The barrel less super gun.

future space operations are going to require building materials to be delivered to space, cheaply. Some of these materials in bulk are a lot more robust than the same stuff in use. For example, a properly wrapped large roll of Mylar will be a lot less liable to damage at launch than a solar sail would be. and beams laid flat on a cargo deck, will be protected from the bending loads that they would experience if they where to be launched in an assembled unit.

So much for the reason why this invention is needed, next comes the principal (but not the method) of the barrel less super gun.

A cavity shaped charge such as a Bazooka shell, fires a jet and slug of Copper forward with sufficient speed that it can force its way through steel armer. As much force as is imparted to the Coper must be imparted to the shell fragments. Equal bur opposite reaction.
Now imagine for a moment a stack of 10 such shells, firing downwards in quick succession. The first one fires, its slug goes down, and the rest of the stack goes up. As each charge fires in turn its recoil adds to the velocity of the pay load, i imagine this as being like a 10 stage conventional rocket. Albeit a very in efficient one.
A word or two about super guns.
Super guns have the same advantage over rockets as rail guns, namely that all of the fuel is burned at or close to the point of launch, rather than most of the fuel going on lifting fuel. Super guns and rail guns share the disadvantage of having there highest velocity at the bottom of there trajectory, where the air is thickest, conventional super guns also have the problem that the conditions inside the barrel are so extrema as to cause erosion of the barrel.

The basis of my variation upon this theme is to build special flat shaped charges that are a couple of cm deep but tens of metres across. A simple cavity shaped charge is longer than it is wide, because it needs room for the detonation head to grow. I would also recommend using ANFO as the main explosive, with a detonation velocity of between 1100 and 2700 metres per second and a peak pressure of less than 4000 atmospheres. The confinement of the larger part, I'm guessing about four fifths, of the explosive force into the vertical direction is a result of the shape of the thin shaped charges.

Huygens principal, that each point on a wave front may be regarded as a sours of new waves, is applicable to the diffraction of sound waves and water waves and as much as it is to light waves. A zone plate is a special type of diffraction grating, that acts as a convex lens For the moment picture plain parallel sound waves, from a distant source, striking the front of a zone plate and being focused to a point be hind it.

The cargo pod will have heat proofing on its roof. It will be in the shape of a short broad inverted truncated cone. There will be a variable number of cargo decks in side, to allow the cargo to be distributed and supported appropriately. It will sit on a pusher plate supported by light weight sabot. To give you an image of this, imagine the Apollo return module, turned up side down, and scaled up to the volume of a large cargo aeroplane, or bigger. With plastic buttresses keeping it steady

the pusher plate will be a section of composite armer, to protect the cargo pod from shrapnel It will have three layer each made of; a hard but brittle layer (like toughened glass), two sheets of bubble wrap which as bubbles full of water, several sheets of kevlar weave, and a metal sheet.

The zone plate shaped charge.

Each zone plate shaped charge is built on a flat metal plate, the same size as the pusher plate. On top of this is an even number ( possibly 10 to 20 ) of concentric rings. The radii of these rings are related to each other in a very specific way. If we define the radius of the smallest centre ring as 1 unit, then the radius of the second ring is 1.4142 units. That is the square root of two units. The third ring is 1.7320 units. That is the square root of three units, and so on.
These concentric rings are filled or not as follows; the centre circle is left empty, the first annular space has explosives, the second and subsequent even numbed annular spaces are empty, and the third and subsequent odd numbed annular spaces are loaded with explosives.
All of the explosives in a given zone plate shaped charge are detonated at the same time. So looking at it from above it it will look like a massive compression wave coming through a zone plate and having the majority of its energy directed upwards and in over. Unlike in a simple explosion, where the blast travels out evenly in all directions. The same effect will be seen from below.

To recap. a large cargo pod ( the bigger the better to take advantage of square cube rule ), is blown ( undamaged ) to an orbital hight, by mean of a stack of thin zone plate shaped charges fired in rapid succession