One of the last ground support items I need is a tube cutter on the fill line to the rocket flight tank. I drew up several elaborate designs using quick disconnect fittings. But the complexity and time required to machine those parts made me think... This should be a simple function, so let's keep it simple and just cut the nylon tube at the surface of the rocket. Some sort of shears powered by either a pyro charge or a cold gas supply. Since this cutting device needs to be activated from a safe distance, I decided a pyro charge would be the easiest way to create the needed gas pressure. Of course, that does mean another firing circuit in the launch sequence.
This would be the side that will face the rocket. It should cut the line within 1/4" of the surface of the rocket.
From this side you can see the end of the aluminum bar that's slotted to clamp the tube into the cutter.
Here I've added a stop on one side of the cutter blade and a bracket to keep the blade tight against the block on the other side.
From another angle.
And a shot from the back side. You can see the bolts that thread into the coupler, they really should be cut to length and some bushings added to keep it positioned in the bracket.
Here the piston has been removed.
The cutter works on the piston principle. I turned down a 1/2" steel rod to fit inside a 3/8" stainless steel high pressure nipple. The steel rod is slotted at the end and is bolted to the cutting blade. The top end of the piston cylinder is a 3/8" high pressure coupler that has been drilled and tapped for 1/4" pivot bolts on either side. To seal the top of the cylinder I used a 3/8" steel plug, the plug is drilled out to allow an e-match wire into the cylinder.
I think you get the idea from the pictures. Pressure in the cylinder drives the piston down, which pushes on the hinged blade cutting the nylon tube.
In use, I'll make an arm to hold the cutter against the side of the rocket. When the cutter is activated and the tube cut, a small spring will pull the cutter and supply end of the tube away from the rocket. This will allow visual confirmation the tube is cut, and also move the cutter away from the rocket prior to launch.
In tests I determined .6 grams of black powder do a nice job of cutting a 1/4" OD nylon tube. I think if I were to do this again, I might make the whole device in-line. That is, bore out the aluminum stock so the piston runs the length of the stock, then cut a slot and fit the cutter blade inside the aluminum stock as well. This would require more machine work, but would make the unit more compact. As it is, it works fine and the size isn't really important since this is ground support equipment and won't fly. It's also a quick and easy to make the way it is now.
I decided to test the line cutter using Red Dot gun powder, rather than black powder. I've done some tests using Red Dot with the PIRM2, and it works very well. One has to keep in mind two things when using something like Red Dot; first, it needs confinement. It won't burn completely in the open. Second, it has about twice the energy per unit mass that black powder has. So the amount needed in a device like this is very small. I use soda straws as a measure, one end is sealed and I simply measure how many inches are needed for a given job. In the case of the line cutter, .5" seems to works fine. The straws I use aren't the real big kind, these are about .23" in diameter with a very thin plastic wall. It's a little hard to make accurate measurements when dealing with such small amounts, but I found the Red Dot weighs about .32 grams per linear inch of straw. So my .5" straw measure is only .16 grams of Red Dot.
Click Here for a short video of the cutter using Red Dot powder.
I did decide to make a new line cutter, below is the content of a new page I started on it.
This was actually my original design idea for a line cutter, but I wasn't sure I could make this without a milling
machine. So my first line cutter was a rather large device and wasn't optimal by any means, but it did work and
was built with just a lathe and drill press.
Here's a picture of the finished cutter.
What I wanted, was a piston driving a cutting blade, the cutting blade would need a slot dead center in 1" square aluminum stock and a 5/16" hole also dead in the center for the piston. A milling machine would have no trouble with the slot, and the 5//16" piston bore hole would be easy if I had a 4 jaw chuck for my lathe. Unfortunately, I don't have either. So to cut the blade slot, I removed the clamp from my 14" abrasive wheel cut off saw, and used a drill vice with X and Y adjustments to hold the stock. I used (8) very strong rare earth magnets to hold the vice in place. Once the stock was centered, I lowed the cut off wheel into the stock until I had a slot 1.35" deep. In hind sight, I probably should have drilled the bore hole first, then cut the slot. Now I had to make a perfect bore down the middle of the stock. I marked the center of the stock from the non-slotted end, and drilled the bore hole on my drill press. The stock is 3.5" long, and I was somewhat pessimistic I'd get a straight hole drilled that length. But when it was drilled out, it was pretty close to perfect. Lucky or good, I'm not sure, but it worked...
For the piston and cutter, I was going to weld a cutting blade to a piece of 5/16" round bolt stock. Then I had another idea. So I took a 5/16" bolt and heated the bolt head cherry red, and hammered the head more or less flat. Then ground a cutting edge in the tip with a bench grinder, and finished up with a whetstone.
Here you can see the piston/cutter out of the device, the back cover plate is also off.
Ok, that was the new design. Unfortunately, I don't think I'll ever use it for cutting a N2O line. After I made it, it dawned on my the gas from the pyro charge would blow right on the cut N2O line. It seems all too likely to me that the pyro charge could ignite the N2O. Since my current designs use a one way valve to allowing filling the tank, I'm afraid if the N2O ignited, the flame could propagate through the one way valve and into the tank. So, while this design works fine, I don't intend to ever use it on a N2O line. Maybe I can find some other use for it in the future. It would also work to cut a rope for deployment of some sort I'm sure, so I'll keep it in a storage bin until some good use for it turns up.