Supergun Launching of Satellites I should say first that I don't believe this is a feasible launch method, otherwise NASA and other space agencies would be using it by now.
It's based on this BBC news story Saddam Hussein's Supergun but, luckily this monstrosity  was never completed or even fully tested.


These giant cylinders are one of the few remaining pieces of a contender for one of the most audacious pieces of engineering ever designed: a “supergun” called Big Babylon, which could have fired satellites into orbit from a 156m-long barrel (512ft) embedded inside a hill.

Rather than thinking of the engineering aspects of the gun, what are the physics based reasons why we cannot arrange a series of linear explosions, with a valve type device to prevent blowback down the barrel at each stage and thereby maximising the upward boost to the payload to escape velocity.
Again, I would stress that I believe there are physical (rather than engineering) reasons this idea is not used today. I just don't know what they are.  Is it as simple as the barrel would need to be unfeasibly long, even using the most powerful explosives we have available today?
The Project Harp Launch Gun was tested in the 1960s but never achieved more than half the escape velocity required.
Merci beaucoup, Jules Verne (1828-1905). From The Earth To The Moon
 A: Other answers don't mention the fact that no single impulse (e.g, like being fired from a gun) can launch a projectile into orbit.  A purely ballistic projectile fired from a gun must either crash back into the planet, or it must escape from the planet altogether.
In order to achieve orbit, at least two impulses must be applied to the projectile.  The first one (from the gun) launches it into an elliptical trajetory that returns to the surface, and then the second impulse must be applied by a rocket motor to "circularize" the orbit at the moment when the projectile reaches the apogee of the initial ellipse.
A: One more nasty factor:  What is the expansion speed of your propellant.  Take the Jules Verne approach and your spacecraft falls far short no matter how much powder you put in the gun because the expansion velocity is too low.  Your craft will never exceed the expansion velocity of the propellant.
Note, however, that you don't have to use explosives (or combustible gas mixtures--the best chemical space-gun designs I've seen used gas, there was no boom) to build a space gun.  Consider the railguns the navy is working on--supergun type speeds, nothing in the launcher goes boom.  You still need gun-hard electronics, though.
For a fixed launcher you can use a linear motor rather than a railgun.
One more headache to keep in mind:  A simplistic aiming of a space gun cuts through a lot more atmosphere than the 14.7 psi you will go through if you went straight up.  This suggests another approach:  Make your launch angle as steep as possible, over 80 degrees would be ideal if you could build the needed vertical velocity.  What's that I hear from the peanut gallery about needing orbital speed??  If your vertical velocity is high enough you can get away with very little horizontal velocity.  Go nearly straight up at 18,000 mph and you'll fall back.  Go nearly straight up at say 24,500 mph and it's another matter--the objective is to go out as far as possible consistent with not getting your orbit wigged by the sun.  You only need enough horizontal velocity to travel 4000 miles during this hop (and you can take days on the hop, that's not much speed at all), come back down just skimming the atmosphere and then do an aerocapture maneuver.  You'll still need a circularization burn at the end but you've encountered far less drag than if you had gone for orbit directly.
Now, if you're on a body without an atmosphere the linear motor really shines.  You still need to circularize but you aren't fighting mega-drag, nor do you have to worry about things like the shockwave of your craft destroying the launch system.  An ejection angle of zero is fine, thus there's no length limit on the booster.  You no longer need gun-hard electronics, such a system is even useable for manned transport.  (Wrap your boost track clear around the moon and your manned speed is only limited by the centripetal force as you go around the track.  If I haven't messed up the math that's enough to provide anything from solar escape to solar impact.)
A: Anything launched into orbit by such a gun needs to travel at orbital velocity (in fact above orbital velocity) in the lower atmosphere.  That's generally undesirable, to put it mildly: there will be really serious heating.
A: Aside from the interior ballistic aspects of these various projects, it was quickly realized that any satellites launched by gun would have to withstand high g-loadings during firing of the gun and the size and mass of the satellite would be greatly constrained by the dimensions of the bore of the gun and the maximum impulse which could be provided by the propellant without damaging the gun.
Special designs for satellites were prepared so that sensitive electronics would not be damaged by being fired from a gun, and recognizing that the gun could not provide sufficient velocity to reach orbit, satellites with booster rockets were designed to fire after being flung aloft by the gun.
The project ended for various reasons, some budgetary, some political.  The escalating war in Vietnam caused funds for a lot of research projects to be cut, and this project was originally a joint effort between the U.S. and Canada.  When relations between the two countries hit a rough patch over differing policies regarding Vietnam, the project became ripe for being eliminated.
A: I think the heart of the question is whether one could arrange a continuous combustion of propellant along the length of the barrel. In that way the acceleration occurs along the length of the barrel in a more gentle way. Since the expanding gases from the propellant in a shell casing expand and the pressure of the expanding gases declines along the way it means the primary force or acceleration loading is not at the start of the projectile motion.
You still have a huge acceleration.  Suppose the barrel is $100$m in length and assume the projectile has orbital velocity ($\simeq 10^4m/s^2$) at the end of the barrel. Then using the elementary equation $2ad = v_f^2 - v_i^2$ the acceleration is then
$$
a = \frac{v^2}{2d} = \frac{10^8m^2/s^2}{200m} = 5.0\times 10^5m/s^2.
$$
This is the average acceleration, which if you design the firing of propellant correctly it might be the actual acceleration that is nearly constant. This is considerable.
There is an additional problem. The projectile as it leaves the gun will be slowed by the large shock wave it produces in the atmosphere. So you would need to fire the projectile at a higher acceleration to account for this loss.
A: Orbital insertion is hard enough with ships where the initial stages can be aimed in directions favorable to the final orbit.
This design calls for a fixed launch direction, which would be a terrible waste of on-board fuel for all but those satellites whose orbits coincide with the gun's trajectory.
Even that can be overcome with fuel.  The real issue is size and scale.  The proposed design is meant to launch lightweight satellites of relatively small volume - maximum 1 meter in diameter including any casing or shell needed for the launch.
Today's satellites are often much larger than cars and buses on the road - easily over 2 meters in diameter, and weighing much more than this small gun could handle.
You could, in theory, make the gun larger and longer, but in addition to the exponentially greater costs of construction and fuel, you find that the energy required to launch goes up exponentially.
At first glance this shouldn't be a problem, since the same issue exists for our current launches, however this design has a particular drawback - the vehicle must be going much faster at the beginning of the shot than at the end, as it will expend significant energy travelling through the dense atmosphere of the lower layers of the atmosphere.
Our current rocket technology can start off relatively slowly, burning less fuel, and ramp up as they get faster and as they start passing through thinner atmosphere.
This means that the exponential increase in energy required affects both, but the gun requires an even greater exponential increase because it's starting out so fast.
A: Let's say you have got such gun. Next logical step will be to install on the satellite a smaller gun that would shot-back several small shells and so accelerate the satellite, indeed? If this small on-board gun would use really many small shells (size of molecula) then your are getting just a traditional rocket. Apparently it is not much difference for energy required if you launch a rocket from traditional platform or gun it, but engineering difficulties for the latter technology are massive.
