They built a balanced piston valve. Very heavy-duty steel, they've run it as high as 500psi.  The barrel is evacuated before the shot.  There's a nice shadowgraph image of the shock wave from the supersonic baseball.
Discussion of the concept:
The shock waves start here:
            
			
									
						Supersonic baseball cannon on 'Smarter Every Day'
- Moonbogg
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I saw that video.  It's completely awesome!  It's the ultimate pneumatic cannon and pushes the performance to the extreme by doing a variety of special things to get there.  I loved it.  I wonder how a hybrid cannon would do.  I'm thinking a simpler and cheaper hybrid could be made that would out perform the pneumatic.  I wonder why they don't explore that route.  They have focused on pneumatic cannons with evacuated barrels.  Maybe for legal reasons?  If that were the case, they wouldn't have made a combustion spudgun video, which they did.  I say they up their game and build a simple, beautiful, sexy looking piston hyrbid cannon out of heat treated 4140 at around 200kpsi yield and bust loose at around 20X.
            
			
									
						- jackssmirkingrevenge
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I wonder why they didn't go with a burst disk. Much simpler to engineer and arguably has an edge in performance.
There's a certain joy in achieving certain milestones purely with compressed air I suppose. Hybrids are almost "cheating" if the goal is to go supersonic.I wonder how a hybrid cannon would do. I'm thinking a simpler and cheaper hybrid could be made that would out perform the pneumatic. I wonder why they don't explore that route. They have focused on pneumatic cannons with evacuated barrels. Maybe for legal reasons?
hectmarr wrote:You have to make many weapons, because this field is long and short life




