How to mitigate a high velocity for main deployment?

[joeyalex]

Hello,

I am planning on implementing an 18" drogue and 96" main on a ~34 lb rocket. With this configuration, OpenRocket has the main deploying at 112 ft/s. Besides resizing drogue, is there any way to ensure main deploys properly? I have already made sure all other components like the quick links and shock cords are rated for the load shock at that speed, the problem is the chute itself

 

[Steve Shannon]

That's really not terrible. You may find that a smaller drogue results in the rocket parts spreading out and falling slower (or not).
The only certain way to reduce the terminal velocity beneath the drogue is to either increase the drogue area or the drag coefficient of the drogue.
But again, that's not a terrible velocity. There's no reason to think the main won't deploy at that velocity. There are things you can do like z-folding the shock cord and taping it with masking tape or lightweight rubber bands so the shock cord extends slightly more gradually, thus spreading the jerk over time.

 

[joeyalex]

Thank you for the reassurance. I had found other threads that mentioned 100 fps being a decent upper bound and although those were just estimates, they were troubling considering the speed we would be deploying at. I will utilize a method similar to the image shown to help spread the load over time. Many thanks, Steve!

 

[teepot]

Is there a reason that you are using an 18" drogue? Do you need to come down quickly because of field size or wind conditions? Why not use the size drogue to get a velocity you want?

 

[joeyalex]

Is there a reason that you are using an 18" drogue? Do you need to come down quickly because of field size or wind conditions? Why not use the size drogue to get a velocity you want?

Yeah that's exactly it. I would like to keep our descent time to under 80s, with a 24" we'd be right at 80s (in OpenRocket, 86.4s with hand calcs). Ideally a 20" would be a great middle ground but there aren't really any online that have images/sufficient information in the product description.

 

[tfish]

I use nylon..it has some stretch to it. Unlike Kevlar. I'm a big fan of "break points' I use electrical tape and like to see some "points" not broken. The rocket in the video it 43# I use Three 60" Aerocon chutes for the main.



Tony

 

[joeyalex]

I believe we had chosen braided Kevlar from Rocketman since they had the length options we desired (5,10,15,20 ft) at the rating we would like (above 1900 lbs, since that is what we calculated our opening load shock for the main parachute which is dubious to be honest). The elasticity of nylon does look appealing, but this is all intended to go into a USLI report which is due tomorrow so it might be too late to go back and swap that. Good to keep in mind for next year's design though.

 

[Titan II]

Yeah that's exactly it. I would like to keep our descent time to under 80s, with a 24" we'd be right at 80s (in OpenRocket, 86.4s with hand calcs). Ideally a 20" would be a great middle ground but there aren't really any online that have images/sufficient information in the product description.

Just some actual numbers that may help. My 38 pounder (after burn) came down at 76 fps with a 24" SkyAngle.

 

[rharshberger]

One thing that will help is using a parachute deployment bag, it allows the shroud lines to fully stretch out before the chute opens. There are a number of ways to rig them, and afaik all are superior to a burrito style chute deployment method and way more reliable.

There are many threads on TRF about Chute Deployment Bags aka D-Bags.

 

[Tractionengines]

D-bag as said above...

Plus, Add a slider ring to the shroud lines. Will slow opening time of main.

Size needs to be sure it will NOT slip up over bottom of chute and prevent opening.

 

[joeyalex]

I forgot to mention we were using deployment bags, whoops! Thank you for mentioning the slider ring, that seems like it would be extremely helpful. I appreciate everyone's input on the matter

 

[Tractionengines]

If you can; "play" with a slider on a smaller rockets and chute. You need to get a feel for what it looks like and how much it slows the inflation time, so you can adjust the deployment altitude.

 

[boatgeek]

Is there a reason you don’t want to use a bigger drogue?

 

[OzHybrid]

Bit late to this... but you can use a reefing ring on your main chute shroud lines. This controls the rate the chute can open at until the slower opening of the chute pushes the ring down. This eliminates a lot of the shock of the chute opening as the initially partially opened chute, partially slows down the rocket a bit.. The usual shock cord taped bundles helps with shock and also controls the cord into tidy bundles for packing. See Georges study on this.



The diameter of the ring you use is important. Not too big, not too small.

Also here's a NASA paper on reefing and reefing line loads.
https://ntrs.nasa.gov/api/citations/20130011075/downloads/20130011075.pdf
You probably don't want to do a full analysis, so short version, controlled release of reefing takes the opening shock out.
Norm