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Author Topic: Why Does Re-Entry Have To Be Fast ?  (Read 9272 times)

Offline neilep

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Why Does Re-Entry Have To Be Fast ?
« on: 05/08/2005 04:04:11 »
Well, I sincerely hope the Shuttle Discovery is ok !....

I know the atmosphere can be rather windy at high altitudes, but why can't the shuttle (or any other welcome guest from far away)just come to a stop above the earth and just drop down slowly ?...you understand why I'm asking don't you ?

I'm so missing the obvious here aren't I ?

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Offline ukmicky

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #1 on: 05/08/2005 04:38:04 »
the shuttle is moving too fast and it would take a large amount of fuel to slow down the shuttle before re-entry.
 

Offline anthony

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #2 on: 05/08/2005 08:04:27 »
The shuttle is travelling very fast in orbit, it is this speed that keeps it remaining in orbit. Remember the thought experiment of firing a canon ball faster and faster off the side of a tall mountain... The shuttle must slow down to come out of orbit and land, the first stage is with rocket boosters in space, then it looses speed in the atmosphere this by "buring off" it's kinetic energy with friction. A steeper descent means burning off that speed quicker and heating up faster, too shallow a descent and the shuttle may bounce off the atmosphere, an old chestnut in SciFi serials. I expect the typical shuttle flight path is about as shallow as they can reliably make it. Discovery has been orbiting at 363km (225 miles), anyone care to calculate how fast it must have been travelling?
 

Offline neilep

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #3 on: 05/08/2005 13:03:36 »
...thanks guys, of course !!..it's obvious now...and I was missing it !!

.....But maybe in the future when we've sorted all that braking-against-fuel-effficiency stuff out, will,we then be able to come to a halt and just drop in.......thanks again.

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Offline simeonie

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #4 on: 05/08/2005 13:17:22 »
Tell me if I am wrong but if it tried to come down to earth really slowly wouldn't it just start orbitting it?

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Offline chimera

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #5 on: 05/08/2005 13:21:17 »
No Neil (and Simeonie, I see) , I'm afraid without antigravity that will never happen.

If you're in space everything 'just hangs there' because of no gravity, but  near Earth, everything tends to fall towards it, faster and faster. Meaning you burn up if you don't keep your distance. Now the cheapest way of doing that, is by falling towards Earth and just miss it by the narrowest margin.

This is what we call an 'orbit'. Circling around is actually falling towards. Now to land on Earth again, you'll find that coming to a full stop will make you fall towards Earth again, so just lowering your speed will suffice. Finding the best average between falling around and falling inwards will get you where you want and is called 'landing'.

Sounds like a much more passive ride than 'normal' spaceflight, but we could not afford to do it otherwise. It takes a HUGE amount of juice to fly those machines, and they're not luxury convenience vehicles that can drop you off at any given location at whim.

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« Last Edit: 05/08/2005 13:25:25 by chimera »
 

Offline gsmollin

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #6 on: 05/08/2005 17:32:20 »
quote:
Originally posted by neilep

...thanks guys, of course !!..it's obvious now...and I was missing it !!

.....But maybe in the future when we've sorted all that braking-against-fuel-effficiency stuff out, will,we then be able to come to a halt and just drop in.......thanks again.

Men are the same as women.... just inside out !!



Right you are, neil. Remember the Startrek episode, where they ran out of fuel in the shuttle craft, and were going to make a re-entry without fuel? The shuttle was burning up, but Sulu found them in time and Scotty transported them safely to the Enterprise. Sadly, we couldn't do the same for the crew of the Columbia.

To hang some ball-park numbers on the re-entry: Typical orbital speed is 25,000 fps (7,620 m/s) at an altutude of around 800,000 feet (241,000 m). The fuel of the SRBs and the external tank was expended to reach that orbit. Now a change of only 200 fps (61 m/s) will lower the perigee of the orbit to about 400,000 feet (121,000 m). Orbital speed is actually slightly higher at this point, and the apogee remains where it was. However, the shuttle encounters atmosphere at the new perigee, and begins to slow. Re-entry ensues.

The re-entry burn is nominally performed by OMS, but if that system is inoperative, the forward or sternward RCS can each perform the re-entry burn. Triple redundancy insures the crew would never be stranded in orbit.

However, I still don't understand why there is no backup heatshield for the carbon-carbon wing leading edge. It would have saved Columbia, and the three remaining shuttles as well.
 

Offline Tronix

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #7 on: 08/08/2005 15:16:48 »
maybe weight is why. im no physcisit, but i know the heavy something is, the more fuel it takes to lift up and the more energy it has when it goes a certain speed. IT would therefore probably take longer going up and coming back down (safely at least). but then again, its probably not a significant differance, except in the pocketbook, as i imagine material that can withstand those speeds and heat are not cheap. but then again, i know little of these things.

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Offline David Sparkman

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #8 on: 09/08/2005 04:53:36 »
The fundimental answer is that the distance the shuttle is from the earth balances out the centripital forces with the gravitational forces. As you go faster, you go higher, as you go slower you go lower. Point to point travel especally when you are fighting a gravity well like the earth is extremely expensive in terms of fuel and just can't be done with what we have today. Perhaps someone can find the approperate equation.

There is an interesting way to get to the moon using chaos theory that is more cost effective than what we have done before. But it would only be so for cargo as it takes about 8 weeks instead of a few days, and the  fuel you would save would be lost to the supplies you would need for a manned flight.

The idea is to first get above the atmosphere, then slowly spirial up to the gravitational boundry between the earth, moon and sun. Push just over the boundry on the moon side, and then slowly spirial down into the moon's gravitational well.

Btw. We have a very interesting relationship with the moon. It is a sister planet and not a captured moon. The attractive forces between the earth and the moon are less than the forces between the moon and the sun. We are both orbiting each other around a point outside of the earth. Kind of doing a little dance or jig around each other. Of course no other plant has a moon so massive compaired to the planet 1/6th. And, the most commonly held theory says that the moom was once much closer to us and has gradually distanced itself from us(gained energy). That would suggest that it once was a true moon.

Of course I don't entirely buy that idea as the energy to lift the moon further away is enormous... But people like theories with holes...

David
 

Offline gsmollin

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #9 on: 09/08/2005 17:21:32 »
The moon and earth both orbit each other around their joint center of mass. This is the same for any other orbit, anywhere. The moon and earth are thought to have been formed from the same material, but separated from each other by a cosmic collision after formation. I suppose you could call that a sister planet.

The changing orbit is a result of the ocean tides. These cause the moon to orbit around a point slightly behind the earth's center of mass, and angular momentum is not conserved. The angular momentum of the earth is transfered to the moon, and its orbit rises. Earth's rotation is slowing. Interestingly, this process is very strong now, and apparently not so strong in the past, else the moon would be in a higher orbit. Apparently, the present continent arrangement gives a high tidal force, and the land mass was in supercontinent form for long periods in the past, which had low tidal force.
« Last Edit: 09/08/2005 17:22:33 by gsmollin »
 

Offline David Sparkman

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #10 on: 09/08/2005 22:12:44 »
It would also be reasonable to suppose that the moon had a rotational velocity at an earlier time. The energy lost as the moon's rotation became locked with the earth could have also added to the increased distance between the two bodies.(such thoughts "occur twixt sleep and awakening")

David
 

Offline Razor

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #11 on: 09/08/2005 22:47:20 »
Im more concerned about the fuel it burns to go into orbit!
Im sure there's a more cost effective and enviromentally friendly alternative!

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Offline simeonie

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #12 on: 10/08/2005 11:56:59 »
Yes there is. Get Superman to fly the ship up there like in the film! DUH!!!!

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Offline Razor

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #13 on: 10/08/2005 14:11:42 »
Uhh....*rolls eyes*....yeah!,sure.....

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Offline Allison

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #14 on: 11/08/2005 04:00:10 »
The fuel used to power the space shuttle is super-cooled liquid hydrogen, rather than some kind of petroleum-based fuel.  The hydrogen is mixed with liquid oxygen inside the engines (the reaction provides the energy required to blast off), and the clouds of "smoke" that appear when the shuttle blasts of is actually water vapor--incredibly dramatic water vapor, but much less harmful than exhaust from a tail pipe.
 

Offline Razor

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #15 on: 11/08/2005 12:50:25 »
Really? Nice!

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Offline gsmollin

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #16 on: 12/08/2005 15:45:51 »
quote:
Originally posted by Allison

The fuel used to power the space shuttle is super-cooled liquid hydrogen, rather than some kind of petroleum-based fuel.  The hydrogen is mixed with liquid oxygen inside the engines (the reaction provides the energy required to blast off), and the clouds of "smoke" that appear when the shuttle blasts of is actually water vapor--incredibly dramatic water vapor, but much less harmful than exhaust from a tail pipe.



Unfortunately, this is only half the story. The three main engines are hydrogen-oxygen powered. The two boosters, that provide ~80% of the lift-off thrust utilize solid fuel. The oxidizer in the Shuttle solids is ammonium perchlorate, which forms 69.93 percent of the mixture. The fuel is a form of powdered aluminum (16 percent), with an iron oxidizer powder (0.07) as a catalyst. The binder that holds the mixture together is polybutadiene acrylic acid acrylonitrile (12.04 percent). In addition, the mixture contains an epoxy-curing agent (1.96 percent). The binder and epoxy also burn as fuel, adding thrust.

As you can see, there are many pollutants available from the SRBs. Once again, the decision to use SRBs was economic. Liquid propulsion was always first choice.

However, Boeing's new Delta IV rocket has an all cryogenic hydrogen-oxygen propulsion system.

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Offline Allison

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #17 on: 13/08/2005 04:42:00 »
quote:
However, Boeing's new Delta IV rocket has an all cryogenic hydrogen-oxygen propulsion system.

Well, I had the right answer, but for the wrong question!

Do you know if anyone has ever attempted to measure or even calculate pollutant levels from a single shuttle launch?
 

Offline gsmollin

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #18 on: 13/08/2005 18:47:26 »
The solid fuel weighs about 1000 tons in both SRBs.

"F = ma, E = mc^2, and you can't push a string."
 

Offline lui

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #19 on: 16/08/2005 14:08:56 »
OK - so back to the original question about re-entry.

The shuttle has wings right?
So why can't it fly all the way down rather than bash through the atmosphere and risk burning up.
As you begin to encounter the (very thin)atmosphere at orbital speed, can't the wings generate enough lift so that you can stay up longer and wash the speed off more slowly? Maybe the relationship between lift and friction is very different at orbital speeds and in such thin air.
 

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Re: Why Does Re-Entry Have To Be Fast ?
« Reply #19 on: 16/08/2005 14:08:56 »

 

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