Volume in a donut avbay

Wayco · Dec 15, 2013

Working on my 5" Jart DD and the avbay has a 3" MMT running through it. So I calculated the volume
within a 5"x9" coupler and got 176.7" then did it again for the MMT 3"x9" and got 63.6, subtracted one from the other and my volume is 113.1 cubic inches, right?
My rule of thumb for static ports is 1/4" for each 100 cubic inches of volume, so 1/4" divided by 3 is .083". If I drill three 3/32" holes, which are .093" each, I'm good right?
As much as I love rocketry, math is my weakness, so could someone check my work?
3/32" holes seem small for this big avbay, I was guessing 1/8" but decided to do the math.....

BLKKROW · Dec 15, 2013

Your volume is 100% correct.

I got 113.0976 cubic inches.

blackbrandt · Dec 15, 2013

Your volume is right.

Not sure on the formula for the hole size though...
What I use is (volume)/(number of port holes/2)

Is that the formula that you used?

Matt

JMZawodny · Dec 15, 2013

Your method for calculating the volume is correct, however, your simple division of 1/4" by 3 is not. You need to divide the area into three equal holes, not the diameter. The simplest method is to divide the diameter by the square root of the number of holes. In your case 0.25/1.732=0.144" holes.

K'Tesh · Dec 15, 2013

[video=youtube;8-4P1WPE-Qg]https://www.youtube.com/watch?v=8-4P1WPE-Qg[/video]

blackjack2564 · Dec 15, 2013

You RAT B@$t@rd$ !

I just got back with a dozen Crispy Cremes & gallon of milk. Hot sign was on thank goodness ......& as usual 4 of 'em didn't make to the house.

That'll teach me to read threads like this!@#$##@!

Wayco · Dec 16, 2013

blackbrandt said:
Your volume is right.

Not sure on the formula for the hole size though...
What I use is (volume)/(number of port holes/2)

Is that the formula that you used?

Matt

I have been using a website called "Vern's Rocketry" for my static port sizing:
https://vernk.com/AltimeterPortSizing.htm

JMZawodny said:
Your method for calculating the volume is correct, however, your simple division of 1/4" by 3 is not. You need to divide the area into three equal holes, not the diameter. The simplest method is to divide the diameter by the square root of the number of holes. In your case 0.25/1.732=0.144" holes.

I knew something was fishy with that small hole size, thanks for the help.

blackjack2564 said:
You RAT B@$t@rd$ !

I just got back with a dozen Crispy Cremes & gallon of milk. Hot sign was on thank goodness ......& as usual 4 of 'em didn't make to the house.

That'll teach me to read threads like this!@#$##@!

At least you still have Crispy Cremes, they aren't available here anymore. Probably a good thing too, I'm like you, four of them disappear on the drive home. Then I get a speeding ticket from the sugar rush!

jus_rockets · Dec 17, 2013

They have a Krispy Kreme in Phoenix, I eat them all the time. Have any pictures of you av-bay? I will be using the NC shoulder as the av-bay on my 5" Jart.

Wayco · Dec 17, 2013

But I live in Goodyear, half way to Kalifornia, they grow Kotton, raise Kattle out here. No donuts.
No pic's of the avbay yet, I'm still trying to fit a sled with batteries and altimeters into the little space around a 75mm tube. Is your Jart DD?

jus_rockets · Dec 19, 2013

How long of a motor are you planning on flying it on? My Jart is DD, same concept of the 4" Jimbo Jart. You use the NC coupler as the av bay and the main chute is in the NC.

Wayco · Dec 19, 2013

As it is now, my avbay is around the fincan/payload coupler with the main chute in the payload tube. The upper avbay bulkhead limits the motor to a 4 grain 75mm motor, which is plenty for a 15 lb. rocket.

Wayco · Jul 23, 2014

I'm revisiting this thread because I never did a build thread on my 5" Jart. Because I chose to build it with a 75mm motor mount tube, there were some specific challenges with this rocket. First was getting the avbay sled to fit around the big MMT extension in the coupler:

I built a redundant altimeter system to practice for my L-3 build which is going on now here:

https://www.rocketryforum.com/showthread.php?66950-Wayco%92s-Dark-Star-Extreme

It took some work, but I got a Stratologger, an RRC2+, two featherweight magnetic switches and two 9v batteries stuffed in there:

I also added an Eggfinder to the N/C:

I have a half dozen flights with this rocket, and recently found a problem with how the avbay was constructed. Following the Gizmo XL instructions, I put heavy fillets on the outside of the bottom centering rings, but the instructions did not suggest that I needed to put fillets on the inside of the avbay, and since it would be very difficult to do that with the limited space, I never did.
Cleaning up "Jughead" after it's last flight, I found this:

The rifle casings were the only way I could fit an ejection charge cup in the limited space on the lower centering ring of my avbay, and the fillets had cracked where both casings were mounted:

I removed all the parts on the CR and dremeled out the fillets so I could remove the CR and MMT extension from the coupler. Then I ordered a CR that fit on top of the coupler and put it all back together with a longer MMT extension:

This time I put fillets on both sides of the MMT extension and inside the avbay where the CR butts up to the coupler. No way is that puppy going anywhere now.

I also replaced the old terminal block with a new one:

I ordered a set of small "Blastcap" charge cups that hold 4 grams of BP. They came in today and I will be mounting them tomorrow. I also ordered some larger (6 gram) "Doghouse" charge cups for the other end of the avbay. Both orders came in today, and despite the difference in price, they are both well made and look durable. I'm using 3.1 grams of BP for the primary ejection system, and 4 grams for the backup.
When I get it all back together, I will post up pictures of both ends.

Wayco · Jul 24, 2014

Got the charge cups installed today, new threaded holes for the Blastcap cups and had to enlarge the holes from 8-32 to 10-32 for the Doghouse cups. Here are the new 4 gram Blastcap cups mounted on the "drogue" end of the avbay:

They have their name etched in the side, but it's hard to see in the picture:

The extra space I gained with the new centering ring was just enough to mount them on.

The Doghouse Rocketry charge cups are larger and hold 6 grams:

They are replacing the smaller 3 gram cups I bought originally. I probably should have filled the original holes and drilled new ones, but they still fit, right at the edge of the bulkhead:

Both of these products are well made and a good upgrade for larger rockets.

Wayco · Aug 10, 2014

I'm revisiting this thread again to remind me how to solve the problem I had sizing static ports for donut avbays. It's not hard to get the volume, simply use this site to get the volume of the inside cylinder and outside cylinder and subtract the small one from the large one:
https://www.onlineconversion.com/object_volume_cylinder_tank.htm

So when Sharon and I reworked the formula for her Stinger, we got 170 cubic inches of volume in her avbay, which is a 6" BT with a 3" MMT down the center. The avbay is 8" long. My 5" Jart was 125 cubic inches, with a 5" BT and a 3" MMT down the center with the length of 10".
Then we went to this other site:
https://vernk.com/AltimeterPortSizing.htm

and used this chart to figure out what size we needed for three holes. To do that, you have to reverse calculate a few of the sizes until you find one that matches the cubic inches you have in your donut avbay. A 3.9" tube 14" long works out to about 169 cubic inches, so we use a 3/16" drill for three holes.

jd2cylman · Aug 10, 2014

Thanks for the post, as my 5" donut av bay has not been completed yet. Nice to have a reference.

Adrian

Volume in a donut avbay

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Volume in a donut avbay

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