Expanding upon the gas gun concept as space propulsion

This piece warrants a speculation alert.

This idea came to me as I was listening John S. Lewis talk about asteroid exploitation.  What he said gave me an idea to put a Hunter gas gun on an asteroid and use it to propel the asteroid to wherever you wanted to put it.  As reaction mass, you would use the asteroid itself.

Now, it isn't exactly desirable to use all of the good stuff in an asteroid to get it to where you want to go.  For example, if an asteroid has 30% water, you'd rather not use that water for propulsion.  No, it would be far better to take the less desirable stuff and shoot it out of this gas gun and that would be your reaction mass.  You can process the asteroid while using its waste as propellant.  How about those apples?

The gas gun need not be so big, as you are not trying to get a large mass to exit the Earth's deep gravity well.

It could actually be no bigger than an artillery piece.  For example, Hunter's gas gun outfit proposed to build a small gas gun as a proof of concept.  It would put a smaller amount of mass into LEO.  I think it was about a hundred pounds.  This gas gun was much smaller than the one that he proposed to use for a commercial venture to put larger payloads into orbit.

The size of the gun for this purpose of moving asteroids could be much smaller because you could use much smaller projectiles.  Let's say a few pounds apiece.

How much thrust could you get from that?  The calculations would have to be guessed by moi, so take it all with a grain of salt.

Okay.  Let's get to basics here.  Force equals mass times acceleration comes to mind.  I need help here, so I consulted a website.  Let's calculate kinetic energy as one of the four ways of calculating joules.  If you have 1 kg mass and you accelerate it 10 meters and it takes one tenth of a second to get to the end of the muzzle ( a guess because I don't know what the actual velocity would be), then it velocity would be 10 divided 1/10 meters sec or 100 meters sec.  Or if it takes only 1 hundredth of a sec, then it would be 1 km sec.  One thousandth of a second would make it 10 km/sec.  The 10 km sec velocity would be near the limit that the gas gun could make a projectile go.  You may need a longer barrel to attain that velocity.

Anyway, from there we could calculate joules, or energy expended:  You multiply 10000 m/sec by itself giving 100,000,000 times the mass of 10 kg which gives  1,000,000,000 .  Then divide by 2 giving joules.  In this case that 500 million joules, or 500 mega joules.  Lots of energy there.  Maybe we don't need so much.  The recoil from such a massive outburst could shatter the asteroid.  Or anything else for that matter.

You could make the projectile smaller and the length of barrel shorter for a slower velocity.  You could also use less energy to heat the hydrogen which would mean a slower muzzle velocity.  At any rate, you could play around with the thing and optimize the size of it in order to get the amount of thrust you want.

Maybe a relatively small gun could fire out small waste projectiles as your reaction mass.  This could propel you where you want to go.  The hydrogen could be recycled ( as much as is practical ), and you would only expend the stuff that you don't want.  By the time you get to your destination, you would have a partially or wholly processed asteroid.

Pretty neat, huh?

Update:

As a comparison, take the muzzle velocity of an AR-15, which is nearly 1 km/sec  ( 1000 meters).  It fires cartridges which are 5 grams.  ( .005 kg ).  The barrel length is about half a meter.

So multiply 1000 times itself giving 1 million.   Multiply 1000000 times .005 giving 5000.  Now divide by two giving 2500 joules.  The fully automatic AR-15 can fire 800 rounds per minute or 13 rounds a sec.  That gives approx 33k joules of power per sec.

Now, if you convert to that to a Hunter gas gun, what have you got?

The hydrogen gas gun could accelerate faster than 1 km/sec.  Using the same size barrel and cartridge size, you may be able to get a lot more out of the gun than this.  But could you save the hydrogen?  Probably not.

You'd have to feed the hydrogen in constantly, which may be a problem.

You could make them fire like a cartridge, releasing the hydrogen while in the barrel.  The hydrogen expands and accelerates the cartridge.

I suppose that it would take a lot of experimentation in order to optimize something like this and make it a practical device.

Update:

I'm going to make a series out of this.  I like the concept.  Consider this part 2 because the original idea began with the post referenced at the beginning of this post.  That will be part 1.

Update:

Oops!  Almost forgot this link.  This will go on the next post.