Jules Verne was the first to work out this idea. From the Earth to the Moon. He knew about recoil, so his Space Gun was entirely sunk into the earth. A hole bored 900 feet deep, IIRC, and stuffed with tons of guncotton. And situated in Florida, of all the crazy places.
While he pretty well eliminated the recoil problem, aiming the thing isn’t so simple. You’d have to wait until the earth was aligned into the exact position you wanted to aim. Re-entry of a projectile wouldn’t be so simple. You’d have to equip the payload with retro-rockets. The only possible re-entry would be a “splashdown” in the ocean. Hey, wait a moment…
Well, I don’t dispute the claims in DanBlather’s post. However, for the purposes here (getting out of the atmosphere) you’d want to be as high as possible above sea level, not from the center of the Earth.
I don’t think so; aero-breaking can only lower the high point of an orbit, not raise the low point, which is what’s needed here. And even then you need rocket motor on board to raise the low point out of the atmosphere once the aero-breaking is done.
The way to decrease the recoil is to distribute the acceleration over time.
The original “Supergun” attempt was by the Nazi’s during WWII. They were trying to build a cannon with a barrel a mile* long. The barrel was buried in the ground and was going to be aimed at London. There were to be multiple firing stages along the barrel to accelerate the projectile up to sufficient speeds.
They were never quite able to get all the bugs worked out, since it requires some nice precision machining and intricate timing. But, had they been able to complete it (and before the Allies gained air superiority), it would have been able to drop a shell on London every thirty seconds, 24 hours a day.
The Bagdad gun was based on this idea. (At least part of the reason we opposed it was that it seemed much better suited to dropping shells on Israel than putting payloads into space.)
*all of this is recalled from a show on the History Channel, and some details may have shifted during transport.
My point is that the recoil effect is somewhat diminished by the multiple firing stages. (Unlike Jules Verne’s cannon in which a single pile of guncotton was used.) The acceleration could be distributed along the mile or two of the gun barrel, thus reducing both the maximum acceleration and maximum jerk.
Obviously, a rocket spreads that acceleration across tens or hundreds of miles, and so would have even less acceleration and jerk. But, a gun projectile augmented by rockets once it reached stratospheric altitudes could conceivably be designed to carry fragile bags of water (such as human beings) or delicate electronics.
Not that I think it is a good idea, or that I think it could come anywhere close to $600/kg. But I think it could be done.
Couldn’t the rate of acceleration also be controlled by using a mix of propellants with different burn rates? Or by encapsulating fuels in substances which would delay their ignition, akin to time-release medications? I think some (most?) solid rocket fuels use a similar technology.
You can enclose just about anything, including a spaceship, in a shell which would fall away after launch.
**I don’t think this would help (well, it might help the survival of the payload, but at the cost of not getting it into orbit. Acceleration has to take place within the length of the barrel (otherwise, it;s a rocket, not a bullet) - as soon as the projectile leaves the barrel, it starts slowing down.
Enclosing the payload does not protect against the effects of acceleration.
When a rocket takes off it is burning a lot of fuel just to lift more fuel so, the higher the acceleration, the more efficient the process but rockets already function at pretty much the maximum acceleration their payloads can withstand because NASA engineers know what they are doing. A cannon firing stuff into space is pretty impossible for the reasons which have been noted but a cannon serving as the initial launch for a rocket, like a catapult on an aircraft carrier, could save fuel and increase efficiency. Suppose the first 3000 meters of altitude and whatever speed is achieved by then were obtained by a catapult, rather than rocket. The savings would be enourmous because it is in the taking off that fuel consumption is largest.
Not for protection against acceleration, Mangetout, but to make it ‘fit’ the barrel of the gun.
The speed does indeed fall off rapidly once the projectile leaves the barrel. But you’ve used much more of the energy available in the fuel, haven’t you? And, with controlled acceleration, provided a much gentler take-off. I was thinking along the lines of what sailor says, using the cannon as a catapult. Hence the rocket inside the shell.
I really know very little about space travel, I’m simply curious.
There is another way – you can distribute the acceleration over time by leaving the “motor” on the earth and only sending the reaction mass up with the payload. Arthur Kantrowitz suggested this several times, and I’ve worked on it – it’s called Laser Propulsion, and it’s distinct from light pressure and solar sails – you pump energy into reaction mass via Inverse Brensstrahlung, creating a Laser-Sustained Detonation (LSD) wave.
The idea has shown up in science fiction in Jerry Pournelle’s High Justice (the original publication in Analog has a beautiful cover painting by Frank Kelly Freas), and in Michael-Kube MacDowell’s The Quiet Pools. MacDowell calls them Kantrowitz-Kare launch beams, giving credit to JPL scientist Jordin Kare.
We did some of this on a small scale, blasting dime-sized payloads around a laboratory, using big COs lasers.
The only one still doing this work that I know of is Leik Myrabo of Renssalear Polytechnic, with his Apollo Lightcraft project. Myrabo’s craft doesn’t even carry reaction mass – he uses ambient air for that, relying on his ability to get to orbit before his air runs out. Myrabo has launched small models for short distances, and I still see him on TV occasionally. He and SF author Dean Ing wrote a book about this, and Ing later wrote a science fiction novel based on it, The Big Lifters.
What’s interesting is that the projected cost per pound to orbit was in the ballpark you listed for cannon, but without the bad effects of big acceleration.
After a quick search of the web for Inverse Bremsstrahlung, I think the laymans definitions would be something like this:
A very strong laser is pointed at a target mass (such as hydrogen).
The heat of the laser converts the mass to plasma.
The plasma rapidly expands, pushing the space craft higher.
This is different from combusting the hydrogen which converts it (and oxygen) to water. Presumably, the phase change to plasma can get more energy per kilogram of hydrogen.
The challege is that ALL of that energy has to come from the laser. Building a laser with that much oomph through that much intermediary air would be quite challenging.
If an expert would care to correct any errors… this is just an effort to head off the obvious next question of “Inverse What?”
