Saturday, November 28, 2020

Comparing instruments...

Today's electronics are wonderful; you can fit a lot of measurement capability into a very small package. There are a goodly number of choices available from various vendors and it is a very natural thing to compare capabilities and accuracy, especially when new products hit the market. That's one reason I built the clone of the Centuri RX-16 and the beefy Big Bertha look-a-like, Beulah. Their large payload sections and interchangeable motor mounts enable me to fly multiple instruments on the same flight, facilitating comparisons.

PocketLab Voyager

I was perusing the Estes website last month, and came across these teacher's bundles, which feature the large 24mm powered Green Eggs rocket and some electronics gizmo called the PocketLab. The prices were a little rich for my wallet, but I was intrigued by the PocketLab - I had not heard of it. A Google search instantly produced the manufacturer's website, where I learned that the PocketLab included in the Estes bundles was the PocketLab Voyager, a 1.5 x 1.5 x 0.6 inch mini laboratory capable of making measurements of the following:

  • Acceleration
  • Angular Velocity
  • Magnetic Field
  • Barometric Pressure
  • Altitude
  • Infrared Rangefinder
  • Internal Temperature
  • Temperature Probe
  • Humidity
  • Light Intensity
Wowzers! This is a lot of capability, and after perusing the online documentation, I figured I ought to get one - in the interest of science, of course. I never play... Anyway, the PocketLab Voyager arrived within a few days, and quick trial and error showed that it would fit - albeit very tightly - in the 1.6 inch diameter Estes BT-60 body tube (the Green Eggs rocket that Estes bundled with the Voyager is 1.8" in diameter). I was happy, as this meant I could use the RX-16 or Beulah to fly the PocketLab Voyager and compare it to other instruments.

So when I attended the HARA launch earlier this month, I carried with me my trusty RX-16, loaded with a Quest Q-Jet D16-4 and 3 devices in the payload section - the PocketLab Voyager, a FlightSketch Mini altimeter, and a Perfectflite PNUT altimeter (the TARC gold standard). I was curious not only to see the data returned by the instruments, but also to see how well my iPhone could configure and handle 2 Bluetooth devices simultaneously (Both the Voyager and the FlightSketch Mini communicate via Bluetooth). I need not have worried about the latter, as the phone was more than up to the task, and as for the former, the instruments agreed fairly well, as you can see from the below plot.

Comparison of altitude data taken on November 14 (Click to enlarge).

Note that the Voyager ceases to return data after 140 feet - that's my fault, as I had not yet figured out how to set the instrument to autonomously record data over a span of time (What? I have to read instructions?). So it was transmitting real-time numbers via Bluetooth, which stopped once the rocket passed beyond range. However, the little bit I received looked good. A club member pointed out that the FlightSketch data is systematically lower that of the PNUT, especially below 300 feet altitude. However, the difference is less than 5 feet, so while curious, I am not bothered by it.

The FlightSketch altimeter also is capable of measuring acceleration, so let's look at that data along with the Voyager's:

FlightSketch Mini and PocketLab Voyager acceleration data for the November 14 RX-16 flight
(Click to enlarge).

Again, the agreement is good until the Voyager data transmission stops. Kind of interesting to see 22 g's at ejection, which is substantially larger than the 12 g's experienced during thrusting.

A note about the PocketLab Voyager - the documentation is not that great, as they encourage you to "experiment" with the device to figure out how it works. However, their email support is awesome, answering questions about the device operation very quickly. They were able to help me to figure out why the iPhone app was not showing the screens that allow you to configure the PocketLab to collect data offline (iPhone has to be set to "light" not "dark" mode).

I did a reflight at last week's launch, but the bad luck pervading that afternoon messed things up. Structural repairs have been made to the RX-16 and it is awaiting a new coat of paint on the fin unit. I'll be trying again at the next launch - whenever that is.

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