Velocity on streamer has a radically different plot appearance than on the chute.

While on the streamer the velocity plots as a jagged saw-tooth going back and forth between -20 and -80 ft/sec. Once the chute comes out the velocity settles into a fairly smooth line until it lands. This surprises me. The decent velocity should be faster of course, but why would it be so erratic surely the rocket's decent velocity can’t be changing 60 ft/sec in fractions of second.

Is this behavior in a barometric altimeter normal? Anyone know why?

My guess is that the airframe was rotating. In doing so it exposed the vents to different air flows causing the anomaly you are seeing. Barometric is questionable for determining velocity.

What your altimeter actually measures.

• A GPS based altimeter measures position.
• An accelerometer based altimeter measures acceleration.
• A barometric altimeter measures air pressure.

Not a single hobby rocket altimeter measures velocity.

Velocity is a derived value based on a calculation based on the change of a measure physical value with time.

For a GPS, delta position / delta time is the velocity.

For an accelerometer, a*t = velocity.

For a barometric altimeter, pressure is converted to a pressure altitude and the change in pressure altitude with time is the calculated velocity. The accuracy of the velocity is dependent on the accuracy of the altitude measurement which requires an accurate measure of the ambient static pressure outside the rocket.

The problem is that the altimeter is located inside the rocket and erratic movement of the rocket during a drogueless or streamer introduces additional dynamic pressure fluctuations that can not be distinguished from real static pressure changes so the calculations produce an altitude profile with a lot of "noise" due to the dynamic pressure fluctuations around the rocket due to the erratic motion of the sampling ports during descent.

The data can be post processed to remove the noise in a program or spreadsheet.

Bob

Sure, you are preaching to the choir. What is surprising me is how "clean" the altitude plot appears relative to the jagged velocity. Simply a visual perception "thing" even though it can be accounted for in the velocity calculation. In other words the velocity appears to be changing to a greater degree than the altitude plot is accounting for, but they are of course on different scales. It's even more surprising that deployment of the main smoothed the plot so much. To Mark's point, the only thing that could account for the change is that the Avbay in the forward body tube is no longer spinning/falling around the aft body tube because it is now suspended by the main.

I don't know if any GPS devices for rockets do it, but GPS systems can measure speed using Doppler shift.

-- Roger