Artillery, and the lack of it

That, is just uncalled for.

Hey, you said it;

Mike

We already have the technology to fire shells at different velocities.
I see no problem with artillery that can reach the other side of a small planet.

Maybe the problem is the word ballistic. Ballistic means an unpowered object. And yet we have Inter Continental Ballistic Missiles (ICBMs) which strictly speaking are not ballistic because they have thrust while in flight. Another fine example of english being . . . imprecise.

Now with thrust, certainly it is possible to hit the far side of the planet. Without it? I seriously doubt it. And I'm not the only one either, or this debate wouldn't have even started.

Keep things plausible. It's better that way.

http://en.wikipedia.org/wiki/Space_gun

This may also help clarify things. Saying we lack the power to fire a projectile into space is inaccurate. It's been done. What we cannot do however, is to get something into orbit that way.

Thrust, is important if you want to reach orbit, and/or hit the other side of the planet.

ICBMs spend the majority of their flight-time adhering to the strict definition of being "ballistic".

Add in to the mix things like this, so whether you like it or not the lines are blurred, go easy on the semantics.

If, mathematically, an artillery piece can hit the exact opposite side of the world - I think it should be.

If there's balance considerations, not any of this wonky rubbish, then you limit the muzzle velocity, and the pitch of the gun.

No, doctorzuber- this very article you link says we lack the power to send a projectile into orbit using a gun. The highest apogee achieved by an actual space gun was only 110 miles (suborbital on a good day), and most likely was not moving fast enough to achieve orbit. The lower your altitude, the faster you have to go to maintain your orbit.

Just to give you an idea of the numbers for common orbits. Low earth orbits are between 100 and 1,200 miles high. Any geosynchronous orbit requires a distance of about 22,000 miles above Earth's surface.

An object in a stable circular orbit at the highest altitude achieved by the best space gun humanity can build (180 km) would, according to Kepler's Third law, need to be traveling...



7526.646 meters per second.

Furthermore, we cannot get a projectile/shell into a stable orbit. Stationkeeping thrusters are required for small objects to maintain a stable orbit indefinitely.

Apologies for many edits- I was re-checking my math, and had an image issue with the equation.

You should read the rest of the article.

Who cares about orbits? I thought we cared about blasting the metallic crap out of robots with giant cannons.

Back.

On.

Topic.

Once again, doctorzuber, the question is whether it is possible for a projectile to hit anywhere on the surface- NOT whether the gun can put an object into orbit such that it will never hit the ground, without escaping the gravity well either.

We want our artillery shells to hit the ground- this is not a good thing for satellites, however.

If you want to argue that you cannot fire an projectile such that it will never hit the planet, and never leave its orbit, I agree with you. But the question is whether you can hit any target point on the planet's surface. The satellite example was to demonstrate that a decaying orbit has many of the same properties as a shell being fired from the surface.

I also think we should go back to discussing possible gameplay for artillery.

On an airless planet, it's really easy. With drag it becomes slightly more difficult to hit stuff on the other side of a planet.

On an airless planet one could shoot a projectile parallel to the suface tangent. Shoot it at the right speed, and it gets a circular orbit. at every point along it's trajectory it's distance to the ground is equal. make the projectile go slightly slowever and it'll eventually hit the ground. it should be obvious that when fired just right, that "hit the ground" happens.....on the other side of the planet.

of course mountains and crap will be in the way. Therefore, one can shoot up higher and cause it to impact at a different angle.

a cannon not capable of putting a sattelite in stable orbit, to me sounds like "the sattelite will eventually crash down" which sounds like "can hit any side of the planet".


The only reason it could look wrong to people if they truly have no idea of ballistics. (which you'll have even when you play silly online gravity games like gravity golf)

The thing you have to realize is that the motion of a (ballistic) shell forms an ellipse around the center of the planet. What this means, is that - assuming no air resistance and non-accelerating planet - no matter how you fire the artillary the shell will (try to) come back to the point of origin and form a closed loop. However, as you might be able to imagine, most of the theoretical paths will go through the earth, which means the shell will strike down.

Finally, there is one setting that let the artillary hit itself - fire parallell to the ground, but with enough velocity to get it into orbit. Then the shell would go all the way around, and form a closed path by arriving parallell to the ground, from "behind"!
With this in mind it is easier to visualize how the artillary can theoretically hit any point on the planet - no probs!

Also, artillary shells travelling part of the way throuh the black of space would be awesome.

I do realize that fully and all the implications it has.
If not directed at me, quote the person you are talking to.

i think planet-to-planet arty is just lame.

I apologize - it wasn't directed at anyone specific. I meant "you" as in "the reader who is interested".


And by "part of the way through space" wasn't meant to imply planet-to-planet, just very high altitude. Hell, I should try to be more clear. Sorry

http://www.lon-capa.org/~mmp/kap7/orbiter/orbit.htm
Simple simulation of a satellite.(First result I got on google with orbit and java.
Click somewhere close to the surface to simulate it being on the surface. You can hit the other side of the planet by varying the speed and/or the angle that the satellite is "launched".

This is fired from space. It's different. Coming from space at exactly the right vector and yes it is possible to fall into a stable orbit. Doing the same thing from the ground and it is completely impossible. Even in newtons example of an impossibly high mountain, it's still impossible. You'll just come around and hit the same mountain you launched from at the end of your very first "orbit".

If by easy you mean require less thrust to launch, sure why not.

But if by easy you mean able to hit the far side of the planet you're dead wrong. Air ironically enough, is your friend here. Air generates drag. You need to lose speed somewhere or you're trapped into a hard choice of either escaping into space, or hitting the ground long before you get anywhere close to the opposite side of the planet. You're cutoff point is closer to 1/4th of the planetary radius. It's nowhere near to the opposite side of the planet as someone diagrammed (and labeled as wrong incidentally).

Depending on how thick the atmosphere is, you may be able to stretch that farther, possibly a lot farther. It's still utterly impossible to get into a stable orbit because that requires further deceleration after you leave the atmosphere, but it might be possible to reach farther around the planet before coming back down again. The opposite side of the planet is still highly unlikely no matter how thick the atmosphere is, but it will give you potentially more range, provided that your gun can provide the necessary thrust and that your bullet can survive the friction from chewing through that much atmosphere at speed.

You don't need to do it like Newton. He fires the cannon horizontally. By tilting the cannon vertically you fire at an angle from the start.
Because of that the projectile can travel at higher altitudes where the atmosphere is thinner so potentially you could even hit the other side even if the planet have an atmosphere.
Remember. Elliptical orbits are the norm. Circular orbits are the exception.

Nobody is talking about shooting the projectile in a stable orbit.

Sorry, but its you who is wrong. As long as projectiles launched with a velocity smaller than the escape velocity always come back to the planet.

I personally think this discussion is important, since I study physics myself and am somehow afraid how less many people in this forum know about physics. But I also think this is the wrong thread for this. So I'd suggest you create either a new one or I'd try to explain it through PM.

If one wishes to talk about orbit, it is important know that the orbiting object will always return to the last point from which it was acted on by a force.
This means that if you built a cannon that had enough velocity to fire a projectile into orbit, and the fired projectile does not exceed the escape velocity; it could leave the atmosphere, circle around, and hit behind the cannon.

I learned everything I know about orbits from kerbal space program.