Underestimating Engine Strength -Estes Prowler (+videos)

This morning the wind conditions were perfect, as were the sky conditions. I thought I'd try launching my Maxi Alpha 3 and my Prowler on single-use composite motors, as conditions were favourable for them land easily within the boundaries of my chosen field. Wind was from the northwest at 4km left hour or less, and the sky was crystal blue clear.

The first launch with the MA3 went very well. I used an Aerotech E15-4W, launch, coast time and deployment went well and as expected the rocket easily landed inside the park. My best estimate is that the MA3 hit its projected altitude of around 950ft. The altimeter didn't take, so it's only a guess. The rocket had weatherocked northward, but stayed in the park.

The second launch with the Prowler went well, albeit with a few unexpected complications. At a loaded weight of 10.42 oz, on an F27-8R, the rocket calculator put its potential altitude at 1155 ft. Again, the launch was very good, straight, albeit with weathercocking North. I had assumed that the very light wind would carry it back down into the park.

Nope.

After a very long coast as expected with a delay of 8 seconds, the parachute deployed nicely, and it came straight down... Way north of the park. In fact, I thought it landed in a construction site. Fortunately, the workers indicated that it landed even further north, landing on the lawn of a senior's residence at the north end of my street. Picture perfect landing, but WAY outside my expected circle. The altimeter said 1455 feet, which means it deployed at that height, as I hadn't prepped the body tube with any sort of holes. The rocket went higher than that. Maybe as much as 1600 feet.

Looking close at the launch rod, the rocket was also pointed northward at launch, plus light weathercocking, and a MUCH higher flight than expected. It tells me that I underestimated a number of things, and especially the Prowler's coefficient of friction. In order to get an altitude of 1455 ft or higher, the Cd would need to be closer to 0.35 than the 0.75 I had expected. Evidently, the Estes E2X Pro2 series have pretty smooth finishes out of the box.

Anyway, videos will follow as soon as I process them... Any feedback or thoughts? It was a fun morning, but I was not expecting the second flight to be 50% higher than predicted!!

What "rocket calculator" are you using? I would expect a 10 oz rocket on a F27 to go much higher than 1100ft. I recently launched an 14.8 oz Estes Ascender, less UBT, weight = 14.8 oz, to 1436 ft with a F26-6FJ.

I'm using an iPhone/iPad app of the same name. The F26 has a burn time of half a second longer than the F27, so I'd expect it to go quite a bit higher. It was a miscalculation on my part, particularly around the coefficient of drag. My Maxi Alpha 3 is pretty draggy, certainly closer to the 0.75 default that this app uses. If it made 1,000ft, I'd be surprised.

The calculations are comparable to what've seen on Thrustcurve. The F27 is a Redline Econojet, so just under 50N total impulse, while the F26 is 70N. If I compensate for a drag coefficient on the Prowler to 0.35, which approximately matches the altitude I measured this morning, the F26 would put it at just shy of 2000ft.

My learning moment is how much a difference the coefficient of drag makes, and how low it must be on the prowler, which is an E2X kit.

Hopefully I'll have the videos up soon.

Here are the videos.

As you can see a very nice day for a launch. Watch these at 720p or 1080p if you can.


Maxi Alpha 3:
[video=youtube_share;Ba4U1hpaG-c]https://youtu.be/Ba4U1hpaG-c[/video]


Prowler:
[video=youtube_share;ykrs90n3zxQ]https://youtu.be/ykrs90n3zxQ[/video]

WRasp32 came up with about the same numbers you did.

With WRasp32, you can put the rocket design and finish into Aerolab and output a drag profile file that can be imported into WRasp32. I have found this gives you a more accurate simulation. It also lets you do good sims past the 80% Mach speed where you would get a warning without the imported drag file.

Did WRasp32 come up with the same initial numbers that I did, at least given my overestimating the coefficient of friction?

I really should do some more experiments with the Prowler, but using some E-strength BP motors and see how they pan out. The F27 launch gave me a bit of a scare, because the F20 that I have is more powerful. I also have an F23-7FJ, which doesn't have the same total impulse as either the F20 or F27...

Those Econojet motors will give you a nice flight under 1,000 feet on the PSII builder kits, like the Leviathan. But on the skinnier, lighter PSII E2X kits, the flights will be much higher. Maybe try an E16.

The Prowler's very first launch was on an E16-6. I finally posted videos of the Prowler's three flights on the original build thread from last year. The E16 will definitely keep it under 1000ft. Assuming a Cd of 0.35, the Prowler should reach a height of about 860ft.

I should put that to the test, and I may even do it in the same field... Now to wait for another calm day.