Pyrotechnic cable cutter modification for vacuum.

As you can see from the previous post, being able to cut away a high altitude balloon is really important, particularly in New Zealand.  The prevailing winds and narrow island means pretty every balloon flight desperately wants to go out to sea.  In fact we have lost a couple of flights this way.

There are several different methods which get used to cut away balloons.  The most common in amateur circles is, I think, the hot wire cutter.  Basically the scheme is to use some nichrome wire to cut something which lets the balloon go.  This is usually a cable tie for simplicity.

We experimented with this approach in our very first balloon flight. There are two problems or challenges with this method.  The first is that it needs a lot of current which can be challenging to supply on a balloon. You also have to wonder, how long do you leave the current running?  You don't want to leave it running too long otherwise you need a separate battery pack to run it least your electronics brown out.  You can test it on the ground but conditions at balloon cut away are going to be very different, cold and a pretty good approximation of a vacuum.  How good a contact does the nichrome have with the cable tie?  How much does convection contribute to getting heat into the cable tie?  There will be almost no convection at altitude.

The second problem is reliability, nichrome is pretty brittle to begin with and fatigues really quickly.  The cold at altitude won't help this.  As the nichrome is in contact with an inherently flexible element and the balloon does bounce around a bit on the way up, it seems like it would be really unreliable.  So then, you double or triple it up but that adds to the requirements of delivering lots of current.

What we ended up going with on our previous flight was a cable cutter.  Nasa uses cable cutter to do things like cut away the bridle on the curiosity mars rover and deploy solar panels on satellites.  They have existed for a long time, here are a couple of patents for these sorts of things from the 60's and 70's.  US 3885484 A,  US 3482484 A  and US 3550274 A.   While these would be easy enough to make, we decided to buy a couple from Archetype Rocketry as they are reasonably affordable there.

These cutters use black powder (BP) and Q2G2 igniters (they are super reliable and very low current, we love these things).   The only problem with this setup is that black powder is very unreliable at high altitude, it's the bane of high altitude high power rocketry folk.    Thankfully we have a vacuum chamber and some reasonable vacuum pumps.  The pumps are supposed to be good down to10^-4 torr but realistically we expect to be getting quite a bit less than that.  For reference 1 torr is about the equivalent pressure at 46km in the 1976 standard atmosphere.

We first tested the igniters in the vacuum chamber by themselves.

The igniter bridge wire burnt but the BP making up the igniter head didn't go up.  The BP has its own oxidiser so that's not the problem.  What happens is that heat isn't transferred well across BP particles.  Sure some of the BP in direct contact with the bridge wire will go and produce some gas, but that will disperse very quickly into the surrounding vacuum, taking its heat away with it. 

There are two ways to fix this, one is to keep the igniter and BP in a pressurised space, doesn't have to be pressurised with air, could be any gas, but its still a pain.  The other option is to enclose the igniter and BP in a small space.  That space can be under vacuum initially but as the small amount of BP in direct contact with the bridge wire goes up, the small amount of gas streaming off can pressurise the small volume a little bit.  This can help some more of the BP to go and then the process can cascade.  Note that the seal around the BP doesn't have to be perfect, it just has to be good enough for the pressure to start rising. Transients are where its at.

Our first attempt was to just try the cable cutters as directed, but under vacuum.  The cable cutter contains the igniter and BP in a small space, perhaps a small enough, sufficiently well sealed enough space for the BP to go.  Sadly this was not the case.

What we came up with next was to make a little cartridge for the BP out of a drinking straw and some hot glue.  Kinda sketchy sounding I know, but it works pretty well.  It is also very convenient that the Archetype rocketry cable cutter internal diameter very nicely matches the diameter of a drinking straw.

We seal one end of the drinking straw with hot glue, this is the side that faces the internal cutter.   What you end up with is this

I may have got a bit carried away CADing this up.  Note to self, don't CAD late at night or you are liable to go on a CAD ramble.   The relative dimensions are only approximate I made this up from memory.  I'm currently in the US and don't have any cable cutters with me.  Or verniers for that matter.

Anyway, we also filled up the hex socket of the bolt with epoxy to help seal the chamber.  It also help keep the igniter wires really secure.  It means we have to replace the bolts every time but they are really easy to make up on the lathe out of a socket screw.  They are imperial sized which is a bit annoying but no big deal.

With these simple modifications the cable cutters work really well under vacuum.  I have a  video of one going off under vacuum,  I can't currently find it but I'll add it when I do. 

These cutters were tested on the last balloon flight, we had a pair of them cutting the same cable tie the balloon payload was suspended from.  They cut the balloon away as we crossed out of the flight zone polygon and saved the payload!  Good job cable cutters.  

This test wasn't perfect as the balloon only got to ~10km.  At this altitude there is a lot more air than at 20km or 30km and there is a good chance the cutters would have worked without any special vacuum mod.  However, it did test the systems and code around the cutters. It was also a test in rough flight conditions, there is a good chance that he balloon was bouncing around a fair bit.

I think we can be reasonably confident the cutter will work in following flights especially if we double them up as we did on this last flight.  Having a reliable flight termination system is really crucial in New Zealand to stop the payload falling in the ocean.  It's also really nice to be able to define an allowed flight area, making sure the balloon doesn't go somewhere we really don't want it to.