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

As most respondents have pointed out, something in orbit has a huge
amount of kinetic energy that has to go *somewhere*. Either you spend
that much energy slowing down with rockets, or you dump it into the
atmosphere as heat.

There is another possibility that hasn't been mentioned, though: you
could transfer that energy to something else in orbit. I'm thinking of
a momentum-exchange tether (http://www.tethers.com/MXTethers.html),
probably of the rotating variety. Here's how it would work:

Space travellers in orbit zip around at, say, 17000 kph. Also in orbit
is a large mass connected to a very long, strong tether, rotating
something like a wheel as it orbits, so that the high end (away from the
Earth) is moving much faster relative to ground than the low end (closer
to the Earth). At the high end, the tip of the tether, is travelling at
17000 kph, but at the low end its ground-relative velocity is only (say)
10000 kph.

So, our space travellers wait for the tether to be in the right position
at the high end, when it's travelling at the same speed they are, and
then hook their craft to it. It swings them down, and they unhook at
the low end. Presto, they're now travelling at only 10000 kph -- which
is less than orbital velocity at that altitude, so they're going down,
but they're doing it much more gently. Even larger or stronger (more
rapidly rotating) versions of the tether could of course have even more
benefit -- even dropping the ship down stationary with respect to the
Earth.

Where does all that kinetic energy go? Into the tether system, of
course. It rises up to a slightly higher orbit. How much its orbit
changes depends on the mass of the tether system compared to the ship;
ideally it would mass a lot more, so its orbit wouldn't change much.

But here's the really cool part: the tether system acts as a "momentum
bank". That energy imparted to it from the ship can be used again to
haul the next ship up to orbit. On launch, the ship only has to attain
the speed of the slow end of the tether, i.e. 10000 kph in my example
above. Then the tether imparts the rest of the energy needed to fling
it up into orbit. Its own orbit is reduced as a result, of course, but
it gets some of that back when it drops the ship back down. What energy
is lost to atmospheric drag, mass ejected from the ship, etc. can be
made up for in more leisurely (and efficient) ways, such as
electrodynamic propulsion.

So, a rotating tether helps with the two biggest problems we have today:
getting to orbit, and getting back down. A spacecraft that by itself is
only capable of suborbital launch and reentry, can nonetheless reach and
leave orbit safely with the help of the tether system, and we're no
longer wasting huge amounts of energy both ways -- much of it is simply
being banked and reused instead.

Best,
- Joe

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Nobody has addressed what seems to me an obvious solution:

- While orbiting at 17000mph or so to dip down into the upper atmosphere -
the very edge -then deploy a large parachute similar to the modern sport
chutes that forms an airfoil.

These things generate lift, just like any other airfoil. It seems to me that
an inclined plane should still generate lift, even when the medium is a very
thin gas - or even independant molecules.

Then just skip along in the upper atmosphere for a long time (maybe as much
as a full "orbit" changing angle of attack - slowly slowing and dropping
into thicker air enough to balance temperatures and lift. If things get too
hot, set the chute to a low drag configuration, and use the lift to lift up
for a bit, then drop back down when you can.

You still have a lot of kinetic energy to dissipate, but the grossly
increased time will allow it to be lost by several mechanisms to the
atmosphere and to radiation.

This may not be a solution for the huge mass of the shuttle, but a future
"X-Prize" orbiter that weighed only a few thousand pounds could use it.

In article ,
Ron Webb wrote:
- While orbiting at 17000mph or so to dip down into the upper atmosphere -
the very edge -then deploy a large parachute similar to the modern sport
chutes that forms an airfoil.

It's been proposed, actually.

These things generate lift, just like any other airfoil. It seems to me that
an inclined plane should still generate lift, even when the medium is a very
thin gas - or even independant molecules.

Correct. The rules are somewhat different up in the region of molecular
flow -- where the molecules are indeed pretty much independent -- and at
hypersonic speeds, but lift is still available... at the usual price of
drag. (See below.)

Then just skip along in the upper atmosphere for a long time (maybe as much
as a full "orbit" changing angle of attack - slowly slowing and dropping
into thicker air enough to balance temperatures and lift.

Alas, here you propose a numerical impossibility. *It can't be done.*

When you buy a certain amount of lift, you pay with a certain amount of
drag. And the L/D ratio of reasonable shapes is not that good at
hypersonic speeds in molecular flow. If you're getting enough lift to
hold you up, you are *not* decelerating very slowly and gradually. Oh,
initially, yes, because at just below orbital speed you don't need much
lift... but the situation doesn't stay that good for very long. At half
orbital speed, "centrifugal lift" is only 1/4 as strong, and aerodynamic
lift must do most of the work, and that means you're getting a *lot* of
drag and slowing down rapidly.

In fact, when you study the details, it turns out that the large surface
area of something like a parafoil doesn't really make any difference to
how *quick* reentry is. That is determined almost solely by the L/D ratio
of the shape, and there are real limits to how good that can be.

A large surface area does buy you something: you decelerate earlier, in
thinner air, and the heat is spread out over a larger area. This lowers
temperatures and makes materials problems much easier. But things still
happen just about as quickly.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

"Henry Spencer" wrote in message
...
In article ,
Ron Webb wrote:
- While orbiting at 17000mph or so to dip down into the upper atmosphere -
the very edge -then deploy a large parachute similar to the modern sport
chutes that forms an airfoil.

It's been proposed, actually.


I assume anything practical has been studied on a simulator, as well as
mathematically, in aerospace engineering classes all over the world as well
as at places like NASA.

That's what's cool about newsgroups like this one. We non-aerospace
engineers get to ask the questions, instead of retaining our misconceptions,
or having to do the calculations ourselves. But who knows, once in a while
the kernel of a new idea may pop out!

Thanks for the reply!




Alas, here you propose a numerical impossibility. *It can't be done.*

snip

In fact, when you study the details, it turns out that the large surface
area of something like a parafoil doesn't really make any difference to
how *quick* reentry is. That is determined almost solely by the L/D ratio
of the shape, and there are real limits to how good that can be.

A large surface area does buy you something: you decelerate earlier, in
thinner air, and the heat is spread out over a larger area. This lowers
temperatures and makes materials problems much easier. But things still
happen just about as quickly.

OK- it seems half my idea is practical (large surface area spreading the
waste energy over a large area, thus making the thermal stress on any given
part a lot less) and half is not very useful (can't slow the re-entry down
much using aerodynamic lift).

How about slowing the re-entry using active thrusters? It would take a lot
less thrust to keep the craft up in the thin air for an extra half hour -
while friction slows us down at reduced temperatures - than it would to try
to actively decelerate using thrusters.

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

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 |

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.

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 |