Supersonic peashooter
Posted: Sun Mar 31, 2019 3:56 pm
I'm thinking about ways to make a self-contained gun where the energy to fire it is delivered manually by pulling a more-or-less constant force across a moderate distance (3 feet). Supposing I can lift about 50lb, and suppose I can make the 'stacking curve' sufficiently flat that I deliver 80% of the nominal F*d work to the mechanism. Then suppose the delivery to the projectile is also about 80% efficient. That yields 100 ft*lb = 136 J muzzle energy.
For a 20g bolt, that's 114 m/s = 375 fps, which is typical for a bow. But what I want to do is transfer the energy to a much lighter projectile, say a 1g lead pellet (.22 cal). No bow can handle this -- it would be essentially a dry-fire, which would destroy the bow. But supposing I could do it somehow, the pellet would be traveling 510 m/s, which is supersonic! That also would not play well with bowstrings.
So, naturally, my thought is to make a spring-gun / light-gas-gun, where the energy is accumulated in a heavier intermediate projectile, which then goes into adiabatically compressing a small volume of air to drive the pellet. The down-side to this concept, compared to the usual spring gun, is the need to minimize the friction on the piston due to the long travel distance. Also, the device will be much bulkier, with the crossbow arms sticking out, etc.
My next thought was, what about using a piston backed by a large reservoir at high pressure as the energy storage mechanism? You could easily get a very-constant 60lb draw weight with 75psi acting on 1" piston. If you have a 4ft x 2" diam chamber, then the working volume changes less than 20% during the cycle, achieving the flat stacking curve I wanted. Now your piston could do double-duty, in the same tube. The remaining issue is how to draw the piston back. If you want a 3ft travel using a draw rod, that means the top of the rod is at 6ft above the ground when drawn, which is clumsy -- I'm imagining the draw action starting from ~ 1ft and going to 4ft off the ground. It's hard to imagine any other way to access the piston mechanically, though.
I suppose the alternative is to use a higher-force piston with a short travel, and then get the draw you want by some type of external mechanical advantage system, such as pulleys or levers. This sort of defeats the point of having a super-simple mechanical system, but it could also allow you to compensate for a non-flat stacking curve from the piston. You wind up with something that converges to a gas-spring-based spring gun. Just for reference, some .22 air-spring pellet rifles claim 50lbs cocking force and 1000fps = 300 m/s, which is probably lower than what I'm aiming at due to the shorter effective draw produced by the break-action style. One potentially nice thing about using external mechanisms: you could include a ratchet mechanism, which would allow you to split the loading effort into a few strokes of the handle, while retaining the nice property of essentially flat stacking on each stroke, unlike pumping up a regular pneumatic. This could also let me get above 500 m/s, into serious velocity territory.
OTOH, if we're going to get into mechanical advantage, it would potentially be simpler to go back to the crossbow-style device. I still think this might be a fun build -- maybe go for a shorter travel, with a much higher draw weight. The piston tube would be slotted (except for the front few inches) to allow the bowstring to attach to the rear of the piston. I think you could build a relatively low-tech gun with very high muzzle velocity this way. And it would look like Chewbacca's crossbow rifle, which is a plus in my book!
I think I'll call it "The Peashooter" if I build it. 
Let's run some numbers: say I want to keep that 100 ft*lb muzzle energy. I'd benchmarked 50lb*3ft*80% = 120 ft*lbs to compensate for losses. Now let's say I want to use a linear stacking curve, so 50% stacking efficiency, and cut the travel down to 6", which puts us at 480lb at peak draw. Yikes! That's too much force on the trigger mechanism, IMO. Increasing the travel to 12" brings us to 240lb, which is comparable to existing crossbow numbers. Using a 4:1 pulley would turn that into 4ft travel with manageable 60lb peak force. I found a crossbow for $80 on ebay with 14", 175lb draw, claiming 111 ft*lb for arrows. This is slightly less than what I was hoping for, but it shows that it's quite feasible, using a cheap commercial crossbow.
Okay, so now I would construct a slotted piston tube, and some type of breach adapter connecting it to the barrel. This might be a break-action style, for simplicity and for minimizing the dead volume. A used Crossman .22 barrel is 18" and $20 on ebay. It would probably require a bit of machining to make a suitable piston & tube, and the breach interface. Then you also need to extend the stock on the crossbow to support the barrel. Supposing I could scrounge some scrap parts & do the machining myself, I could probably build this for $100 total. This is not great considering you can buy a 1400fps .177 air rifle for $150-ish, or 1100fps at .22 for the same price (however, the muzzle energy is only 29 ft*lb, so they are using a lighter round than I am benchmarking, too).
So I guess my point is, I could get 3x the muzzle energy for roughly the same price, and it would also have a big novelty factor. And in fact, it might be possible to do even better on the muzzle energy at reduced price, with a DIY bow. Plus, you have the novelty factor.
Going down in energy, there are some pretty cheap ($10) 50lb, 6" draw toy crossbows. With 12.5 ft*lb at 80% conversion, you could accelerate a 0.1 gram projectile (airsoft BB) to ~450m/s -- so this would be a true supersonic peashooter. 8)
For a 20g bolt, that's 114 m/s = 375 fps, which is typical for a bow. But what I want to do is transfer the energy to a much lighter projectile, say a 1g lead pellet (.22 cal). No bow can handle this -- it would be essentially a dry-fire, which would destroy the bow. But supposing I could do it somehow, the pellet would be traveling 510 m/s, which is supersonic! That also would not play well with bowstrings.
So, naturally, my thought is to make a spring-gun / light-gas-gun, where the energy is accumulated in a heavier intermediate projectile, which then goes into adiabatically compressing a small volume of air to drive the pellet. The down-side to this concept, compared to the usual spring gun, is the need to minimize the friction on the piston due to the long travel distance. Also, the device will be much bulkier, with the crossbow arms sticking out, etc.
My next thought was, what about using a piston backed by a large reservoir at high pressure as the energy storage mechanism? You could easily get a very-constant 60lb draw weight with 75psi acting on 1" piston. If you have a 4ft x 2" diam chamber, then the working volume changes less than 20% during the cycle, achieving the flat stacking curve I wanted. Now your piston could do double-duty, in the same tube. The remaining issue is how to draw the piston back. If you want a 3ft travel using a draw rod, that means the top of the rod is at 6ft above the ground when drawn, which is clumsy -- I'm imagining the draw action starting from ~ 1ft and going to 4ft off the ground. It's hard to imagine any other way to access the piston mechanically, though.
I suppose the alternative is to use a higher-force piston with a short travel, and then get the draw you want by some type of external mechanical advantage system, such as pulleys or levers. This sort of defeats the point of having a super-simple mechanical system, but it could also allow you to compensate for a non-flat stacking curve from the piston. You wind up with something that converges to a gas-spring-based spring gun. Just for reference, some .22 air-spring pellet rifles claim 50lbs cocking force and 1000fps = 300 m/s, which is probably lower than what I'm aiming at due to the shorter effective draw produced by the break-action style. One potentially nice thing about using external mechanisms: you could include a ratchet mechanism, which would allow you to split the loading effort into a few strokes of the handle, while retaining the nice property of essentially flat stacking on each stroke, unlike pumping up a regular pneumatic. This could also let me get above 500 m/s, into serious velocity territory.
OTOH, if we're going to get into mechanical advantage, it would potentially be simpler to go back to the crossbow-style device. I still think this might be a fun build -- maybe go for a shorter travel, with a much higher draw weight. The piston tube would be slotted (except for the front few inches) to allow the bowstring to attach to the rear of the piston. I think you could build a relatively low-tech gun with very high muzzle velocity this way. And it would look like Chewbacca's crossbow rifle, which is a plus in my book!
I think I'll call it "The Peashooter" if I build it. Let's run some numbers: say I want to keep that 100 ft*lb muzzle energy. I'd benchmarked 50lb*3ft*80% = 120 ft*lbs to compensate for losses. Now let's say I want to use a linear stacking curve, so 50% stacking efficiency, and cut the travel down to 6", which puts us at 480lb at peak draw. Yikes! That's too much force on the trigger mechanism, IMO. Increasing the travel to 12" brings us to 240lb, which is comparable to existing crossbow numbers. Using a 4:1 pulley would turn that into 4ft travel with manageable 60lb peak force. I found a crossbow for $80 on ebay with 14", 175lb draw, claiming 111 ft*lb for arrows. This is slightly less than what I was hoping for, but it shows that it's quite feasible, using a cheap commercial crossbow.
Okay, so now I would construct a slotted piston tube, and some type of breach adapter connecting it to the barrel. This might be a break-action style, for simplicity and for minimizing the dead volume. A used Crossman .22 barrel is 18" and $20 on ebay. It would probably require a bit of machining to make a suitable piston & tube, and the breach interface. Then you also need to extend the stock on the crossbow to support the barrel. Supposing I could scrounge some scrap parts & do the machining myself, I could probably build this for $100 total. This is not great considering you can buy a 1400fps .177 air rifle for $150-ish, or 1100fps at .22 for the same price (however, the muzzle energy is only 29 ft*lb, so they are using a lighter round than I am benchmarking, too).
So I guess my point is, I could get 3x the muzzle energy for roughly the same price, and it would also have a big novelty factor. And in fact, it might be possible to do even better on the muzzle energy at reduced price, with a DIY bow. Plus, you have the novelty factor.
Going down in energy, there are some pretty cheap ($10) 50lb, 6" draw toy crossbows. With 12.5 ft*lb at 80% conversion, you could accelerate a 0.1 gram projectile (airsoft BB) to ~450m/s -- so this would be a true supersonic peashooter. 8)
Hydrogen would definitely help, although it would be a waste to use it this way without igniting it.