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Brilliant idea for re-entry

Posted by: SuperShuki - Fri Aug 26, 2005 8:50 am
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Brilliant idea for re-entry 
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Post Brilliant idea for re-entry   Posted on: Fri Aug 26, 2005 8:50 am
Instead of taking a few hours to re-enter, why not take a few days, or weeks? Also, Use a huge parachute in the upper atmosphere. You can use a low angle of approach that way.

Anybody?

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Post    Posted on: Sun Aug 28, 2005 4:00 pm
When you say "huge" with regards to your parachute, you have no idea just how big it would have to be. Try something the size of a small continent, and it just might work -- of course, the weight of the thing would keep you from ever getting off the ground in the first place. Traditional parachutes just don't work that well.

Taking days or weeks to reenter the atmosphere is something that groups like JP Aerospace are doing great on, and you'll want to check out their section of this forum for more information. Just remember, a traditional vehicle will always reenter quick and hot -- it's highly impractical to make them reenter "slowly" (via parachutes or ballutes or any other such device), because of the weight they'd have to drag along.

Note that I won't say that there's no solution to this problem of slowing a "traditional" re-entry vehicle, just no practically implementable one that's been devised yet....

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Post    Posted on: Sun Aug 28, 2005 8:39 pm
spacecowboy wrote:
When you say "huge" with regards to your parachute, you have no idea just how big it would have to be. Try something the size of a small continent, and it just might work -- of course, the weight of the thing would keep you from ever getting off the ground in the first place. Traditional parachutes just don't work that well.

Taking days or weeks to reenter the atmosphere is something that groups like JP Aerospace are doing great on, and you'll want to check out their section of this forum for more information. Just remember, a traditional vehicle will always reenter quick and hot -- it's highly impractical to make them reenter "slowly" (via parachutes or ballutes or any other such device), because of the weight they'd have to drag along.

Note that I won't say that there's no solution to this problem of slowing a "traditional" re-entry vehicle, just no practically implementable one that's been devised yet....


Have you done the math? Even objects in LEO eventually degrade their orbits. The 'degredation' only takes a few years!
Also, if the vehicle can be slowed down fast enough without smearing the passengers, it won't fall very far before its speed is low enough to use traditional wings.
I always thought that there was something weird about JP Aerospace. Balloons filled with a gas can't go out of the atmosphere! Anyway, maybe you can enlighten me as to how JP's concepts apply to this idea.

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Post Parachute   Posted on: Sun Aug 28, 2005 9:44 pm
To make clear, I am talking about entering the upper atmosphere (defined by whatever altitude works) before the parachute comes into use, but using the parachute before the air is too thick.
Also, the thinner the air, the thinner the parachute. If solar sails can work, why not air sails?
By the time the air gets thick, hopefully the vehicle will be going slow enough so that heating won't be as much of a problem.

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Post JP   Posted on: Sun Aug 28, 2005 10:07 pm
I looked at their website, I understand JPs concept. In any case, the idea of a parachute applies to a vehicle that is not able to stay stationary by itself at a non orbital velocity.

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Post Re: Parachute   Posted on: Sun Aug 28, 2005 10:24 pm
SuperShuki wrote:
To make clear, I am talking about entering the upper atmosphere (defined by whatever altitude works) before the parachute comes into use, but using the parachute before the air is too thick.
Also, the thinner the air, the thinner the parachute. If solar sails can work, why not air sails?
By the time the air gets thick, hopefully the vehicle will be going slow enough so that heating won't be as much of a problem.


One problem is you can still only shed a relatively small amount of energy before you will start to enter the more dense stuff so the reduction in heat shield needed might be rather small. It will of course decrease the time required for something to deorbit by some fraction. There may be a slightly different technique possible to lower orbits quickly without using much fuel by sending a kite into the upper atmosphere. Not sure if that's ever been attempted.

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Post    Posted on: Mon Aug 29, 2005 1:15 am
I like the kite idea -- that might be extremely useful for an emergency escape vehicle. As a matter of fact, the more I think about it, the more I like it. Score one for you!

Again, Shuki, remember that re-entry takes a very very short amount of time: once you lose the stable system of orbit where centripedal force and gravitational force balance (not perfectly; all orbits degrade over time), gravity gets really powerful really quick. Entropy takes over, and the object attempts to find a stable state -- aka on the ground. Kinetic energy (motion) and potential energy (height) is exchanged for thermal energy (Entropy in its truest form), and the object quickly finds its new home of stability at the bottom of the Pacific or in a small crater in a Nebraska wheatfield. If the object happens to have some sort of wings (which the Shuttle most certainly does NOT, those things sticking out of the sides are stabilizers that provide negligible lift: the things got all the lift of a brick at pretty much any speed below Mach 6 or so), then it can find another semi-stable state in gliding, where potential energy (height) is exchanged for kinetic energy (forward motion). However, gliding always goes down, so that doesn't last very long. And you need big wings and very little weight, which is almost impossible to put on a vehicle that just had to survive the beating of slamming into the atmosphere at Mach 18.

There's really no passive way to slow yourself down in the upper atmosphere, because the upper atmosphere itself is practically non-existant. You just keep falling faster and faster, without being able to do anything about it, until you drop into the atmosphere (at those speeds, the effect is probably somewhat akin to ditching a small aircraft on water), and then you start to slow down again. You have to go at insane speeds (over 17,000 miles per hour) to keep a stable orbit , and then your speed increases as you drop.

The only way I see to slow yourself down is actively, such as firing your main engines directly into your flight path, and practically "standing" on your engine thrust until you reach enough of an atmosphere to use parachutes or wings. The two problems with this is that A) you need to keep those engines pointed in the EXACT right direction, or you either fly off into deep space or slide sideways into the atmosphere at a still-much-too-high velocity; and B) you will use an INSANE amount of fuel doing this. You're probably looking at a fusion drive to make this feasible.

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Post kite   Posted on: Mon Aug 29, 2005 6:03 am
Dunno exactly what you mean by kite. Do you mean dropping a line down to a lower altitude, and releasing a parachute or slowing down device at that altitude?

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Post    Posted on: Mon Aug 29, 2005 7:09 am
spacecowboy wrote:
You just keep falling faster and faster, without being able to do anything about it, until you drop into the atmosphere (at those speeds, the effect is probably somewhat akin to ditching a small aircraft on water.
.


The case of the airplane hitting the water is the extreme situation: absolutely no intermediate leveling off between the low density atmosphere (the air) and the high density atmosphere (the water). This is opposed to the situation of a gradual atmospheric dedensitification (what a word :D). I have no idea where the atmospherics of earth lie in between these two.

The original idea was about slowing a mars orbiter into a lower altitude, where it could glide over mars and take pictures (and of course do science stuff). This is better than a sattelite that has a fixed orbit., and can't go lower to take higher resolution pictures.

In any case, the situation of a plane hitting the water is the optimal one for this parachute idea. What does a seaplane do? It uses the friction of the bottom of its fusalage to skim the water and slow it down. That's the principle. What's the problem with spacecraft? if they skim the atmosphere, they will skip out of orbit into never never land. This problem would be eliminated if the spacecraft skipped at a low angle of attack- but in the example of the space shuttle, the surface area might not be enough to slow it down- kinda like a speedboat not slowing down on the waves.

So in comes my idea- lower a parachute into the water and use the parachute to skim. It has alot of surface area, and it only has to be in the atmosphere for a liittle bit. Then it burns up, because you've reached a lower atmosphere, which it isn't strong enough for. So the you open another, stronger, smaller, parachute, built for that altitude. Just like normal parachutes have a small parachute to pull them out, so will this too.


Ahah! The perfect example. An arresting wire on an aircraft carrier. The airplane, going at almost unlimitless speed, uses its hook (parachute) to consort with the arresting wire (lower atmosphere) and bring it to a complete halt in a matter of seconds.

In any case, I thank you for for pondering my introspections with your glorious minds .

:P [/code]

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Post    Posted on: Mon Aug 29, 2005 1:58 pm
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So in comes my idea- lower a parachute into the water and use the parachute to skim. It has alot of surface area, and it only has to be in the atmosphere for a liittle bit. Then it burns up, because you've reached a lower atmosphere, which it isn't strong enough for. So the you open another, stronger, smaller, parachute, built for that altitude. Just like normal parachutes have a small parachute to pull them out, so will this too.
A parachute doesn’t generate lift, only drag. What keeps the space craft up during the time that it’s speed is slower than orbital but still faster than you want to enter the lower atmosphere? It would seem to me that instead of a parachute you would need giant wings that could generate lift in the ionosphere without producing too much drag. The wings would have to be extremely large and light weight. Maybe a parasail design made from that new carbon nanotube sheet.


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Post    Posted on: Mon Aug 29, 2005 4:02 pm
What about using Ball'S ballutes? I saw a graphical illustration - it looked as if the ballute will be huge compared to any vehicle and to ecah parachute used up to now.

And what about installling the ballute permanently in orbit? It simply is designed to drop a payload down to the surface.



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Post    Posted on: Mon Aug 29, 2005 7:44 pm
Ballute's don't do any good in orbit, only during the re-entry phase. They're neat, alright, but they're still pretty massive compared to parachutes.

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Post Re: kite   Posted on: Tue Aug 30, 2005 8:48 am
SuperShuki wrote:
Dunno exactly what you mean by kite. Do you mean dropping a line down to a lower altitude, and releasing a parachute or slowing down device at that altitude?


Dropping a line down to a lower altitude.. You have small rocket propel the 'kite' down into the upper atmosphere. The kite is flying upsideown so it keeps the line which holds it taut. Basically you're just reaching down to get a lot of drag, the kite and most of the line would probably be made from ceramic fibre and would get pretty hot though you would still be deorbiting over probably a few days. As the crafts orbit becomes lower the kite gets reeled in to prevent it generating too much force and either destroying itself or breaking its line.

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Post    Posted on: Tue Aug 30, 2005 8:48 am
Hello, spacecowboy,

concretely I had in mind to make them go back into orbit after a vehicle they are connected to reentered safely. They could be undocked to return to space and wait for the next vehicle to reenter. Would that work?

Their walls are 7 micrometers thick if I remember correct Ball's informations but I don't know their complete dimensions.



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Post Re: kite   Posted on: Tue Aug 30, 2005 2:20 pm
nihiladrem wrote:
As the crafts orbit becomes lower the kite gets reeled in to prevent it generating too much force and either destroying itself or breaking its line.
What keeps the craft from dropping below the kite?


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