Line Cutter for 50k feet attempt

[TimothyG]

I've purchased, examined and tested four different line cutters for our groups first black Rock project and used what I learned to make a non user friendly high performance cutter. Tested it with zip ties and the wire from used igniters so far with great success. Final weight is just over 3 grams.

 

[Speaknoevil]

Thanks for sharing. Where did you source this one?

 

[TimothyG]

I made it myself. Two pieces make it light and small but a pain to reload. Our group is going to crank off 20 or so in the next couple weeks so we can do more testing and then we're hoping to have them ready for our use. Anybody is free to make something similar if they feel the inclination I have no intention of producing them.

 

[dhbarr]

What would you say are the main differences between this version and the ones you sampled?

 

[TimothyG]

The ones I sampled are tubes with a plug threaded into place holding the initiator. Mine is a barstock rod drilled out which in turn eliminates the length of threads required as well as the "plug" needed. A more popular design captures the piston on this design the cap is the piston and no O-rings are needed. I basically faced off the rod, drilled through with a hole sized for the wires, drilled to a set depth for my guess at a suitable charge, the parted the piece off with minimal length used. The main housing is 1" on the money and the plug is a plug. If I felt ok with a single use cutter and a projectile being thrown from the cutter I'd make it even simpler but I don't like randomly pointed projectiles on my recovery system.

 

[OverTheTop]

Nice. I assume the cap is kept on a string too?

Actually, if you threaded the eMatch wires through the cap too you won't need any extra tether for that part.

 

[TimothyG]

Yup strung up with kevlar loop. And that's not a bad idea I think I'll try it out on a smaller flight.

 

[dhbarr]

Ahh, I see it now -- the item to be cut goes through both the wall and the base of the cap. One less moving part, I like it!

 

[TonyL]

This is a very nice simple design. I would suggest vacuum testing if you intend to use it for high altitudes, or one could just add the o-ring and seal the wires like some other designs do. Either way a nice compact system.

br/

Tony

 

[Cameron Anderson]

Can I ask why you decided to make a cable cutter instead of using one that's already commercially available?
Did the commercial ones not perform to your standard or did you just want the experience of making your own?

 

[mbeels]

What makes it:

non user friendly

?

Is it threading the ematch through a longer cylinder? Looks like a slick design!

 

[TimothyG]

I epoxy in my igniters so every firing I chuck up the body and drill out the old ignitor. And these will be used on main so function at high altitude not required. Plus these are smaller in diameter all around making handling components a challenge for even those gifted with dexterity.

 

[steveh.jae]

TimothyG
This is very interesting and given you’re embarking on a 50k’ attempt with your crew I don’t want to trouble you with a bunch of questions ... just one. Where would I go to learn about what it is that you are discussing/describing? Thanks and good luck!

 

[Kelly]

Where would I go to learn about what it is that you are discussing/describing? Thanks and good luck!

Your question is about a cable cutter? It's used for dual deployment; a cable of some sort keeps the main parachute undeployed until the rocket gets close to the ground, at which point the cable cutter, well, cuts the cable (usually with a blackpowder charge) and deploys the main for final descent.

https://aeroconsystems.com/cart/launch-recovery/cable-cutter-by-archetype-rocketry/

 

[steveh.jae]

Your question is about a cable cutter? It's used for dual deployment; a cable of some sort keeps the main parachute undeployed until the rocket gets close to the ground, at which point the cable cutter, well, cuts the cable (usually with a blackpowder charge) and deploys the main for final descent.

https://aeroconsystems.com/cart/launch-recovery/cable-cutter-by-archetype-rocketry/

Kelly, Yes. Thanks. The video was enormously helpful in understanding the process.

 

[MClark]

With the low temperatures at 50,000’ will there be an issue with the zip ties being brittle?
M

 

[TimothyG]

There could be. I decided to test out this cutter on wire and zipties. It can very easily cut through a set of igniter wires. Next test will be cutting kevlar cord just to see if it works. But I think I'll limit myself to zipties on the actual flight. I'm not overly concerned about the cold temps enbrittling my zipties as for one they will only be up there for a short period of time and two this is a dust up flight designed to get my friends and I up to speed on larger two stage projects, from a safety perspective and ground prep perspective, so that we can confidently work on a bigger project over the next two years. This flight is a stepping stone.

 

[tfish]

Ive been using home made line cutters for about 10 years. A few flights to mid 30k. Mine are made from 357 cases.

Tony

 

[OverTheTop]

Next test will be cutting kevlar cord just to see if it works.

I am betting this would not work. If you have tried to cut Kevlar with anything but the sharpest of sharp scissors you will know why . Personally I would probably do the experiment but never fly that way anyway.

 

[rocket_troy]

I am betting this would not work. If you have tried to cut Kevlar with anything but the sharpest of sharp scissors you will know why

No arguments there

You'd likely need to construct your line cutter from a hardenable steel like Silver Steel (Drill Rod in the US?) and ensure the tolerances on the cutting edges are tight, sharp and dag free. That might also mean a final cleanup in the hardened state - which is actually not difficult provided you have the right inserts. I've found that ISCAR IC907 grades are fine for it - well, for turning ODs. As for IDs, that small... maybe a micro carbide boring rod if you actually needed to go that far.

 

[Walter Longburn]

If you have tried to cut Kevlar with anything but the sharpest of sharp scissors you will know why

I thought Kevlar was bad until I got some UHMWPE.

 

[rocket_troy]

We do a lot with ultra high PE (>9.2 million g/mol MW) and I don't find it terribly difficult to cut. Sure, it's a lot more PITA than your other GP engineering plastics but nothing as challenging to cut as Kevlar or Spectra; although we use engineering grade solid rod material and turn it, so we might be talking apples and oranges.

 

[rocket_troy]

We do a lot with ultra high PE (>9.2 million g/mol MW) and I don't find it terribly difficult to cut. Sure, it's a lot more PITA than your other GP engineering plastics but nothing as challenging to cut as Kevlar or Spectra; although we use engineering grade solid rod material and turn it, so we might be talking apples and oranges.

Duh, I was talking oranges. Spectra is a form of UHMWPE which *IS* very difficult to cut. Nothing like the engineering grade UHMWPE rod material we deal with at work.

 

[TonyL]

on Mark's question about cable ties: not all cable ties are equal. The ultra cheap ones are significantly more brittle than the good ones. Thomas and Betts are considered to set the standard for quality and performance at least in auto racing. I did some comparisons by just folding one over on itself several times. The ones from NAPA perform well, as did the orange construction ones from Grainger [neither broke]. I did find that any discount store or Harbor Freight ones I tried tended to snap on the first fold.

br/

Tony