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samkent
2007-Aug-15, 04:32 PM
Things are slow at work so my mind is wandering.

Would a childs balloon properly inflated, left outside the shuttle, burn up as it's orbit decayed?
Or would it make it through in a pretty deflated state many months later?

Ignor any deteration from age, UV damage, or the possibility of impact damage.

I suspect that the low mass compared to it's surface area would allow it to slow down in the trace atmosphere of 120 or so miles up without the severe heating effects.

NEOWatcher
2007-Aug-15, 04:49 PM
I suspect that the low mass compared to it's surface area would allow it to slow down in the trace atmosphere of 120 or so miles up without the severe heating effects.
I wouldn't know what the mass effects would be, but I see the problem as a difficult one to picture, because the amount of gas in the balloon to keep it inflated would be virtually nothing compared to when it would have serious atmospheric affects.

My guess would be that the balloon would be deflated enough to have a high enough density to be affected in some way.

Aiwe
2007-Aug-15, 05:00 PM
Off the top of my head: the balloon would expand and burst before entering the atmosphere, because the pressure inside the balloon would have no resistance outside from the vacuum of space. Assuming it was given a bit of magical ability to last in space until reaching the atmosphere, the same would happen because of big pressure differences between the higher parts of the atmosphere and the inside of the balloon. So, if you're assuming a regular kid's balloon blown up in an environment with a pressure around that of sea level there wouldn't be much chance to observe what happens with orbit decay.

antoniseb
2007-Aug-15, 05:28 PM
Ignor any...
Assuming it was made of something that could withstand the internal pressure with a near vacuum outside, the question becomes one of how rapidly would it decelerate at the ISS's altitude, and whether it would drop down to a subsonic speed before deorbiting into a denser atmosphere.

Based on the behavior of the Echo Satellites, I'd say the answer is that it would take quite a while (months) to deorbit. I suspect that when it did so, it would have a fiery moment, but that is a guess.

pghnative
2007-Aug-15, 05:29 PM
Off the top of my head: the balloon would expand and burst before entering the atmosphere, because the pressure inside the balloon would have no resistance outside from the vacuum of space. Whether it would explode or not would depend on how and where it was filled.

Consider a balloon filled at sea level. Ambient pressure is ~ 15 pounds per square inch (psi). The inside of the balloon will be higher than that, maybe 18 psi. So the difference in pressure ("delta P") is 3 psi.

So if an astronaut filled a latex balloon to 18 psi while inside the shuttle, it would look about the same as the 18 psi balloon at sea level. (I presume that the shuttle atmosphere is also ~ 15 psi). If he/she takes it on a space walk, it would almost certainly explode.

But if the astronaut took the empty balloon out on the space walk (along with a small helium cylinder), and filled the balloon to 3 psi, then it would pretty much look the same as the 18 psi balloon at sea level, since the delta P is the same. So no explosion.

Now if that balloon was able to re-enter gently, it would eventualy encounter pressures above 3 psi, so it would shrink (which allows the pressure in the balloon to increase. If it gently reached sea level, it would be ~ 1/5 it's original size (if not smaller), and be 15 psi or more inside the balloon (how much more depends on the elasticity of the latex).

Still, the balloon is travelling wicked fast (~ 5 miles/sec, I think) --- hard for me to believe that you can decelerate that much speed slow enough to not burn the balloon up.

mugaliens
2007-Aug-15, 05:42 PM
Off the top of my head: the balloon would expand and burst before entering the atmosphere, because the pressure inside the balloon would have no resistance outside from the vacuum of space.

Um, no.

All balloons with an exterior shell that's under tension has a pressure differential between the inside gases and the outside gases (or vacuum).

For kicks and grins, let's say it's 1.0 psi. Thus, at sea level, achieving a 1.0 psi differential might expand the balloon to a 12 inch diamter. However, the gas inside would be at 15.7 psi.

But the question was, let's take a balloon already in space. Thus, to achieve a 12 inch diameter, now all you need is a 1.0 psi inside to counter the 0 psi of space's vacuum.

The problem is that by the time the balloon reaches 10,000 feet, it's half it's original volume, or 10.5 inches in diameter (someone please check my math, that sounds large...) to maintain the 1.0 psi differential.

Another factor is that as the balloon shrinks as it descends, it won't be able to maintain 1.0 psi.

Back to the question of whether not it would survive...

...No.

The space shuttle, at just 1/100th of a G during reentry, is already exhibiting the characteristic ionization of superheated gases. Unless the balloon is made of pure carbon, it would melt and rupture when it encountered any appreciable atmosphere.

"But what if it was at the very edge, not reentering from orbit, and was simply dropped? Wouldn't it reach terminal velocity?"

Good question!

Yes, it would. However, long before it reached terminal velocity, it would reach a high enough velocity to melt.

Since there is no distinct boundary between our atmosphere and outer space (merely progressively less air pressure), let's use the Kármán line (http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line). This arbitrary boundary is the altitude at which an aircraft would have to be going travelling faster than orbital velocity in order to generate enough lift to maintain its altitude. Thus, at this altitude, the wings are fairly pointless, even though a propulsion system is still required to counter drag.

This altitude is 62 miles (about 100 km).

By comparison, to obtain a true boundary for space, where the density below the altitude is greater than the density above it, you'd have to travel about 100 times as high, say, 6,200 miles.

If you let a balloon go at the Kármán line, there's sufficient density that the balloon would reach terminal velocity lower than that required to heat and melt rubber.

However, if you were to release the balloon at 6,200 miles altitude, within several miles, long before it encountered enough of an atmosphere to cause any appreciable heating, it's velocity would have surpassed that required to heat the rubber to melting. Thus, by the time it did encounter enough atmosphere to cause heating, it would melt and burst before it slowed down enough that heating was no longer a factor.

That's for a typical latex toy balloon.

However, there are expandable plastics with higher melting points which could be formed into a balloon with thin enough walls such that it would slow to a velocity less than that required to melt it if released even at 6,200 miles, or even if it followed the Shuttle's reentry trajectory.

samkent
2007-Aug-15, 06:02 PM
I was assuming the balloon was inflated outside of a pressurized cabin.

Using http://www.vernier.com/caliper/spring02/balloon.html as a guide, I assume the balloon can hold 15 psi above the ambient pressure. Which in orbit is zero.

I realize the amount of compressed gas used will be smaller. Therefore it should not pop instantly.

Mugaleins post is interesting. But what about skipping off of the upper atmosphere until alot of the orbital speed is bled off? Also wouldn't the cold of space keep the heating below the melting point in the case of such a lo mass object?

NEOWatcher
2007-Aug-15, 06:08 PM
Mugaleins post is interesting. But what about skipping off of the upper atmosphere until alot of the orbital speed is bled off? Also wouldn't the cold of space keep the heating below the melting point in the case of such a lo mass object?
I thought his post was excellent... Well; at least from the point of being impressed at the work that was done.

But; skipping off an atmosphere will start to reduce the speed, but only a miniscule amount of speed will have been lost before it stops skipping. So; the skipping does not do much good.

I'm not good at thermodynamics, but the simple fact that the pressure is enough to heat up the cold air tells me that there's not enough potential for the gained heat to radiate away.

Aiwe
2007-Aug-15, 08:07 PM
Whether it would explode or not would depend on how and where it was filled.

Consider a balloon filled at sea level. Ambient pressure is ~ 15 pounds per square inch (psi). The inside of the balloon will be higher than that, maybe 18 psi. So the difference in pressure ("delta P") is 3 psi.

So if an astronaut filled a latex balloon to 18 psi while inside the shuttle, it would look about the same as the 18 psi balloon at sea level. (I presume that the shuttle atmosphere is also ~ 15 psi). If he/she takes it on a space walk, it would almost certainly explode.


I should have clarified I was making this assumption. :doh:


Um, no.

All balloons with an exterior shell that's under tension has a pressure differential between the inside gases and the outside gases (or vacuum).



Sorry, my reply was poorly worded. I was thinking in the back of my mind about what your post explained, but it didn't end up on the keyboard; I'll refrain from posting without coffee in me from now on ;) Sorry for any confusion I caused, Samkent.

Now to post something constructive, I hope:


The problem is that by the time the balloon reaches 10,000 feet, it's half it's original volume, or 10.5 inches in diameter (someone please check my math, that sounds large...) to maintain the 1.0 psi differential.

Halving the volume of a balloon with an originally 12 in diameter would result in a balloon of about 9.5 in diameter, so 10.5 is close.

neilzero
2007-Aug-15, 11:02 PM
Instead of inflating the balloon in the space shuttle or ISS = international space station, You could squeeze out most of the air as you tie off the neck of the balloon with perhaps 1/10 cubic inch of air inside. When moved to the vacuum of space, the bit of air inside would expand to about 100 cubic inches inflating the balloon to near full size. As the air cooled (it is cold outside) it would shrink to about half size, but it would expand again as it spiraled lower over perhaps the next hundred hours. I'm not sure that it would get hot enough to damage the rubber on re-enty. If it survived, it would shrink as the pressure increased outside gradually back to the 1/10 cubic inch, or 1/15 th cubic inch since the Shuttle and ISS typically have a bit less than sealevel air pressure.
My guess is toy balloons can only tolerate about 1/5 th psi difference between inside and outside pressure and this falls to about 1/10 th PSI as the balloon shrinks to half volume after being fully inflated. Neil

joema
2007-Aug-16, 08:45 PM
...Would a childs balloon properly inflated, left outside the shuttle, burn up as it's orbit decayed? Or would it make it through in a pretty deflated state many months later?...I suspect that the low mass compared to it's surface area would allow it to slow down in the trace atmosphere of 120 or so miles up without the severe heating effects.
This is a good question. You said "properly inflated", so we naturally assume the balloon is pressurized for a round shape in a vacuum.

The question is would such a balloon, made of typical material, survive reentry from a circular orbit of about 170 nautical miles.

We tend to think of the heat of reentry and high pressure, and considering the fragile nature of a balloon, it must burst.

However a balloon has an extremely high ratio of surface area to mass. Many similar objects have already survived reentry. From Columbia STS-107, Skylab, etc. we know individual sheets of paper, lightweight fragile insulation sheets, etc. can survive reentry largely undamaged.

The key is those objects (like balloons) all have a high ratio of surface area to mass.

Why weren't sheets of paper, etc burned up by the heat of reentry? Because their low mass and large surface area allowed them to decelerate rapidly before heat buildup damaged them.

A balloon can actually take a fair amount of heat, provided it's spread out and not localized. A candle flame will burst a balloon if applied to one spot, but high ambient temperature applied all over might not.

Likewise a balloon can take a fair amount of atmospheric dynamic pressure, provided it's not localized. E.g, a thumb tack will burst a balloon, as this applies a lot of pressure to a tiny area. But you can press a balloon between your hands pretty hard before it bursts. The balloon's low mass and high surface area would cause rapid deceleration from minimal drag, long before dynamic pressure ("q") became very high.

The two key reentry effects - heat and atmospheric dynamic pressure can damage many objects.

However the fact other fragile objects with high surface area to mass ratios have survived reentry implies a balloon might possibly also survive.

There are various types of child's balloons -- rubber of various thicknesses, mylar, etc. If one type didn't survive, another might. Metallic mylar is probably more heat resistant than rubber.

schlaugh
2007-Aug-16, 08:50 PM
Why weren't sheets of paper, etc burned up by the heat of reentry? Because their low mass and large surface area allowed them to decelerate rapidly before heat buildup damaged them.

But weren't those objects - papers, videos, etc. - contained within a larger craft for the most substantial portion of reentry? IOW they were protected by the shell of Skylab or Columbia through the worst part of re-entry, thus the comparison doesn't seem to work.

mugaliens
2007-Aug-16, 10:14 PM
But weren't those objects - papers, videos, etc. - contained within a larger craft for the most substantial portion of reentry? IOW they were protected by the shell of Skylab or Columbia through the worst part of re-entry, thus the comparison doesn't seem to work.

Quite correct. In fact, they were spread throughout the atmosphere after most of the compartments reached their terminal velocities in the lower, denser atmosphere.

And many of the papers and materials were never recovered. Some were with partial burning, indicating they were released higher.

joema
2007-Aug-17, 02:09 AM
But weren't those objects - papers, videos, etc. - contained within a larger craft for the most substantial portion of reentry? IOW they were protected by the shell of Skylab or Columbia through the worst part of re-entry, thus the comparison doesn't seem to work.
Not necessarily. At breakup, Columbia was moving at Mach 18.3 and 39 miles high when this happened, which abruptly disgorged the vehicle contents.

This was actually a more severe case than if lightweight objects reentered by themselves from orbit. When Columbia disintegrated, they were almost instantly thrust into a hotter, higher dynamic pressure environment. Also the surrounding heavy structure (which had a lower surface area to mass ratio) was very hot, and any proximity would scorch the lightweight objects.

By contrast if pieces of paper (or a balloon) reentered from orbit, they'd decelerate rapidly at a much higher altitude, before either dynamic pressure or heat became severe.

At entry interface (about 400,000 ft), dynamic pressure is about 2 psf.

A typical 8" dia. latex balloon might weigh 2 grams, and have a cross-sectional area of about 50 square inches. From f=ma we can crudely approximate the deceleration at entry interface.

.34 ft^2 * 2 psf = 0.68 lbf, or 3 Newtons

a = f/m
a = 3 N / 0.002 kg
a = 1,500 meter per second (153 g)

Now 153 g's sounds like a lot, but for a 2 gram balloon it's only about 3/4 of a pound. They can easily survive that much force. In reality the ramp-up would be more gradual and begin at a higher altitude. Why?

Entry interface is an arbitrary definition, and at even higher altitudes (and lower dynamic pressures) a balloon would begin deceleration.

Heating effects are extremely low at entry interface. Yet the faint atmospheric drag would rapidly decelerate objects with a large ratio of surface area to mass. All these facts make me think a balloon might survive reentry.

publiusr
2007-Aug-24, 06:49 PM
A ballute--made a little tougher would be good to use for aerobraking.

hhEb09'1
2007-Aug-25, 03:44 AM
Based on the behavior of the Echo Satellites, I'd say the answer is that it would take quite a while (months) to deorbit. I suspect that when it did so, it would have a fiery moment, but that is a guess.Echo was the first thing I thought of when I read the OP. Let's see, ... nothing in google about re-entry I can see. Hey, what about a this (http://www.aero.org/publications/crosslink/fall2006/backpage.html), a big balloon of aluminized Mylar polyester film, filled with aerogel and vented? :)

PS: This wiki page (http://en.wikipedia.org/wiki/PAGEOS) discusses PAGEOS re-entry, sorta.

PSS: Echo IA (http://en.wikipedia.org/wiki/Echo_satellite) "reentered Earth's atmosphere and burned up on May 24, 1968."