| Date: | 28th May 2023 |
| Location: | Whalan Reserve, Australia |
| Conditions: | light wind ~5km/h, 12C Sunny |
| Members: | GK, PK |
Today we wanted to do an experiment to see if we can see the difference in air pressure between the ground and the rocket at apogee.
The Experiment For this we decided to use a sealed and partially air-filled freezer bag. This bag would be contained inside of a fairing with small holes that would allow pressure to equalize. The fairing would protect the bag from any air flow over the rocket. We would then use a camera on the inside of the fairing to see if we could see the bag inflate as the rocket ascended, and then deflate again as it returned back to the ground.
We first drew a grid on the bag to make it easier to see any movement. Then after it was filled, we placed a sheet of baking paper over the bag and used the edge of an iron to heat weld the bag shut. We did 3 of these welds to make sure the bag was sealed. We then cut off the rest of the bag.
The whole bag was then taped inside the fairing. The fairing had clear sides so that we could also see outside the fairing with the camera and we would know what portion of the flight we were in at any time.
We prepared a couple of bags with different fill amounts.
One of our 808 #16 cameras was placed in a block of foam and inserted into the top portion of a bottle that formed a nosecone on top of the fairing. This allowed us to easily remove the camera when needed. We also adjusted the camera to focus on the bag.
With the camera nosecone attached to one end of the fairing, we attached the other end to the existing nosecone on the rocket itself. This would have completely sealed the inside of the fairing, and so we made a number of holes around the periphery of the bottom of the fairing to allow the pressure to be equalized with the ambient pressure outside the rocket.
For the first test, we used a bag that was only slightly filled with air. The rocket was launched at 110psi and the expected altitude was around 120m. The flight was nominal with parachute deployment right around apogee. When we reviewed the video of the bag, it didn't show any movement. We were expecting at least some movement, but we didn't see any. This was somewhat disappointing, but there could have been a number of reasons for this.
For the second flight we replaced the bag with a more inflated one. The thinking here was having more volume of air would mean there was more to expand. The rocket was filled to 110psi again and launched. The flight was nominal with the parachute deploying right at apogee. On first review of the video it looked like the bag didn't inflate again, but upon close inspection, there was a very slight movement in the bag as if it had inflated slightly. This was particularly evident when toggling between the ground and apogee images. The bag looked a little more deflated again just before landing.
So it looks like the experiment worked, though we had been expecting a bigger difference in the bag inflation. When you do the calculations the difference in air pressure is only 0.21psi and the volume increase is only about 1.41%. That little increase spread across the entire bag means it is only going to move a little bit. At 200m that would have been 2.35% and at 500m that would have been 5.8% increase.
We did a couple of test flights of Brian's flight computer. This was mostly for him to gather data on actual flights and see if it worked. Here is the data from his flight computer. There is still quite a bit of work to do on the software side of things, but the hardware seems to have good capability. I believe that Brian would like to sell these eventually.
Both flights were nominal with good recoveries.
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