21st March 2012, 06:51 AM
BT and avbay vents
Well into my Madcow Super DX3 4" 54mm fiberglass build. This rocket is dual deploy. I am looking for advice or information on the correct size and locations for venting holes in both the upper and lower BT sections, as well as the 8" long avbay coupler. Upper BT is 16" inside length and lower BT is 22" inside length.
From what I have read it seems the recommendation is to go with 2 vent holes for the avbay to help reduce the chance of wind gusts creating high pressure that may effect the altimeter. So vent hole sizes and ideal locations to place them would be appreciated.
21st March 2012, 04:35 PM
Read over Crazy Jim's build thread of a 4" starting at post #56 it should help.Its not the same kit but will give you a good idea for hole placement.
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Give us a brake,an slow down!!
21st March 2012, 04:37 PM
So, be prepared to open up a can of worms with your vent hole question. In my experience, vent holes are really not necessary to relieve pressure from altitude change. Unless you are talking about extreme altitudes (over 30k feet or so), you really don't need them. Even fiberglass rockets leak enough that the change in pressure is unlikely to cause a premature separation.
Perhaps a better way to ensure you don't have early separation (that will protect against both pressure change and drag separation) is to use shear pins. I use #2 nylon screw (available at McMaster Carr) on small rockets, #4 or #6 on larger rockets. These keep things together until your ejection charge fires. Just be sure you ground test your charges with shear pins in place to ensure your charge size is adequate.
For the altimeter bay vent hole, I have found this page from Vern Knowles to be quite helpful: http://vernk.com/AltimeterPortSizing.htm. I use either three or four vent holes. Two can work, but most folks use more.
21st March 2012, 11:57 PM
Your av-bay sounds the like it's the same size as mine. I use four 1/16" holes evenly spaced around the bay. That has worked just fine, the drogue pops at apogee instead of on the way down like what happens if you have too small of holes.
As for vent holes in the BT, I use one 1/16" hole in each tube. I don't know if it does anything during flight, but it makes assembly easier when you're pushing on the nosecone or shoving the motor in the hole.
22nd March 2012, 12:15 AM
Thanks for the feedback all. Was able to gleen info from each of you.
Rocdoc there will definatley be sheer bolts installed into the NC to keep the main chute in place. I too aquired them from McMaster. I'll have to say they have one of the fastest turn around times I've ever seen. It only took 1 day to receive various hardware on my door step. Amazing!!!
Handeman just curious where along your main BTs did you place the vent holes...bottom, middle or near the top on each section?
22nd March 2012, 12:22 AM
I put mine near the av-bay on both sections. I haven't bothered with vent holes on my motor deploy rockets because they usually stay below 3,000 ft.
Originally Posted by Windeavor
I have never used shear pins either, just masking tape on the shoulders to get a snug fit. I've never had a main come out when it isn't supposed to. That is much easier to achieve if you don't use the blow it out or blow it up method. Properly sized charges and not overly long shock cords make the shock load less.
23rd March 2012, 01:53 PM
Ahh, the old "Blow it out or blow it up" debate. Nice! I use properly sized and ground tested primary ejection charges and back them up with a 1.5x larger "blow it out or blow it up" charge. My thought is that if the first charge doesn't do it, I want the Hail Mary charge to have the best chance of getting the laundry out. At that point, I don't much care if I pop the main at apogee due to an overly zealous drogue ejection event. The idea then is to just get the thing down safely, both for the rocket and for spectators, vehicles, prairie dogs, etc. And if the laundry is already out, the only thing the backup charge does is make a nice bang.
Originally Posted by Handeman
I don't understand the comment about not using overly long shock cords to reduce shock load. Can you explain that? In my experience, the longer the shock cord, the more energy that is dissippated before the two pieces reach the end of the line.
As for the tape around the nose cone, I HATE that. I have PTSD from the time I had a difficult time getting the perfect fit for my son's Junior L1 rocket nose cone. Ended up too tight, so I watched his cert rocket fly beautifully followed by lawn darting in. It's one thing to lose your own rocket. It's quite another to look at your kid's face as his rocket augers in. Screw motor eject, screw friction fit. Give me shear pins and electronics every time. Can't wait for him to hit 18. I will throw away my delay drilling tool and never calculate another optimal delay again.
Speaking of tape, however, I have seen several people use the foil ducting tape (used for connecting rigid metal ductwork) instead of shear pins. It tears reliably and keeps the nose cone on until the ejection charge fires. I still prefer shear pins, but if it's a choice between the foil tape and friction fit, my rocket will be the one with the little pieces of foil tape around the nosecone.
23rd March 2012, 11:50 PM
You are right, the longer the cord, the more ejection charge energy it will dissipate. If you need that much cord, that usually means you have too large of ejection charges. I agree with larger charges for backup. If the initial charge didn't get the laundry out, you definitely want the back to work.
Originally Posted by rocdoc
The problem with long cords, and I see it at every launch, is they are usually used without a drogue. Eventually you'll get a flight where the payload will lead the fin can down. If it's really bad, the payload will go ballistic and pull the fin can down behind it. When the main blows, it inflates fully and almost stops because the payload is the only weight on it. The fin can will do one of two things, fall through the main, taking it down with it, or miss the main and fall to the end of the shock cord where it jerks to a stop and puts a huge shock load on the recovery system. Even if the fin can is at the same height as the payload when the main deploys, it still falls a long ways before hitting the end of a long cord and still puts a large shock load on the recovery. I watched a L3 cert fail when the fin can hit the end of the long shock cord like that, it never even slowed down, just ripped the anchor out and kept on falling. Just at the last launch I saw three fin cans hit the mains or tangle in the upper shock cords. I don't know how many DD flights there were, but there was only 36 HPR flight that day and most were H motor deploys.
I understand your dislike with friction fit after that. I have to say, I've never had an issue with it. I usually make sure I can lift the rocket by the nosecone without it coming apart, but if I shake it good, the nosecone at least comes out half way. It's worked with supersonic flights and WARP-9 loads and everything in between. Nothing wrong with sheer pins though, I just haven't use them.
Originally Posted by rocdoc
I also agree with the electronic deployment. Much more reliable and accurate then motor deployment.