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JLCR and OR

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ActingLikeAKid

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Has anyone figured out a good way to simulate the Jolly Logic Chute Release in OpenRocket? It looks like there's nothing in OR to simulate "the chute came out so the rocket is now tumbling, but not under chute". With a little experimentation on a simple rocket design, I found this:

No chute, no ejection charge, ground hit at 165 mph.
No chute, ejection charge, ground hit at 165 mph.
30cm chute, ground hit at 22 mph.
1cm chute (I was trying for "there's theoretically a chute out, but it's not really doing anything"), ground hit at 197 mph. So it looks like this is one of those "near the end of the curve, things get weird" things, fair enough.
7cm chute, ground hit at 97 mph.

I don't have a ton of data on "rockets falling separated but without a functioning chute", but I would guess that ~100mph is a maximum, maybe less? Of course, it depends on body tube width and rocket weight.... So I think my strategy will be to create a pseudo-drogue parachute, enough to slow the rocket to about 80 - 100 mph, with an overridden mass of zero, that comes out at apogee, then have the real chute open at whatever the JLCR would be set to.

Open to input on this, and particularly interested if anyone has data on how fast a rocket falls under JLCR (i.e. nose or upper body separated, parachute "bundle" out)
 

mccordmw

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Here's a post I did on calculating how the airframe works as a drogue when tumbling separated.

https://www.rocketryforum.com/showt...ute-Size-Decision-Aaaah&p=1622655#post1622655

I always include this now in my sims. Gets my calculated descent rates very close to the observed rates. For JL chute release, I'd add airframe tumble deploying at apogee and have your real chute open at the altitude you set.

Can you attach your ORK file?
 

ActingLikeAKid

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Here's a post I did on calculating how the airframe works as a drogue when tumbling separated.

https://www.rocketryforum.com/showt...ute-Size-Decision-Aaaah&p=1622655#post1622655

I always include this now in my sims. Gets my calculated descent rates very close to the observed rates. For JL chute release, I'd add airframe tumble deploying at apogee and have your real chute open at the altitude you set.

Can you attach your ORK file?
That is EXACTLY what I was looking for, thank you!! Going to tweak my ORK with a faux-drogue per your specs and then I"ll post.
 

ActingLikeAKid

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Here it is. Using your calculations, if my math is right:

BT diameter = 54mm
BT length = 991mm
Cylinder surface area = 168114 mm^2
sqrt(168114/pi) = 231 mm

That gives me a "faux drogue" chute of 231 mm. With that, I get a descent of about 65 mph under Chute Release (maybe less if the bundle flopping around contributes to drag). While quick, that's not "zipper a bluetube" quick.

View attachment mvf rebuild.ork
 

ActingLikeAKid

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Nope, I'm an idiot.
231mm is the radius, so the drogue would be 462mm. That slows it down to about 32 mph under tumble. Which seems slow?
 

mccordmw

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I came up with an 18.5" diameter drogue chute. Matches your 231mm radius. I looked at your ORK. You put in a diameter of 9". The formula gives a radius, so you should double it. Change your drogue to 18.5". You'll tumble at 13.3 m/s (about 30 mph).
 

mccordmw

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If it tumbles flat, that's about right. My scratch built dropped at that calculated rate as long as it doesn't up-end and tail slide. Then it drops faster until it tumbles sideways again.
 

ActingLikeAKid

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I came up with an 18.5" diameter drogue chute. Matches your 231mm radius. I looked at your ORK. You put in a diameter of 9". The formula gives a radius, so you should double it. Change your drogue to 18.5". You'll tumble at 13.3 m/s (about 30 mph).
Yeah, I caught that after I posted, I'm an idiot :) Now if there only were a way to "remove" the body-drag drogue once it's under main, since the body tends to hang vertically once the main is out....
 

mccordmw

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Think of it as just a drogue. Those hang after the main is out and don't contribute much after that. The same goes for the body drogue. Delete it and see what the final descent is under main. It won't be too much different.
 
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