Center of gravity and barometric pressure readings

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bobtheshortstop

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I’m building my first dual deploy rocket and a thought came into my mind in the form of a question. If the electronics bay is right at the center of gravity, will it affect and barometric pressure readings required by the altimeter? I tend to build my rockets with the CG a little bit more forward than usual. I adjust nose weight or exchange nose cones if I have to adjust the CG. Any info helps. Thanks!
 

Voyager1

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Others will no doubt offer more detailed answers to your query, but the short answer is no. Typically, the requirement for placement of your altimeter's barometric sensor sample ports on the airframe are that they are at least a couple of calibers aft of the nosecone shoulder and similarly forward from the rocket base - and anywhere in between. This will determine to an extent where you place your altimeter, as the sample ports would typically be near the altimeter. The avbays in a dual deploy configuration are usually near the center of the airframe, unless you are employing HED. Where your center of gravity is should not effect the operation of the barometric sensor.
 

bobtheshortstop

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Others will no doubt offer more detailed answers to your query, but the short answer is no. Typically, the requirement for placement of your altimeter's barometric sensor sample ports on the airframe are that they are at least a couple of calibers aft of the nosecone shoulder and similarly forward from the rocket base - and anywhere in between. This will determine to an extent where you place your altimeter, as the sample ports would typically be near the altimeter. The avbays in a dual deploy configuration are usually near the center of the airframe, unless you are employing HED. Where your center of gravity is should not effect the operation of the barometric sensor.
I didn’t think so. I’m not a physics major or highly intelligent person by any means. I try to look at the big picture of everything and then ask questions. Thanks for the reply.
 

manixFan

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The biggest issues isn't where the electronics are located, but where the baro sensing hole(s) is placed. Ideally the hole(s) that the altimeter uses to measure the barometric pressure is placed away from things that would affect a good reading. The rule of thumb is to place it far enough below a body tube joint, whether a nose cone or coupler, to avoid that junction from disturbing the airflow. You'll hear a variety of 'rules', most of them would say at least one caliber of body tube below a disturbance, and two for good measure. Things below the holes don't have any effect, so it's only the distance above the sampling holes to a joint or other disturbance.

It's important to remember that the A/V bay needs to be sealed from the rest of the rocket and the ejection charges. The altimeter(s) needs separate holes that vent the atmosphere to the sensor that are unaffected by protuberances from the rocket's airframe or from ejection charges.

The great thing about modern altimeters is they are designed to be immune from 'mach effects', which is when pressure may actually increase during flight due to Mach shockwave effects. This allows even greater flexibility than older altimeters provided. As a result flyers can now mount altimeters in nose cones, which used to be impractical because many nose cones see a temporary increase in pressure, depending on speed, which older altimeters would interpret as descending from apogee. So the good news is location isn't as sensitive as it used to be, but it is still important. Don't mount it below a fin, camera shroud, launch lug or rail guide; or within one caliper of a body tube joint or the nose cone, and you should be fine.


Tony
 
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GlenP

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Local surface pressure on the rocket will be more likely to be different from local atmospheric pressure in areas where the geometry is changing. At the nose cone, at a transition or diameter change, and near the fins, or even a slightly uneven seam between two tubes, for example.

Along a portion of the rocket where you are not near those things, purely cylindrical and smooth constant cross section with no seams, you will get a better measure of the local atmospheric pressure and that is a good place to put the vent or port for an altimeter.

if you have a wobbly rocket, then yes any motion related changes in pressure would be somewhat less near the CG as that is the center of rotation, but most rockets fly straight and any rotational or pitching and yawing effects are small, the front end, middle, aft end would not have much difference in local surface pressure as long as they are not near large changes in geometry.

Airplanes for example have pressure ports on the sides, usually near the front but these may have some errors at different flight speeds as the plane pitches up and down and flies in different orientations, those calibrations are analyzed and documented.

No such thing as a bad question if you are learning along the way.
 

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