Spacecraft Question.

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Spacecraft Question.

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?

Wouldn't break through the atmosphere I think. Just bounce off otherwise.

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

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

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)

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.

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

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.

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

Terror Scrotum wrote

It was a joke

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

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.)

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.

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)

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...

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.

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 sheildingHowever I reserve the right to be totally wrong

{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..

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:

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!

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 sheildingHowever 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.

WiseBuddha wrote

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

As I wrote pretty much the same thing I'm going to say this is all right .However, the first sentence is wrong. To get into free fall you don't have to break gravity, in fact at that level (LEO - Low Earth Orbit), gravity isn't much different than it is down here. You just need enough altitude to be where the atmosphere is thin enough it doesn't give you any friction, so free fall is attained.I'm not a rocket scientist either, but I'm right on this one at least .

WiseBuddha wrote

vehicle is travelling at upwards of 25000mph,

Pah, my zx10 is quicker

all this assumes that we have been in space,didnt you know its all just a nasa con, no stars see

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)

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 sheildingHowever I reserve the right to be totally wrong

I recon both those are goodn's... ...although "boundary of the upper atmosphere" isn't all that meaningful... the atmosphere just gets thinner and thinner...You don't, of course, have to go into orbit if you can reach escape velocity... (have enough energy to escape the earths gravitational pull)

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..

Yes they do, I was referring to the thread starter who was enquiring about whether you could 'drop' back into the atmosphere, which would require a insane amount of slowing down, hence my reply..The question was whether you could get gravity to bring the craft back to earth more sedately, which is most unlikely.

The tiles of the heatshield aren't teflon are they ?I thought it was some kind of advanced silicone material

The shuttle orbits the earth at 17,500 miles per hour. Same as the Space Station.

londongal76 wrote

The shuttle orbits the earth at 17,500 miles per hour. Same as the Space Station.

How does it catch it up then?

I've often wondered aswell, and after reading the answers I'm still not sure; what if a spacecraft were coming back from the moon, and therefore not in orbit, couldn't it re enter at 0 degrees very slowly and just change it's angle of approach once it's passed through? or is the speed of the revolving earth something to do with why it cannot? Sorry for being a dingbat.

banana boy wrote

The tiles of the heatshield aren't teflon are they ?I thought it was some kind of advanced silicone material

Usually carbon based for the ablative ones. (A heat pulse creates an insulating foam at a certain distance below the surface - the foam is more insulating than the solid material...but means that they have to be replaced).Silicates tend to get dense and glassy IIRC.(Further googling says I'm wrong and that lots of the shuttle is covered in silica based material...I'm out of date)Edit: The really hot bits (nose, leading edge of wings) are mainly carbon...so I wasn't completely wrong)

arti46 wrote

I've often wondered aswell, and after reading the answers I'm still not sure; what if a spacecraft were coming back from the moon, and therefore not in orbit, couldn't it re enter at 0 degrees very slowly and just change it's angle of approach once it's passed through? or is the speed of the revolving earth something to do with why it cannot? Sorry for being a dingbat.

Another uneducated guessThis all comes down to speed, the earth rotates at around 1000mph, in order to get to the moon and back you will be travelling at 20k mph+, so you you have to loose 19k mph in speed, this takes fuel without external assitance, fuel it heavy (see posts above)

{Buddha} wrote

Every gram of weight is crucial when it comes to launch ...

Just a quick one that's puzzled me for ages, if the above statement is true, which I don't doubt, how is it the later apollo moon landings were able to fit a car on board and where did they stow it I realise it could be very light due to the lack of gravity, but come on....

Kebab The Cat wrote

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

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 sheildingHowever I reserve the right to be totally wrong

Yeah, sorry, I guess it couldn't possibly be in orbit for reasons mentioned. I suppose it's just a sightseeing vehicle anyway. It's an amazing acheivement none the less but not really designed for interplanetary travel

Thanks for all the replies everyone. I understand the orbital facts now.The link from Pirahna is very interesting as well, so I've book marked that for further study.

WiseBuddha wrote

Another uneducated guessThis all comes down to speed, the earth rotates at around 1000mph, in order to get to the moon and back you will be travelling at 20k mph+, so you you have to loose 19k mph in speed, this takes fuel without external assitance, fuel it heavy (see posts above)

I think I get it; slowing down is the problem! Cheers, wise one.

arti46 wrote

Just a quick one that's puzzled me for ages, if the above statement is true, which I don't doubt, how is it the later apollo moon landings were able to fit a car on board and where did they stow it I realise it could be very light due to the lack of gravity, but come on....

It was only a little car. I think it was a Hillman Imp.

WiseBuddha wrote

Another uneducated guessThis all comes down to speed, the earth rotates at around 1000mph, in order to get to the moon and back you will be travelling at 20k mph+, so you you have to loose 19k mph in speed, this takes fuel without external assitance, fuel it heavy (see posts above)

also...Remember, to get to the moon (see hint above) you need to - almost - reach escape velocity... around 8000mph. So, equally, if you where at the moon traveling at 0mph away from the earth and 'let go' (so to speak) you'd be doing around 8000mph by the time you hit the atmosphere... but your right, as there's no atmosphere most the way, the craft will be in 'free fall' and accelerating... and it'd take fuel to stop that.... and then there's the problem of touching down (landing) in the right place on earth... which requires getting synced with earths orbit a bit...

arti46 wrote

Just a quick one that's puzzled me for ages, if the above statement is true, which I don't doubt, how is it the later apollo moon landings were able to fit a car on board and where did they stow it I realise it could be very light due to the lack of gravity, but come on....

IIRC the lunar rover only weighed 200kg, which in the grand scheme of the entire launch vehicle coming in at 3 million kg, was nothing.On that subject, that 3 million kg, only gave a rocket burn time of around 10 minutes, which brings us back to the feasibility of taken sufficient fuel into space to provide enough power to reach and maintain geosynchronous rentry

arti46 wrote

Just a quick one that's puzzled me for ages, if the above statement is true, which I don't doubt, how is it the later apollo moon landings were able to fit a car on board and where did they stow it I realise it could be very light due to the lack of gravity, but come on....

Not sure, but its quite possible they took it up bit by bit and assembled it on the moon in the way they are cobbling together the space station. There were quite a few missions to the moon in the Apollo program.

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.

Somewhat different scenario....Spaceship 2 (as it's called) apparently isn't an orbital type of spaceship. I think it just goes staright up, ya get 10-15 minutes or so of floating about in space before plummeting back to earth like a rock. It won't be up very long as it's designed to land about 1/2 mile from where it will take off from ...just up the road from here as it happens, I watched the previous attempt from my back yard....& it's nowhere near big enough to carry food etc for the self loading cargo etc.Therefore it won't be going like a bat outta hell to get itself into orbit. Bad enough when the bloody shuttle lands here.. if you think a jet fighter makes a loud sonic boom you'd shit yourself when you hear a shuttle coming back....

{Buddha} wrote

Not sure, but its quite possible they took it up bit by bit and assembled it on the moon in the way they are cobbling together the space station. There were quite a few missions to the moon in the Apollo program.

Apollo 11 was the first manned mission that landed on the moon, and short of the odd probe that had been crash into the the surface this was the first thing on the moon.They took everything they needed with them.

Count Steer wrote

So Beardy Branstons space ships won't actually go into space?Strangely, this doesn't surprise me.

Bransom was/is just sponsoring an entry in a competition to get a vehicle into 'space' more than once. I think the prize was £10m. A NW schoolteacher was also an entrant. 'Space' was 60,000 feet IIRC

ZRX61 wrote

Bad enough when the bloody shuttle lands here.. if you think a jet fighter makes a loud sonic boom you'd shit yourself when you hear a shuttle coming back....

I would love to see a shuttle take off and landing, I would have to do it alone though as I am sure that I would probably come a little bit.

I'm amazed. There's no such thing as a mandatory "escape velocity" nor any necessity to attain geosynchronous anything to re-enter. And as for the comments about "free fall"... You got enough fuel you can do anything you want. Mention my name.

This is all fascinating!It's easy to forget the very simple principles of physics that have such massive ramifications when leaving the influences of gravity and friction. If you wanna slow something down you need more fuel, and more fuel means you need more fuel...What we need is weightless energy to provide massless thrust. We're currently doing something akin to throwing ballast off a hot air ballon to gain height Space flight is amazingly impressive yet strangely archaic at the same time.What became of the ion drive? If you could accelerate mass through an electrical field then surely the exit veocity of the mass would be directly related to the charge applied? If you could harvest charge in space (from solar sails or whatnot) then the amount of acceleration that could be garnered from x amount of onboard mass is variable and only limited by the capacity for storing charge. Genuine question btw, I've always wondered about this since my lecturer explained how a mass spec machine worked. Similar thing I guess.

Oaky wrote

I'm amazed. There's no such thing as "escape velocity" nor any necessity to attain geosynchronous anything to re-enter. And as for the comments about "free fall"... You got enough fuel you can do anything you want. Mention my name.

go on then, "my name", tell it like it is...

Mr Rooty Tooty wrote

What became of the ion drive?

ESA just used it on SMART-1www.esa.int/esaCP/SEM2S8WJD1E_index_0.htmlwww.esa.int/esaSC/SEMY9ES1VED_index_0.html

WiseBuddha wrote

On that subject, that 3 million kg, only gave a rocket burn time of around 10 minutes, which brings us back to the feasibility of taken sufficient fuel into space to provide enough power to reach and maintain geosynchronous rentry

Mate of mine works out at the Rocket Site on Edwards AFB buggering about with the next gen of rocket motors. Right now he is working on a fuel pump that pumps liquid hydrogenIt spins at about 100,000rpm & pushes fuel at 9000psi.... & it has no bearings in it.. at least not in the conventional sense..They measure flow in (olympic size) "swimming pools per minute".. ..right now they are at 7sppm.... If they put this pump in a big enough lake & activate it, you would end up with a 1 mile high fountain.....The fuel is delivered in 40ft tankers.. that pump is capable of emptying one of them in 1.5 seconds.. which would crush it like a coke can.... Some impressive shit going on around here... Just picture an olympic size pool being emptied (or filled..) in a tad over 8.5 seconds..& as they are fond of saying:.... "Rocket science is more fun with real rockets"

Copperhead wrote

...until it reaches the point where gravity takes over, and then drop naturally?

Gravity doesn't take over, when a shuttle is in orbit, it's essentially always falling towards earth, but missing the edge Imagine it this way:1.) Throw a tennis ball and it curves back to earth.2.) Throw it hard enough (cannon?) then it will come back to earth as the earth begins to curve away.3.) Throw it even harder and it will fall *around* the earth as the earths surface drops away at a similar rate as the tennis ball falls. That's essentially orbit by NyseriA, aged 13 3/4

WiseBuddha wrote

Apollo 11 was the first manned mission that landed on the moon, and short of the odd probe that had been crash into the the surface this was the first thing on the moon.They took everything they needed with them.

It was only the latter manned missions that had the lunar rover thing, there were about 7 manned mission in all

One never leaves the influences of gravity or friction, not on my watch. The inverse square bit diminishes the former pretty quickly in the empty ether of space, and the empty ether does what it can to friction without being totally empty, but they're fighting a bigger enemy. Chuck Norris.

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