My understanding of science is pretty much limited to Mystery Science Theater 3000, and I’m not afraid to admit it. This makes me the PERFECT person to ask all these annoying questions that everyone else would be embarrassed to ask.
Elsewhere on this site, it’s been claimed that shooting stars are actually particles roughly the size of a grain of sand “burning up” as they enter our atmosphere. I’ve also read about the “heat shielding” that the Shuttle and Lunar Re-Entry craft needed to avoid “burning up” upon re-entry.
This makes no sense to me. But then, I’m a science dummy, and pretty much figure the world is flat because I can’t see the curve.
Here’s the thing: As I recall from my skydiving days, a body hits terminal velocity in about 150 feet. The fastest anyone has ever managed to go in free-fall was a Nasa jumper that did an extremely high Altitude jump- he hit something like 700 mph.
Yet he didn’t “burn up”. So how can a grain of space dirt “burn up” just by entering our atmosphere? Now, you might say that it had all kinds of excess velocity BEFORE hitting our atmosphere (relative to Earth’s movement), which immediately converted to energy once friction slowed it down. But how does this explain the Space Shuttle? Since one would imagine that you could just DROP the damn thing from about any height, and it’d never pick up much more speed that our aforementioned Skydiver (because, after all, St. Newton showed that bodys fall at the same rate regardless of mass) why does the Shuttle need “heat shielding”? Especially considering that it’s DAMN COLD in the upper atmosphere.
Now, I realize that there’s something I’m missing, being, as I am, a Science Dummy. I’m trying to reach enlightenment on what that “something” is. Does the shuttle end up in it’s own orbit, with it’s own relativistic speeds?
Things in space can be travelling incredibly fast (the space shuttle is travelling at an orbital velocity of several thousand miles per hour, so it doesn’t just drop in like a skydiver). There is no terminal velocity in space if there’s no atmospheric drag to balance the acceleration of gravity. When an object in space hits the atmosphere, it has to slow down to terminal velocity. The heating is primarily caused by the compression of the air in front of the moving body.
Well, for one thing, meteors (it’s not a meteorite until it hits the Earth) ravel considerably faster than 700 MPH–more on the order of 17,000 to 25,000 MPH. At this speed, the air can’t move out of the way fast enough and it “piles up” in front of the object, becoming highly compressed. When you compress a gas, you cause it to heat up, just like in a refrigerator or air conditioner. This compressive heating is what is responsible for causing meteors to burn up as they enter our atmosphere.
As Q.E.D. points out the objects are moving fast initially. Normally on the order of 30 kmph. Now once they hit an atmosphere they will encounter atmospheric drag trying to reduce their velocity to whatever terminal velocity the particle would have. Since they’re moving so fast, the friction is tremendous and they literally vaporize.
Hypersonic is a velocity that exceeds Mach 4; supersonic is Mach 1 - Mach 4. Supersonic aircraft also have heat considerations because of air compression. One of the early problems was the heat drawing the temper of the glass in the cockpit. At one point, the windshields were made from quartz in order to handle the heat; synthetics and a better understanding of glass today allow more conventional materials to be used.
The guy who did the high altitude balloon jump actually went transonic for a short period, but also needed a drogue chute to keep him stable. He didn’t spontaneously combust because he never exceeded the terminal velocity for the given altitude.
‘Atmospheric friction’ is a deliberate lie perpetuated by the press, because they think the general public is too stupid to understand something as basic as Boyle’s (or is it Charles’ ?) law.
This is not a hard and fast rule. There is no clear cut boundary between supersonic and hypersonic. Hypersonic is defined as a flow in which certain effect become more significant than they are at lower supersonic speeds. These effects include very thin shock layers (shock waves close to the body), relatively thick boundary layers because high temperatures increase viscous effects, chemically reacting boundary layers (because of high temperature), very low density where you might even get temperature and velocity slip at the surface and some other effects I don’t recall at the moment.
There’s a relatively famous (at least within the field) quote from some old smart guy pointing out that most people agree that hypersonic flow starts at Mach number 5 or 6, but it’s possible to make the argument for a boundary as low as 3 or as high as 12. The point is, the change from supersonic to hypersonic is not a clear boundary like subsonic to supersonic. It’s a gradual change where some effects that are negligible at lower speed become more important.
I think “atmospheric friction” is appropriate for a general public perspective. The gas laws themselves are only statistical abstractions that quantify the average kinetic energy of bunches of perfectly elastic ideal particles. Whether the increase in heat energy of the meteor/spacecraft is caused by the accumulated kinetic transfer of energy of gas particles bouncing off of the object or not, the word “friction” is still an appropriate description of the process, because at the small scale “friction”, “heat”, “pressure”, and “temperature” become very semantically hard to discern from each other and are often effectively equivalent when talking about gasses.
So I think “deliberate lie” is unduly harsh. “Deliberately simplified” is more like it.
I may be wrong about this, so someone please correct me if so.
I think that using the term “burning up” is incorrect. I do not think their is any “burning” in the sense of moelcular conversion involving oxygen. I believe the meteor actually vaporizes or perhaps the temperatures are high enough to atomize it (are we dealing with a plasma?).
You seem to have this idea that the Space Shuttle in orbit is at rest. It’s not. If it were at rest, it would not be in orbit–it would simply fall down to Earth. As I mentioned in another thread this morning, the Earth’s gravitational force of attraction 500 km up is 86% of the value at the surface; you do not magically “escape” gravity once in you reach space.
The upshot is that any object in low earth orbit has a characteristic speed in the 17,000 to 18,000 miles/hour range (and must have this speed to be in orbit in the first place), and this is the speed that the Shuttle must shed in order to land.
P.S. While Newton was knighted, he was never canonized.
A little from column “A”, a little from column “B”. With a dash of column “C”. There’s a little of everything you mentioned there: vaporization, oxidation and formation of plasma. The temps surrounding a falling meteor are intense; in the order of several thousand degees–plenty high enough to form plasma.
I’ve read that meteorites that are picked up right after they hit the surface, such as those that come through the roof or hit the car. aren’t particularly hot. The main body stays relatively cool by ablation which is the thing that keeps space capsules from burning up on reentry. The surface of the protective skin, tiles or what-have-you, vaporizes and also erodes away taking with it most of the heat.
If you want to discuss the microscopic mechanics of friction, you could argue that they are the same, but they are certainly not the same in the aerodynamic context we’re discussing. Friction effects happen in the viscous boundary layer around a body. The pressure effect mentioned by me and several other posters above is the compression of the air by the shockwave in front of the body. A blunt body at supersonic speeds will have a detached bow shock, a rounded shockwave in front of the leading edge rather than the oblique attached shocks that appear on sharper bodies designed for these speeds. The pressure increases dramatically across the shock (or, more accurately, the shock exists as a discontinuity in the pressure and density caused by the velocity change in the fluid due to the moving body). The temperature increases with pressure, so you get this hot, high-pressure envelope of air around the body outside the viscous boundary layer where friction effects are happening. Heating due to friction in the viscous layer certainly contributes, but the temperature change due to the compression across the shock is the dominant effect.
I would broadly define kinetic friction as any force which tends to reduce the speed of a moving object in a non-reversible way. This definition applies equally well to pushing a crate on sandpaper and to a meteor entering the atmosphere, and it’s perfectly consistent with the mechanism of the friction being compression of air on the leading edge, without recourse to any microscopic analysis. What rival definition of friction would you use?
I’m not arguing your definition of friction, but I think if you lump every force which opposes the motion of the body into the title “friction”, you lose some important distinctions. In aerodynamics it is typical to separate drag into different components and treat viscous drag (which I would call friction, but that’s just me) separately from form drag (drag caused by pressure differentials, which I would not call friction).
When you talk about something heated by friction, it’s because two things are in contact. The viscous boundary layer is caused by the no-slip condition at the surface of the body and intuitively this is friction. In an inviscid flow where there is no boundary layer, you still have the bow shock and the compression of the air in front of the body. I would consider an inviscid flow “frictionless” in that there is a slip condition at the body surface (fluid in contact with the body can have a velocity different than the body) but you still have a drag force caused by the shock compression opposing the motion of the body.
Perhaps my definition of friction is simply wrong or not general enough, or perhaps it’s a matter of semantics.
