Low Altitude Deployment

Discussion about deployment systems including altimeters, timers, air speed flaps, servo systems, and chemical reactions.
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Brian
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Low Altitude Deployment

Post by Brian »

The higher the rocket flies, the larger the launch area needs to be. Therefore I wish to know of ways of SAFELY deploying a parachute at low altitude to reduce drift?

I first thought of deploying a streamer at apogee (deploying a parachute after a free fall could damage the parachute or rocket and leaves little room for error) and then deploying the parachute at low altitude.

But now I'm thinking about deploying a parachute at apogee but keeping it closed till low altitude.
I think it could be possible if holes were made around the base of the parachute and a string woven through them and when this string is under tension, the parachute will remain closed.

Just asking for input or other ideas?
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air.command
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Re: Low Altitude Deployment

Post by air.command »

Brian wrote:The higher the rocket flies, the larger the launch area needs to be. Therefore I wish to know of ways of SAFELY deploying a parachute at low altitude to reduce drift?

I first thought of deploying a streamer at apogee (deploying a parachute after a free fall could damage the parachute or rocket and leaves little room for error) and then deploying the parachute at low altitude.

But now I'm thinking about deploying a parachute at apogee but keeping it closed till low altitude.
I think it could be possible if holes were made around the base of the parachute and a string woven through them and when this string is under tension, the parachute will remain closed.

Just asking for input or other ideas?
Hi Brian, this is a classic dual deploy situation. I guess the problem can be broken down into two parts. The first is the detection of the two events, apogee and the desired "low altitude". The other is what you do during those events. Have you given thought to what you want to use to detect the events? Whether time based (mechanical or electronic), direct altitude measurement (barometric/accelerometer), or a combination of other sensors like a MAD? When you choose your event detection technique then you can see what options you have to activate your recovery system(s). You will also need two actuators (or multiple positions of the same actuator) to release your recovery systems at the two events. Did you have any plans for the type of actuators you want to use?

You also need to choose what your desired descent rates are for your rocket. This will affect what you do to achieve the desired rates. The other factor affecting your decision is how much of a weight budget you have for your system.

The tied-off parachute seems like a reasonable approach, the one potential problem is that while the rocket is descending the chute will twist up the shroud lines along with the release string preventing it from being pulled when you reach the low altitude. (A swivel may be problematic if you are running the release line past it from the rocket.) It would be very interesting though if you built it to see how it performs.

You may be able to build a backgliding rocket so that at apogee it falls sideways and then at the low altitude you deploy a parachute as normal.

The streamer/drogue chute at apogee and larger chute at low altitude is a well proven design that works well. With two separate chutes you also have partial redundancy built in. If the main fails to open you don't hit as hard, and if the drogue fails to open you run the risk of a high speed deploy, but if built well should not be an issue.

Alternatively there are also multiple techniques for opening parachutes at high speed such as elastic shock cords, making your parachute and shroud lines from stronger materials, using a reefing ring (similar to the technique you are suggesting), and taping up the main line into zig-zag sections so that as the parachute deploys the tape rips releasing a longer section of cord and absorbing the shock. Have seen this used on model rockets and works well.

- George
http://www.AirCommandRockets.com
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Brian
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Re: Low Altitude Deployment

Post by Brian »

Thanks for your input.
I'm considering modifying my current Tomy timer recovery system. Both deployments are released by the timer just at different times.The backgliding idea isn't a bad one. I'm not sure how to change the decent rates of my parachutes and am not sure if it would get tangled but my current system will require little modification so I can build and launch soon.
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Re: Low Altitude Deployment

Post by Brian »

Proud to say, it crashed.
The parachute was deployed and was stopped from opening but the release string got caught on the parachute and kept the parachute closed. Minimal damage.
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air.command
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Re: Low Altitude Deployment

Post by air.command »

Brian wrote:Proud to say, it crashed.
The parachute was deployed and was stopped from opening but the release string got caught on the parachute and kept the parachute closed. Minimal damage.
Sorry to hear about the crash. A slightly different approach that might work is to tie a line to the center of the parachute and pull it lower than the rest of the shroud lines so that it is essentially turned inside out so that when the parachute is deployed it just flaps in the wind. To open it you just cut the string which then allows the parachute to inflate. If you used a swivel with a hole in the middle then you could pass this line through the middle of it. You could have a second swivel on this line to stop it from twisting up.

- George
http://www.AirCommandRockets.com
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Brian
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Re: Low Altitude Deployment

Post by Brian »

As long as the parachute isn't held too far down, this method should be less likely to tangle than my previous atempt. Thanks for the suggestion, I hope to try this soon.
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Re: Low Altitude Deployment

Post by U.S. Water Rockets1 »

Brian wrote:As long as the parachute isn't held too far down, this method should be less likely to tangle than my previous atempt. Thanks for the suggestion, I hope to try this soon.
Reefing parachutes is very difficult to do on models rockets because of the small scale. A full size parachute is on a very large scale which makes the friction of the rigging much less significant. The lines move against one another a lot differently than they do in the small scale. If you were to reproduce a reefing system in small scale perfectly, you would find that the miniature version performs much differently than the large scale counterpart because the friction is not scaled accordingly.

If you look further, you might be able to scale the friction component by using a smoother material for the rigging, like thread or fishing line, but even on those situations, you have to also take into consideration that the miniature lines will not have the same flexibility as the large scale lines, so the friction may work out but the lines will not be as flexible.

A reefing system that works in miniature is very difficult to get right, so if you manage to get something like that working well, then you will really have accomplished something. Keep us posted on how you do with this concept.
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