Where do altimeters go in the electronics bay and how to size vent holes?

[jayrocketry]

I did a recent test launch and the deployment charges seemed to go off at least 6 seconds after what appeared to be apogee. I'm not sure if this has to do with vent hole sizing or where the altimeters were placed in the electronics bay. Additionally, the main deployed at apogee which I assume was due to using too much black powder and not enough shear pins, and I believe the drogue deployment was so strong it sheared the main shear pins. It was 4 2-56 shear pins. I will upgrade from 2-56 to 4-40 shear pins.

My electronics bay is about 6 inches in diameter and 17 inches long. I sized the vent holes as stated in the Stratologger user manual and created four ports/holes each being 0.5" in diameter. The bay has 2 key switches but the switches fit snuggly and are epoxied so there should be little to no leak. Are my vent holes sized correctly? Additionally, I placed the altimeters a few inches away from the vent holes since I want them to stay away from heat and sunlight, and from any wind gusts that happen to enter the vent hole. Are the altimeters supposed to be placed exactly near the vent holes or they can be placed inches away from the vent holes? Thank you. I am flying to 10k

 

[tim cubbedge]

Did you do any ground testing before your flight?

 

[rocketgeek101]

Vent holes don't need to be very big. On my 6" diameter av-bay I use three 1/4" vent holes and it works fine. Your four 1/2" vents should be more than plenty. It sounds like either your charges are oversized or something is wrong with the altimeter.

Is there any chance you wired the main and apogee charges backwards?

 

[Buckeye]

I did a recent test launch and the deployment charges seemed to go off at least 6 seconds after what appeared to be apogee.

How do you know this?

 

[Handeman]

My av-bay that size has four 1/4" dia. vent holes and it works fine. I'm sure your vent holes are plenty big. The altimeters can be any place in the av-bay, it doesn't matter.
Apogee can be very deceiving when the rocket starts to tilt over and starts to look like it's going sideways . I suspect the altimeter fired right at apogee and your observation of actual apogee is off.

I assume the answer to @tim cubbedge question is No, or you would know if your charges are too big.

Since you plan to fly a 6" rocket to 10K feet, which will have to be a L3 sized motor, and are asking these elementary questions, I assume you are with a student group planning on entering SLI, IREC, or similar competition. Since you are asking these simple questions on the forum, I also assume you do not have an experienced rocketry mentor working with your team.

GET A MENTOR! Your chances of success, not getting hurt, and not wasting money on damaged rockets and equipment will go way up!

 

[jayrocketry]

Did you do any ground testing before your flight?

Yes, we did black powder separation testing but will now be looking into smaller-sized charges. The charge may have been oversized. We performed altimeter testing using a vacuum chamber

Vent holes don't need to be very big. On my 6" diameter av-bay I use three 1/4" vent holes and it works fine. Your four 1/2" vents should be more than plenty. It sounds like either your charges are oversized or something is wrong with the altimeter.

Is there any chance you wired the main and apogee charges backwards?

I suspected that was what happened but post-analysis photos showed that everything was wired and assembled correctly.

How do you know this?

Apogee can be very deceiving when the rocket starts to tilt over and starts to look like it's going sideways . I suspect the altimeter fired right at apogee and your observation of actual apogee is off.

I assume you are with a student group planning on entering SLI, IREC, or similar competition. Since you are asking these simple questions on the forum, I also assume you do not have an experienced rocketry mentor working with your team.

I honestly do not know. It was my first time seeing a rocket launch to 10k and I was extremely excited to witness it so determining apogee was deceiving. Both altimeters did measure very similar results they began to ascend and descend at the same time so I guess I just have to trust the altimeter data.

Yes, I'm part of a student group planning to compete and I'm in the recovery subteam. We do have an L3 mentor who witnessed the launch and gave feedback and mentioned some design changes we should consider. I am here on the forum looking for any additional input. Thanks for the responses.

 

[cerving]

Vacuum chamber testing is not a good quantitative test... at best it's a functional test of the baro sensor and the outputs. The problem is that the vacuum tends to be drawn and released MUCH faster than it would in a real flight. Typically, it looks like your rocket is going about Mach 3 on the way up with very little coast time to apogee, and it "falls" so fast that either both the apogee and main charge appear to fire at the same time or the main doesn't fire until the vacuum has pretty much subsided. If you get an "altitude" out of the test and the charges fire at all, your altimeter is fine.

 

[kramer714]

uh huh, a lot to unpack here...

First thing I gotta ask, you have a 6" diameter 17" long av bay? That is really big, room to pack a lunch?

Ok on to serious stuff, here are my thoughts, (and yes, ask your mentor)

1) Shear pins. How long is the rocket? I have seen this happen (heck I have actually done it) using small diameter shear screws in a large rocket it is VERY easy to shear the pins just from handling (walk to the pad rocket wiggle is a symptom). Plus it is very hard to actualy tap the small holes in fiberglass, Lots of people just kind of, sort of, 'thread them in' and hope for the best. I dont use pins like that I use https://www.mcmaster.com/90136A532 they fit in well, shear cleanly on fiberglass, and you can remove them by just cutting the head off and pushing them into the bay. When you say thathe drogue deployment was soo strong it sheard the pins, i wonder how well were they installed...

1.5) Here is how you install these. FIrst of all , buy the ACTUAL drill that McMaster calls out, if they call out a #32, use the number #32, not something close. A clean hole is a good hole, the way I 'mentored' the last student team on how to drill the holes was to;
A) mark the spots for the holes and make a mark from the coupler to the body tube so you know how to line it up. Slide the body tube over the coupler and tape it in place.
B) Have at last (2) drills each of a 3/32, and a #32 Carbide, Cobalt or TiN coated. These should be sharp. Fast and wet... (we are talking drilling here), when you drill use a little water (um, or spit) on the drill bit, and 2,000 RPM minimum, at the day job (aircraft) we use 4,000 rpm or higher.
C) Drill a 3/32 hole in one of the locations, LEAVE THE DRILL BIT IN THE HOLE AS A PIN (option, use a Cleco to pin the hole)
D) WITH THE BIT IN THE FIRST HOLE, drill the remaining holes to 3/32
E) WITH THE 3/32 bit still in place, Use the #32 bit and open up one of the previously drilled 3/32 holes, LEAVE THE DRILL BIT IN THE HOLE AS A PIN (option, use a Cleco to pin the hole)
F) open up the remaining holes with the #32 drill, the last hole to do is the one with the 3/32 drill in it, remove it and open the hole up to the #32 size.
If you did it right, you should have very clan holes with minimal (or no) splintering of the inner diameter.

2) Big rocket, long shock cord with 'shock absorbing' folds I did a post on this Shock Cord Folding

3) Do you have an altimeter setup sheet that you, A) have figured out what you want the altimeter to do, B)Know how to set the altimeters (program, dip switches etc) and listed it on your setup sheet, C)actually use it when you assemble the rocket, D)actually use it when you arm the avionics, and E) have it as part of your checklist? Think about everything you want on this, even things like what is the lowest voltage you will launch with for example.

3.5) As part of your setup sheet have different colored wires for the main, drogue, backup main, backup drogue. I use the extra wire from e-matches, to get some different colors, or at least mark the wires. Put the colors down on your setup sheet (see #3)

4) Talk to your mentor about your black powder test, did you have all of the 'laundry in the rocket? What should the backup charge be relative to the primary? What do you learn from the test, and from a vacuum chamber test (and to what Eggfinder Cris said) what you don't learn from it.

4.5) For vacuum testing think about the pressure change. For 10,000 from sea level you should see a little over 4 psi change, but it isnt linear vs time when a rocket launches. I had a High School team make a poor mans vacuum chamber using a gallon pickle jar, to simulate apogee they uses a push pin an poked a small hole in the nylon hose. Slow leak not a woosh...

Mike (I never wired an av bay wrong..... sure.....) K