The Ice Gun

[farmerben]

Please correct me if this is way off.  A gas gun with between 300-400 psi has in the ballpark the thrust of a falcon 9 or similar.  Supposing you can maintain that pressure through the length of a barrel.  

Three huge problems arise.  1) You need reasonably tight gas bushings on the vessel.  2) You need an exponential increase of gas behind the vessel inside the barrel. 3) the barrel must be maintained with absolute precision.  

 

Possible solutions: 1) The vessel wadding is made up primarily of CO2 dry ice.  It could have fibers to control cracking, have hex tiles,...  It could have pneumatic pistons like self adjusting brakes to push the shoes against the barrel with constant force and diameter as the dry ice ablates.  The shoes can be discarded at the muzzle of the barrel and will evaporate before they hit the ground 

2) Rather than use foul timed explosives or sit on the wrong end of a rocket nozzle, we could use N2 as the driving gas.  The heat and driving energy can all be kept external to the barrel.  We can dump an exponential amount of heat into liquid nitrogen and get an exponential volume of cold gas.  This is very clean and beneficial to the system compared to hot corrosive gasses.   

3) To make the barrel absolutely precise we construct it out of H20 water ice.  Specialized Zamboni robots can machine an entirely fresh barrel surface in only a few hours.  Reserves of liquid N2 help ensure the special glacier remains stable throughout the year.  

 

[magnemoe]

You want the projectile to be softer than the barrel this tend to be solved with copper on normal guns even artillery who operates with far higher pressure and is rifled. 
Gas guns for target shooting tend to have an cone in the rear, the gas pressure expands the rear of the bullet giving an thigh seal even with the pretty low pressure this increases accuracy. 

Your problem would be that the projectile will go supersonic, adding more pressurized gas will not help as you will need the gas push to be supersonic to help. 
Would rater go railgun or coil gun. perhaps use gas to get it up to speed and if barrel is in vacuum who you want if going orbital you the gas in barrel will prevent an shock then atmosphere enter the barrel. 

[Zeiss Ikon]

There are a number of supersonic gas-pressure gun designs around.  One I'm aware of is used to do things like shoot ping pong balls through ping pong paddles (leaving a neat, round hole, at least on the impact side).  Another uses hydrogen to accelerate tiny projectiles to six or seven (or ten?) times the speed of sound to study high velocity impact (like micrometeoroids hitting the ISS).  All of these have in common that the  barrel is held in a vacuum before a rupturing diaphragm releases the high pressure driving gas.

Now, ordinary spring-piston air rifles can reach modestly supersonic velocities by compressing the air at the time of the shot, and using the compression to multiply velocity (the piston is, say, ten times the area of the barrel, so the barrel can drive at tein times the piston's velocity on the same volume flow) -- but this would require some massive apparatus to produce the needed pressure/flow on an instantaneous basis.  You can't make it from liquid nitrogen on demand; it takes time to transfer the heat needed to boil off the liquid into gas.

And all that said, you have the same problem with this kind of launcher that Jules Verne did with his underground cannon in Florida, written to fire men at the Moon in the 1860s: the acceleration needed to build up useful velocity in a manageable barrel length would be enough to liquefy bone and flesh.  It's possible to build electronics that can be fired from a gun (they did it with vacuum tubes in the Second World War) -- but humans couldn't survive the tens of thousands of G that would come with launching to orbital speed in a barrel short enough to actually build.

[farmerben]

A 10 km launcher throws humans at 5g up to about 1000m/s.

That is potentially a dV of 1000m/s more than you could possibly achieve without a ground assist.  

In practice though the best thing to shoot is fuel.  The launcher will shoot consistent orbits... very useful for fuel depot rendezvous.  

 

[Gargamel]

8 hours ago, Zeiss Ikon said:

  All of these have in common that the  barrel is held in a vacuum before a rupturing diaphragm releases the high pressure driving gas. 

I made a 10" diameter spud gun using this technique once, minus the vacuum of course.  It'd put a nerf ball 50+ yds at a good velocity, and more ballistically oriented projectiles quite a bit farther.   Built it for a high school production of The Pirates of Penzance, it was a cannon on the ship.  I had to lower the pressure a bit as the during the first show, the "cannon ball" knuckle balled down into the ,orchestra pit, flipping the trombone player over backwards.    Compressed air at around 70 psi would rupture 12 layers of tin foil (the diaphragm), sending the ball, baby powder and gold and red glitter flying out of the barrel.  Loud boom too. 

[farmerben]

A 10 km launcher throws humans at 5g up to about 1000m/s.

That is potentially a dV of 1000m/s more than you could possibly achieve without a ground assist.  

In practice though the best thing to shoot is fuel.  The launcher will shoot consistent orbits... very useful for fuel depot rendezvous.  

If aluminum foil can be used to hold vacuum this becomes much easier than I thought.  There is talk about a plasma gate.. but that’s fairly unproven at scale.

[wumpus]

Unsurprisingly, the Oracle of of all things Kerbal, Scott Manley, has already had his say on this: https://www.youtube.com/watch?v=Moo5nuLWtHs

 

As mentioned above, acceleration-limited loads (such as astronauts and passengers) require completely unfeasible gun lenghts.  Either limit cargo to fuel and similar (fuel would be sufficient to justify such a thing) or have an additional stage (presumably SRB for ruggedness), and of course you will always need a circularization stage to keep anything in orbit.  That said, if Musk ever builds a NY-Chicago hyperloop, that's roughly the size needed to fling passengers into orbital velocity (except that it would require adding power the entire length of the track, which is probably vastly more expensive than a standard hyperloop).

As originally noted: "maintaining constant pressure" is the kicker.  Traditionally, this involved carefully firing additional powder in cylinders the projectile was passing in order to maintain such pressure.  If you wanted an "ice gun", I assume that it would be possible to do both this and build turbopumps pumping in liquid nitrogen that would quickly boil and push the projectile.  My guess is that one look at the price for this would send you back to using artillery powder.

[sevenperforce]

Have you thought about doing a cold launch pressurized liquid-gas gun?

Cold-launched missiles use a powder cartridge to eject them from their silo or launch tube, ensuring they will fall clear if they fail to ignite or otherwise suffer problems. This is in contrast to a hot-launched missile, where the rocket engine is located inside the tube (and you need an exhaust port to allow the rocket exhaust to leave). Here's what one looks like:

 

Why not combine the two?

Build your giant space launch gun. Instead of putting in powder, put in feed lines that can pump enormous amounts of liquid nitrogen into the chamber at ridiculous pressures. Instead of using a bullet-shaped projectile, add in a standard conventional solid rocket engine. You would, of course, want to evacuate the barrel of atmospheric gas and use a frangible diaphram on the barrel end.

Pump liquid nitrogen into the chamber just before igniting the engine. The expanding exhaust from the rocket engine fills the chamber, beginning to push the payload and attached rocket forward with far greater force than the thrust of the rocket engine alone. As the exhaust mixes with the liquid nitrogen, it flashes the nitrogen from a liquid to a gas, exponentially increasing the amount of gas in the chamber and overcoming (at least initially) the gas-expansion limits on projectile launch.

As the rocket continues to fire, the expanding gases continue to propel it forward as with a normal gun, but are constantly being given additional heat and pressure due to containing the exhaust from the rocket engine. Eventually, the rocket's own thrust is producing more of a propulsive effect than the expanding gases, which should be right about the time that the rocket blows through the diaphram and exits into the atmosphere. The rocket engine need only continue firing as a sustainer to push it out of the atmosphere and circularize, and the liquid nitrogen will have protected the interior of the barrel from the immense heat of the rocket exhaust.

[magnemoe]

3 hours ago, farmerben said:

A 10 km launcher throws humans at 5g up to about 1000m/s.

That is potentially a dV of 1000m/s more than you could possibly achieve without a ground assist.  

In practice though the best thing to shoot is fuel.  The launcher will shoot consistent orbits... very useful for fuel depot rendezvous.  

If aluminum foil can be used to hold vacuum this becomes much easier than I thought.  There is talk about a plasma gate.. but that’s fairly unproven at scale.

1000 m/s is an decent speed however you would still need an significant rocket to reach orbit, less than half the speed of an falcon 9 first stage. 
And because low attitude you face lots of air resistance. 
It would be way cheaper to build an large mach 3 carrier plane (Valkyrie is one template) to put an same size payload at the same speed at 15 Km or higher. Put an rocket engine on it and you can reach 6km/s this let you drop payload in space so no supersonic separation or fairing. 

on the moon you might want to use something like this in the future, magelv track with linear accelerator, not human rated say 50-200 g but you can put raw materials at L1 cheap then land the thing for reuse. An simpler version might be an mix of an centrifuge and an trebuchet, spin your payload and an rock around fast, real out wire while accelerating then drop both   

[magnemoe]

5 minutes ago, sevenperforce said:

Have you thought about doing a cold launch pressurized liquid-gas gun?

Cold-launched missiles use a powder cartridge to eject them from their silo or launch tube, ensuring they will fall clear if they fail to ignite or otherwise suffer problems. This is in contrast to a hot-launched missile, where the rocket engine is located inside the tube (and you need an exhaust port to allow the rocket exhaust to leave). Here's what one looks like:



 

Why not combine the two?

Build your giant space launch gun. Instead of putting in powder, put in feed lines that can pump enormous amounts of liquid nitrogen into the chamber at ridiculous pressures. Instead of using a bullet-shaped projectile, add in a standard conventional solid rocket engine. You would, of course, want to evacuate the barrel of atmospheric gas and use a frangible diaphram on the barrel end.

Pump liquid nitrogen into the chamber just before igniting the engine. The expanding exhaust from the rocket engine fills the chamber, beginning to push the payload and attached rocket forward with far greater force than the thrust of the rocket engine alone. As the exhaust mixes with the liquid nitrogen, it flashes the nitrogen from a liquid to a gas, exponentially increasing the amount of gas in the chamber and overcoming (at least initially) the gas-expansion limits on projectile launch.

As the rocket continues to fire, the expanding gases continue to propel it forward as with a normal gun, but are constantly being given additional heat and pressure due to containing the exhaust from the rocket engine. Eventually, the rocket's own thrust is producing more of a propulsive effect than the expanding gases, which should be right about the time that the rocket blows through the diaphram and exits into the atmosphere. The rocket engine need only continue firing as a sustainer to push it out of the atmosphere and circularize, and the liquid nitrogen will have protected the interior of the barrel from the immense heat of the rocket exhaust.

Think cold launch uses an gas generator, think something who generates lots of pressure, steam would be perfect. 
Now back then i was young we found that launching an firework rocket from an closed tube increased its speed significantly. Discovered then dropping tiny budget firework rockets into launch tubes. Scaling up worked well i assume the increased pressure also made the rocket engine burn faster. 
Friend made an gun for launching rockets an simple cut rifle stock, an pvc pipe and an lighter as trigger worked well enough but only a bit more accurate than normal firework luckily enough. 
 

[sevenperforce]

8 hours ago, magnemoe said:

Think cold launch uses an gas generator, think something who generates lots of pressure, steam would be perfect. 
Now back then i was young we found that launching an firework rocket from an closed tube increased its speed significantly. Discovered then dropping tiny budget firework rockets into launch tubes. Scaling up worked well i assume the increased pressure also made the rocket engine burn faster. 
Friend made an gun for launching rockets an simple cut rifle stock, an pvc pipe and an lighter as trigger worked well enough but only a bit more accurate than normal firework luckily enough. 
 

Cold launch GG is still pyro.

[kerbiloid]

Spoiler

15 hours ago, magnemoe said:

Now back then i was young we found that launching an firework rocket from an closed tube increased its speed significantly. 

Since then a sign is there.

 

[p1t1o]

How does it outperform a standard kerolox rocket? Can it do something we cannot already do?

[wumpus]

3 hours ago, p1t1o said:

How does it outperform a standard kerolox rocket? Can it do something we cannot already do?

I would assume that any "rocket tube" would have to be base on kerolox (or methalox) as you are unlikely to gain any effect once the thing goes supersonic.  Of course, going straight up with a TWR<1.5, this might take a few km, but I'd have to assume that you would lay pipe at a steep angle up the western face of a mountain so you would accelerate a bit faster.  It would take significant TWR to run out of mountain before you went supersonic.

It might make sense to use pressurized water tanks to allow high pressure to build up at much lower temperature, although I'd be curious how you eject things likely stored above the engine once you clear the tube (probably the same way rocket labs ejects spent batteries).

It would also take an enormous fleet of rockets to justify this before building an air-augmented rocket.  I strongly suspect that air augmented rockets are the next step, and should improve things to 20km, not just up a mountain ramp.

[p1t1o]

To be honest, since we can sustainably produce methane and to a degree, kerosene, (and LOx), Occams razor just says scale up standard rockets.

That will work for quite some time into the future.

What you send, and what you do with it in space, thats the thing. IMHO.