Scott, it's not the velocity. The orbit of an unpowered satellite will pass through the point it was last powered. For a cannon, that means the Earth's surface. Unless you lift your cannon several hundred miles up, the projectile will either return one orbit later to the cannon and strike the surface, or leave the Earth's vicinity altogether.
The great big gun...
- Thread starter NHBL
- Start date
Scott Rosenthal said:
My understanding is that orbital velocity is obtainable, but it would be a very eccentric orbit--perigee would be below the Earth's surface. If, once the round was up there, a rocket made a course correction, then you're in a stable orbit. If you use an electromagnetic catapult, the satelite will still need rockets to change its trajectory--at the desired altitude, the rockes fire and you're in a stable orbit
I believe that your understanding of orbital dynamics is perhaps a bit off. Yes, the projectile will return to its original point of departure, but (and this is the key) the Earth won't be there anymore, as it is moving as well. That is the whole point of orbital velocity, the projectile is moving fast enough RELATIVE to the point of departure, that their orbits are 'coordinated' (more precisely, they are moving relative to one another, in a stable relationship), and hence in a stable or quasi-stable relationship. In point of fact, it IS all about velocity, since the satellite doesn't 'know' what imparted its delta-vee
A launch from a cannon, electromagnetic catapult, BIG trampoline, rocket, what-have-you all show the same behavior, and all have the same requirements. A satellite lauched from a rocket makes secondary delta-vee changes during flight to alter its orbital path, but it need not do so. Yes, a course correction is often desirable to provide some orbital paths, but there is no necessity that this be done. Satellites, I might point out, often have highly elliptical orbits (the Soviets did this with communications satellites for years, as circular obits were poor choices since their country lay so far to the north), but even circular orbits can be obtained by arranging for the originally elliptical orbit to brush the top of the atmosphere (yes, I know...terrible imagery), which would slowly adjust the orbit to become less elliptical with time. This technique was used with several of the problems sent to Mars (the Odyssey, for instance), to minimize the fuel necessary to bring along....
A launch from a cannon, electromagnetic catapult, BIG trampoline, rocket, what-have-you all show the same behavior, and all have the same requirements. A satellite lauched from a rocket makes secondary delta-vee changes during flight to alter its orbital path, but it need not do so. Yes, a course correction is often desirable to provide some orbital paths, but there is no necessity that this be done. Satellites, I might point out, often have highly elliptical orbits (the Soviets did this with communications satellites for years, as circular obits were poor choices since their country lay so far to the north), but even circular orbits can be obtained by arranging for the originally elliptical orbit to brush the top of the atmosphere (yes, I know...terrible imagery), which would slowly adjust the orbit to become less elliptical with time. This technique was used with several of the problems sent to Mars (the Odyssey, for instance), to minimize the fuel necessary to bring along....
To interject my vastly superior knowledge....
You can use a rocket on the shell to attain a reasonably stable and circular orbit. Remember, the velocity of the shell depends on the molecular weight of the propulsion gases. Hydrogen gives you four kilometers per second, and conventional artillary propellants give you two kilometers per second.
You can use a rocket on the shell to attain a reasonably stable and circular orbit. Remember, the velocity of the shell depends on the molecular weight of the propulsion gases. Hydrogen gives you four kilometers per second, and conventional artillary propellants give you two kilometers per second.
Tom, please read a bit before you make yourself look even more ignorant...
All of these bodies are moving relative to one another. This is really essential Newtonian physics, not rocket science...
All of these bodies are moving relative to one another. This is really essential Newtonian physics, not rocket science...
Scott, I know that. I am literally reading what you said about "Earth not being there anymore". YOU are the one who said it. If moving relative to the Earth meant that the Earth's own orbit had to be taken into account, you could not drive a car without needing life support. Then wkwillis says that you need a rocket on the projectile. OF COURSE! This is what I said. You CANNOT launch an unassisted satellite into orbit with a cannon, which you seem to be saying you can do. Either I am grossly misunderstanding YOU, or you are the ignorant one. You could theoretically launch a rocket into space with a cannon, and then have it fire into orbit in space (I don't know if there is a propellant strong enough, or if the atmosphere would fzzt the projectile...my degree is in astronomy. This is why I said theoretically). But WITHOUT A MIDCOURSE CORRECTION it will either crash or go off into a solar orbit at least.
Tom, whatever clown college sold you your degree should be ashamed of itself....
As for midcourse corrections, it isn't essential that one takes place, though certainly it makes the whole thing easier. As the projectile returns to its original starting place, if it's trajectory brings it close enough to the earth to graze the upper edges of its atmosphere, the friction will produce more than enough delta-vee to 'correct' the orbit without the use of a secondary propulsion system. As I mentioned earlier, NASA has used exactly this technique for more than one of the Mars Orbiters, and the I beleive (though am not sure) that the EUnicks have done the same with the Mars Express.
Now your original quote was that 'the perigee would have to be suborbital', which is clearly incorrect. Even without midcourse correction, you still get an orbit, albeit a somewhat unstable one.
As for midcourse corrections, it isn't essential that one takes place, though certainly it makes the whole thing easier. As the projectile returns to its original starting place, if it's trajectory brings it close enough to the earth to graze the upper edges of its atmosphere, the friction will produce more than enough delta-vee to 'correct' the orbit without the use of a secondary propulsion system. As I mentioned earlier, NASA has used exactly this technique for more than one of the Mars Orbiters, and the I beleive (though am not sure) that the EUnicks have done the same with the Mars Express.
Now your original quote was that 'the perigee would have to be suborbital', which is clearly incorrect. Even without midcourse correction, you still get an orbit, albeit a somewhat unstable one.
It was CWRU, and it was not "sold".
Scott, the delta v is gonna be negative unless the shape is so screwy that it would never get out of the atmosphere in the first place, even theoretically. It would slow the projectile down, and surely crash it.
I am on a work break, so I will allow that I scrambled up my quote and not check. I meant subsurface. Thank you...I was probably thinking suborbital path intersecting the surface.
Finally, just assume that by a miracle it gets out of the atmosphere and by another miracle hits it just right one orbit later. After this double miracle, it will hit the atmosphere AGAIN, and either skip totaly into space forever or (more likely) burn up.
Scott, the delta v is gonna be negative unless the shape is so screwy that it would never get out of the atmosphere in the first place, even theoretically. It would slow the projectile down, and surely crash it.
I am on a work break, so I will allow that I scrambled up my quote and not check. I meant subsurface. Thank you...I was probably thinking suborbital path intersecting the surface.
Finally, just assume that by a miracle it gets out of the atmosphere and by another miracle hits it just right one orbit later. After this double miracle, it will hit the atmosphere AGAIN, and either skip totaly into space forever or (more likely) burn up.
Last edited:
Doing some research, I came across a mention of a Canadian idea of the 60s to use a cannon to aid in launching a satellite. This was probably to launch a rocket that would fire in space, although there is not nearly enough info on the web (that I can find) to figure out how it was supposed to work.
Playing Devil's Advocate, I came up with two other scenarios that might give you a non-powered satellite from a cannon:
1) It is hit in space by a meteroid of the right mass, velocity and trajectory to put it into orbit.
This is not off the wall, it is off the ceiling. Enough said
2) The satellite is propelled *almost* fast enough to escape completely, putting it in a position to have its path modified by the Moon.
This one actually has a chance...Shoemaker-Levy was an existance proof!
It would have a ridiculously high eccentricity, and almost certainly eventually hit the Earth (as Shoemaker-Levy hit Jupiter). No-one would try this, even as a crazy stunt - you would have to calibrate the cannon to the nearest kilometer per hour, AND know the weather patterns along the rising part of the trajectory in advance. Of course, my problem here is that I was thinking of just the Earth, and not the Earth-Moon system.
Playing Devil's Advocate, I came up with two other scenarios that might give you a non-powered satellite from a cannon:
1) It is hit in space by a meteroid of the right mass, velocity and trajectory to put it into orbit.
This is not off the wall, it is off the ceiling. Enough said
2) The satellite is propelled *almost* fast enough to escape completely, putting it in a position to have its path modified by the Moon.
This one actually has a chance...Shoemaker-Levy was an existance proof!
It would have a ridiculously high eccentricity, and almost certainly eventually hit the Earth (as Shoemaker-Levy hit Jupiter). No-one would try this, even as a crazy stunt - you would have to calibrate the cannon to the nearest kilometer per hour, AND know the weather patterns along the rising part of the trajectory in advance. Of course, my problem here is that I was thinking of just the Earth, and not the Earth-Moon system.
How about in 1920?
From what I've seen here a "Berlin Gun in France, or a "Paris Gun" in Germany would have been possible in the pre-Great War years--but ery impractical.
Now, I'm working on another ATL for the 1920's. Could an aimable gun be built to fire from deep in France to Berlin or vice-versa--a gun that could also be swiveled around to shoot at other cities or any battlefield. I'm assumeing a large scale effort, not just a minor gun development project. There are other heavy artillery projects underway as it is.
Requirements:
Range--500 miles
Fully trainable over 360 degree arc--no more than one week for major changes in aiming point
Rate of fire--1 per hour minimum
Accuracy-able to hit a city-sized target at maximum range consistantly
Calliber--12" shell minimum, larger is better
Can this be done at all?
Note on the cost--in this ATL, there is a budget for military development--but severe treaty limitations on what can be built, and the size of the army, navy, and air force for the building power. Money is there to build anything that might be able to be built.
From what I've seen here a "Berlin Gun in France, or a "Paris Gun" in Germany would have been possible in the pre-Great War years--but ery impractical.
Now, I'm working on another ATL for the 1920's. Could an aimable gun be built to fire from deep in France to Berlin or vice-versa--a gun that could also be swiveled around to shoot at other cities or any battlefield. I'm assumeing a large scale effort, not just a minor gun development project. There are other heavy artillery projects underway as it is.
Requirements:
Range--500 miles
Fully trainable over 360 degree arc--no more than one week for major changes in aiming point
Rate of fire--1 per hour minimum
Accuracy-able to hit a city-sized target at maximum range consistantly
Calliber--12" shell minimum, larger is better
Can this be done at all?
Note on the cost--in this ATL, there is a budget for military development--but severe treaty limitations on what can be built, and the size of the army, navy, and air force for the building power. Money is there to build anything that might be able to be built.