Puncturing the iSi Cream Charger

[ghost]

I'm trying to make a new non-pyro deployment system, and I need advice on how to puncture the small iSi container. It contains 10cm3 of N20. There is a thin seal disk at the top. How should I puncture this (without Pyros)? Is there a way to test-puncture it (to see how many lbs of force I need)? What is a safe way to do this? Thanks

 

[heada]

What about punchuring it on ground, containing the N2O behind a burst disc and then melting through the disc with some nichrome?

For example, if your container has 50 psi of N2O and when you release it into your burst disc area it drops to 40 psi (increase in volume, drop in pressure) and you have a 55 psi burst disc, you only need to lower it below the 40 psi that the burst disc will hold back. Nichrome heated up will nearly melt through most plastic burst discs in that range in less than 1 second I would think.

-Aaron

 

[ghost]

Where do I get burst disks? Can I make them?
Wouldn't the burst disk have to be contained in some chamber?

 

[heada]

You can make them yourself from plastic sheet. Thinner the sheet, the less pressure they'll contain. May have to experiment with some (air pressure and a regulator to show what pressure it bursts at)

They'd need to be held between two plates. (think plate, o-ring, burst disc, o-ring, plate)

-Aaron

 

[ghost]

How do I break the metal seal on the steal container, though?

 

[...]

I, personally, would stick with using CO2 whippets, rather than N2O, even if simply because you won't get weird looks when people think you are trying to get high

In any case, it isn't all bad as there are a multitude of designs for exactly what you are trying to do online, just google 'N2O cracker'. You would just have to add a way to move the pin with an electrical solenoid or something, or make one that seals well and have a burst disk on the end (I would prefer to see the first one done personally).

 

[m85476585]

What about punchuring it on ground, containing the N2O behind a burst disc and then melting through the disc with some nichrome?

For example, if your container has 50 psi of N2O and when you release it into your burst disc area it drops to 40 psi (increase in volume, drop in pressure) and you have a 55 psi burst disc, you only need to lower it below the 40 psi that the burst disc will hold back. Nichrome heated up will nearly melt through most plastic burst discs in that range in less than 1 second I would think.

-Aaron

Those are probably like CO2 cartridges, so the pressure will be a lot higher than 50 psi. Depending on the temperature it will probably be somewhere over 700 psi.

I don't think anyone has come up with a reliable way to puncture CO2 or other cartridges in a rocket that is completely non-pyro, but it would be great given the current regulations on e-matches and BP.

The metal seal shouldn't take a lot of pressure if you puncture it with something sharp (Don't just try to poke something in without securing it because all that pressure can create some thrust). A geared motor might be a good way to puncture a cartridge in the air. You could also try a strong spring with a pin holding it back (but then you have to figure out how to remove the pin. You could try to make something that works like a mousetrap). Puncturing the cartridge on the ground would be a little more difficult because you have to contain the pressure while you are puncturing it, and until it is time to release it in the air.

 

[heada]

https://www.aeroconsystems.com/motors/Screamer_folder/screamer.htm

The burst disc is broken by means of an igniter in this case, but I think it could be modified so that it is broken or weakened enough by means on a bit of nichrome .

-Aaron

 

[ghost]

I, personally, would stick with using CO2 whippets, rather than N2O, even if simply because you won't get weird looks when people think you are trying to get high

In any case, it isn't all bad as there are a multitude of designs for exactly what you are trying to do online, just google 'N2O cracker'. You would just have to add a way to move the pin with an electrical solenoid or something, or make one that seals well and have a burst disk on the end (I would prefer to see the first one done personally).

The box says N20... I'll look up N2O cracker. Thanks

 

[ghost]

Hmmm... I want to make a PVC cracker, but I need to know how much PSI there will be. THe box says the cylinder holds 8 grams of N20 and when not compressed, is 10cm3. Any idea of the PSI?

Thanks

 

[billspad]

Hmmm... I want to make a PVC cracker, but I need to know how much PSI there will be. THe box says the cylinder holds 8 grams of N20 and when not compressed, is 10cm3. Any idea of the PSI?

Thanks

The 700 psi mentioned in a previous post around the middle of the range you can expect at normal temperatures.

 

[ghost]

So using a pVC cracker wouldn't work? Any other ideas?

 

[heada]

Imagine the base of a rivet. (wide, flat but having a cylinder type protrusion in the center) Cut the protrusion at a 45 so that you now have a needle like point but a hole to allow the contents to flow past and yet still have a wide base to push against. Make that out of stainless steel and you'd almost be able to push through with your thumb (not recommended) You could also take an actual needle (16 gauge or so I would think) and just use that as it is cut at closer to 60 degrees (I think, might be more)

-Aaron

 

[ghost]

Imagine the base of a rivet. (wide, flat but having a cylinder type protrusion in the center) Cut the protrusion at a 45 so that you now have a needle like point but a hole to allow the contents to flow past and yet still have a wide base to push against. Make that out of stainless steel and you'd almost be able to push through with your thumb (not recommended) You could also take an actual needle (16 gauge or so I would think) and just use that as it is cut at closer to 60 degrees (I think, might be more)

-Aaron

But how would you contain the gas?

My concept is attached. On the ground, you puncture the N2O with a screw. In the air, the nichrome melts the plastic burst disk.

View attachment n20.PNG

 

[m85476585]

PVC might not be able to contain the pressure, depending on the volume. P1V1=P2V2. If you get the pressure low enough to be safe in PVC, it might not be enough to separate the rocket (depending on how quickly the burst disk can release the gas). Make sure the final pressure it 15psi below the safe pressure for PVC since the atmospheric pressure will decrease as you go up, and atmospheric pressure is about 14.7psi. (to get a 14.7psi relative increase, your rocket would have to leave the atmosphere, but it's better to be safe.)

 

[ghost]

Is there a way to calculate what the pressure will be on a 1 foot long PVC tube that is 1" OD (like .75" ID)?:

 

[m85476585]

The volume of the PVC is about 58cm^3, the pressure of the N2O cartridge is about 700psi, and I think you said the volume of the cartridge is 10cm^3
700 x 10 = 58 x P
p=120.

According to this:
https://www.engineeringtoolbox.com/pvc-cpvc-pipes-pressures-d_796.html
the maximum operating pressure for 1" schd40 PVC is 270 psi (the burst pressure is much higher).

If the pressure is going to be that low you might as well just pressurize the PVC with an air compressor (I think there was a thread about air pressure ejection somewhere).

How big of a rocket are you going to put this in? 8g might not be enough to get reliable separation.

 

[heada]

But how would you contain the gas?

My concept is attached. On the ground, you puncture the N2O with a screw. In the air, the nichrome melts the plastic burst disk.

Flip the N2O around, put the piercing device on top of the burst disc, and put the screw device on the aft end of the N2O so that when you screw it down, it pushes the N2O onto the piercing device which pierces the N2O and pressurizes the container. Apply 12V to the nichrome that is laid across the burst disc and POP!

I would use CO2 and not N2O for a few reasons. 1) they'll be cheaper and easier to source in larger sizes. 2) CO2 decomposes into C2 + (2)O2 at a higher temp than N2O decomposes into (2)N2 + O2 and therefore would be safer if the nichrome where to heat up too high or if this were placed too close to the motor.

There is always the option of using compressed air. Using a quick disconnect setup like found on pretty much all air-tools you could pressurize a container very quickly and for almost free.

-Aaron

 

[...]

The volume of the PVC is about 58cm^3, the pressure of the N2O cartridge is about 700psi, and I think you said the volume of the cartridge is 10cm^3
700 x 10 = 58 x P
p=120.

You can't use pv=pv for this application, because the N2O in a cartridge is in liquid form, the 700psi number is the vapor pressure of the liquid N2O.

In fact, with a 58cm^3 container, you will not be able to convert all of the liquid N2O to gas. You will still be at 700psi, with a few grams of liquid N2O in the bottom of the tube.

For reference, there is about 5L of gas in an 8g cartridge (measured at 1atm/25C), so if you wanted to get the pressure down to 270psi (20atm), you would need a volume of .25L, or 250cm^3.

 

[Sailorbill]

oops

 

[m85476585]

You can't use pv=pv for this application, because the N2O in a cartridge is in liquid form, the 700psi number is the vapor pressure of the liquid N2O.

In fact, with a 58cm^3 container, you will not be able to convert all of the liquid N2O to gas. You will still be at 700psi, with a few grams of liquid N2O in the bottom of the tube.

For reference, there is about 5L of gas in an 8g cartridge (measured at 1atm/25C), so if you wanted to get the pressure down to 270psi (20atm), you would need a volume of .25L, or 250cm^3.

I thought 120 psi sounded a little low. You could use pV=nRT, but it is a bit of work since p is in pascals, v is in m^3 and t is in kelvin. You can also use the constant 22.4l/mol at stp, but you have to change the temperature in Kelvins (V1/t1=V2/t2). For reference there are .1817 moles of N2O in 8g.

 

[ghost]

The actual cartridge is very small... only about 1.5 inches long and .5 inches in diameter. Here is what the box says:

1 charger whups up to 0.5L of liquid... Non aerosol. Volume: 10cm3. Contents: net approx 8 g pure N2O under pressure.

 

[...]

What exactly are you asking?

The numbers on the back are what I am using for all of the calculations, 8g of N2O is .187moles of it, which at STP is a little over 4 liters.

If you squeeze that into a 10cm^3 container, the pressure would be about 400atm (6000psi), but N2O turns into a liquid long before you get to that high of a pressure, so in reality you have mostly liquid with a bit of gas.

The pressure of the mix depends on the temperature (the equilibrium between N2O(l)<-->N2O(g) is pushed towards N2O(g) at higher temperature/volume, towards N2O(l) for higher pressure), and at STP it is about 700psi. However, you should you decide to store liquid N2O in your own container, you should design for at least 1000psi, as the pressure of N2O hits 1000psi at about 80f. Really, you should design for 1500-2000psi.

Needless to say, unless you are planning to have some spun aluminum containers built, you are going to want keep the N2O in gas form at a more reasonable pressure, or keep it inside the cartridge until you are ready to use it.

Really, it shouldn't be that hard to design a system to puncture the cartridge, I would estimate that about 5J of capacitors (a capacitor/charger out of a disposable camera in parallel would be ideal) into a 20-100turn electromagnet would puncture it with no trouble. Google 'coilgun' for more info about the type of topology I am thinking of, but you would want to redesign the coil such that instead of launching a projectile you would poke a rod into the canister. You would want the coil to be thin, but wide (ie, lots of layers of wire), with a plate the diameter of the coil a few mm away from the coil, such that when you energize it it will move towards the cartridge, and have a rod going through the center of the coil with a point on the end to fire the cartridge. At this point, having the container sealed wouldn't matter at all, since you are just venting into the rocket...

sorry if that didn't make any sense...

 

[ghost]

This makes more sense. My local electronic store sells electric solenoids which might work if they are strong enough (https://www.allelectronics.com/cgi-bin/item/SOL-58/search/24_VDC_PULL-TYPE_SOLENOID_.html).
(yes i know it is 24 volts... it is just an example of the type of solenoid...)

 

[m85476585]

If a solenoid isn't strong enough, you could use it to hold back a larger spring. Compress the spring (with something to puncture the seal at one end) on the ground and hold it back with a solenoid. Then have the solenoid release the spring.

 

[heada]

Seloniods are used to "hold back" the N2O in most hybrid GSE and so can handle the pressure, temprature and are designed for 12V. The problem is that they're not small or light. Smaller and lighter seloniods are available (just search a place like McMaster-Carr) but I don't know what pressures they can withstand.

-Aaron

 

[...]

You are confusing solenoids (as in a coil of wire) with a solenoid valve (as in a valve that is actuated with a solenoid).

You do not want to use a solenoid valve, as then you are back to the problem that you need to contain the liquid N2O in your own container, and once you have that you are better off with a burst disk.

When you use the foil on the creamer cartridge as the valve, and only open it when you are ready to use it, you can eliminate the need to store excessive pressures in your own container.

On that note, the voltage that you use with a solenoid is arbitrary, you can design one that will output the same force for a 1v input signal (although you would need to run an insane amount of current through it) or 1,000v signal (but insulating the wire would be a challenge). If you have a supply of high current 12v (I am guessing you would need 10+ amps, although I don't know how hard that foil on the creamer cartridge is to puncture), you could use less turns and run at 12v.

I would also recommend against using a solenoid to hold a spring compressed (using some type of latch), as after you have punctured the canister the puncture device will be spring loaded into the cartridge, and the N2O won't be able to get out, so you would need some type of hollow needle.

 

[ghost]

Hmm... this is getting more complicated than I thought.

So using a solenoid (which just moves an arm back and forth) and attaching it to a needle wouldn't work?

 

[...]

Sure it would, you just need to be able to turn the solenoid off after you puncture the canister. When you use a spring/latch you need a fancy needle, but if you can turn the solenoid off (you might also need a return spring to pull the needle back out), you can just pull the needle out and let the N2O out through that hole.

But really to speculate to much more, we need more data on what exactly needs to be done to the foil on the cartridge. If you just need to poke it with a needle that is .020" in diameter, the design is a lot easier than if you need to poke a hole that is .1" in diameter. Perhaps it would work better to use something like an razor blade to poke the hole. And how thick is the foil to begin with?

 

[ghost]

The foil is about .2" in diameter