Spacecraft Question.

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26/11/2007 at 20:46
Can some knowledgeable person(s) answer this question for me, as it is something that I've thought about whenever I've seen a space mission.
Upon re-entry, spacecraft such as the Shuttle, rely on heat shields or tiles, to protect the craft from burning up.
Now, I understand that the vast speed of re-entry causes the friction of the atmosphere on the craft to reach high temperatures.
So my question is this. Why do spacecraft have to re-enter at such high speed?
Why can't the craft be slowed down in space prior to re-entry, 'nudged' downwards gently with it's thrusters until it reaches the point where gravity takes over, and then drop naturally? As it's my understanding that any object that is just falling, such as a parachutist, prior to opening his chute, will travel downwards at approximately 125MPH. Or is that wrong?
26/11/2007 at 20:50
Wouldn't break through the atmosphere I think. Just bounce off otherwise.

26/11/2007 at 20:54
Chr!s wrote
Wouldn't break through the atmosphere I think. Just bounce off otherwise.


Thats to do with a angle of re-entry, not speed. The answer to the question is here:

google.com/answers/threadview?id=587531
26/11/2007 at 20:57
Copperhead wrote
Why do spacecraft have to re-enter at such high speed?


Because if they didn't, teflon would never have been invented and fried eggs would stick to frying pans

26/11/2007 at 20:59
To be in orbit, you have to travel at speed. Orbit is a permanent, balanced state of fall. For re-entry you slow down to the point that the forces don't balance, and gravity starts to win. At this point, you're still traveling hellish fast forwards, and you need to slow down a lot before you hit the planet. Hence the friction, heat shields, etc..

I think to be in orbit around this planet, you need something like a minimum speed of 11000km/hr (though I might have forgotten a decimal point there somewhere)

Faster, faster, faster, until the thrill of speed overcomes the fear of death...
Hunter S. Thompson

www.KebabTheCat.com
26/11/2007 at 21:00
Terror Scrotum wrote
Because if they didn't, teflon would never have been invented and fried eggs would stick to frying pans


Hah!

Like they did before teflon was invented?

Oh yus. People never fried eggs before man went to the moon.

Or made custard.
26/11/2007 at 21:02
Count Steer wrote
Hah!

Like they did before teflon was invented?

Oh yus. People never fried eggs before man went to the moon.

Or made custard.


It was a joke

26/11/2007 at 21:04
Kebab The Cat wrote
To be in orbit, you have to travel at speed. Orbit is a permanent, balanced state of fall. For re-entry you slow down to the point that the forces don't balance, and gravity starts to win. At this point, you're still traveling hellish fast forwards, and you need to slow down a lot before you hit the planet. Hence the friction, heat shields, etc..

I think to be in orbit around this planet, you need something like a minimum speed of 11000km/hr (though I might have forgotten a decimal point there somewhere)


I see. Never realised that speed was needed to remain in orbit. I always assumed it was purely a 'distance from Earth' thing only, if you see what I mean.
Ta for that KTC.
26/11/2007 at 21:05
Serious [strike]Answer[/strike] Guess Alert*

In order to get into "space" in the first place a considerable amount of speed is required to break the grasp of gravity, this speed places the craft into an un-powered orbit where the vehicle is travelling at upwards of 25000mph, and this speed is required to maintain orbit (otherwise orbit slowy decays and you are pulled back into earth).

In order to "drop" into the atmosphere, the craft would have to be slowed to geosynchronous speeds (i.e. the same speed as the rotation of the earth). Unfortunately the volume and weight of fuel required to slow the vehicle to this speed and maintain it would be collosal, and would obviously have to be taken into space in the first place.

As technology has beaten the fast re-entry issue (with the exception of the odd accident here and there), there is little point in carrying the extra fuel required.

* Do I look like a rocket scientist? No, so this answer is based on my own application of logic and therefore reserves the right to be constructed, in its entirity, with bollocks and belly button fluff
26/11/2007 at 21:05
Terror Scrotum wrote
It was a joke


I realised that but forgot to put 'Ho ho ho'.

26/11/2007 at 21:07
Copperhead wrote
I see. Never realised that speed was needed to remain in orbit. I always assumed it was purely a 'distance from Earth' thing only, if you see what I mean.
Ta for that KTC.


No worries.. Have a think about buckets attached to bits of string, being twirled about your head. There's a minimum speed needed to keep the string tight..

string=gravity, you=planet.. (in a very simple soon to be corrected by someone kind of way.)

Faster, faster, faster, until the thrill of speed overcomes the fear of death...
Hunter S. Thompson

www.KebabTheCat.com
26/11/2007 at 21:07
Doesn't the reusable spacecraft that Branson is sponsoring use a form of low speed drift reentry? It certainly doesn't need heat shielding.

Not sure how high up it is (ie if it is in orbit or not). It's probably only in the upper atmosphere or sommat.
Pax
26/11/2007 at 21:08
Pirahna wrote
Thats to do with a angle of re-entry, not speed. The answer to the question is here:

google.com/answers/threadview?id=587531


This is the right answer. You don't have to be dropping at mental fast speeds - if you had infinite fuel you could do it in a much more sedate fashion. But as Kebab says you'd have to maintain a synchronous orbit as you closed with the surface. With infinite fuel and powerful enough engines, I don't see why not.

I reckon Kyot's the man for this one.

(edited out some drivel there)

"Gentlemen! You can't fight in here! This is the War Room!" Where can I get fud?
Where can I sign up as an organ grinder?

26/11/2007 at 21:08
I'm gonna take a stab at this one..

If you watch NASA TV when there is a shuttle mission taking place, you get the map showing where the craft is in relation to the world, you can track it bezzing around the globe at a hell of a rate (someone may be along shortly to tell me how long it takes for the shuttle to complete one orbit I have a feeling it isn't much over an hour), the problem with trying to take something out of orbit is that you need to slow it down enough for it to start dropping.

For those of you who don't understand how 'orbit' works, think of firing an air pistol at 90 degrees to the ground, the pellet leaves the gun, gradually looses velocity and drops to the ground... move up to an air rifle and it fires a bit more forcefully and it drops further away from you.. now work your way up through things that fire more and more forcefully, and eventually (given a huge amount of US government hand-outs) you'll get something that never drops to the ground as it follows the curvature of the earth, thats 'orbit'..

Now when something is in 'orbit' it is hauling some serious ass (if you ever follow a re-entry you hear 5 figure mph speeds quoted even after it has started to re-enter), to make it drop towards earth again requires slowing it down, by quite a lot..

To slow it down enough to make it 'drop' as mentioned in the thread starter, would require a LOT of opposite thrust (possibly a dangerous amount in G forces, and once you've started the craft will begin to drop and the thrust engines may have less effect once you are out of the outer atmosphere), meaning you would need to burn a lot of fuel, which you'd have to carry through the launch and during the mission. Every gram of weight is crucial when it comes to launch and anything extra costs more and would lead to smaller payloads being taken up each mission, which may lead to the whole program being a waste of time.

I think that sums it up...

"It'll be alright mate"
26/11/2007 at 21:09
Mr Rooty Tooty wrote
Doesn't the reusable spacecraft that Branson is sponsoring use a form of low speed drift reentry? It certainly doesn't need heat shielding.

Not sure how high up it is (ie if it is in orbit or not). It's probably only in the upper atmosphere or sommat.


It's nice, I like it, but it get no-where close to orbit.. It just goes very very high.

Faster, faster, faster, until the thrill of speed overcomes the fear of death...
Hunter S. Thompson

www.KebabTheCat.com
26/11/2007 at 21:09
Mr Rooty Tooty wrote
Doesn't the reusable spacecraft that Branson is sponsoring use a form of low speed drift reentry? It certainly doesn't need heat shielding.

Not sure how high up it is (ie if it is in orbit or not). It's probably only in the upper atmosphere or sommat.


I might be wrong, but I believe the virgin ships are sub orbital and get no where near the speeds required to a) break the boundary of the upper atmosphere, b) require heat sheilding

However I reserve the right to be totally wrong
26/11/2007 at 21:11
{Buddha} wrote


To slow it down enough to make it 'drop' as mentioned in the thread starter, would require a LOT of opposite thrust (possibly a dangerous amount in G forces, and once you've started the craft will begin to drop and the thrust engines may have less effect once you are out of the outer atmosphere), meaning you would need to burn a lot of fuel, which you'd have to carry through the launch and during the mission. Every gram of weight is crucial when it comes to launch and anything extra costs more and would lead to smaller payloads being taken up each mission, which may lead to the whole program being a waste of time.

I think that sums it up...


Umm.. so how do they slow it down to get out of orbit?

I Think they fire thrusters at the right time and slow it down a little bit.. gravity does the rest..

Faster, faster, faster, until the thrill of speed overcomes the fear of death...
Hunter S. Thompson

www.KebabTheCat.com
26/11/2007 at 21:14
Kebab The Cat wrote
Umm.. so how do they slow it down to get out of orbit?


Wait until they hit a pre-determined angle then yank the handbrake :burnout:

26/11/2007 at 21:14
Kebab The Cat wrote
Umm.. so how do they slow it down to get out of orbit?

I Think they fire thrusters at the right time and slow it down a little bit.. gravity does the rest..


Thrusters are used to manouever the craft into a rapidly decaying orbit, the further it decays the greater the grasp of gravity. However the atmosphere itself (via the friction it creates) does the vast majority of the slowing down.

The heat generated on the face of the sheild it massive, IIRC 3000 degress celcius+, so hot that plasma is created!
26/11/2007 at 21:16
WiseBuddha wrote
I might be wrong, but I believe the virgin ships are sub orbital and get no where near the speeds required to a) break the boundary of the upper atmosphere, b) require heat sheilding

However I reserve the right to be totally wrong


So Beardy Branstons space ships won't actually go into space?

Strangely, this doesn't surprise me.
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