3 ms·

Any kind of ballistic shot? That doesn't seem right. If you can make the initial impetus arbitrarily powerful and the projectile arbitrarily light, surely at

by pg 17y ago

Any kind of ballistic shot? That doesn't seem right. If you can make the initial impetus arbitrarily powerful and the projectile arbitrarily light, surely at some point you cross the threshold of being able to get it into orbit.

yread 17y ago
obviously you are right, that's why we can see Earth as a blue planet from the space - the photons bounce of the ground and leave the atmosphere without tangential thrust. But theoretically if we were the only body in space they would eventually fall back. It's needed only when you want to escape the gravity field by exceeding the escape velocity.
- ivankirigin 17y ago
  Thrust for massless photons? Photons escape earth's gravitational pull because the gravity is not large enough to bend space to collapse the photon back onto the earth.
deleted 17y ago
[deleted]
jws 17y ago
One orbit. Consider the projectile just as it leaves the barrel, if it is in orbit then this point is on its orbit. It will return. The earth will have rotated the gun away, but the projectile is still going to come back into the atmosphere where it will lose too much energy to continue. Any useful orbit needs to be above the atmosphere and would have to be shot from there. You could use a secondary nudge of some sort to round out the orbit once it is in space and keep the projectile up, but making such a device that will survive the initial acceleration is unlikely.
- astine 17y ago
  What if we took advantage of the drag that the atmosphere exerted on the object as it ascended to change it's orbit or to move it into a useful orbit? Edit: Never mind, that would only work if we wanted to lower the orbit.
  - randallsquared 17y ago
    Lowering the orbit isn't the problem, if it's already fast enough. The problem is that the end of the velocity change is still going to be in the atmosphere.
  - jws 17y ago
    I think this can get you most of the way there, using some form of atmospheric steering. Best case you could get your nearest approach tangent to the highest spot of atmosphere. You still need a bit more to avoid the atmosphere entirely, but your are mostly there.
- sho 17y ago
  "making such a device that will survive the initial acceleration is unlikely" Bear in mind that using an electromagnetic launcher you could accelerate (relatively) gradually over a length of track, the so-called "Mass Driver" concept.
  - ivankirigin 17y ago
    The friction with the air at its highest speed (and where the air is densest) would still be a problem.
    - rjurney 17y ago
      Which is why you run it up the Himalayas, right? :)
DanielBMarkham 17y ago
Nope. At least where I see from the simulator. Either it's gone completely -- escape velocity -- or it falls right back down. I think the problem is that orbiting bodies are continually "falling over the horizon", ie, there is a high degree of speed parallel to the ground underneath them for orbiting to work. (Or a low speed with a small pull of gravity, etc. But always some significant forward motion) You can do some interesting things with packages that have thrusters on them. If I remember correctly you could have a 300-400km railgun shoot boxcar-sized payloads into orbit with very little additional thrust. It's a helluva cheap way to send mass into space -- once you spend a couple hundred billion for the railgun setup (which is still a good deal, btw)
- pg 17y ago
  The arguments about why it wouldn't work to shoot a projectile into orbit sound convincing, so I suspect they're right. But what actually happens as you launch a projectile at a 45% angle with increasing velocities?
  - DanielBMarkham 17y ago
    I couldn't find the simulator, but as I recall, from any point on the surface, whatever initial velocity you give a projectile will impact the surface at some time later, if it doesn't achieve escape velocity. A good way to think of it is this way: if you were able to point the cannon tangential to the ground, ie horizontal, and you were to fire the projectile at exactly orbital speed for that elevation, then you would achieve orbit at ground level. If, however, you elevate the cannon at all from horizontal, you create an oblong orbit that intersects the surface at some point (even if the orbit extends thousands of miles up into space) That's because all orbits are elliptical to some degree. No matter what the angle or power, you're just creating an orbit with some degree of stretch that eventually comes back around to where you launched. A 45-degree launch from one place would just be the same as launching at 90-degrees from another place on the globe. BTW, this makes for a nice hobby program to write over the weekend. I ended up adding angle, initial velocity, and projectile thrust. I also added scaling where you could either go very small, like a model rocket launch, or zoom out to include the moon. I was very surprised at the sizes of the earth and moon and the distances between them.

by pg 17y ago

yread 17y ago

obviously you are right, that's why we can see Earth as a blue planet from the space - the photons bounce of the ground and leave the atmosphere without tangential thrust. But theoretically if we were the only body in space they would eventually fall back. It's needed only when you want to escape the gravity field by exceeding the escape velocity.

ivankirigin 17y ago
Thrust for massless photons? Photons escape earth's gravitational pull because the gravity is not large enough to bend space to collapse the photon back onto the earth.

deleted 17y ago

[deleted]

jws 17y ago

One orbit. Consider the projectile just as it leaves the barrel, if it is in orbit then this point is on its orbit. It will return. The earth will have rotated the gun away, but the projectile is still going to come back into the atmosphere where it will lose too much energy to continue. Any useful orbit needs to be above the atmosphere and would have to be shot from there. You could use a secondary nudge of some sort to round out the orbit once it is in space and keep the projectile up, but making such a device that will survive the initial acceleration is unlikely.

astine 17y ago
What if we took advantage of the drag that the atmosphere exerted on the object as it ascended to change it's orbit or to move it into a useful orbit? Edit: Never mind, that would only work if we wanted to lower the orbit.
- randallsquared 17y ago
  Lowering the orbit isn't the problem, if it's already fast enough. The problem is that the end of the velocity change is still going to be in the atmosphere.
- jws 17y ago
  I think this can get you most of the way there, using some form of atmospheric steering. Best case you could get your nearest approach tangent to the highest spot of atmosphere. You still need a bit more to avoid the atmosphere entirely, but your are mostly there.
sho 17y ago
"making such a device that will survive the initial acceleration is unlikely" Bear in mind that using an electromagnetic launcher you could accelerate (relatively) gradually over a length of track, the so-called "Mass Driver" concept.
- ivankirigin 17y ago
  The friction with the air at its highest speed (and where the air is densest) would still be a problem.
  - rjurney 17y ago
    Which is why you run it up the Himalayas, right? :)

DanielBMarkham 17y ago

Nope. At least where I see from the simulator. Either it's gone completely -- escape velocity -- or it falls right back down. I think the problem is that orbiting bodies are continually "falling over the horizon", ie, there is a high degree of speed parallel to the ground underneath them for orbiting to work. (Or a low speed with a small pull of gravity, etc. But always some significant forward motion) You can do some interesting things with packages that have thrusters on them. If I remember correctly you could have a 300-400km railgun shoot boxcar-sized payloads into orbit with very little additional thrust. It's a helluva cheap way to send mass into space -- once you spend a couple hundred billion for the railgun setup (which is still a good deal, btw)

pg 17y ago
The arguments about why it wouldn't work to shoot a projectile into orbit sound convincing, so I suspect they're right. But what actually happens as you launch a projectile at a 45% angle with increasing velocities?
- DanielBMarkham 17y ago
  I couldn't find the simulator, but as I recall, from any point on the surface, whatever initial velocity you give a projectile will impact the surface at some time later, if it doesn't achieve escape velocity. A good way to think of it is this way: if you were able to point the cannon tangential to the ground, ie horizontal, and you were to fire the projectile at exactly orbital speed for that elevation, then you would achieve orbit at ground level. If, however, you elevate the cannon at all from horizontal, you create an oblong orbit that intersects the surface at some point (even if the orbit extends thousands of miles up into space) That's because all orbits are elliptical to some degree. No matter what the angle or power, you're just creating an orbit with some degree of stretch that eventually comes back around to where you launched. A 45-degree launch from one place would just be the same as launching at 90-degrees from another place on the globe. BTW, this makes for a nice hobby program to write over the weekend. I ended up adding angle, initial velocity, and projectile thrust. I also added scaling where you could either go very small, like a model rocket launch, or zoom out to include the moon. I was very surprised at the sizes of the earth and moon and the distances between them.