Rocket Ship 27: An L3 Build Thread

[JLebow]

Packing up at the launch site still. Pictures, gps tracks, and hopefully video to come. I had a successful L3 cert flight this morning. The altitude was closer to 13,500 feet. The fins are more draggy than the sims. My first attempt on Friday had the main pop at apogee. Rocket was still in perfect shape. Acquired another motor from the onsite vendor trailer, built it and adjusted my apogee deploy charge. Today's flight was about perfect. Landed about 0.5 miles from the pad. I really want to give my thanks to my TAPS. Mentors like Joe Bevier and Jim Wilkerson are who make this a fun and rewarding hobby. Thank you.

 

[kbRocket]

Beautiful flight Joey. You earned it. Thanks for sharing your experience by posting all of the details. Here's a movie of the results.

 

[OverTheTop]

Congratulations on your L3 certification .

 

[Nytrunner]

Joe's a good guy, congratulations!

 

[JLebow]

Beautiful flight Joey. You earned it. Thanks for sharing your experience by posting all of the details. Here's a movie of the results.

Fantastic video. Thanks for posting it.

 

[JLebow]

Joe's a good guy, congratulations!

Both my TAPS were awesome. Generous with their time reviewing my plans, offering suggestions, and most important - how to develop a safe and capable design. The process was an incredible experience.

 

[JLebow]

This series of photographs was taken by the Oregon Rocketry President, Gary Goncher. Thanks Gary, they look great.

 

[JLebow]

It is clear that the fat fins were not correctly modeled. I used the average fin thickness of 0.75 inches, which should capture the cross sectional area just fine, but the exaggerated facets of the fins seemed to have created even more drag. I didn't expect the model to be exact, but I also didn't expect the Cd to be greater than the prediction of 0.67 by 75%.

This picture shows the concept of dual deploy. The up is very nearly identical. Guess which GPS track was from the flight with the main open at the top. 4.6 miles drift vs. 0.5 miles.


And finally, the logger data plots:

It looks like the drogue could be a little smaller. 85 ft/sec is a good target.

Stats:

 

[Nytrunner]

What equivalent fin thickness gives you a closer altitude result?

And have you ruled out other factors like surface roughness, etc?

 

[JLebow]

What equivalent fin thickness gives you a closer altitude result?

And have you ruled out other factors like surface roughness, etc?

That is an excellent question. The rocket was finished with 400 grit and a clear coat. Everything gets coated in a fine layer of volcanic pumice out in Brothers, Oregon. I selected camouflage paint (a 5 of out 8 with 1 being smooth) in RASAero II, all turbulent flow, and Rogers modified Barrowman as original sim options.

To Match the sim (within 100 feet), I increased the fin thickness to 1.5 in and changed surface finish to the max setting of rough cast iron. If changing just fin thickness, the sim restricts max fin thickness to 2.25" and the sim still comes in high at 14,660 ft. If changing just the surface finish to rough cast iron, the sim is 15,500 ft.

Usually my open rocket sims for sub sonic flights come in within 15%, and RASAero II has been spooky close at times for some flights, but is good to within 10%. This is over the course of 40 high power flights. This fin design strays pretty far from anything I expect would have been wind tunnel tested.

 

[Nytrunner]

Fascinating.
I guess something about the fin geometry (although super cool, unique, and impressive to fabricate) may not behave well when it comes to high speed drag

 

[JLebow]

Fascinating.
I guess something about the fin geometry (although super cool, unique, and impressive to fabricate) may not behave well when it comes to high speed drag

I'm wondering if the prediction would be closer on a larger diameter rocket. It is possible that because the fins are very close together on this 3" diameter airframe that they interact and behave as if they are larger. This is beyond my area of knowledge. Notice how the distance between fins reduces along the fin root. This would be less dramatic on a larger diameter rocket. If it is fin geometry, I'm guessing that is close fin spacing. The wetted out difference between my fins and flat (g10) fins is only 2.2%.

 

[OverTheTop]

There is a thing in aerodynamics called the area rule. The design of the fins as it stands is directly against that particular rule, with the thickest base also at the location of the most fin area. Around Mach that will pay a penalty.
https://en.wikipedia.org/wiki/Area_rule
Better explanation here:
https://www.aerospaceweb.org/question/aerodynamics/q0104.shtml
I don't have a feel for how much it would affect your flight, but it will.

 

[boatgeek]

I'm guessing a bit, but the aft end of the fins are blunt enough that you might have gotten flow separation there rather than a clean exit. That would add quite a bit to the drag.

But all's well that ends well. Congrats on the cert!

 

[JLebow]

There is a thing in aerodynamics called the area rule. The design of the fins as it stands is directly against that particular rule, with the thickest base also at the location of the most fin area. Around Mach that will pay a penalty.
https://en.wikipedia.org/wiki/Area_rule
Better explanation here:
https://www.aerospaceweb.org/question/aerodynamics/q0104.shtml
I don't have a feel for how much it would affect your flight, but it will.

The sim does have enough information to compute this cross section area at the largest point. Another interesting data point. My friend certed on a similar rocket last year- same mass to within 0.1 lbs, same motor, same length, diameter, same launch tower, but it was a Wildman intimidator. It simmed to 24,000 and flew somewhere around 23,500 ft. The only practical difference is the fin style and 600 ft pad elevation difference between Brothers, Oregon and the Black Rock Desert. I'm quite sure it is the fins.

I'm guessing a bit, but the aft end of the fins are blunt enough that you might have gotten flow separation there rather than a clean exit. That would add quite a bit to the drag.

But all's well that ends well. Congrats on the cert!

The length of the fin is 12.25". The widest part of the fin (sweep length) is 10". The the aft of the fins have an 18.4 degree half angle (relative to the centerline). For supersonic boat tails, I hear that 8 degrees is a good target to avoid separation - but in reality I'm sure the fluid dynamics are quite complex and rule of thumbs are just to keep you sane. The important thing was I knew the fin shape was not optimal going in. I just liked the way it looked, and it was up to the job of getting a cert.

Reviewing the flight is nearly as much fun for me as the actual flying. Thanks for your comments and ideas.

 

[Nytrunner]

Hey, you had a fantastic build and flight, successful cert, and dome extra learnings in rocket science along the way. Sounds like a win on all counts.

I'm sure the fluid dynamics are quite complex and rule of thumbs are just to keep you sane.

You're not wrong

 

[OverTheTop]

Yep. Nice job.

 

[Handeman]

Hey, you had a fantastic build and flight, successful cert, and done extra learning in rocket science along the way. Sounds like a win on all counts.

Totally agree! Great job and congrats. I'm sure you'll have many more flights on this rocket and have the fin dynamics down pat.