Warning

 

Close

Confirm Action

Are you sure you wish to do this?

Confirm Cancel
Member Login
Site Notices
9/22/2017 12:11:25 AM
Posted: 8/9/2005 5:07:14 AM EDT
Hey guys, I've always wondered something about the Shuttle. Maybe someone out there can help.

When landing, why does it have to deccelerate so quickly? In other words, as it starts to reenter the atmosphere, it gets into thicker and thicker air until eventually the adiabatic compression of the air causes it to heat up.

Well if it's flying that fast through air that's that dense, don't the wings have enough lift to keep it at that altitude ? So in other words, why doesn't it just skip along the top of the atmosphere and then slowly decend as it slows down rather than drop like a rock?

Link Posted: 8/9/2005 5:13:04 AM EDT
Not an AeroE, but don't the wings look more optimized for speed rather than lift? (think SR71 vs U2)

Any idea what the stall speed for the shuttle is? Would think it would be comparitively high...
Link Posted: 8/9/2005 5:16:54 AM EDT

Originally Posted By Merrell:
Not an AeroE, but don't the wings look more optimized for speed rather than lift? (think SR71 vs U2)

Any idea what the stall speed for the shuttle is? Would think it would be comparitively high...



They look awfully thick (i.e. drag) to me.
Link Posted: 8/9/2005 5:18:57 AM EDT
The forces are pretty much all the same with all aircraft...Once the shuttle gets back into the earths atmosphere, it is an airplane with no thrust, free falling or a heavy glider.

The Shuttle heats up due to friction and particles.

They need her to slow down so much for handling and also for weight reasons, the brakes can only do so much and they only have so much runway.

AOA, angle of attack and gravity gets her back down.

The stall speed is dependant upon weight.

Link Posted: 8/9/2005 5:19:06 AM EDT

Originally Posted By TexasEd:
Hey guys, I've always wondered something about the Shuttle. Maybe someone out there can help.

When landing, why does it have to deccelerate so quickly? In other words, as it starts to reenter the atmosphere, it gets into thicker and thicker air until eventually the adiabatic compression of the air causes it to heat up.

Well if it's flying that fast through air that's that dense, don't the wings have enough lift to keep it at that altitude ? So in other words, why doesn't it just skip along the top of the atmosphere and then slowly decend as it slows down rather than drop like a rock?




No AeroE here either, but the Shuttle is basically a giant brick hurling through space at 17,000+mph. It doesn't have much of an ability to steer as it is basically a glider. It has to hit a certain point in its orbit and enter the atmosphere precisely so that it can make a clean approach to the runway. If it misses its re-entry point, there is no going back and it will overshoot its landing area.
Link Posted: 8/9/2005 5:19:28 AM EDT
Not an aeronautical engineer, but I am guessing that the shuttle wings do not have enough lift in the upper atmosphere to skip along. Heat build up occurs because they use atmospheric friction to slow down from 17500?? mph in orbit to 200??mph landing speed. IIRC the glide path is 5000 miles long.
Link Posted: 8/9/2005 5:24:27 AM EDT
Slowing down enough to avoid the high temps of re-entry would mean it wouldn't have enough speed left to glide to the runway. Skipping is what they want to avoid, if you use up to much speed skipping you won't "fly" back to Earth, you'll just fall.

Also, the only way to slow down other than using the atmosphere is to use engines, which means more fuel, or more weight for takeoff.
Link Posted: 8/9/2005 5:56:26 AM EDT

Originally Posted By jchewie:
Not an aeronautical engineer, but I am guessing that the shuttle wings do not have enough lift in the upper atmosphere to skip along. Heat build up occurs because they use atmospheric friction to slow down from 17500?? mph in orbit to 200??mph landing speed. IIRC the glide path is 5000 miles long.



That's what confuses me. If there's not enough air density or forward speed to have enough lift, then how could there be that much heat (by compressing the air by the way, NOT friction)
Link Posted: 8/9/2005 6:00:23 AM EDT
not an AE, but:

Is it a time issue? Perhaps it *could* have a shallower approach but that would result in an extended period of high temp?
Link Posted: 8/9/2005 6:05:48 AM EDT

Originally Posted By NoVaGator:
not an AE, but:

Is it a time issue? Perhaps it *could* have a shallower approach but that would result in an extended period of high temp?



I thought about that too. That could be it. Maybe the Shuttle can get real real hot....just not for too long.
Link Posted: 8/9/2005 6:11:43 AM EDT
[Last Edit: 8/9/2005 6:12:59 AM EDT by ZitiForBreakfast]

Originally Posted By TexasEd:

Originally Posted By jchewie:
Not an aeronautical engineer, but I am guessing that the shuttle wings do not have enough lift in the upper atmosphere to skip along. Heat build up occurs because they use atmospheric friction to slow down from 17500?? mph in orbit to 200??mph landing speed. IIRC the glide path is 5000 miles long.



That's what confuses me. If there's not enough air density or forward speed to have enough lift, then how could there be that much heat (by compressing the air by the way, NOT friction)



They are not relying on lift. For reentry, it is all gravity.

The point that is confussing you canx's each other out. The 17k+ Speed breaking through the atmosphere make up for the lack of 'heavy' air. The speed alone if forcing the shuttle to move through the air, at those speeds the wings get what they need to work. The heat is 100% caused by particles and friction due to the speed and what the atmosphere is made of at those altitudes. If you think it has nothing to do with friction you are wrong.

Look at some old HUD video, they keep the nose pointed down at the numbers...a brick with movable flight control surfaces can do that.
Link Posted: 8/9/2005 6:14:36 AM EDT
As has been said the shuttle is no more than a glider that can't glide. it falls thru the upper levels of the atmosphere and only when the air is heavy enough to support it's weight can it glide down to earth. keep in mind the more you glide the more buffeting and problems you run into. the shuttle can't afford to "skip" or sally around when it starts it's landing. it has to cut the upper atmosphere like a knife and plunge thru a sequence of windows to get to the runway. miss a window... especially one of the first and it will be way off course.
Link Posted: 8/9/2005 6:17:39 AM EDT
... Not an astronautics engineer, but atmospheric reentry is a very violent. The conditions can burn up solid chunks of iron the size of large cars. Sub, or low-earth orbital spaceflight requires considerable lateral velocity (relative to earth) to remain in low-earth orbit. Transition and deceleration from this speed occurs high in altitude and where there are very thin gasses. Aerodynamic properties over lifting bodies behave differently than they do where conventional wings are designed to perform below say, 45,000 ft. Think of t this way, what kind of materials and in what form would you design wings so that chunk of iron the size of your car would make it to the ground without breaking an egg riding on its back?
Link Posted: 8/9/2005 6:28:42 AM EDT
[Last Edit: 8/9/2005 6:31:04 AM EDT by ZitiForBreakfast]
Here

and

Here

A large turn is not only required for alignment, but also to bleed speed.

Link Posted: 8/9/2005 6:42:01 AM EDT

Originally Posted By TexasEd:

Originally Posted By jchewie:
Not an aeronautical engineer, but I am guessing that the shuttle wings do not have enough lift in the upper atmosphere to skip along. Heat build up occurs because they use atmospheric friction to slow down from 17500?? mph in orbit to 200??mph landing speed. IIRC the glide path is 5000 miles long.



That's what confuses me. If there's not enough air density or forward speed to have enough lift, then how could there be that much heat (by compressing the air by the way, NOT friction)



If it is pressure causing the heating, then why doesn't the air ABOVE the shuttle freeze due to the extremely low pressure area created.

If the underside gets really hot, then the upperside must have the opposite reaction.

No, I really think it is friction.

I do understand your question though, and I have thought about it myself. The Apollo capsules had to enter at a precise angle to avoid skipping off the atmosphere, so even a depth charge like the Apollo capsule can get enough lift in the low gravity situation of a decaying orbit to get lift. A good way to brake a craft would be to skip it off the surface of the planet a series of times, but that may be more applicable to interplanetary flight, where the speeds are FAR higher.
Link Posted: 8/9/2005 6:56:46 AM EDT
Regarding heating due to friction vs compression:

From NASA Question and Answer site:

quest.nasa.gov/aero/chats/09-19-00gr.html

RE: [Andy] When the shuttle re enters the atmosphere why doesn't it come in as a streamlined shape like a high diver ??

Hi Jim; T hat's an excellent question, and the answer isn't at all obvious. Solving that problem was a major breakthrough. You would think that the easiest way to get a vehicle through the atmosphere is to make it very pointed. That's true from an aerodynamic st andpoint--it reduces resistance against the air. That's why fighter jets tend to be pointed. But while a fighter jet takes 3 or 4 seconds to fly one mile, the shuttle entry speed is about 10 miles per second! At those speeds, atmospheric heating is the hardest engineering problem. A very pointy surface generates a shock layer close to it, which heats up. But the pointed surface can't conduct heat away from the point rapidly, so it gets extremely hot. A blunt surface sets up the shock wave further away, and also is better at conducting heat away from the surface. So for thermal reasons, a blunt surface is easier to manage than a sharp or pointed surface.

See the word 'friction' anywhere? As the air is compressed it heats up. They use a blunt nose so that the shock wave is father away and not in direct contact with the shuttle.

Also discussed in Science Facts People Get wrong

www.mcrosolv.demon.co.uk/getwrong.html#t1

Link Posted: 8/9/2005 7:02:58 AM EDT

Originally Posted By TexasEd:
Regarding heating due to friction vs compression:

From NASA Question and Answer site:

quest.nasa.gov/aero/chats/09-19-00gr.html

RE: [Andy] When the shuttle re enters the atmosphere why doesn't it come in as a streamlined shape like a high diver ??

Hi Jim; T hat's an excellent question, and the answer isn't at all obvious. Solving that problem was a major breakthrough. You would think that the easiest way to get a vehicle through the atmosphere is to make it very pointed. That's true from an aerodynamic st andpoint--it reduces resistance against the air. That's why fighter jets tend to be pointed. But while a fighter jet takes 3 or 4 seconds to fly one mile, the shuttle entry speed is about 10 miles per second! At those speeds, atmospheric heating is the hardest engineering problem. A very pointy surface generates a shock layer close to it, which heats up. But the pointed surface can't conduct heat away from the point rapidly, so it gets extremely hot. A blunt surface sets up the shock wave further away, and also is better at conducting heat away from the surface. So for thermal reasons, a blunt surface is easier to manage than a sharp or pointed surface.

See the word 'friction' anywhere? As the air is compressed it heats up. They use a blunt nose so that the shock wave is father away and not in direct contact with the shuttle.

Also discussed in Science Facts People Get wrong

www.mcrosolv.demon.co.uk/getwrong.html#t1




Isn't heat the same as friction between particles? (which increase as a result of compression)
Link Posted: 8/9/2005 7:06:12 AM EDT

Isn't heat the same as friction between particles?


Yes. 100%.

Ask about drag index's if you do not want answers....
Link Posted: 8/9/2005 7:09:16 AM EDT
[Last Edit: 8/9/2005 7:11:20 AM EDT by Torf]
PV=NRT I guess, but does the upper half of the shuttle freeze proportionately?
Link Posted: 8/9/2005 7:10:38 AM EDT

Originally Posted By ZitiForBreakfast:

Isn't heat the same as friction between particles?


Yes. 100%.

Ask about drag index's if you do not want answers....



Absolutely wrong!! Heat is the total molecular kinetic energy. Temperature is the average molecular kinetic energy.

Show me anywhere where heat is defined as friction between particles.
Link Posted: 8/9/2005 7:12:48 AM EDT

Originally Posted By TexasEd:

Originally Posted By ZitiForBreakfast:

Isn't heat the same as friction between particles?


Yes. 100%.

Ask about drag index's if you do not want answers....



Absolutely wrong!! Heat is the total molecular kinetic energy. Temperature is the average molecular kinetic energy.

Show me anywhere where heat is defined as friction between particles.



Why does a molecule get more energetic just because the space between them is less?

It couldn't be because of molecular collisions could it?
Link Posted: 8/9/2005 7:20:12 AM EDT

Originally Posted By TexasEd:

Originally Posted By jchewie:
Not an aeronautical engineer, but I am guessing that the shuttle wings do not have enough lift in the upper atmosphere to skip along. Heat build up occurs because they use atmospheric friction to slow down from 17500?? mph in orbit to 200??mph landing speed. IIRC the glide path is 5000 miles long.



That's what confuses me. If there's not enough air density or forward speed to have enough lift, then how could there be that much heat (by compressing the air by the way, NOT friction)



It's all relative. In the upper atmosphere, there isn't enough air to "fly" ... there isn't enough air to even heat up a slow moving object either. Now, if an object is moving 5 miles per second (around 17,500MPH), then there is enough air to cause high-heat friction on an object moving that fast. The shuttle cannot actually "fly" until it is well into the atmosphere. If it never slowed down when it reached the lower atmosphere, it would simply melt.
Link Posted: 8/9/2005 7:20:37 AM EDT
[Last Edit: 8/9/2005 7:22:24 AM EDT by ZitiForBreakfast]

Absolutely wrong!! Heat is the total molecular kinetic energy. Temperature is the average molecular kinetic energy.

Show me anywhere where heat is defined as friction between particles.





I guess you are too smart for me. I know that heat is the result of friction...

Go on list all the different types of heat there is and what causes them.

Ask your nasa buddies, what all the plasma stuff is around the shuttle...
Link Posted: 8/9/2005 7:27:30 AM EDT
[Last Edit: 8/9/2005 7:27:53 AM EDT by metroplex]
You try decelerating from 18,000 mph with nothing but wing!
Link Posted: 8/9/2005 7:30:27 AM EDT
You do NOT want to skip on hte atmosphere. that is what gets you lost outside of earths orbit, then youre screwed.
Link Posted: 8/9/2005 7:32:23 AM EDT

Originally Posted By metroplex:
You try decelerating from 18,000 mph with nothing but wing!



Flat corkscrew or figure 8 or tear drop pattern from FL1890
Link Posted: 8/9/2005 7:35:55 AM EDT

Originally Posted By x5060:
You do NOT want to skip on hte atmosphere. that is what gets you lost outside of earths orbit, then youre screwed.



You're thinking about back in the Apollo days when they were going much faster than the shuttle. The shuttle is going just the correct speed to stay in orbit and not fall back to earth. Skipping on the atmosphere is not going to make it gain speed so as to escape orbit.

I didn't mean for this to go off on a friction vs compression debate.

I was just wondering if it will ever be possible to do anything such as the 'shuttlecock' aerobraking as is done on Burt Rutan's Spaceship One, (which is indeed going much slower than the shuttle) rather than dive through the atmosphere in a ball of fire.

Link Posted: 8/9/2005 9:17:07 AM EDT
[Last Edit: 8/9/2005 9:30:57 AM EDT by ArmdLbrl]

Originally Posted By TexasEd:

Originally Posted By x5060:
You do NOT want to skip on hte atmosphere. that is what gets you lost outside of earths orbit, then youre screwed.



You're thinking about back in the Apollo days when they were going much faster than the shuttle. The shuttle is going just the correct speed to stay in orbit and not fall back to earth. Skipping on the atmosphere is not going to make it gain speed so as to escape orbit.

I didn't mean for this to go off on a friction vs compression debate.

I was just wondering if it will ever be possible to do anything such as the 'shuttlecock' aerobraking as is done on Burt Rutan's Spaceship One, (which is indeed going much slower than the shuttle) rather than dive through the atmosphere in a ball of fire.



You'll have to ask Rutan about that. He is supposedly working on a larger version to win the Americas Space Prize contest- 50 Million this time for the first privately built spacecraft to complete two consecutive orbits of the Earth at a altitude of 400km or higher, return safely, and repeat the flight within 60 days. All before January 10th 2010.

www.space.com/spacenews/businessmonday_bigelow_041108.html

www.bigelowaerospace.com/prize.html

Now is this the way to really pioneer space travel or what!

Thing is, could you imagine what it would be like if someone put one years worth of NASAs budget up as a prize?
Link Posted: 8/9/2005 9:36:11 AM EDT
[Last Edit: 8/9/2005 9:38:11 AM EDT by Keith_J]
The object of shuttle reentry is transition from orbit to flight. This is started by retro burn which decreases orbital velocity, allowing the upper reaches of the atmosphere where drag can further degrade the velocity to the necessary altitude where the shuttle can fly.

Yes, it does glide but at a very poor glide slope compared to most small aircraft but that same wing area allows for a reentry using the foamed silica tile as a heat shield (with SiC protection and radiative dissipation).

Most of the heating happens at altitudes over 250,000 feet. Nothing can fly in that density.

Link Posted: 8/9/2005 9:45:40 AM EDT
The simple answer is that there is a boat load of total energy to dissipate, and the only mechanism is to generate heat. The total energy consists of the kinetic energy caused by the airplane's speed and weight plus the potential energy from the vehicle's weight, altitude, and gravity.

The shuttle orbit speed is about 12500 miles per hour; I have no idea what the range may be depending on altitude and mission, but it can't vary much, there's no way to add much energy to the vehicle after the boosters and main tank are gone. What's there is for simple maneuvers.

Re-entry is way more complex than compressible aerodynamics, and in fact, the first part of the process is described by aerothermochemistry (a fancy name for redox problems).

There are shuttle operating manuals on-line that give every detail about operation of the vehicle.
Link Posted: 8/9/2005 9:51:17 AM EDT
I took a class on atmospheric reentry about 20 years ago but I don't remember much about it. I think the answer has as much to do with orbital mechanics as aerodynamics. If the shuttle comes in too shallow, it runs the risk of "skipping" off the atmosphere. Obviously, it can't "skip" into a more energetic orbit than the one it's already in, but it could bounce back up and, because it has lost energy (due to drag), when it comes back down it'll be coming back down at a steeper angle than last time. That would be bad. Imagine looking down at the planet coming up at an angle you know is too steep with your mind screaming "pull up," but since there's no atmosphere, you can't pull up until it's too late.


If it is pressure causing the heating, then why doesn't the air ABOVE the shuttle freeze due to the extremely low pressure area created.

If the underside gets really hot, then the upperside must have the opposite reaction.

No, I really think it is friction.



With regard to the friction versus compression debate, I believe that the vast majority of the heating occurs due to the shock wave (i.e, because of compression). The reason that the air on the top of the shuttle's wing isn't freezing is because that air is also behind the shock wave.
Top Top