A Space & astronomy forum. SpaceBanter.com

Go Back   Home » SpaceBanter.com forum » Space Science » Space Science Misc
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

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



 
 
Thread Tools Display Modes
  #11  
Old July 26th 04, 03:36 PM
Trakar
external usenet poster
 
Posts: n/a
Default Could a bullet be made any something that could go from orbit to Earth's surface?

On Fri, 16 Jul 2004 06:15:40 -0700 (PDT), (Derek
Lyons) wrote:

"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.


Isn't this more or less equivalent to the speculative Thor KE orbital
weapon system? Admittedly, a standard bullet may not have the KE to
maintain goodly velocity all the way to the ground (gonna have to buy
some envelopes ;-).
For a little additional information you might want to check out;
http://www.ucsusa.org/documents/CM_apF-J.pdf
It is more about doing heat shield calculations for small re-entry
warheads (bomblets) of different shapes, but, it does seem to cover
the basics involved here pretty well.
With the light mass (what around a 100grains?) of a rifle bullet, drag
is going to be a bigger factor than it would with these larger masses
(10 Kg ) but the initial velocity is some 2.5x what they are working
with also.
"Democracy is when the indigent,
and not the men of property,
are the rulers."
Aristotle

  #12  
Old July 26th 04, 03:36 PM
Scott T. Jensen
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

"Alain Fournier" wrote:
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.


Please elaborate. I don't see why it wouldn't hit the Earth.

Scott Jensen
--
Got a business question, problem, or dream?
Discuss it with the professionals that hang out at...
misc.business.consulting, misc.business.marketing.moderated
misc.business.moderated, and misc.entrepreneurs.moderated

  #13  
Old July 27th 04, 05:01 AM
Alain Fournier
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

Scott T. Jensen wrote:
"Alain Fournier" wrote:

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.



Please elaborate. I don't see why it wouldn't hit the Earth.



It is moving around the Earth at a speed of a little over
3 km/s (it goes around Earth once every 24 h at a distance
from the center of the Earth of about 42 164 km). Most riffles
have their bullets exit at less than 1 km/s. So no matter what
direction you shoot the bullet still has a substantial part
of its original velocity. Enough to stay in orbit. Don't let
yourself be fooled by the fact that a geo-stationary satellite
stays always above the same point of Earth, the satellite is
still moving around Earth at high speed.

As the bullet goes down towards Earth its angular velocity will
go up, because with the same speed you can go around Earth in
less time when you are closer to Earth and because of the way
orbital mechanics works, when you fall towards Earth you gain
some speed. So even though at first you stay above the same
point over Earth while you go down you will start moving east.


Alain Fournier

  #14  
Old July 27th 04, 02:00 PM
Scott T. Jensen
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

"Alain Fournier" wrote:
Scott T. Jensen wrote:
"Alain Fournier" wrote:
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.


Please elaborate. I don't see why it wouldn't hit the Earth.


It is moving around the Earth at a speed of a little over
3 km/s (it goes around Earth once every 24 h at a distance
from the center of the Earth of about 42 164 km). Most riffles
have their bullets exit at less than 1 km/s. So no matter what
direction you shoot the bullet still has a substantial part
of its original velocity. Enough to stay in orbit. Don't let
yourself be fooled by the fact that a geo-stationary satellite
stays always above the same point of Earth, the satellite is
still moving around Earth at high speed.

As the bullet goes down towards Earth its angular velocity will
go up, because with the same speed you can go around Earth in
less time when you are closer to Earth and because of the way
orbital mechanics works, when you fall towards Earth you gain
some speed. So even though at first you stay above the same
point over Earth while you go down you will start moving east.


Please bear with me on this. I'm trying to understand the mechanics of what
you're saying.

I naturally agree that even at geo-stationary orbit, the rifle would also be
orbiting at a great speed. However...

The bullet is fired directly down at the Earth. Rounding things to make it
easier, let's say it is traveling at 1 km/s, from geo it is 42,000 km from
Earth's center, Earth's radius being 6,000 km (yes, I know I just shrunk the
Earth but I'm going for simplicity here so bear with me), and thus the
bullet has to travel 36,000 km to hit the equator. That means it will take
the 1 km/s bullet 36 seconds to strike IF it can go down. There is no real
drag on the bullet until the last hundred miles. There will be some
gravitational pull but that's pulling it down as well. I do not see how it
would then go into orbit and not enter our air and, saying it is made of the
right stuff, hit the surface.

Scott Jensen
--
Got a business question, problem, or dream?
Discuss it with the professionals that hang out at...
misc.business.consulting, misc.business.marketing.moderated
misc.business.moderated, and misc.entrepreneurs.moderated

  #15  
Old July 27th 04, 08:10 PM
dave schneider
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

"Scott T. Jensen" wrote:
[...]
bullet has to travel 36,000 km to hit the equator. That means it will take
the 1 km/s bullet 36 seconds to strike IF it can go down.


hmmm, 36000/1 = 36?

Mayhap we need to track a decimal point a wee bit better....

/dps

  #16  
Old July 28th 04, 05:27 PM
Scott T. Jensen
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

"dave schneider" wrote:
"Scott T. Jensen" wrote:
[...]
bullet has to travel 36,000 km to hit the equator. That means
it will take the 1 km/s bullet 36 seconds to strike IF it can go
down.


hmmm, 36000/1 = 36?

Mayhap we need to track a decimal point a wee bit better....


Oops! *laugh* Yes. Sorry about that.

Scott Jensen
--
Like a cure for A.I.D.S., Alzheimer, Parkinson, & Mad Cow Disease?
Volunteer your computer for folding-protein research for when it's idle.
Go to http://www.distributedfolding.org/ to sign up your computer.

  #17  
Old July 28th 04, 05:28 PM
Filip Larsen
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

Scott T. Jensen wrote

Please bear with me on this. I'm trying to understand the mechanics

of what
you're saying.


Don't try to understand orbital mechanics using the normal concept of
speed and distance, like you know them from everyday travelling
situations. Orbital mechanics are often counter-intuitive in this
regard.

If you want something in circular orbit around earth to enter the
atmosphere, you must change its velocity so that it enters a elliptical
orbit where the lowest point is inside the atmosphere, say at 100km just
to keep the numbers simple.

The most effecient way of doing this (i.e. requiring lowest change of
speed) is to "break" by altering the velocity directly opposite the
current velocity wrt to earth. If you are in a normal prograde
equatorial orbit the orbital velocity vector is pointing east and you
therefore need to change the velocity towards west, which means you want
to shoot your rifle towards west if you want the bullet to go low.

However, beside direction, your bullet also need a certain minimum speed
in order to be able to reach the atmosphere. The higher the orbit the
more speed is required. For instance, at 300km altitude the bullet only
needs around 60 m/s, but at 36000 km altitude (GEO) the required speed
is around 1500 m/s. So depending on the gun, it should be able to
re-enter a bullet at least up to some fairly high altitude, but probably
not so high as geostationary altitude.

The math is as follows: If you are in a cirular orbit at altitude h, you
are r = h + R from the center of earth (with R = 6378 km) and you are
moving with respect to the center of earth at a speed of

v = sqrt(k/r),

where k = 398601 km^3/s^2 is the gravitational constant for earth (also
called GM). Now you want to change your orbit into an elliptical one
with the highest point at radius r, and the lowest at s = 100km + R. In
such an orbit you would at the highest point need an orbital speed of

w = sqrt(k*(2/r-1/s))

which means that you must change your speed with

dv = v - w

in order to change from the circular orbit to the elliptical orbit. If
you use the numbers as I have written them above, then the speed unit
will be km/s.


Regards,
--
Filip Larsen

  #18  
Old July 29th 04, 06:56 PM
Greg D. Moore (Strider)
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

"Scott T. Jensen" wrote in message
...

The bullet is fired directly down at the Earth. Rounding things to make

it
easier, let's say it is traveling at 1 km/s, from geo it is 42,000 km from
Earth's center, Earth's radius being 6,000 km (yes, I know I just shrunk

the
Earth but I'm going for simplicity here so bear with me), and thus the
bullet has to travel 36,000 km to hit the equator. That means it will

take
the 1 km/s bullet 36 seconds to strike IF it can go down. There is no

real
drag on the bullet until the last hundred miles. There will be some
gravitational pull but that's pulling it down as well. I do not see how

it
would then go into orbit and not enter our air and, saying it is made of

the
right stuff, hit the surface.


Keep in mind it's still moving "sideways" with it's original orbital
velocity.

So, 1,000 seconds later it's also moved eastward several thousand km. And
as a result it "misses" the Earth.

It starts moving "up" again until it reaches its perigee and then starts
falling "down" again. Of course it's moving "eastward" the entire time so
it keeps missing the Earth.


Scott Jensen
--
Got a business question, problem, or dream?
Discuss it with the professionals that hang out at...
misc.business.consulting, misc.business.marketing.moderated
misc.business.moderated, and misc.entrepreneurs.moderated


  #19  
Old July 29th 04, 06:56 PM
Scott T. Jensen
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

Filip,

Thanks for the explanation. I think I'm getting it now. I want to slow
down the bullet's speed and the way to do that is fire it backwards. The
lower the orbit, the slower this backfire needs to be.

Have I've gotten it yet?

Scott Jensen
--
Got a business question, problem, or dream?
Discuss it with the professionals that hang out at...
misc.business.consulting, misc.business.marketing.moderated
misc.business.moderated, and misc.entrepreneurs.moderated

  #20  
Old July 30th 04, 12:33 AM
Alain Fournier
external usenet poster
 
Posts: n/a
Default Could a bullet be made something that could go from orbit to Earth's surface?

Scott T. Jensen wrote:
Filip,

Thanks for the explanation. I think I'm getting it now. I want to slow
down the bullet's speed and the way to do that is fire it backwards. The
lower the orbit, the slower this backfire needs to be.

Have I've gotten it yet?

Scott Jensen


You are getting close. You are right that the best way to de-orbit
is to fire backwards. But you are wrong about the lower orbit needing
slower backfire. The lower the orbit the faster the orbit, so you
need a bigger backfire. This is offset by the fact that if you are
in a low orbit you only need to get the orbit a little elongated for
it to intersect Earth's atmosphere. So in a very low orbit you need
only a small push to de-orbit, in a very high orbit you also need only
a small push to de-orbit (but this time it has to be more precise
if the push is too strong you will end up orbiting in the other
direction or even escaping, if the push is too weak you will will
stay in orbit just change the shape of the orbit). It is if you are
somewhere between very low orbit and very high orbit that you need
the biggest push to re-enter.

Alain Fournier

 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT +1. The time now is 05:57 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 SpaceBanter.com.
The comments are property of their posters.