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Could a bullet be made any something that could go from orbit to Earth's surface?



 
 
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  #1  
Old July 14th 04, 10:40 PM
Scott T. Jensen
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Default Could a bullet be made any something that could go from orbit to Earth's surface?

I'm up on the Space Station and I go out for a nice little space walk. I
take along my specially designed hunting rifle and fire a bullet down at the
planet. What would the bullet need to be made of for it to make it all the
way to the surface and not burn up on entry to our atmosphere? Is there
anything the bullet could be made of that would be able it to make the trip?
And would the speed of the bullet affect its chances? Would also the bullet
go weird like how bullets do when you shoot them into a pool of water?

Idly curious,
Scott Jensen
--
Peer-to-peer networking (a.k.a. file-sharing) is entertainment's future.
If you'd like to know why, read the white paper at the link below.
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  #2  
Old July 16th 04, 02:15 PM
Doug...
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Default Could a bullet be made any something that could go from orbit to Earth's surface?

In article , says...
I'm up on the Space Station and I go out for a nice little space walk. I
take along my specially designed hunting rifle and fire a bullet down at the
planet. What would the bullet need to be made of for it to make it all the
way to the surface and not burn up on entry to our atmosphere? Is there
anything the bullet could be made of that would be able it to make the trip?
And would the speed of the bullet affect its chances? Would also the bullet
go weird like how bullets do when you shoot them into a pool of water?

Idly curious,
Scott Jensen


Hmmm... I know you're asking a materials question, but the hypothetical
you're using is flawed.

If you're in orbit, hanging in your suit outside of the ISS, you're
traveling at about 17.5kmph. If you fire a bullet (and you'd better
hope that you're braced against the station when you fire, or else the
recoil might seriously test the design limits of your tether) straight
"down" at the Earth, that bullet leaves the barrel with its muzzle
velocity (several hundred mph, IIRC) but it's still going a LOT faster
in the direction of orbit than it's heading "down."

Since you're adding energy to the bullet's orbit perpendicular to its
orbital direction, you're not going to raise or lower its *overall*
altitude. Only adding energy into or against the orbital motion vector
will raise or lower the overall orbit.

What you *will* do is change the *shape* of the bullet's orbit. I
haven't done the math, so I don't know how much the shape would be
altered by an "average" gun and round's muzzle velocity. But let's say,
for sake of argument, that the energy would push the bullet 50 miles
closer to the Earth at its perigee. Because you haven't reduced the
bullet's orbital speed, the 50-mile decrease in the orbit's lowest point
is then matched by a 50-mile increase in its highest point. So, a
bullet that started in a circular 250-mile orbit might end up in an
elliptical 200x300-mile orbit.

Oh, and the bullet's orbit will intersect ISS's orbit every time it
crosses the 250-mile point, twice per orbit. During some of those
intersections, ISS will be where the bullet is passing through.

Now, the reality is that atmospheric drag (yes, there is still a very,
very thin, but measurable, amount of atmosphere at 250 miles) will
probably slow the bullet down a lot faster than it slows down ISS -- the
bullet has a lot less mass. So it will probably re-enter in a few days,
or weeks. At which point your original materials question comes into
play...

--

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distributed right." -Mark Twain

  #3  
Old July 16th 04, 02:15 PM
Derek Lyons
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Default Could a bullet be made any something that could go from orbit to Earth's surface?

"Scott T. Jensen" wrote:

I'm up on the Space Station and I go out for a nice little space walk. I
take along my specially designed hunting rifle and fire a bullet down at the
planet. What would the bullet need to be made of for it to make it all the
way to the surface and not burn up on entry to our atmosphere?


Unobtanium.

D.
--
Touch-twice life. Eat. Drink. Laugh.

  #4  
Old July 17th 04, 01:41 AM
Alain Fournier
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Default Could a bullet be made any something that could go from orbit to Earth's surface?

Doug... wrote:

If you're in orbit, hanging in your suit outside of the ISS, you're
traveling at about 17.5kmph. If you fire a bullet (and you'd better
hope that you're braced against the station when you fire, or else the
recoil might seriously test the design limits of your tether) straight
"down" at the Earth, that bullet leaves the barrel with its muzzle
velocity (several hundred mph, IIRC) but it's still going a LOT faster
in the direction of orbit than it's heading "down."

Since you're adding energy to the bullet's orbit perpendicular to its
orbital direction, you're not going to raise or lower its *overall*
altitude. Only adding energy into or against the orbital motion vector
will raise or lower the overall orbit.

What you *will* do is change the *shape* of the bullet's orbit. I
haven't done the math, so I don't know how much the shape would be
altered by an "average" gun and round's muzzle velocity. But let's say,
for sake of argument, that the energy would push the bullet 50 miles
closer to the Earth at its perigee. Because you haven't reduced the
bullet's orbital speed, the 50-mile decrease in the orbit's lowest point
is then matched by a 50-mile increase in its highest point. So, a
bullet that started in a circular 250-mile orbit might end up in an
elliptical 200x300-mile orbit.



The average orbital altitude will change. The apogee will raise
more than the perigee will go down. The total energy of the bullet
does increase when it is fired. Not as much as if it was fired
in the direction of motion but it still does increase.

I computed that if the original orbit (orbit of ISS) is circular
at 300 km; that the gun is pointed vertically towards Earth;
and the muzzle velocity is 1km/s then the perigee would be
480 km below sea level. At that altitude drag is very important :-)
The theoretical apogee (never reached because the bullet first
hits Earth) would be at 1300 km.


Oh, and the bullet's orbit will intersect ISS's orbit every time it
crosses the 250-mile point, twice per orbit. During some of those
intersections, ISS will be where the bullet is passing through.

Now, the reality is that atmospheric drag (yes, there is still a very,
very thin, but measurable, amount of atmosphere at 250 miles) will
probably slow the bullet down a lot faster than it slows down ISS -- the
bullet has a lot less mass. So it will probably re-enter in a few days,
or weeks. At which point your original materials question comes into
play...


The fact that the bullet has less mass is irrelevant here. What
is important is the density and the shape of the objects. The bullet
would likely be much denser than ISS, that gives it less drag. But
the lower perigee makes that not important.

Alain Fournier

  #5  
Old July 17th 04, 03:01 PM
Ian Stirling
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Default Could a bullet be made any something that could go from orbit to Earth's surface?

Doug... wrote:
In article , says...
I'm up on the Space Station and I go out for a nice little space walk. I
take along my specially designed hunting rifle and fire a bullet down at the
planet. What would the bullet need to be made of for it to make it all the
way to the surface and not burn up on entry to our atmosphere? Is there
anything the bullet could be made of that would be able it to make the trip?
And would the speed of the bullet affect its chances? Would also the bullet
go weird like how bullets do when you shoot them into a pool of water?


Hmmm... I know you're asking a materials question, but the hypothetical
you're using is flawed.

If you're in orbit, hanging in your suit outside of the ISS, you're
traveling at about 17.5kmph. If you fire a bullet (and you'd better
hope that you're braced against the station when you fire, or else the
recoil might seriously test the design limits of your tether) straight
"down" at the Earth, that bullet leaves the barrel with its muzzle
velocity (several hundred mph, IIRC) but it's still going a LOT faster
in the direction of orbit than it's heading "down."

Since you're adding energy to the bullet's orbit perpendicular to its
orbital direction, you're not going to raise or lower its *overall*
altitude. Only adding energy into or against the orbital motion vector
will raise or lower the overall orbit.

What you *will* do is change the *shape* of the bullet's orbit. I
haven't done the math, so I don't know how much the shape would be
altered by an "average" gun and round's muzzle velocity. But let's say,


Ballpark.
Say 600m/s.
1/4 of an orbit is 1350 seconds.
1/2 of time * velocity = 400Km.

This would seem to indicate that it's going to go in.

for sake of argument, that the energy would push the bullet 50 miles
closer to the Earth at its perigee. Because you haven't reduced the
bullet's orbital speed, the 50-mile decrease in the orbit's lowest point
is then matched by a 50-mile increase in its highest point. So, a
bullet that started in a circular 250-mile orbit might end up in an
elliptical 200x300-mile orbit.

Oh, and the bullet's orbit will intersect ISS's orbit every time it
crosses the 250-mile point, twice per orbit. During some of those
intersections, ISS will be where the bullet is passing through.


There is very little chance of this.

For an equatorial orbit, the chances are higher.
However, for ISS-like orbits, the plane of the orbit is usually wrong,
which makes it several orders of magnitude less likely.

Now, the reality is that atmospheric drag (yes, there is still a very,
very thin, but measurable, amount of atmosphere at 250 miles) will
probably slow the bullet down a lot faster than it slows down ISS -- the
bullet has a lot less mass. So it will probably re-enter in a few days,
or weeks. At which point your original materials question comes into


  #6  
Old July 17th 04, 03:01 PM
Ian Stirling
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Default Could a bullet be made any something that could go from orbit to Earth's surface?

Alain Fournier wrote:
Doug... wrote:

Oh, and the bullet's orbit will intersect ISS's orbit every time it
crosses the 250-mile point, twice per orbit. During some of those
intersections, ISS will be where the bullet is passing through.

Now, the reality is that atmospheric drag (yes, there is still a very,
very thin, but measurable, amount of atmosphere at 250 miles) will
probably slow the bullet down a lot faster than it slows down ISS -- the
bullet has a lot less mass. So it will probably re-enter in a few days,
or weeks. At which point your original materials question comes into
play...


The fact that the bullet has less mass is irrelevant here. What
is important is the density and the shape of the objects. The bullet
would likely be much denser than ISS, that gives it less drag. But
the lower perigee makes that not important.


Not quite.
The important bit is sectional density - mass per unit area.
On this measure, the bullet is lots less sectionally dense than ISS,
it will decellerate faster.

  #8  
Old July 17th 04, 04:36 PM
Alain Fournier
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Posts: n/a
Default Could a bullet be made any something that could go from orbit to Earth's surface?

Ian Stirling wrote:

Alain Fournier wrote:

Doug... wrote:


Oh, and the bullet's orbit will intersect ISS's orbit every time it
crosses the 250-mile point, twice per orbit. During some of those
intersections, ISS will be where the bullet is passing through.

Now, the reality is that atmospheric drag (yes, there is still a very,
very thin, but measurable, amount of atmosphere at 250 miles) will
probably slow the bullet down a lot faster than it slows down ISS -- the
bullet has a lot less mass. So it will probably re-enter in a few days,
or weeks. At which point your original materials question comes into
play...


The fact that the bullet has less mass is irrelevant here. What
is important is the density and the shape of the objects. The bullet
would likely be much denser than ISS, that gives it less drag. But
the lower perigee makes that not important.



Not quite.
The important bit is sectional density - mass per unit area.
On this measure, the bullet is lots less sectionally dense than ISS,
it will decellerate faster.


Right. I wasn't thinking straight there. But I'm not quite sure
that the bullet is lots less sectionally dense than ISS. ISS is
mostly hollow. It weighs 187 tons, has about 1000 m^2 of cross
section area. That is 187kg/m^2 or 19g per cm^2. That is more
than normal bullets, but only by a factor of a few, so they are
less sectionally dense than ISS but not really *lots* less.

Alain Fournier

  #9  
Old July 25th 04, 11:01 PM
Alain Fournier
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Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

Scott T. Jensen wrote:

"Doug..." wrote:

Since you're adding energy to the bullet's orbit perpendicular
to its orbital direction, you're not going to raise or lower its
*overall* altitude.



Oh, I see. Hmmm. What if it was fired from a geo-stationary (fixed over a
location on Earth) orbit straight down at the Earth?


It would enter an elliptical orbit. The bullet would never get
close to Earth.

Alain Fournier

  #10  
Old July 26th 04, 03:35 PM
alfred montestruc
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Default Could a bullet be made any something that could go from orbit to Earth's surface?

"Scott T. Jensen" wrote in message
...
I'm up on the Space Station and I go out for a nice little space walk. I
take along my specially designed hunting rifle and fire a bullet down at

the
planet.


Depending on the altitude of your orbit, and the muzzle velocity of the
rifle, it probably will not hit the atmosphere at all and will wind up in a
higher average orbit than you were in the first place.

If you are in a very low orbit, and have a very high muzzle velocity, you
might manage to hit the atmosphere.

Rifles have muzzle velocities from (on the low end of the scale) 1000 foot
per second to on the really high end of the scale 4000 foot per second.
Orbital velocity is about 8000 meters per second or roughly 24,000 foot per
second. If youm fire straight down with a 4000 foot per second rifle, the
total velocity of the bullet with respect to the earth is going to be about
V=(24000^2+4000^2)^.5=24,331 ft/sec. An orbit in low earth orbit takes
about 90 minutes or about 5400 seconds, 1/36 orbit which will move you 10
degrees around the earth will take 150 seconds. In that time the bullet
from a 4000 ft/sec rifle will travel 4000*150/5280=113.6 miles. But you
would natually have moved about 61.7 miles in that direction in that time
anyway for a 100 mile orbit.. If you were in a ~100 mile high or lower
orbit, then probably the bullet will burn up. The higher up, the less
likely.



What would the bullet need to be made of for it to make it all the
way to the surface and not burn up on entry to our atmosphere?


Something that does not burn, and that stays solid at very high
temperatures. A ceramic probably.



Is there
anything the bullet could be made of that would be able it to make the

trip?
And would the speed of the bullet affect its chances? Would also the

bullet
go weird like how bullets do when you shoot them into a pool of water?

Idly curious,
Scott Jensen
--
Peer-to-peer networking (a.k.a. file-sharing) is entertainment's future.
If you'd like to know why, read the white paper at the link below.
http://www.scottjensenshow.com/P2PRevolution.pdf


 




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