Have you ever seen a shooting star? If you have, then you've seen why aeronautic engineers don't take hyper-sonic speed lightly. Anyone who's felt wind on their skin knows what wind chill is. A common misconception is that it's the wind itself that's causing the cooling.
Human skin is usually warmer than the surrounding air, and as such slightly heats the air around it. Any given person walking around is carrying their own thin bubble of hot air. When the wind blows, your protective blanket of heat is blown away with it, and you're exposed to the real air temperature. In higher humidities this is more pronounced because water is a very efficient thermal conductor.
In commercial airliners, the ice that can form on their wings isn't due to wind, as some people believe, but due to altitude. The same temperature gradient that allows snow to fall on mountains while it's 80 (F) degrees in the valley continues as altitude increases, getting as low as 40 degrees (F) below zero at the altitude commercial airliners fly at. It's not surprising ice will form when it's that cold out.
Now we get to the part those two paragraphs were leading up to; frictional and compressive heating. When air moves, it's just like any other moving object, and there's friction involved. In the form of air movement it's called "wind resistance" and at high velocities it can translate into a lot of heat. Wind rubbing over an aircraft hull can be just like rubbing your hands together, and at supersonic speeds generates just as much heat.
Compressive heating works slightly different and is best compared to a diesel engine. Diesel engines don't use spark plugs to ignite their fuel, instead they use incredibly high compression ratios and very rapid compression speeds to heat pockets of air until the fuel vapors in it ignite from the heat.
What's happening is that any given volume of air has a certain amount of thermal energy in it. When you compress the air, that same energy is now taking up a smaller space, meaning the energy density is now higher, and in diesel engines this is high enough to set things on fire. This heating, while significant and very useful in industry, has its limits, and these limits are actually used to protect spacecraft on re-entry.
Ever since Project Gemini heat shields have been used to take the brunt of that atmospheric heating, exploiting the limits of compressive heating to allow them to handle re-entry with slightly less cooked astronauts. These heat shields have deliberately high drag, compressing air in front of them. This compressed air forms two distinct layers called the shock layer and the boundary layer.
The shock layer is where the bulk of heat takes place. Incoming air is displaced violently by the supersonic vehicle. In the case of the now retired NASA space shuttle and the ill-fated Buran shuttle, this heating was intense enough that it actually broke down atmospheric molecules into their base atoms.
There's no denying that the shock layer is a violent place, in fact it's the pressure wave created by the shock layer around supersonic planes that causes the sonic boom you hear as they rip by overhead. Hypersonic re-entry vehicles are able to survive this through a combination of heat insulation and the compressive heating. By making themselves have a high drag, they drag a pocket of air with them, this is called the boundary layer.
The boundary layer acts as a buffer between the vehicle and the intensely heated shock layer, like the oil in a pan that keeps cooked food from scalding; yeah, it's hot, but it's not as hot as the metal pan below.
So.
This will probably make you ask the question; Why do they have to enter going so fast?
Well that's a matter of orbital mechanics. As The Hitchhiker's Guide To The Galaxy famously states: "There is an art to flying, or rather a knack. Its knack lies in learning to throw yourself at the ground and miss. ... Clearly, it is this second part, the missing, that presents the difficulties."
Orbit is achieved by moving sideways so fast that by time you fall down, that direction is no longer down because the planet has moved beneath you. In Low Earth Orbit This takes moving at 15,000 miles per hour (24,140Kph) which translates to mach 20, 20 times the speed of sound.
Getting up to that speed is one thing. Getting up to that speed and still having enough fuel to slow back down is another. Orbital spacecraft have to rely on chemical thrusters to operate outside of our atmosphere, and these engines, while powerful, are far less fuel-efficient than those that take advantage of the readily available oxygen in the air around us.
When spacecraft re-enter, they can slow themselves down a lot, but they're still typically travelling above Mach 10, and at those speeds heat in the shock layer around an aircraft causes the atmosphere to heat to incandescence, the separation of molecules into atoms energizing the atmosphere so much it forms a glowing charged plasma around it (this is the "fire" you see around them)
Now comes the fun part. Surely, you must be asking, if something were light enough and wind resistant enough, it could slow down to safe speeds long before hitting the thicker lower atmosphere and thus avoid burning up.
This is not only possible, but it happens all the time. If you have ready access to a rooftop that doesn't see any foot traffic and happen to have a high powered magnet available, you can collect your own meteorites. Most meteorites contain iron, and tiny, nearly microscopic ones are impacting earth all the time. These meteorites are so tiny they slow down long before heating up and fall harmlessly to the ground. Large, flat, impermeable surfaces make ready-made collectors for these.
On smooth metal roofs, rain water will wash them away, but if you live in a desert there's no rain to wash them off, and if you have tar shingles, they can become wedged among the pieces of gravel embedded in the tar. A few passes with a magnet and you've got a fistful of stardust.
As propulsion technology continues to advance, there exists a real possibility of making spacecraft that slow down just as efficiently as these bits of space debris, removing astronaut's worries of a very toasty home trip, and opening the door to colonization of space. Indeed, the day may come when our great great grandchildren look at pictures of a space shuttle and ask "What kind of crazy person would think that was a safe way to land?"
Wise Words Of The Day:
"Later, I realized that the mission had to end in a let-down because the real barrier wasn't in the sky but in our knowledge and experience of supersonic flight."
~Chuck Yeager
Human skin is usually warmer than the surrounding air, and as such slightly heats the air around it. Any given person walking around is carrying their own thin bubble of hot air. When the wind blows, your protective blanket of heat is blown away with it, and you're exposed to the real air temperature. In higher humidities this is more pronounced because water is a very efficient thermal conductor.
In commercial airliners, the ice that can form on their wings isn't due to wind, as some people believe, but due to altitude. The same temperature gradient that allows snow to fall on mountains while it's 80 (F) degrees in the valley continues as altitude increases, getting as low as 40 degrees (F) below zero at the altitude commercial airliners fly at. It's not surprising ice will form when it's that cold out.
Now we get to the part those two paragraphs were leading up to; frictional and compressive heating. When air moves, it's just like any other moving object, and there's friction involved. In the form of air movement it's called "wind resistance" and at high velocities it can translate into a lot of heat. Wind rubbing over an aircraft hull can be just like rubbing your hands together, and at supersonic speeds generates just as much heat.
Compressive heating works slightly different and is best compared to a diesel engine. Diesel engines don't use spark plugs to ignite their fuel, instead they use incredibly high compression ratios and very rapid compression speeds to heat pockets of air until the fuel vapors in it ignite from the heat.
What's happening is that any given volume of air has a certain amount of thermal energy in it. When you compress the air, that same energy is now taking up a smaller space, meaning the energy density is now higher, and in diesel engines this is high enough to set things on fire. This heating, while significant and very useful in industry, has its limits, and these limits are actually used to protect spacecraft on re-entry.
Ever since Project Gemini heat shields have been used to take the brunt of that atmospheric heating, exploiting the limits of compressive heating to allow them to handle re-entry with slightly less cooked astronauts. These heat shields have deliberately high drag, compressing air in front of them. This compressed air forms two distinct layers called the shock layer and the boundary layer.
The shock layer is where the bulk of heat takes place. Incoming air is displaced violently by the supersonic vehicle. In the case of the now retired NASA space shuttle and the ill-fated Buran shuttle, this heating was intense enough that it actually broke down atmospheric molecules into their base atoms.
There's no denying that the shock layer is a violent place, in fact it's the pressure wave created by the shock layer around supersonic planes that causes the sonic boom you hear as they rip by overhead. Hypersonic re-entry vehicles are able to survive this through a combination of heat insulation and the compressive heating. By making themselves have a high drag, they drag a pocket of air with them, this is called the boundary layer.
The boundary layer acts as a buffer between the vehicle and the intensely heated shock layer, like the oil in a pan that keeps cooked food from scalding; yeah, it's hot, but it's not as hot as the metal pan below.
So.
This will probably make you ask the question; Why do they have to enter going so fast?
Well that's a matter of orbital mechanics. As The Hitchhiker's Guide To The Galaxy famously states: "There is an art to flying, or rather a knack. Its knack lies in learning to throw yourself at the ground and miss. ... Clearly, it is this second part, the missing, that presents the difficulties."
Orbit is achieved by moving sideways so fast that by time you fall down, that direction is no longer down because the planet has moved beneath you. In Low Earth Orbit This takes moving at 15,000 miles per hour (24,140Kph) which translates to mach 20, 20 times the speed of sound.
Getting up to that speed is one thing. Getting up to that speed and still having enough fuel to slow back down is another. Orbital spacecraft have to rely on chemical thrusters to operate outside of our atmosphere, and these engines, while powerful, are far less fuel-efficient than those that take advantage of the readily available oxygen in the air around us.
When spacecraft re-enter, they can slow themselves down a lot, but they're still typically travelling above Mach 10, and at those speeds heat in the shock layer around an aircraft causes the atmosphere to heat to incandescence, the separation of molecules into atoms energizing the atmosphere so much it forms a glowing charged plasma around it (this is the "fire" you see around them)
Now comes the fun part. Surely, you must be asking, if something were light enough and wind resistant enough, it could slow down to safe speeds long before hitting the thicker lower atmosphere and thus avoid burning up.
This is not only possible, but it happens all the time. If you have ready access to a rooftop that doesn't see any foot traffic and happen to have a high powered magnet available, you can collect your own meteorites. Most meteorites contain iron, and tiny, nearly microscopic ones are impacting earth all the time. These meteorites are so tiny they slow down long before heating up and fall harmlessly to the ground. Large, flat, impermeable surfaces make ready-made collectors for these.
On smooth metal roofs, rain water will wash them away, but if you live in a desert there's no rain to wash them off, and if you have tar shingles, they can become wedged among the pieces of gravel embedded in the tar. A few passes with a magnet and you've got a fistful of stardust.
As propulsion technology continues to advance, there exists a real possibility of making spacecraft that slow down just as efficiently as these bits of space debris, removing astronaut's worries of a very toasty home trip, and opening the door to colonization of space. Indeed, the day may come when our great great grandchildren look at pictures of a space shuttle and ask "What kind of crazy person would think that was a safe way to land?"
Wise Words Of The Day:
"Later, I realized that the mission had to end in a let-down because the real barrier wasn't in the sky but in our knowledge and experience of supersonic flight."
~Chuck Yeager
No comments:
Post a Comment