Parachute ground disconnect

[slh56]

Hey there, I thought I would start a new thread rather than hijacking the other one. I think it would rock to solve the problem of disconnecting the parachute upon landing so I’ll post my findings.

Here are some used by the military, they look really expensive and oversized:
http://www.airborne-sys.com/pdfs/_AS-Website_Marketing_PDFs/10-PARACHUTE%20GROUND%20DISCONNECTS/GD500%20and%20GD-5K/GD-500%20and%205K.pdf

Here are the ones from the skydiving website, they are really expensive:
http://www.para-gear.com/templates/parachute.asp?group=429&level=2&parent=337

Other ideas include an altimeter activated disconnect either via altitude or acc based. The altimeter could activate a black powder charge blowing the parachute off or it could activate a mechanical release.

What are your ideas?

-Steve

[Doug GerrardParticipant]

I’m not sure about using an altimeter. How would you take into account the difference in elevation from the launch area to the landing area? I suppose if it were sensitive enough you could detect that it was no longer falling but that would take a few seconds. The last thing you want it for an automatic device to release too early. I’m still thinking of using the WRC and manually releasing it. For larger projects you usually have a dedicated recovery crew that can see it lay down. The problem is for larger rockets that a parachute designed for 100 to 200 pound rocket can play havoc for one man to bring down under control.

Doug

[slh56]

I like the idea of having the altimeter detect that is was no longer falling. I’m not sure on this and pardon my computer code translated to english but I think that the way the altimeters detect apogee is by asking themselves: “am I sill going up?” so couldn’t you have it ask itself “am I still going down?”

In order to prevent it from releasing early it could use the mach delay idea and start a timer once the main is deployed. Sure it doesn’t have to release the moment it hits the ground, but any release after it lands is better than no release.

Another thing would be to have the accelerometer look for the jolt from the landing. The timer idea still applies to prevent premature separation. Once the main is deployed the timer starts and if the timer ends after the rocket has landed the jolts from being dragged and tossed into fences would trigger the release. If once it lands it is not being dragged or tossed around then no release is necessary no release would occur. But figuring out the timer duration in this case is another story.

-Steve

[Ed Dawson]

If I had to choose, then I’d prefer the manually based Missile Works remote system.

I’m not sure it would ever happen but what if there were thermals, or an odd puff of wind that made it look like a stop in the descent rate. I’d hate to see that happen over the flight line.

[AdrianParticipant]

I’ve been checking out the data from John’s K shot around touchdown, and I agree that any response based on the the baro sensor would have to be delayed long enough that it wouldn’t protect anyone’s paint job. But I do see a really strong accel signal right at impact. I think that could be combined with an altitude threshold that’s set below the main deploy altitude. The proposed logic woudl be to deploy when apogee has already happened and if the altitude is less than, say, 400 feet above the launch pad altitude and the accel registers > 10 Gs upwards, then release. John’s rocket had an 80 G reading followed by 6 more accel readings over 5 Gs. After the main deploy and before the impact, there weren’t any readings above about 3 Gs:

One thing this points out is that the main deployment altitude needs to take into account landing on top of a hill, and the parachute release would have to be inhibited at an altitude a good deal below that so that the main chute deployment dynamics are done.

By the way, the funny bumps in the altitude trace are from the pressure transducer facing toward the sun in an av-bay that doesn’t block out all the sun’s infrared energy. For this application, since we would be controlling the properties of the container the altimeter would be in, this would be easily prevented. But filtering out the rest of the noise (and avoiding thermals) would require several seconds of constant altitude to prevent a premature release.

[AdrianParticipant]

Here’s a zoomed in view:

[Doug GerrardParticipant]

Let’s not put the cart before the horse. The method one chooses to activate the device is secondary (and a personal choice) to actually building a unit capable of holding the recovery lines and breaking apart upon demand.

Doug

[Warren B. MusselmanModerator]

What about some sort of accelerometer based deal? Shouldn’t be too hard to work out the code for it. If Main Deployed AND speed = 0 for period > 30 seconds, release chute. Of course you’d need a bit more capable flight controller than just a dual deploy altimieter. Something like the Defy Gravity or RDAS or similar.

Warren

[muleParticipant]

How about RC?
I know you would have to be in a somewhat close area on this one, but, they make micro Rx units that would work to set off a charge, servo activated release or….. I have a mini Rx that is only about 1.25x1x.5″, coupled w/ a LiPo battery it would be pretty small. The Rx could just trigger a small charge .

[Murdock]

I thought I had posted this yesterday, but guess not.

Could you use the Defy Gravity tether to release the chute? Just have your electronics trigger it once its on the ground?

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