Deployment Method

[jqavins]

I was just a moment ago struck by an idea that is probably a re-invention: parachute deployment by a spring.

Make a floating piston. Attach it by a spring to a vented bulkhead above the motor mount, with a spring in between. The back of the piston and the front of the bulkhead would have small screw eyes or some other kind of attachment points for a string that holds the piston down close to the bulkhead, one attachment centered in the piston and two on the bulkhead diametrically opposite. The ejection charge melts or burns through the string and the piston pushed out the 'chute. The piston probably should pop out rather than hit a stop, so it'd have to be tied to the shock cord.

Here's a rough schematic of the bulkhead:

I haven't worked out how to pull the piston back and attach the string, but I'm sure that's solvable.

Has anybody done this or seen it done?

 

[Chad]

I've been thinking about something like this too. Only instead of using a charge to release the spring hook up a small linear actuator to the altimeter and use that to release the deployment mechanism. Some of the actuators out there use an electro-magnetic for push/pull so a good pulse of power (like what's needed to fire an e-match) would open or close it.

the main thing i can't get past is 1 gram of BP handles deployment with authority, i don't see any mechanical deployment coming close to that level of oomph at 1 gram of mass.

 

[Bat-mite]

It's good to have alternatives in mind if the day comes when we can't get BP any more. But until that day comes, I'm sticking with BP. There is CO2 deployment, and I guess something with springs could work, but it would be hard to pull off.

BP charges create a uniform gas pressure, whereas a spring would press unevenly on the piston. I could see jams happening unpredictably and regularly.

But if you can overcome that, it would probably be a safer method outside the ATF's purview.

 

[jqavins]

I was thinking of lower power rockets, so it's only the motor's ejection charge that parts the string.

It would certainly be heavier than just some wadding. The only advantages I can see are 1) the 'chute is better isolated from the ejection charge and B) it's new and different and worth a try. Some outside-the-box ideas are fun even if they're not revolutionary.

 

[mbeels]

It could be a fun idea to try.

The piston would experience the motivation of both the ejection charge, and the spring. So it should add some "oomph" to getting the parachute out.

I like the idea of a piston for deployment, it seems like an elegant way to more or less completely isolating the parachute from the ejection blast. It seems in practice, I hear that they can have a tendency to stick unless adjusted and fit just right.

 

[kuririn]

The vintage Estes cold power models that were powered by Freon used a spring/piston method for chute ejection.
And no pyro charge.
https://www.oldrocketplans.com/estes/est1111/est1111.pdf
I think the pressurization of the motor with the Freon pushed the rubber grippers out and secured the motor and piston, but I'm not sure on that. Never had one.
I remember reading that some water rockets use spring eject as well, without a pyro charge.
I think they used a timer.

 

[Dugway]

The ejection charge melts

I guess my first reaction to the idea is that if you have an ejection charge, you might as well use it to eject your parachute. In fact, I'm not sure how you would be able to tell if the spring did anything, the ejection charge is going to eject the piston at any rate. Am I just missing something???

 

[jqavins]

No, more likely I missed something. Like needing to have a vent in the body tube in order for the spring to really mean anything.

Oh well, it was just a thought.

 

[mbeels]

I can imagine some situations where it might be useful. For example, your method has positive retention, so it would reduce the chance of drag separation. And I was reading in another thread about Big Daddy lawn darts, it sounds like that rocket could benefit from a bit of an extra kick to get the nose cone off. There are other ways to accomplish the same thing, but an interesting idea nonetheless.