way to *stay up* in thin air that long.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Mike Lepore wrote:
Someone please tell me why spacecraft are designed to reenter the earth's
atmosphere at high speed. Isn't there some way to come down slowly,
so the heat shields wouldn't be needed? Has anyone modeled the idea of
unfolding some large wings to add a lot of surface area, or using
propellers to resist falling, or parachutes? Thank you.

A spacecraft enters the atmosphere at such high speed because its
orbital speed is so high to begin with. On orbit a satellite has a
certain (large) orbital speed, and if some of that speed is lost, for
instance by firing its rocket motor forward, then the shape of the
orbit changes so that it dips downward, closer to Earth. If the
satellite loses enough speed, then the path dips right down deep into
the atmosphere. This is, in fact, how a returning spacecraft is made to
lose enough of its speed to allow it to land safely, by plowing through
the atmosphere.

One may ask, "So why not just lose more speed before even entering
the atmosphere, eliminating the need for heat shielding?" First, think
of how much energy the spacecraft loses by its passage through the air.
In the case of the space shuttle it goes from the neighborhood of
17,000 mph to only a few hundred mph by aerodynamic drag alone. That's
nearly as much change in orbital speed as it aquired at launch. But
launch required an enormous quantity of propellant. In order to lose
all that speed by rocket would require the shuttle to have onboard
nearly as much propellant as it took to get it aloft to begin with.

But that original investment of propellant was just enough to get
the shuttle to orbit *without* all that extra fuel. To place it on
orbit with enough fuel to lose all or most of its orbital speed would
then require VASTLY more propellant at launch. It's quite possible it
couldn't even be gotten off the ground. The most practical & economical
method is for the shuttle to have just enough propellant available to
change its orbital path to dip down into the atmosphere, whereat it
loses most of its speed by drag.

Keeping this in mind, it's possible to see that folding wings won't
do much in the spacecraft's favor. The shuttle would still have to plow
through the air at the same speed, and in fact extra large wings might
even be a liability, since they'd increase the lift, keeping the ship
from dropping downward. All the extra wing area would do is increase
the orbiter's weight at liftoff, reducing the payload it can carry.

-Mark Martin

In article .com,
wrote:
Different re-entry profiles can help, too. The original civilian
designs for the US space shuttle used metallic heat shields. When the
USAF signed on, it had requirements for the shuttle that included more
demanding re-entries...

It wasn't quite that simple. The original ideas for metallic thermal
protection were complicated, and nobody was really all that sure that they
could be made to work. They required very thin layers of exotic metals
that were difficult to work with and tended to be brittle; it wasn't clear
that they were going to be very durable. They certainly looked like
they'd be heavy. And one big snag was that the metals required protection
against oxidation, and the coatings needed for that were poorly developed.
People had been *talking* about metallic heatshields for a long time, but
the reality wasn't so rosy.

Ceramic tiles -- lightweight, inherently oxidation-resistant, and good
insulators -- looked like a huge improvement in many ways.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article .com,
wrote:
Another idea, that I don't know enough about to speak to, is to drop
down into the atmosphere and then pitch up so that you fly out of the
atmosphere like a rock skipping on a pond. You're still on a sub
orbital trajectory though, you don't fly off into space, you come back
down into the atmosphere and repeat the process.

Skipping doesn't really help with the fundamental problem, that there
isn't *enough* aerodynamic lift available to stay up in the thin air where
deceleration is gradual. Sure, the heating is concentrated in brief
periods, but so is the lift -- there is no net gain.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |