Payload release / tether cutter

Hey all.

I basically joined this to hopefully get some options. I'm not a rocketry guy by trade or hobby.

I launch helium balloons that carry a 7lbs payload. The balloon needs to reach and maintain an altitude of about 27k feet.

Because of restrictions governed by 14 CFR 101 I need to have a device integrated into the system that will be able to cut a fiber twine tether after a predetermined amount of time.

The company that sells the balloons can make a device that works off a Cypress AAD line cutter with a timer built into it. Problem is, the balloons/payload is a one-time use and not recoverable. So I'm looking for a much cheaper alternative.

I have looked at the Archetype Cable Cutter and it looks like a pretty solid system. I think the guy that came up with it might be a member on this forum. I shot him a message to get a few more details on that product.

I'd like to find out what other options are available for cutting a tether.

I thought about using an e-match by itself to burn the twine. Run it off a spring wound timer, a digital kitchen timer, or possibly a solid state circuit using a 555 timer. I'm not sure whether altitude would effect electronics.

Any recommendations and input would be greatly appreciated.

Thanks in advance
Matt

Matt,

Most rocket stuff is designed to be reusable and reliable even under violent force so not sure you will find it cheaper. Can you give us an idea as to what price you are looking at? As for altitude I believe 27k should not be an issue, I think it's around 40k that you need to start worrying about electrolytes boiling off etc.

Matthew

Matthew,

Depending on my options, I'm giving thought to putting together devices using 2 e-matches to burn the tether. I'yym not 100% in love with the idea of pyrotechnics (black powder), mainly for storage. If I end up making these in-house I'd like to keep as many as I have balloons on hand, which can be as many as 20-30. I know it's only a small amount of pyro, so it might not be a huge issue.

I'd like to just do a solid state 555 timer with either a battery or capacitor. I need to figure out if a charged capacitor would gradually bleed off while in storage.

I'd need to figure out how small of a board I could get and a small confinement to put the package in... maybe like one of these or these. Basically to keep it as weather proof as possible.

If we launch a balloon in the rain I don't want the tether to get so wet it wouldn't burn. Might need to either consider using a different tether line that doesn't soak wick water so easy or figure out a way to water proof it prior to releasing it.

Right now I'm just kind of thinking out loud and spitballing ideas.

If I can keep them units >$20 I'd probably be pretty good.

For 20 I think your on the right track with ematchs and a timer. Keep in mind though going to 27k you maybe be in and out of clouds, there will be a large temperature drop and its likely that condensation will form in and outside of your enclosure at some stage. Have you though about a mechanical option such as using a small motor to undo a screw thread?

Ematches are regulated items in the US. You can use other igniters like those made by Estes. They have different electrical requirements, but you will have to test this anyway when you design your own electronics. Another option would be a small heating element, e.g. NiChrome wire and melt through some nylon fishing cord. The Chute Tamer is an example of this approach.

Reinhard

https://www.jollylogic.com is close to producing a reefing mechanism that might be suitable. There's some chatter over in the NARCON 2014 thread. A search for "ChuteReefer" will find what little info we currently have. It uses a servo instead of pyrotechnics.

John's on the forum and has expressed an interest in working with people on new applications for his products. Hint hint...

Thanks guys. All great info.


I'll read up on the Jolly.

How are e-matches regulated? Are there different' "strengths" of e-matches?

I don't think I'm familiar with the Estes... I'll have to look that up as well.

We had considered the heating element idea... kind of like what's in a toaster.

Normally we tie off directly between the balloon to the payload. So I'm trying to figure out a way to just connect the device to to the line that's holding the payload. The payload is 7lbs but I'm not sure what additional amount of force is put on the line when it first takes off and "yanks" the line and payload.

more to follow.

Another option would be using a military squib. I can probably get these without much problem. Would still require the timing circuit though.

Is this payload going down with a parachute, if so you can recover the system, if not, why you drop a 7 pounds payload without a chute ?

Area66 said:

Is this payload going down with a parachute, if so you can recover the system, if not, why you drop a 7 pounds payload without a chute ?

Click to expand...

Payload uses no parachute.
No need or way to recover the payload. It ends up in the ocean.

What's the smallest battery that could fire an e-match? or the minimum volt requirement?

We routinely fire matches with tiny, 1S 150mAh LiPo batteries (3.7-4V), but you'd need to consider the cold-soaking that happens on a balloon payload at high altitude.

As you probably know, the balloon community, especially the near-space, amateur radio balloon community is pretty good about sharing techniques on forums and mailing lists. I receive the GPSL Yahoo group, and that would be a good start. The arhab.org site is also useful for contact and other info.

Mark


Mark Joseph
TRA/NAR L2
Central Illinois Aerospace

Awesome. Thanks mark. I'll head over there as well. Much appreciated.

Looks like I wasn't given all the information I needed up front.
The cutting device needs to be completely non-metallic. That's just short of impossible.

Thanks for all the help and input on this. It's definitely appreciated.

even the wires? I'm thinking something like the following. A loop of 50# fishing line passed through a short piece of surgical tubing. Use zip ties to seal. add black powder and e-match and seal that end with zip tie.

I played around years ago 'cutting' zip ties with ematches and they liked to melt and not cut. You can even make your own ematches that will work...PM me off list.

I'm very interested in your project. My grand daughters and I are doing a Balloon next summer. I've just begun to wade through the FAR 101 stuff.

Tony

Father said:

Looks like I wasn't given all the information I needed up front.
The cutting device needs to be completely non-metallic. That's just short of impossible.

Thanks for all the help and input on this. It's definitely appreciated.

Click to expand...

Your idea in post #3 is still a viable one. You don't need to use e-matches. If you used mono-filament fishing line for a tether you could melt it by it wrapping in thin nichrome heating element wire. When your timer times out it can apply power and stay locked in. It would be inexpensive and easy to build and test.