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  1. #1
    Join Date
    26th January 2010
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    Altimeter vent holes

    One thing I almost always forget to do, for some reason, is to drill vent holes for my AltimeterThree. Jolly Logic recommends three holes around the body tube. But I've seen rockets with two or three holes drilled in a line on the back. Would three inline 1/8" holes suffice? The volume in the three rockets I need to drill are 3" x 23"; 3" x 19"; and 4" x 10".

    NAR 91107, Level 2

    I really, really hate bugs.

  2. #2
    Join Date
    6th September 2009
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    1,648
    Circumferential holes are best to average out static pressure and reduce the effect of bias/fluctuations. Kinda like how you need 3 fins 120 deg apart, and not one giant fin!


  3. #3
    Join Date
    9th October 2013
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    Buckeye has the reason, though I will note that this could be considered less important for an AltimeterThree (or One or Two) than a typical deployment altimeter. For the A3 the worst that would do is log a bad flight and you wouldn't get good data for it, which is very different than a deployment altimeter that might end up firing charges on the pad, or airstart a motor on the ground, etc. So I think the risk is much less with an A3 since it's not handling any charges, but it would still best to spread them around.

    For me, I usually fly my A2's/A3's in a parachute section, not an actual av-bay. I don't vent it like an av-bay, but I do still have a vent hole or two to prevent pressure separation (depending on the volume of the airframe section). I typically keep those holes towards the back for cosmetics, so it would face the rail. The rail might help block (or at least disturb) a direct wind, but I'd wonder if a cross-wind wouldn't be the worse thing for pressure fluctuations anyhow, much like blowing across a bottle opening. And I'm not sure if the A2/A3 uses its accelerometer to actually do the launch detect, so it might be immune to sudden air pressure changes anyway. But even with where I put/vent my A2/A3's, I generally get readings that match my altimeters in their properly-vented av-bays.
    Will Ferry (Launches & Videos) NAR #96512 (L2) / TRA #15328 (L2) / LUNAR #2759
    L1: 9/2013 @ XPRS, GLR T-Bolt "Thunderbolt" (R.I.P.), H148R
    L2: 4/2016 @ TCC Helm, Binder Design Excel w/DD "dd2.xls", J315R
    Impulse flown (flights): 2013: 767Ns (2), 2014: 4298Ns (8), 2015: 7486Ns (16), 2016: 11693Ns (18), 2017: 11138Ns (16)

  4. #4
    Join Date
    5th June 2010
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    We recommend a circumferential vent hole pattern to counteract any side-to-side unbalanced pressure waves (e.g. a breeze). You don't want a breeze to ram air into your vent, or "coke bottle" the pressure out. Adding additional holes prevents those imbalances.

    In a payload section, the more holes the merrier. No sense not venting the bejeezus—if you believe in bejeezus—from a payload section.

    If you are a fuselage flyer, without vent holes you'll probably get a fairly good apogee reading, unless the delay is really late. Most fuselages aren't that airtight, but some really are, and those are the cases we see the most false-triggering of both altimeters and Chute Releases as the nose cone or top section is forced on or off at the pad. If you've got a rocket that feels springy when you put the nosecone on, it's possible to false trigger Chute Release on the ground. Not super easy, but possible. Add vents in that case.

    Without venting, you're also much more prone to weird pressure artifacts. Effects like "pistoning" where during high boost the inertia of the internal air in the altimeter section presses downward, which can lead to weird bumps in your altimeter data. These manifest as an odd jump upward (top of the bay) or downward (bottom of the bay) in the altitude graph during the boost phase. I have seen graphs where the altitude goes negative at launch, as well as ones with a weird upward hump in that same location. And when you don't see those you always you can observe "pressure lag" where the altitude graph only sluggishly syncs with the acceleration graph.

    The best reference for venting I think is by Vern Knowles here.

    And I have heard that there can be induced base drag from the rear of the rocket that tries to suck the air out of the fuselage during high speed, though I have not observed that one in modern rockets myself. That's something Tommy Billings of Adept seems to focus on in his reference guide (link here) for venting, despite the fact that he warns against "too big" vent holes (I don't think that's ever an issue unless they are ginormous). I feel he over-hypes the aforementioned base drag (most rockets are well sealed at the rear), and his disdain for altimeters in the fuselage (hey, the vast majority of rockets don't have a payload section; let's not let the perfect be the enemy of the good).

    If you're still reading, one little physics thing that should be appreciated (in the same spirit that we appreciate center of pressure, center of mass, and stability) is that vent holes should be sized based on how much air is inside the section, and how fast it needs to escape. If you have a very tightly packed compartment with little air, you don't need as much venting. Bigger sections with lots of air need larger vents. Rockets that "jump up" to high altitude need larger vents than slow and low rockets. It's all a question of how much + how fast.

    Not that anybody on this forum cares, but these effects are very pronounced in water rockets, since they accelerate much more violently than most of the rockets the folks on this forum fly.

    UPDATE: in the first version of this I co-mingled Vern and Tommy's guides. Fixed.
    Last edited by John Beans; 16th December 2017 at 02:03 AM.

  5. #5
    Join Date
    1st October 2014
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    Boise Idaho
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    Quote Originally Posted by John Beans View Post
    ... That's something Vern Knowles seems to focus on in what I consider to be one of the best references (link here) for venting, despite the fact that he warns against "too big" vent holes (I don't think that's ever an issue unless they are ginormous).
    John,
    You might be confusing me with Adept Rocketry. Your link goes to an Adept publication.
    Vern

    Vern Knowles
    www.multitronix.com
    www.vernk.com
    TRA 4061 L3
    NAR 45236 L3

  6. #6
    Join Date
    5th June 2010
    Location
    Danville, CA
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    Quote Originally Posted by VernK View Post
    John,
    You might be confusing me with Adept Rocketry. Your link goes to an Adept publication.
    Vern
    Yes! Sorry. You're right, I fixed it in my post above.
    I had both documents open and I confused them.
    I always point folks to your guide, which once again is:
    http://www.vernk.com/AltimeterPortSizing.htm

  7. #7
    Join Date
    26th January 2010
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    Thanks for the information, John. My only reluctance to drilling three circumferential holes is looks. Although I don't know why I care so much about that, my rockets aren't the prettiest, and you couldn't see them from 25' away anyway. The last time I used the AltimeterThree was in an unvented rocket, and got some pretty wonky results. This rocket simmed at over 1000', but the altimeter said 690'. Plus there's a weird descent in the graph. Could that have been when the Chute Release released? I'll try inline vent holes in my next rocket, and see what happens.
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    NAR 91107, Level 2

    I really, really hate bugs.

  8. #8
    Join Date
    26th January 2010
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    Now that I think about it, I did have the CR set for 500', which is right around where the weird looking descent happened.
    NAR 91107, Level 2

    I really, really hate bugs.

  9. #9
    Join Date
    5th June 2010
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    Danville, CA
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    725
    I think that blip is your ejection (late). The curve is so smooth before that, just like a smooth ballistic arc.
    As far as apogee goes, one thing to keep in mind is that apogee is most drastically affected by trajectory. I remember hearing Ken Biba talk about high altitude record attempts at Black Rock. The biggest difficulty? Going straight up.
    The rule of thumb I use is that there can be 40% variation in apogee due to trajectory. That is a made-up number from me, but I believe it. If you have a windy day and an over-stable rocket, it can sometimes appear to boost and then head sideways. I like to think that it was pursuing an orbital trajectory.

    As for the appearance of vent holes, that's been discussed in past threads. For cardboard tubes (which are the hardest to make look clean) before you paint drill the hole, then coat with super glue, then sand after drying, then re-drill. You can make very clean holes that way.

  10. #10
    Join Date
    8th October 2016
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    Quote Originally Posted by John Beans View Post
    We recommend a circumferential vent hole pattern to counteract any side-to-side unbalanced pressure waves (e.g. a breeze). You don't want a breeze to ram air into your vent, or "coke bottle" the pressure out. Adding additional holes prevents those imbalances.

    In a payload section, the more holes the merrier. No sense not venting the bejeezus—if you believe in bejeezus—from a payload section......
    Many thanks for the info and the links.


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