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Compressed air launch

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Winston

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In my search for something technically interesting and possibly useful to do with hobby rockets, I considered compressed air (for non-pyrotechnic safety) tube launched, aerodynamic, instrumented probes to attach to one of Reed Timmer's tornado intercept vehicles to be launched into the outer and inner circulation of tornadoes. Reed had previously done something similar with more crude, compressed air launched, non-aerodynamic instrumented projectiles, then didn't continue the practice for unstated reasons, perhaps because of cost, poor results, etc.

Anyway, after reading up on the large(ish) R/C drones now being used by professional tornado researchers, I abandoned the idea. However, the compressed air tube launch idea stuck with me. Drop the "useful" bit and it's still technically interesting.

What I'm thinking of: a vertical 2" PVC pipe launch tube with a horizontal 3" PVC pipe compressed air reservoir (2" Schedule 40 PVC pipe has a 280psi rating; 3" Schedule 40 PVC pipe has a 260psi rating; max to be used: 100psi) with a 6" PVC (I'd much prefer 6" HDPE, but try to find it) pipe surround around the reservoir for burst safety with no-go zones at both ends of the reservoir; a sabot (split pool noodle - fits nicely when slightly compressed into 2" PVC pipe); 3D printed sabot base "pusher" (if needed); 3D printed dart or rocket; modified sprinkler valve for rapid air dump using large orifice 12V truck air bag suspension/air horn solenoid valve ($18 shipped on eBay) as the modified sprinkler valve's upper chamber pressure release instead of the manually actuated air nozzle release used in the mod video below and typically used for spud guns:

Sprinkler valve theory of operation

http://www.spudtech.com/content.asp?id=21

Fast Hacks #4 - Sprinkler Valve Modification for an Air Cannon

[video=youtube;H4uT0wUzvK0]https://www.youtube.com/watch?v=H4uT0wUzvK0[/video]

Reservoir pressurization would be supplied by a 12V tire inflation pump with pressure gauge. Streamer or parachute deployment would be accomplished via compression spring released by altimeter. If an 18mm composite motor is used, the launch air valve would be triggered by an IR flame sensor port after ?? delay (perhaps combined with an acoustic and/or launch tube pressure sensor as confirmation of actual motor ignition). I wonder what effect a 100 psi ambient pressure jump and extremely high acceleration would have on a just-ignited 18mm composite motor during the instant it spends travelling down the launch tube. ;-)

Launch force (thrust) at 100psi: pi*r^2*100psi = 314 pounds.

Additional safety considerations:

http://www.spudtech.com/content.asp?id=16

Anyone know of anyone doing this sort of thing before? Comments and suggestions?
 

rstaff3

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I built myself a non-optimized air launcher for the grand rocketeers...yeah, that is, it's for them. I used a sprinkler valve but opted for a pneumatic release vs electronic.

Here's one that reportedly hit 660 G's and 1000 mph.

Many years ago, there was a guy who was building an air launcher with an air-started rocket. He flew at MDRA but I wasn't there to see it. Here's a link with a photo. His blog is dead but, IIRC, he used two Spudtech Supah valves.

Here's my biggest air rocket, a HoJo by Mike Bauer:
stomp hojo launch 003 (Large).jpg

Which is dwarfed by his Saturn V!
PICT0100 (Large).JPG
 

Winston

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I built myself a non-optimized air launcher for the grand rocketeers...yeah, that is, it's for them. I used a sprinkler valve but opted for a pneumatic release vs electronic.

Here's one that reportedly hit 660 G's and 1000 mph.

Many years ago, there was a guy who was building an air launcher with an air-started rocket. He flew at MDRA but I wasn't there to see it. Here's a link with a photo. His blog is dead but, IIRC, he used two Spudtech Supah valves.

Here's my biggest air rocket, a HoJo by Mike Bauer:
View attachment 297449

Which is dwarfed by his Saturn V!
View attachment 297450
Cool! Thanks for those links! I wish his blog was still active. But your link allowed me to find him on YouTube and I may be able to contact him via email:

[video=youtube;mPGLPVG2RF8]https://www.youtube.com/watch?v=mPGLPVG2RF8[/video]

Unfortunately, no live launch with air start motor shown. I wonder what the extensions at the barrel muzzle are for in the photo. Muzzle velocity sensing? Rocket barrel departure detection and motor ignition? (actually, you could do all of that with the same electronics hardware and two conductive strips; prior to seeing this, I was thinking about using photoelectric velocity measurement). Looks like he's using an unmodified sprinkler valve. The sabot looks like it might even be a pool noodle.



If I don't end up doing this in a serious way, it would make a very interesting (to me anyway) NAR research project for someone - finding optimum mass for max altitude at a given psi; measuring muzzle velocity; determining optimum aerodynamic shape for velocities obtained; sabot design; high speed videography, etc. Lots of variables, so maybe dial in one and play with another.

EDIT: the key for a cheap and reasonably sized system, I think, being smallish 3D printed darts or rockets.
 
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rstaff3

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The Supah valves are triggered by standard sprinkler valves, which is what you are seeing. You reminded me that the motor ignition was initiated as it exited the barrel. I don't remember any details, but the electronics were custom.
 

rstaff3

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You can find some of his posts on the wayback machine. Could not get that link to copy on my phone. I probably am remembering wrong, but I don't think the launch detector description was the final version.
 

rstaff3

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You should also search for Dave Hall's gas gun simulator.
 

cerving

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I think you could use a microswitch that's triggered on exiting the tube to airstart your motor. You'd want to have the igniter power off until you're a safe distance away from this thing, of course... an Eggtimer WiFi Switch would be ideal for that. (I know, microswitch-actuated airstarts are prohihibited by TRA... I don't think NAR has the same rule, however).
 

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