Will the AltitudeTwo work OK if you don't drill the holes in the body tube? Anyone try that?

I think you are asking about the Jolly Logic "AltimeterTwo" .

Check out page 4 of the user guide - "The altimeter needs to sense outside atmospheric pressure at all times. Drill at least three 1/16” to 1/8” diameter holes evenly spaced around
the payload bay or the fuselage in such a way that they will not be blocked once the rocket is assembled for launch."

http://www.jollylogic.com/wp-conten...uideV18Web2.pdf

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- Scott, NAR 91621
Woosh #558

I once drilled a hole in a road cone and mounted the altimeter to my cat...it seems to work fine.

Yes I have read the manual. I am looking to hear from someone who has launched it in the main body tube without drilling the holes, and to find out what measurements were compromised. The acceleromete measurements won't be affected, and the post ejection air pressure measurements won't be affected. There will be leakage around the nose cone and engine mount, so the body tube won't stay completely pressurized. If apogee occurs somewhat before ejection, some accuacy may be lost because there are not air holes drilled, but the altitude (and therefore pressure) changes are relatively low at that point, so there may be little if any downside to not drilling the holes.

I wouldn't automatically assume that...

They made a point of saying that the altimeter one could be attached directly to the shock cord and would read properly despite being exposed to the ejection charge and not having depressurizing ports or being in a dedicated payload compartment or altimeter bay.

If the instructions say to drill the holes or the thing won't work as advertised, then it's probably a good idea to do it. Maybe they changed the altimeter part of it and the pressure sensor is different and won't read properly.

I DOUBT they'd have changed the instructions for NO reason... obviously there's an advantage to having an altimeter that DOESN'T need air pressure holes in the vehicle, so it would be counterproductive to say "drill holes" in the instructions if there was no need for them.

Not like you're depending on them for ejection, but you might well want to experiment for yourself and see what it does... if you have an altimeter one, fly it side-by-side with the altimeter two and compare readings...

Or of course you could always go straight to the source and ASK TIM VAN MILLIGAN! He does answer questions you know, and he's usually pretty knowledgeable about the products he sells... or will find out for you...

Later and good luck! OL JR

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Yeah, I am constantly surprised by how "hand on" Tim is. I had a problem wit Rocsim a couple months ago and expected yo hear back from a software tech or customer service rep, but, no - Tim responded to the question personally. That company has bodacious customer service!

Greg

Which is exactly why I was asking for anyone who had actual experience with using this device in that way.



I am a mechanical engineer. All designs are based on trade-offs. In this case there is a an obvious trade-off between accuracy of the max altitude in some situations, versus drilling holes in a nicely finished rocket body. I started this thread to see if someone had actual experience with the reduction in accuracy caused by not drilling the holes. If the accuracy suffers by 10 feet in the situations where it will be affected (ejection after apogee), then the trade-off now has been quantified, and a better decision on whether to drill or not can be made.




I plan on flying the AltimeterTwo in the same rocket, before and after drilling the breather holse, but I though I would use the possible previous real experience of this group to increase my knowledge before doing so.

Dan

I think you've summarized it pretty well already. Ejection altitude will be compromised, but the effect will be hard to quantify without specific knowledge of the configuration ... i.e., leak rate, compartment volume, etc. If the compartment were perfectly sealed, the barometer would record no pressure change at all and thus no altitude increase until ejection .... the disparity between that and what the accelerometer is sensing might cause a problem if the device has some error trap built into its internal logic. If not, it should "work ok" if you're not expecting an accurate ejection altitude reading.

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Tim Wilson

I hypothesize that ejection altitude will be compromised only if ejection occurs much before or after apogee. Near apogee, there is not much altitude change, and therefore not much pressure change, allowing the air trapped in the rocket body to leak out/in around the nose cone and engine mount and stabilize with the outside ambient pressure.

I am more interested in peak altitude for the flight, and I hypothesize that peak altitude will not be affected at all from the absence of breather holes as long as ejection is before, or very slightly after, apogee. Once ejection occurs, the instrument is in the ambient, and if ejection occurs before apogee, then a good max altitude reading will be made.

I don't expect to see problems with the instrument getting confused by the slightly higher pressure inside the body tube from no breather holes being drilled. If the tube was sealed very well, the relative pressure difference that develops during ascent would soon push the nose cone off as the rocket slowed near apogee. If the instrument logged all the pressure readings (something like 30 per second), I suspect there would be very signficant error on the way up, as the air in the body seeks to find its way out during a time of rapid ambient pressure reduction. But since this instrument only reports a few min/max pressure events, there is no real issue there.

It has been an interesting technical exercise. I might receive my AltimeterTwo today, although with winds gusting to 30 mph today, and remaining high the next few days, it will be a few before I play around with it in a test rocket I will be using. I'll launch it three or four times without breather holes, and then add the holes and launch 3 or 4 times again, to see if I can detect a difference between measurement sets.

Dan

Why the resistance to drilling the holes anyway ?
Is this just for an experiment ?
If not, why compromise the accuracy of the instrument ?
I would be surprised if the accuracy is acceptable without the holes.

I'm an engineer too (EE)....know plenty of good ones and plenty of lousy ones....several I know if taken out of their area of expertise could not even figure out how a mousetrap works.

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