Altimeter Three accuracy

I used my new Altimeter Three for the first time yesterday, and got unexpected results. Open Rocket predicted 1500~ feet on my 5 Times A Fool with an H165. But the Altimeter Three measured 987 ft. OR predicted 1200~ feet on my Meaner Machine, but the A3 measured 690 ft. I weighed 5 Times, and it does weigh 10oz more than what OR figured. But even after overriding to the correct weight, and changing the finish to "rough", OR sims at 1151 ft. That's quite a difference. I can't imagine the A3 is that unaccurate. What am I missing?

qquake2k said:

I just realized that I didn't drill vent holes in either rocket. Could that do it?

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You mean the holes that allow the avionics bay to equalize to the same outside air pressure that the altimeter is trying to measure?

Yeah, that would do it.


Steve Shannon

Holes should help.

I've compared my Altimeter Three to a UBlox GPS and another altimeter on several high power launches. They're always very close to each other. I had the Altimeter Three along for the ride on my L2 cert recently. You can see what that graph looked like here: https://www.rocketryforum.com/showt...Release-for-L1-L2-or-L3&p=1687894#post1687894

Well that seems weird!!! - The instructions for the AltimeterTWO are clear and specific about venting to the outside atmosphere, however that little detail seems to be missing from the AltimeterTHREE user manual.

On a recent flight using both an Altimeter Two and a Perfectlite Stratolgger I got almost identical results. The Altimeter Two said 3,811 feet and the Stratologger said 3,800 feet.

Since AltimeterTwo and AltimeterThree have similar (I believe) hardware with respect to pressure sensors and accelerometers, they would both need the same sorts of installations, including vents for the static pressure measurements.

I have also flown JL devices (not just A3s) alongside other brands (PerfectFlite in particular) and have had VERY similar results when the devices were in the same compartment on a give flight.

Without vents the altimeter is going to depend on leakage into/out of the compartment it's in for pressure changes and if it's fairly tight the pressure decrease on ascent will lag what's outside the rocket. With those way late ejections in your graphs the pressure altitude inside the rocket's payload compartment may never have gotten up to what the rocket did if there were no vents.

Mass, finish, and fin edge treatment (square, rounded, streamlined) all play an important part in simulated altitude, as does making sure that the time-thrust curve the simulator is using matches the motor you're actually using. This can be a problem if there's more than one set of data for a motor "out there".

Yes, you definitely need venting for AltimeterThree.
The definitive guide is probably:
https://www.vernk.com/AltimeterPortSizing.htm
(But it really should say that its suggestions are minimum sizes.)
Also, don't forget that trajectory can affect apogee by as much as 40% between flights. Some flights are straight up, some arc a lot.

John Beans said:

Also, don't forget that trajectory can affect apogee by as much as 40% between flights. Some flights are straight up, some arc a lot.

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Does the A3 report apogee from the pressure sensor, integrating the accel, or both? If baro only, then trajectory doesn't matter, and the apogee will be measured accordingly.

Buckeye said:

Does the A3 report apogee from the pressure sensor, integrating the accel, or both? If baro only, then trajectory doesn't matter, and the apogee will be measured accordingly.

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I think he was referring to simulator predictions.

qquake2k said:

I think what John was saying is that the trajectory affects altitude, but that the A3 will measure the actual distance from the ground regardless of trajectory.

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Yup.

Yup.

Duplicate post

I used a MissileWorks RRC3 dual deploy altimeter for the first time yesterday, and I've been using Jolly Logic altimeters the past 3 years. On my first DD flight, the RRC3 and JL Altimeter3 returned identical altitude readings. On the second flight the indicated readings were only 1' different. Each of these devices had their own compartment, with venting. If you have your static ports are adequate in size and properly located, you should get very accurate results.

qquake2k said:

I think what John was saying is that the trajectory affects altitude, but that the A3 will measure the actual distance from the ground regardless of trajectory.

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I think we were discussing the same situation, from different perspectives. I meant the simulated altitude makes certain trajectory assumptions, whereas the A3 doesn't care if it SHOULD be doing something, it just tells you what it IS doing.

John Beans said:

Also, don't forget that trajectory can affect apogee by as much as 40% between flights. Some flights are straight up, some arc a lot.

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Donnie said:

I think he was referring to simulator predictions.

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Errors in simulated wind, rod angle, or motor thrust (ie, trajectory), even big ones, are not enough to explain 40% differences in apogee. Something much bigger is going wrong. In this case, the altimeter is not correctly sampling the ambient pressure in flight because the vent holes are missing.

Where did you mount the A3? Was it in a sealed AV compartment or just clipped to part of the recovery harness like the nose cone eye bolt? If it was just clipped to the nose cone eye bolt, then as soon as your deployment went off and the NC separated, the A3 would be in free air and would read the correct altitude.

Your simulations are probably correct. The altimeter is doing funny things. In your first chart, there is a local maximum altitude at t=2, then falls down, then peaks again at t=9. That makes no sense.

Buckeye said:

Your simulations are probably correct. The altimeter is doing funny things. In your first chart, there is a local maximum altitude at t=2, then falls down, then peaks again at t=9. That makes no sense.

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I wondered what caused that. It's almost like it slid backwards, but it didn't. I'll have to look at the video again.

qquake2k said:

This may be a stupid question, but will vent holes affect separation? Will I need to add BP to the ejection charge?

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Not really, if you're doing dual deploy it's always useful to ground test though. I've never had to take my vent holes into consideration when sizing my charges though.

qquake2k said:

I wondered what caused that. It's almost like it slid backwards, but it didn't. I'll have to look at the video again.

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I've seen weird data like that too when I stuffed the A3 in a payload bay that didn't have vent holes.

Buckeye said:

Your simulations are probably correct. The altimeter is doing funny things. In your first chart, there is a local maximum altitude at t=2, then falls down, then peaks again at t=9. That makes no sense.

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We normally associate such effects with high speed, but high acceleration does it too. It's common with barometric altimeters.

qquake2k said:

I wondered what caused that. It's almost like it slid backwards, but it didn't. I'll have to look at the video again.

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No need to look at video. It is safe to say that your rocket did not go up, fall backwards 100 ft, and then continue going up to apogee.

Larry Curcio said:

We normally associate such effects with high speed, but high acceleration does it too. It's common with barometric altimeters.

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The OP's flight was neither high speed nor high acceleration. His A3 altimeter trace does not look very common to me.

It would be interesting to see the first flight's data in the OP's first post with the acceleration traces showing on the graph to see what's really going on there. Or just the "total acceleration" trace. That spike on the ascent is odd.

Here is the flight report for 5 Times. It won't let me post the html file.

View attachment flightReport (3).xlsx