Is it possible to re-enter the earths atmosphere without heating up?

[Mr. Pedantic]

You could have a gun in space, and if it had its own oxygen supply, it would work perfectly too.

 

[Rubycon]

You could have a gun in space, and if it had its own oxygen supply, it would work perfectly too.

Propellants have their own oxidizer so they will work. But that is something completely different.

Imagine firing a .50 cal during a spacewalk!

 

[Pepsei]

Propellants have their own oxidizer so they will work. But that is something completely different.

Imagine firing a .50 cal during a spacewalk!

well, a space .50 cal will need to have it's own exhaust hollow tube like rpg.

 

[Cattlegod]

There are a couple ways to not increase in heat at all.

1 - Have a tube from the ground to the docking station in space where the inside of the tube is a vacuum.

2 - Sustain a reaction that absorbs heat from the re-entry vehicle that is greater or equal to the heat transferred from the atmosphere.

3 - Re-enter in another re-entry vehicle.

4 - Develop a way to control the atmosphere (maybe some type of man-made tornado) so that a vacuum is created on the surface and creates a tunnel of air moving downward from the top of the atmosphere - just move relative to the air moving downward until you get to the surface.

5 - Use the heat to generate reverse thurst

 

[DrPizza]

Ok I have figured out a way. Create a vaccum field around your space ship. Then you won't heat up.

Except that your space ship would still heat up due to radiation of the heat. (not from conduction through.)

As Dr. Pizza mentioned, it's not "pure friction".

"Reentry heating differs from the normal atmospheric heating associated with jet aircraft, and this governs TPS design and characteristics. The skin of high-speed jet aircraft can become hot from atmospheric friction, but this frictional heating is similar to rubbing your hands together. The Orbiter reenters the atmosphere as a blunt body by having a very high (40-degree) angle of attack, with its broad lower surface facing the direction of flight. Over 80% of the heating the Orbiter experiences during reentry is caused by compression of the air ahead of the ultrasonic vehicle, in accordance with the basic thermodynamic relation between pressure and temperature."

http://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system#Reentry_heating

(But yes, take care of speed and you take care of both issues)

Thanks. I hadn't looked anywhere to see what percent was actually from friction. Only 20%. Nice. You could grossly simplify it to say "it's friction", and you'd mostly do that because the average person in the US has no clue that when you compress air, the pressure is going to increase. The reality is that the air can't move fast enough to get out of the way of the shuttle which is moving at mach xteen. i.e. saying the space shuttle slows by friction is like saying that when you jump off a building and hit the ground, it's friction that slows you down. Except rather than hitting a single ground, the space shuttle re-entering the atmosphere is sort of like Jackie Chan jumping off the top of a building and crashing through 2 awnings before hitting the ground, each of which slow him down a little. But in the case of the shuttle, it's more of a continuous function, rather than a step function.

 

[PieIsAwesome]

The shuttle could be loaded with nukes and fitted with a shield in front of it. Upon reentry the shuttle could successively fire the nukes ahead of it to kill the air and avoid heat due to friction and compression of the air.

 

[gwai lo]

I'm not an astrophysicist so I don't know just how hot the SSME and SRBs get, but it could potentially be hotter than the heat from atmospheric friction.

Heat fluxes can get as high as 8 kW/cm², so you can compare that to boiling a pot of water in 0.125 seconds.

Which, on a very fundamental level, is based in friction. On a very simple level the action is similar to rubbing your hands together, and then rubbing them together harder

i think he's getting at shocks causing a large amount of the heating.

just looking up my normal shock table, at Mach 15, the temperature ratio is 44.69, so the temperature is getting heated up about 45 times. obviously the shuttle doesn't come in at just one Mach number, but I just picked something high to demonstrate the point.

so if the ambient temperature around the shuttle is pretty hot, then there should be quite a bit of convective heat transfer...i guess friction gets bundled into that somewhere..

 

[MrDudeMan]

Mach 3 is roughly 1500 mph, a satellite in low earth orbit would have to be traveling at around 17,000 mph to keep from falling back into the atmosphere. The farther away from the planet the slower the speed required to hold a stationary orbit. Of course, you'd need more fuel to get to that orbit and more fuel to slow your decent from the higher altitude.

Yeah, I'm back to the practicality thing again...sorry.

I just wanted to point out that Mach 1 is approximately 761 mph, so Mach 3 is ~2281 mph.

 

[Electric Amish]

I would think you could as long as you go really, really slow.

 

[Mr. Pedantic]

Imagine firing a .50 cal during a spacewalk!

Imagine being lost in space forever!

1 - Have a tube from the ground to the docking station in space where the inside of the tube is a vacuum.

Then how do you slow the thing down?

Thanks. I hadn't looked anywhere to see what percent was actually from friction. Only 20%. Nice. You could grossly simplify it to say "it's friction", and you'd mostly do that because the average person in the US has no clue that when you compress air, the pressure is going to increase. The reality is that the air can't move fast enough to get out of the way of the shuttle which is moving at mach xteen. i.e. saying the space shuttle slows by friction is like saying that when you jump off a building and hit the ground, it's friction that slows you down. Except rather than hitting a single ground, the space shuttle re-entering the atmosphere is sort of like Jackie Chan jumping off the top of a building and crashing through 2 awnings before hitting the ground, each of which slow him down a little. But in the case of the shuttle, it's more of a continuous function, rather than a step function.

I was still closer...

 

[marvdmartian]

He does it all the time!

As to the question posed by the OP......let's say you have an anti-gravity device on your ship, that can make you go up or down (in and out of the atmosphere) at any rate of speed you choose. In that case, yes, you could enter the atmosphere at a slow enough rate to not heat up.

It's not the presence of air that causes the heat build up, it's the friction as your 17000mph spacecraft moves through the air that causes the heat. Since you're not firing engines to continue moving that fast, the other effect is that your spacecraft slows down, again, due to friction against the air.

 

[dighn]

Then how do you slow the thing down?

induction. a reverse mass driver. you could probably get some power out of it too.

 

[Rubycon]

Imagine being lost in space forever!

I can think of a few posters here that are.

 

[gwai lo]

I just wanted to point out that Mach 1 is approximately 761 mph, so Mach 3 is ~2281 mph.

only at sea level though.

 

[drinkmorejava]

Which, on a very fundamental level, is based in friction. On a very simple level the action is similar to rubbing your hands together, and then rubbing them together harder

I'm not sure I would try to describe discrete air particles hitting a surface and a plasma sandwiched between a shockwave and an orbiter as fundamentally the same. Yes, they're both convection, but no one would look at it and say, "Hey guys, look what happens when I rub my hands really frickin hard."

 

[Cattlegod]

Think we can use our earthquake machine to create a mountain that goes into space for a landing? Then use it again to bring the mountain back down?

 

[zinfamous]

The reason why is due to fuel constraints. To control the speed, you'd need so much fuel that the size of the craft would be too big to launch in the first place.

I think a gram or 2 of unobtanium would be sufficient, no?

 

[dighn]

Think we can use our earthquake machine to create a mountain that goes into space for a landing? Then use it again to bring the mountain back down?

that's the most awesomest idea i've ever heard. my mind is blown.

 

[Rubycon]

well, a space .50 cal will need to have it's own exhaust hollow tube like rpg.

That would defeat the purpose though. A gun has to closed (or chambered) in order to propel the round forward. Now if we're talking about a "rocket bullet" that's different.

 

[Mr. Pedantic]

That would defeat the purpose though. A gun has to closed (or chambered) in order to propel the round forward. Now if we're talking about a "rocket bullet" that's different.

That's not true. An open-ended gun just means that the propellant wasted half its momentum.

 

[Rubycon]

That's not true. An open-ended gun just means that the propellant wasted half its momentum.

Throw a bullet in a fire and see how far the slug goes.

 

[AeroEngy]

Throw a bullet in a fire and see how far the slug goes.

The .50 round is approximately 117 grams (45g bullet, 14.2g powder and about 67.8g for case and primer). This varies quite a bit depending on loading and round type but is probably about average.

Assuming the bullet and the case are imparted with equal energies from the propellant. Then the Velocity of the bullet is sqrt(Mc/Mc*Vc^2) which works out to be 1.23 times the velocity of the case.

So it will probably go farther than the case ...

 

[alkemyst]

If you're as cool as me, you'd only generate heat when you want too.

 

[yh125d]

I'm not sure I would try to describe discrete air particles hitting a surface and a plasma sandwiched between a shockwave and an orbiter as fundamentally the same. Yes, they're both convection, but no one would look at it and say, "Hey guys, look what happens when I rub my hands really frickin hard."

Sure you could. It's analogous to sand-blasting, except on an atomic level instead of particle level. Either way, it's being bombarded by matter, whether it is in plasma form or not is irrelevant (to the strict question of is friction involved


What I'm saying is that the heat caused by the compression of the air surrounding is caused by, ultimately, friction. In fact, when kinetic energy directly causes gains in thermal energy it's always due to friction in some way I think.

 

[skyking]

What causes the heat is your re-entry speed vs. the atmosphere. If you were able to use retro rockets to keep you at a slow reentry speed, you would not heat up. You would need an incredible amount of fuel to do this however.

That's it in a nutshell. You could descend right back onto the launch pad if you had enough thrust. Possible yes, practical no.