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A question about air holes and altimeters

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T.J.Bones

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Most altimeters are basically fancy barometers. I am not asking about gps based systems. Altimeters measure air pressure and correlate pressure changes with altitude. So, to get an altimeter to work it needs to be exposed to the atmosphere during rocket flight. The conventional/recommended way of doing this is by having holes in the rocket’s body tube to expose the on board altimeter to the outside atmospheric air pressure.

Now for question 1. Are holes in the body tube necessary if the only purpose of the altimeter is to record altitude (not for some other E-bay function)? In this case, couldn’t the altimeter be tethered to the nose cone and conduct its atmospheric reading at ejection? I know ejection does not always occur at apogee, but it is supposed to.

Now for the second question. Holes in the body tube will have air rushing by them during rocket flight. According to Bernoulli’s principle this rushing air will create a low pressure zone at the body tube hole. This in turn will lower the air pressure inside the body tube, wherein the altimeter is located. Since lower air pressure is interpreted as higher altitude, would not this make the altitude reading of the altimeter inaccurate? If the effect I am describing is so small as to be negligible, then could not the same conclusion be drawn about the necessity of the air holes in the first place?

If you are using a gps based altimeter, then the air holes should not needed for that system either.

Am I missing something here?
 

Buckeye

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1. Holes are not always necessary, but they are a good practice. Most rockets have enough natural leakage to detect barometric changes. Jolly Logic Chute Release (maybe the other JL altimeters too, not sure) does not even mention the need for holes. It only starts sampling pressure in earnest after ejection (like you are suggesting). Yes, you can start sampling pressure any time in the flight, as long as the altimeter is not depending on previous data to make decisions, like when to fire deployment charges.

2. Static ports are considered accurate. If you have some weird airflow going past them on the way up, then your altitude vs. time plot may be out of whack. At apogee when the velocity is nearest zero, then the reading will always be very good.

Yes, if you are super-serious about accurate altitude, then use GPS!
 

gtg738w

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Sure, the air is moving relative to the rocket going up but its not moving relative to the rest of the atmosphere. If you have a curved surface like a nose cone or an airfoil on a wing, then the air is being accelerated and does have a change in speed and a change in static pressure. That's why static ports need to be on a straight section of tube for useful readings. Even still, if the rocket is at any angle of attack it will have a non-uniform pressure distribution on the surface. Most of the time you will see multiple vents around the circumference to try and average out things like that.
 

Bat-mite

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I don't know what velocity of air will overwhelm the baro sensors on our favorite alts, but I'm guessing there's a reason they tell you to drill holes and put it inside.
 

AllDigital

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For dual-deployment altimeters, proper porting is critical, as failure to deploy can be life threatening. For an altimeter just measuring altitude on a low altitude single deployment rocket, as @Buckeye says, your rocket might have enough leakage for it not to matter. That said, if your rocket is air tight then the baro might not get a good baseline reading, so might report inaccurate.

Port holes serve two purposes. One, for altimeters. Two is for pressure equalization of the inside airframe. If you are airtight and go up to altitude without proper vents then you risk premature deployment due to pressurization.
 

rklapp

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My understanding is that the holes are necessary if you want an accurate reading at apogee before the ejection and exposure to the atmo. Here's one of Russ' mini altimeters doing what it do.

a.png
 
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