On Thu, 2009-04-23 at 12:50 -0700, rq1...@q7.com wrote:
> I didn't go back and look, but iirc, psas flight data has the same
> pressure glitch at motor burn-out. Previously we have attributed the
> glitch to changes in the pressure field over the vehicle surface due to
> the presence or absence of motor plume.

Yeah, we're not sure what's causing this, but we do know that this
particular airframe has a bad seal between the lower recovery bay and
the avionics -- the other altimeter showed a huge spike downwards at
apogee caused by the recovery charge leaking into the avionics bay.

> This explanation may be wrong. In particular at supersonic velocities
> motor plume cannot effect the pressure field on the forward part of the
> vehicle. Perhaps motor exhaust is somehow leaking into the interior of
> the airframe and pressurizing the entire structure? If that were the
> case then running a hunk of tubing from the pressure sensor directly to
> the outside would eliminate the effect.

This rocket did not fly above mach 1, the highest recorded velocity was
about 170m/s. We'll see if this same artifact occurs in other vehicles.

> The (calibrated) pressure sensor is probably more accurate in
> determining altitude at apogee than the accelerometer.

Yeah, for differential altitude, the pressure sensor is pretty good. Of
course, we get a lot of noise with our sensor as it is very sensitive to

>  You could rescale
> the filtered accelerometer data to match the pressure determined apogee.
> That should provide a better calibrated view of the burn-out pressure
> glitch.

At this point, I mostly want to collect a lot more data; having the same
electronics flown in a lot of different configurations should show us
most of the variations in data.

> I'm a little confused as to why all the accelerations are positive. Even
> accepting that the acceleration on the pad reads as +1 gee it really
> seems like the initial post-burn-out acceleration should be negative.

The scales on that plot weren't ideal; I've re-plotted the data and set
the pad acceleration to zero. With that graph, the minimum acceleration
is about -1.7g. As this rocket only flew to 170m/s, .7g of drag doesn't
seem unreasonable to me.


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