How to decrease shootup

[GatoLoco]

This is just a thought. If shootup is caused by heat in the marker, why not incorperate either A) venting or cooling (seems a little too complicated), or B) some sort of wadding akin to that used by model rockets? Even if it does not eliminate shootup, by venting, cooling, or having something absorb heat, it could be reduced. I realize that AGD is busy and this is probably not at the top of their list, however when I was walking to buy milk, the idea popped into my head.

[GatoLoco]

anybody have a comment?

[Wat]

The problem is that its not the chamber heating up thats the problem, its that the gas itself is heating up as its being compressed. There's really no effective way to quickly cool off the gas, especially when your firing away at 15bps.

[xatle]

i think about this problem often.
my current idea is that the problem isn't the shoot up caused by the gas heating up when it recompresses, i see this as an indication of increased efficiancy during rapid fire, better efficiancy is a good thing. the problem is the shoot down that you get when the gas is allowed to cool off before you fire.
reading what i just wrote, my first thought was to not let it cool down. insulate the dump chamber and add an internal thermocouple and heating element to keep the air from cooling down. a sensor detecting the position of the seer can cut power to the element during the actual fireing cycle. this is a new idea for me, needs more thought to really flesh it out.
ill have more to say later, when im not so tired.

[Redkey]

Being a non mag person...

When is the gas compressed? I assume you are talking about the "empty" dump chamber being repressurized?

What happens if the dump chamber was larger and ran at a lower pressure? At some point I would assume the cooling of the expanding gas would equal the compression heating of the residual gas.

But, I'm not a mag person so what do I know.

[nerobro]

Reduce the input pressure. that will reduce the speed the chamber is filled. and reduce the heat generated by forcing air through small passages.

the lower the input pressure, the lower the shootup. :-)

So long as your'e not seeing shootdown, continue reducing your input pressure untill you get a gun that preforms the way you want.

[athomas]

Its not the volume of the air or the passages that cause the heat. It is the compression/recompression of the gas.

Even a larger lower pressure chamber would suffer from heat generation because of the rate of compression not because of the amount of pressure. Think of heat generation due to compression much like heat generation caused when you slam a hammer on a block of steel. A light tap and you hardly notice the temperature change. Slam the hammer down hard and you will feel significant heat at the point of contact. The retro valve charges so fast that the air rushing into the front chamber compresses/recompresses the air so quickly it heats up.


The only way to prevent the heat would be to prevent the loss of pressure. Deliver the entire front chamber pressure in one burst in an instant. The retro valve as it stands, takes 750psi input pressure which provides the force neccessary to push the air through the tiny passages of the valve in a timely maner so that the valve fully charges before the next shot is fired. The tiny passages provide the restriction that makes it neccessary for such a high input pressure to maintain a good recharge rate. The high recharge rate provided by the high pressure is the "hammer" and the front chamber air is the "steel". If we could open up the passages then we could provide the same recharge rate with less pressure differential. There would be less heating as a result.

So, two things could fix the problem. One, allow the air to heat and then maintain the heat as was previously mentioned. Two, prevent the air from heating by either providing better cooling, or by preventing the generation of heat in the first place.

[Redkey]

more volume

About that hammer... if the swing is the same and the surface area (point of contact) is larger the heat energy of the impact will be spread out over a larger area which will reduce the measured temperature gain.

The thought behind the more volume relates back to the lower chamber pressure idea. If you are going reduce the pressure in the dump chamber then you will need to make the chamber larger so the amounts of stored energy are similar.

Since, as you mentioned, high pressure is needed to overcome the flow restrictions... a lower pressure trying to navigate the same restrictions will definately slow the recharge rate.

We had a similar problem with compression/decompression cycles changing the air temperature in a cylinder which then changed the spring rate of the assembly. The solution was clever although I don't think it would work for this problem since our system was running at about 100 psi. Is there a diagram showing the operation of the mag somewhere?

[athomas]

The hammer and heat thing was used just to help convey a concept. The area and size of the head definately affects the heat generated.:)

I'm thinking that a larger volume front chamber at a reduced pressure would work if the air passsages were opened up. The on/off pin could also be made bigger due to the reduction in required pressure. If you lower the input pressure and maintain the flow rate then you reduce heat due to recompression.

Heck, maybe this thought process has been overlooked, but:

Previous thoughts - high pressure to allow for adequate air flow through valve. Keep on/off pin size(large head) small to reduce reactivity for tournament legality.

New thoughs - open valve ducts to allow more flow. Make on/off pin head larger. Use lower input pressure. The lower input pressure would still keep reactivity down because the force felt by the on/off top could still be the same as when it was smaller. Now combine that with a larger front chamber and you really reduce the heat problem.

I don't know if the valve has the capability to have larger air passages, and making the front air chamber larger would take a complete redesign.

I wonder if the regulator on the back of the mag could be made into a two stage design where most of the flow was regulated by the main reg and a more sensitive reg was used to top the valve off. The pressure would never go above a preset value based on the normal heated charge. If the air cooled, then a more sensitive part of the reg could gently leak a bit of air into the chamber as needed to bring the chamber back up to the regulated value. That way there will be no difference between single shot and rapid fire shots accross the chrono. Maybe pipe dreams.:)