Hot wire cable cutter system

[matthewdlaudato]

I've hinted at this elsewhere:
https://www.rocketryforum.com/showthread.php?61344-Mega-Alpha-Rebuild-thread and
https://www.rocketryforum.com/showthread.php?65896-Speculation-why-my-cutter-deploys-at-higher-than-expected-altitude

Happy to say that I have a working, reproducible, flight tested approach that works well.

The inspiration came from the endless threads about BP and the headaches around procuring it. Saw one comment about a discontinued commercial product that used monfilament (fishing line) and the wheels started turning.

Design constraints:
- Motor ejection at apogee required. If you sim your rocket correctly and know how to drill a delay, it should be possible to get an ejection event near apogee.
- No BP required for main deploy
- Must be easy to construct
- Must be reliable

Here's a closeup of the result, and a second shot showing the (typical) cutter attachments to the nose section.



The cutter consists of:
1-1/4" of 32g nichrome wire (easily obtained from Amazon and other sources)
3' of discarded igniter wire or equivalent
A 'long' bread package tie
2 shrink wrap wire covers

I thread the bare igniter wire through two holes I punch in the bread tie, slip the shrink wrap over them for later use, and then solder the nichrome wire in place. I also use some electrical tape to secure the wire assembly to the bread tie, and to secure the assembly to the outside of the nomex blanket.

The cables I use are thin beaded ties from McMaster-Carr (https://www.mcmaster.com/?orderview=new#7567k43/=sfn73d). They are more than strong enough to hold most chute bundles together, can be linked to wrap bigger bundles, and most importantly, the beaded design yields positive contact between the nichrome wire and the thin part of the cable (between the beads).

In 3 bench tests and 5 flights, the hot wire successfully severed the cable tie when powered off the 'Main' port of a Stratologger SL100 every time. Once I got the rocket design straightened out (bundle was getting stuck in a 'too long' upper payload section), smooth flying and recovery. I have another rocket on the bench (a DD mod Partizon) that will use the same approach, and can't see why it wouldn't work in nearly any DD flight that can use motor ejection at apogee.

So, if you are looking for an easy DIY project and don't want to mess with BP and its problems, might be worth giving this approach a try.

 

[azzie]

Hi Matt,

Nice idea! I've been playing with nichrome wires a while ago, in my experience repeatable behavior depends on specific battery and electrical circuit that the wire is plugged into.

1) Too low current and it does not heat up sufficiently to break the zip tie (or takes longer than altimeter's output duration (1 second for Stratologger)
2) Too high current and nichrome wire melts before breaking the zip tie
3) If nichrome wire is coiled, elements might touch each other, and resistance will be lower (i.e. higher current and see #2). Your choice of beaded ties, though, seems to avoid this (nice!).

What I'm getting at, not every battery+altimeter+wire+zip combination will work as easily as you are describing it But, I have to say, I really like the idea of moving away from BP.

 

[matthewdlaudato]

Thanks Alex and good points.

I specifically have only used the Stratologger (I own two of them). During testing I tried several gauges of nichrome and settled on the 32 gauge, and specifically 1 1/4" of it. I also deliberately used batteries of various age and voltage, to convince myself that I would get consistent results.

The nice thing about the combination I arrived at is that the wire has a nice 'heat curve', so that well before and after the current peaks, it is pretty darn hot - I'd estimate around 2 seconds of 'red' heat, more than enough to cut the tie I use. It also has the advantage (again, with the Stratologger) of not burning through. I've used the same rig on three flights now (I just thread a new cable tie through it). Of course my luck will run out and I will run into Laudato's 1st law (simply stated, things work the number of times you need them to work, minus 1). So for now, I'm sticking with 3 times and then building a new one.

The beaded cable was the only real choice there - as you say, it keeps the nichrome from touching itself, and also makes it easy to thread a new cable through an existing wire unit.

 

[matthewdlaudato]

Here's a closeup of what remains of the cable after successful operation. This is from today's flight of the Partizon. You can see that the wire cut two of the three spots that it could have cut (I wrap the cable tie 3 times around the nichrome wire, so there are always 3 touch points).

 

[azzie]

Hi Matt,

Do you replace nichrome wire after every flight?

I also wonder how long it takes to cut through the zip-tie, do you have any data from stratologger that can show that?...


-Alex

 

[matthewdlaudato]

I haven't replaced it yet, since the 32g wire doesn't burn through - so far the same unit has gone 4 flights. I expect to use it one more time and then replace it just to be safe Built another unit this morning so that the two rockets I use with this configuration each have their own cutter 'head'.

In my bench tests (using the 'fire main' action in the PerfectFlight tool) the cable is cut nearly instantaneously, almost certainly before the 1s of current finishes flowing through the wire. The rigs I've built have around 2 ohms of resistance, so I'm getting around 4.5A through the wire, more than enough to heat it red hot but not enough to burn through.

So far two successful flights - flew my Partizon yesterday to 1500', flawless dual deploy using this technique.

 

[RochonRockets]

Could you show a picture of the burrito with the cutter in place. I am liking the idea.

 

[matthewdlaudato]

Could you show a picture of the burrito with the cutter in place. I am liking the idea.

Sure - I'm at work now but will wrap one when I get home and post a pic. The wrapping technique is pretty standard though - see for example the video on the Archetype Rocketry site. I own their cutter as well and use the same wrapping method.

 

[richh]

Has this been tried with a rubber band as the object being cut? I'm thinking of using this as a dual deploy for smaller rockets..

 

[matthewdlaudato]

Has this been tried with a rubber band as the object being cut? I'm thinking of using this as a dual deploy for smaller rockets..

Believe the answer is yes - someone a a recent launch where I was using this rig mentioned a commercial product that used fishing line, also heard of rubber bands being used. In my early tests I used fishing line but settled on the beaded cable tie.

 

[matthewdlaudato]

(Bump, to add pics of the fully wrapped and ready to fly configuration as requested long ago.)

 

[RochonRockets]

Thanks, better late than never. :-D

 

[woferry]

I can't quite tell from the pictures, is the nichrome wire attached to the package tie in any way (other than the tape at the base obviously)? How is the beaded tie passed through the wire (is the wire wrapped around it or something)? Just wondering what guarantees a good contact to melt the tie, without risking breaking the wire during the shock of the apogee ejection. Does putting the package tie hole on the nichrome wire end serve any purpose or was that choice arbitrary?

Thanks for the thread and details, I have the cable ties and nichrome wire on-order and have collected a few of the larger package ties and the rest of the bits already.

---

Will Ferry
NAR #96512, L1
TRA #15328, L1
L1: 9/2013 @ XPRS, GLR T-Bolt "Thunderbolt" (R.I.P.), H148R
Impulse flown: 2013: 767N, 2014: 4298N
Launches & Videos

 

[matthewdlaudato]

The beaded tie is wrapped around the wire, typically 3 times - that has routinely created several contact points and I've yet to have a misfire where the cable isn't cut. The bread tie serves as a mounting point for the shooter wire and keeps the whole assembly tidy, but the nichrome wire "floats free" above it.

As for breaking during ejection, that's never happened. There's no tension on the wire or tie - all tension is on the shock cord.

Here's the best closeup pic I have - hope this helps!

 

[Buckeye]

Well done! No ematch. No BP. I like it.

 

[matthewdlaudato]

Well done! No ematch. No BP. I like it.

Thanks. That was the design constraint I imposed - had to work with motor ejection and be regulation-free. I find it a good solution especially for high flying MPRs on smaller fields.

 

[matthewdlaudato]

Should add, I generally tape the bread tie to the chute protector - this keeps the assembly from flapping around during 'drogue' descent. I suspect this reduces the risk of the cable tie or nichrome wire snapping prematurely.

 

[woferry]

So I finally got the bits in the mail and gave it a ground test. I have a few questions / comments.

First off, I had no idea that nichrome wire doesn't stick to solder. How do you attach the ends of the nichrome to whatever wire you use? After some web searching I seem to have found a method that passes at least a basic tug test, but it takes me probably 20 minutes to prepare one piece , and involves very fine (40AWG) wire in addition to the wire I'm soldering to (to wrap the two different types of wire together, then add solder to surround everything).

My ground testing was definitely not an accurate setup, I simply soldered the nichrome to two leads, wrapped it around the zip tie, then connected the two leads to a 2S 950mAh battery ("25-50C"). With a 1 1/4" piece of 32AWG nichrome it got so hot that it fused the nichrome, barely even leaving a burn mark on the zip line (it didn't cut it, or even really weaken it). That was a total of ~1.2 ohms, and wouldn't account for the resistance of the FET/transistor that actually energizes the deployment on any altimeter so it was more current through the nichrome than would happen in a real firing.

So I did a bit more research and came across a useful PDF online here, looks like it was put together from pages here. Using the tables there, I determined that with my 2S battery I should target more like 4 ohms, to get ~2A and around 800C, staying clear of the ~1400C melting point of the wire. So I re-did my setup with longer lengths of nichrome.

Both times I actually tested with two different cable ties. One was a beaded tie I ordered from eBay (specifically this one, I wanted 8" to deal with my ~2" chute bundle). But when I received the package it was thicker than I'd hoped. The other one I tried is a more traditional zip tie I stumbled across at my local Harbor Freight, it's an "Identification Cable Tie", which is also 8" long but I could see it's very thin (18 lbs working load), and also includes a large flat portion at the top that I think can (mostly) take the place of the bread wrapper clip. I can't find this tie at HF's website (the store had it marked as clearance, I need to buy the rest of them I guess ), but it looks the same as this part here.

Now because I had to use ~4" of nichrome to get the right resistance for my test setup, I had a lot more wire to wrap around the ties. When I re-tested with the beaded tie it melted a section of the tie within a few seconds, but then the nichrome wire tightened around the beads and held the wrap in place (it didn't let the parachute open). With the more traditional zip tie there was nothing to snag on, so it melted through and let go, dropping the parachute.

So I did get a successful ground test with a direct connection to a battery, I will have to re-characterize with a real altimeter to see how much resistance it adds. Right now I only have a StratoLogger SL100, which as far as I can tell doesn't support ground testing other than trying to induce a vacuum around it. But I have an Eggtimer TRS on order and I know it can do ground testing (plus it's the altimeter I'd actually fly in the rocket I want to use this deployment scheme with), so I'll wait until I build that to do more testing. The next question for me is whether the deployment power connection is long enough to melt through this cable tie for me, plus re-setting my nichrome length to deal with the altimeter's deployment circuitry. The zip tie I used melted noticeably faster than the beaded tie I bought, but it still felt like a few seconds so if the circuit only energizes for a second or two it may not be long enough (maybe I need to run it a bit hotter or something).

So at least from my very limited experience so far it seems I'd want to target around 2A through the 32AWG nichrome (taking into account the battery voltage, nichrome wire resistance (length) as well as the rest of the resistance in the path of the deployment circuit), and I think this "identification" cable tie is another reasonable thing to consider (it at least provides a point to terminate the nichrome leads that will move with the cable tie, something to sit between the actual coil of nichrome and the chute protector would probably still be wise, I'm sure even nomex or kevlar don't like seeing ~1500F).

But I am curious what techniques people are using to connect the nichrome to other wire to get back to the altimeter.

Thanks!

 

[matthewdlaudato]

I'm still perfecting the connection to the shooter wire, but silver solder plus Ruby flux or equivalent works. I've used both a soldering iron and my small Portasol torch successfully.

I may try using a mechanical connection - crimping the two wires into a ferrule.