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Reentry at high temperature



 
 
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  #11  
Old July 27th 05, 03:45 PM
Joe Strout
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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

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
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  #12  
Old July 27th 05, 08:03 PM
Ron Webb
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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.



  #13  
Old July 29th 05, 05:36 PM
Anni
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OK if the shuttle is going the same orbital velocity required to get at into
orbital velocity.
Then cannot it be slowed down while in orbit where you would not need a
massive amount of energy to slow it down from 22700mph if done over a few
days which I take it is its approximate speed while in orbit
Could this work? And if it was slowed down could they not use parachutes to
keep it from reentry problems

wrote in message
oups.com...
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.



  #14  
Old July 29th 05, 11:32 PM
Icarus
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wrote:
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.


How much could the Shuttle decelerate with its own engines, given an
unlimited supply of fuel, before it encountered significant
atmospheric drag? I was thinking that if they built a (relatively)
cheap and cheerful rocket for the sole purpose of getting rocket fuel
into orbit, the Shuttle could use that for its descent, and the same
orbiting refueling stations could also be used for spacecraft leaving
Earth orbit, making (say) missions to other planets cheaper and
quicker.

Of course, as far as the Shuttle is concerned, the easiest solution
would be just to make its heat shield 100% reliable...


  #15  
Old July 30th 05, 04:37 AM
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Icarus wrote:

How much could the Shuttle decelerate with its own engines, given an
unlimited supply of fuel, before it encountered significant
atmospheric drag? I was thinking that if they built a (relatively)
cheap and cheerful rocket for the sole purpose of getting rocket fuel
into orbit, the Shuttle could use that for its descent, and the same
orbiting refueling stations could also be used for spacecraft leaving
Earth orbit, making (say) missions to other planets cheaper and
quicker.


Let's look at it this way. Suppose that the space shuttle could just
"magically" lose all of its orbital speed in an instant. The pilot
throws a switch and, presto, the spacecraft no longer has any
horizontal speed. What happens next? What does happen is that the only
component of motion remaining is the ship's downward acceleration
towards Earth; It falls like a rock. That's no good, because this alone
will also subject the ship to a significant air speed with high
temperature. (Not as high as with the re-entry trajectories that are
actually used. But high enough to be a problem still.)

So you see it's not strictly a problem of having enough fuel to slow
down. Slowing too rapidly can actually be counterproductive. The
spacecraft needs a way to travel slowly both horizontally and
vertically. It might be conceivable at least hypothetically to deliver
a large tank of propellant to the space shuttle, which could then
retrofire its motors to slow it waaayy down horozontally. But to keep
it from falling too quickly vertically would require a high *upward*
thrust for the entire trip down. There's just no way to do that. The
mass ratio required would be impossibly high.

Of course, as far as the Shuttle is concerned, the easiest solution
would be just to make its heat shield 100% reliable...


Probably the best thing would be to abandon the whole space shuttle
paradigm of carrying both cargo & people all in one package. It'd be
much safer, much more reliable, to send them up separately on
conventional rockets. The Russians have been doing it this way for
decades, and it works very well, and more cheaply. They built an entire
space station (in fact, several space stations) by boosting the
components and then rendesvousing crews with them later, in the Soyuz.
The Soyuz is a very highly optimised configuration for a disposable
orbital ferry. It's designed so that the ablative heat shield is a
minimal size, to keep its mass down. It works nicely. I think that in
45 years there's never been a genuine failure of an ablative heat
shield, either Russian or American.

-Mark Martin

  #16  
Old July 30th 05, 05:44 AM
Henry Spencer
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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 |
  #17  
Old July 30th 05, 06:18 AM
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Anni wrote:
Then cannot it be slowed down while in orbit where you would not need a
massive amount of energy to slow it down from 22700mph if done over a few
days which I take it is its approximate speed while in orbit
Could this work?


No, it won't work, because "orbital velocity" is the velocity you need
to stay in orbit. If you slow down, your orbital altitude drops. For a
vehicle like the shuttle, that means the vehicle ends up in the
atmosphere.

And you still would need a lot of fuel to slow down a significant
degree.

Henry Spencer wrote:
It wasn't quite that simple. The original ideas for metallic
thermal protection were complicated,


Hey, hey. No fair blocking my rosy-tinted 20/20 hindsight.

They required very thin layers of exotic metals
that were difficult to work with and tended to be brittle;


Where's the requirement for "very thin" layers come from? Just weight
requirements alone?

Mike Miller, Materials Engineer

  #18  
Old July 30th 05, 11:22 AM
Ian Stirling
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Henry Spencer wrote:
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


To elaborate - aerodynamic lift doesn't work nearly as well at orbital
speeds. Gliders can get lift/drag of 40. A reentry L/D of 5 is astoundingly
good.

And as lift on average has to be the same as the weight of the vehicle,
then drag (which creates heat) has to be 1/5 of this value on average.
This doesn't change the total heating if you go deep enough to accellerate
upwards at 2G, so you can skip.
deceleration is gradual. Sure, the heating is concentrated in brief
periods, but so is the lift -- there is no net gain.


There may be gain though.
Some proposals have been to skip, and dissipate the heat over the whole
vehicle while it's back in space.
However, nobody has come up with numbers saying that this is astoundingly
good.

It turns out that shedding heat from a heatshield that's glowing white-hot
is really efficient, and other schemes tend to get a bit messy as they
have to absorb and store or radiate at low temperatures the heat.
  #19  
Old July 30th 05, 08:50 PM
chosp
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"Anni" wrote in message
...
OK if the shuttle is going the same orbital velocity required to get at
into
orbital velocity.
Then cannot it be slowed down while in orbit where you would not need a
massive amount of energy to slow it down from 22700mph if done over a few
days which I take it is its approximate speed while in orbit
Could this work?


No. It would take far too much fuel. Far more than could be carried
aboard the shuttle. The shuttle may appear weightless because it
is in freefall, but it still retains its 100 tons of mass. Momentum
is mass times velocity and 100 tons of mass times 17,500 mph
(or thereabouts) is a hell of a lot of momentum and would use
an entire external tank's worth of fuel to counteract.

And if it was slowed down could they not use parachutes to
keep it from reentry problems?


The problem here is that the parachutes would be useless until the shuttle
was deep into the atmosphere. If the shuttle slowed down sufficiently it
still drop like a rock and continue accelerating until it entered the
atmosphere
at excessively high speeds - far too high to deploy a parachute. Besides,
try, for a moment, to imagine the stresses on parachutes trying to slow
down a supersonic 100 ton space shuttle. Imagine the size and weight of
the parachutes involved.


  #20  
Old August 1st 05, 02:23 PM
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Anni wrote:
OK if the shuttle is going the same orbital velocity required to get at into
orbital velocity.
Then cannot it be slowed down while in orbit where you would not need a
massive amount of energy to slow it down from 22700mph if done over a few
days which I take it is its approximate speed while in orbit
Could this work? And if it was slowed down could they not use parachutes to
keep it from reentry problems


I'm having trouble reading that, but I think this is the core of your
question:

"while in orbit can it be slowed down over a few days?"

The answer is no. This is one of those things that's very easy to
explain with a diagram, but very difficult to explain with just words.

Imagine a nice circular orbit. If you turn your spaceship so that it
faces in the direction you're going, and you fire your engines, that's
called a prograde burn. Prograde means, "in the direction you're
going." If you turn your spaceship so that it faces opposite the
direction you're going, and you fire your engines, that's called a
retrograde burn.

Once again, imagine that nice circular orbit. Pick a point on the
circle. That's where your spaceship is. If you make prograde burn at
that spot, do you know what happens to the orbit? Nothing at all
happens at the spot where your spaceship is. Instead, the part of the
orbit on the opposite side of the planet moves outward. If you make a
retrograde burn, nothing at all happens to the spot in the orbit where
you made that burn. Instead, the part of the orbit on the opposite
side of the planet moves inward.

This is rule number one of orbital maneuvering. When you make a
prograde or a retrograde burn, nothing at all happens to your orbit at
the spot where you made the burn. Instead, the other side of your
orbit moves in or out.

And that gets us to the reason why you can't slow down gradually over
several days. Once you slow down enough that the opposite side of your
orbit touches the atmosphere, you *will* hit the atmosphere in exactly
one half of one orbit. At the altitude that the shuttle orbits, it
only takes 90 minutes for a complete orbit, so once it slows enough to
touch the atmosphere, it is coming home in 90/2 minutes. End of story.
It doesn't have several days left to slow down gradually. Once it
touches the atmosphere that's it.

If you had a lot of thrust, you could make a very powerful retrograde
burn and lower your speed as much as you want, all the way down to zero
if you wanted to, but you have to do all of that in one half of one
orbit.

Like I said, this is very easy to understand if someone can draw it on
a chalkboard.

 




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