Every object I’ve ever seen entering through the earth’s atmosphere whether in real life or on TV and movies begins to burn up. I know that it is because the atmosphere is thick, and friction with dense gases creates the necessary eruption in flames like rubbing two sticks together to make fire.
However, if an object travels slow enough, couldn’t it drop through the atmosphere from space with no burning or ill effects? How slow would such an object have to move in order to not burn up? Suppose that the object has some way of slowing its decent and correcting for changes in gravity, couldn’t they do this with space shuttles and pods entering the atmosphere so they wouldn’t have to risk being engulfed in flames?
They don’t burn up when going UP through the atmosphere, so just go at that speed…
The problem is the fuel burn. It takes a TREMENDOUS amount of fuel to offset gravity which is why the space shuttle and other orbital vehicles have such huge fuel tanks. Trying to slow yourself down takes almost as much fuel as trying to completely overcome gravity. They would need to haul more than twice the amount of fuel up into space (fuel to slow the decent + extra fuel to haul the extra fuel).
I think the way you might rephrase the question is something like this:
Could you “drop” something from “space” and not have it “burn/burn up”
Assuming zero horizontal or vertical velocity to start with, with respect to earth.
Of course “space” and “burn/burn up” would have to be defined.
My WAG would be that things would only get “warm” for reasonable definitions of space.
The real? problem is things in space are moving at very high speeds and the best/worst that happens is that that they loose (or gain) only a tiny fraction of that speed which then results in them entering the earths atmosphere at a still very high speed…
Again, I’m kicking myself for never having taken a compressible fluids course, but isn’t the heat created not because of friction, but due to compression of the gas at the boundary layer of the craft? PV=nRT, and all that?
Yes. This starts to occur at the local speed of sound; I’m not sure at what speed the heating effects start to become s9ignificant, but it’s probably not until several times local Mach 1.
Yeah, friction drag is pretty slim compared to compression from what I remember. Everywhere behind the bow shockwave gets compressed as the air transitions from hypersonic to subsonic speeds (relative to the re-entry vehicle).
I can’t answer your question directly, but would it make a difference whether the heat is caused by friction or compression? In either case, wouldn’t slowing down (relative to the atmosphere) reduce the cause (friction/compression) and therefore the heat?
The issue here is that just about all the objects are accelerating due to gravity. Any deceleration an unpowered object exhibits means that the acceleration must have been turned into another form of energy, usually heat. For an object to re-enter without heating up means that it must do something - likely expend energy - to mostly counteract gravity. This is not trivial.
Well the heat is caused not by friction but by ram pressure (shock wave) which doesn’t manifest until the speed of sound. In normal atmospheric pressure, terminal velocity tends to keep this from happening if the object starts out in freefall, but higher up at lesser pressures there tends to be less drag and one can build up a much greater speed just from falling than the terminal velocity anwhere with normal pressure. Captain Kittinger did a free fall from 31km and reached about 600mph after four minutes. While he needed a pressure suit, it doesn’t seem he needed any special heat sheilding.
According to wikipedia:
“An approximate rule-of-thumb used by heat shield designers for estimating peak shock layer temperature is to assume the air temperature in kelvins to be equal to the entry speed in meters per second - a mathematical coincidence. For example, a spacecraft entering the atmosphere at 7.8 km/s would experience a peak shock layer temperature of 7800 K. This is unexpected, since the kinetic energy increases with the square of the velocity, and can only occur because the specific heat of the gas increases greatly with temperature (unlike the nearly constant specific heat assumed for solids under ordinary conditions).”
“Burning up” depends obviously on the melting and/or flashpoint of your building material. More important is probably temperatures amenable to the human body. Assuming an optimal human temperature of less than 300K, you wouldn’t want to enter the atmosphere at more than 300 m/s (.3 km/s = 1080 kmph or 671 mph)
I would guess that that rule of thumb isn’t applicable at speeds or temperatures that low. After all, when I drop a feather at a small fraction of a meter per second, it doesn’t instantly cool to near absolute zero.
I’m not sure what the cut off is. I would guess it’s around where the maximum speed hits the sonic barrier since that’s the point at which ram forces become evident. Note that it’s specifically a rule of thumb for objects entering Earth’s atmosphere.
I think you need a math dullard like me to put this in layman’s terms…
Of course you could slow an object down to the point where it would not burn up–in theory.
In practice, slowing down a massive object like the space shuttle with thrusters would require a lot of fuel–fuel that you would have to get up there in the first place.
The shuttle is orbiting the earth at about 23-25 Mach, say, in order to keep it in orbit. Give it an altitude of 220 miles for the sake of this example (not quite as high as the space station). Suppose you could slow it down to a relative halt (and as pointed out, you’d need as much fuel as you used to accelerate it to that speed) without letting it’s altitude degrade (again, just for example; in practice slowing it down would cause it to start falling back to earth instead of falling around the earth).
Now you essentially drop the sucker from a point 220 miles high, but stationary relative to a point below on the earth. Would it still burn up?
Well, if my arithmetic is correct (highly unlikely, so do your own math), and without accounting for wind resistance, gravity would accelerate such an object to about 5800 miles/hr. In round numbers, let’s call that 2600 meters/second. Enough to make it warm up to 2600 Kelvin (see the post above). Sounds warm to me.
Objects burn up when they hit the atmosphere because they are already going pretty fast when the earth’s gravity catches them and accelerates them toward earth. But even a dead start at only a few hundred miles up is going to heat up pretty bad.
For lesser sonic speeds, you probably want to use the standard equations for normal shockwaves instead of relying on a rule-of-thumb intended for extreme hypersonic (i.e, Mach 25) speeds. Also note that just because the air temperature is >300K, if the air is pretty thin (as it would be in the extremely high altitudes typical of re-entry), you wouldn’t necessarily even feel warm surrounded by it. (Of course, this is complicated by the fact that air density does increase behind the shock wave, and the convective heat transfer increases from the speeds involved too)
Woudn’t something like SpaceShip One furfill the OPs criteria? It used a “feathered wing” system for reentering the atmosphere without the thermal problems.
yeah…i thought about that in the middle of the night last night :smack:
That was pretty much the equivalent of dropping something from space, or at least near space.
But actually, that design solved a different problem.
You want a blunt body like an apollo capsule for rentry…
yeah, yeah, the shuttle is flying machine…not an apollo capsule.
But it RE ENTERS like an apollo capsule…belly/bottom aimed towards the direction it is reentering…
The problem is a “flying machine” like that doesnt like it. Not a stable flight profile at all. Computers and/or in addition to damn good pilots are required.
What Burt Rutans design did was allow one configuration that was blunt yet inherently STABLE during the rentry phase, and another that was flyable during the landing phase.
So, something like an apollo capsule during rentry. More like the shuttle during landing.
Also, I dont think Spaceship One got particulary “hot?” from rentering the atmosphere.
You’re right that the heat still needs to be dissipated somehow. However, the OP asserted (and many people believe) that the heating up is due to friction, which is not the case.
One of my most memorable parts of a textbook was from a vector analysis text. It stated an axiom, and said, “the laborious proof is left to the reader (or you can read section 2.13).” There was no section 2.13! :smack: The class sucked, but the book was written in a humorous but professional way. I wish the authors of other tedious texts approached their writing that way.
