OpenRocket altitude measurements

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amalli0

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From which point on the rocket does OR calculate apogee? Is it measured from the tip of the nose cone, the center of the rocket? Is it possible to change it? Thanks.
 

Zeus-cat

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Rockets tend to arc over at apogee, so the nose, body and tail will all pass pretty darn close to apogee. Unless your rocket is REALLY big and backslides after hitting apogee instead of arcing over I think the question is moot.
 

amalli0

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Rockets tend to arc over at apogee, so the nose, body and tail will all pass pretty darn close to apogee. Unless your rocket is REALLY big and backslides after hitting apogee instead of arcing over I think the question is moot.
Ok, so putting the altimeter anywhere in the rocket should produce a similar apogee measurement right?
 

gtg738w

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Ok, so putting the altimeter anywhere in the rocket should produce a similar apogee measurement right?
I would guess it tracks the center of mass but as long as the altimeter is in the rocket that won't be anywhere close to the largest source of error. Make sure you are correcting the altimeter AND the sim for your local weather, that is usually the biggest miss.
 

BEC

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Unless the rocket is really really big, it won’t matter. The length of 1/2 a rocket will fall well within the accuracy, and probably also the precision, of whatever altimeter you choose to use.

It’s much more important to provide adequate static pressure venting for the altimeter.

As for getting it to match the simulation…if you do everything right — component weights in the simulation match the real parts, finish quality matches (and its weight is accounted for), the fin cross-section matches the simulation setting, the fins on the actual rocket are straight, the flight is straight (or matches the altitude/temperature/wind conditions of the actual launch) and so forth. Getting measured altitude within 5% of that calculated by a simulation is considered really good. So again, unless the rocket is huge, it won’t matter.

I expect OR and RockSim probably calculate to the CG, but that’s just a guess on my part. Ah…it looks like Russ thinks the same. And since he makes altimeters, his thoughts are worth noting.
 

Buckeye

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Ok, so putting the altimeter anywhere in the rocket should produce a similar apogee measurement right?
Yes. The barometric altimeter measures the difference in air pressure from where the rocket started to its highest point. So any point on the rocket should see this same difference.
 

Maxwelljets

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Here's a one second excerpt of real world flight data from a stratologger CF right around apogee:
1641438532841.png

As you can see, there's ~2 ft of variance on the actual altitude between adjacent measurements. It's even higher on other flights.

In short, the noise in the sensor is greater than the difference in height between the various points on most rockets. It doesn't matter in the slightest.
 

BEC

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...and as @Buckeye pointed out, what the altimeter reports is the delta between its initial altitude before flight and its apogee, pretty much irrespective of what it's riding in (well, as long as the static vents are adequate so there's an adequately short lag between pressure changes outside and those wherever the altimeter is riding).
 
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