Supersonic Model Rocket Fin Alignment Questions

[OzHybrid]

It is properly referred to as “matte”, not “matt”. It is a non-glossy finish, not appreciably different than a flat finish.

You can either use flat colors, or when you select a clear coat, choose one which is matte (as opposed to gloss or semi-gloss).

Depends which country you're in. Matt is correct in Europe and UK. Matte is more common in the Americas. As diyaerospace has not said which country they're in, lets not force American english on them yet.
The paint finish also known as a flat finish is used by the military for their camouflage painting. Light reflections from military things are nor encouraged.....

 

[Alan15578]

That may be the best insult I've ever heard. Up there with calling my mother a hamster.

Surely not. The craftsmanship shown is probably better than average, however the the craftsmanship shown in pictures posted on TRF is exceptionally high.
I don't know what hamster means in this context, but there is nothing derogatory about being an amateur radio buff.

 

[Alan15578]

Thanks for all the suggestions!
The fins are 1/8 balsa with paper skins. I was confident that this would be good enough because the apogee aspire another supersonic model rocket uses 1/11 balsa with paper skins. However my design will experience 9 more gees of acceleration than the aspire. I added epoxy clay fillets and the fins are much stronger than any I have build before.
I don't think the fin flutter calculator you posted has the option for balsa wood.
The stability margin on that flight was 1.73. And yes the fins were very small compared to the rocket body. I did fly the same deign with an E30T motor and it archived 660 miles an hour which is transonic. The flight did have a tiny wiggle but other than that it was perfect.
Also would it help if I uploaded my rocksim design here?
Slowed down video of it's flight

The photo helps. You should still have fins with a thin sharp leading edge, and perhaps some sweep as well. However, I think the problem is mostly in the body tube.

 

[diyaerospace]

I have been hearing a lot of people mentioning air foiling and rounding fins. Rocksim tells me that this will only increase the speed by 1 mile an hour at most. Personally I don't find air foiling the fins worth the extra mile an hour. Is there some other reason that air foiling or rounding the fins is helpful?

 

[watheyak]

I have been hearing a lot of people mentioning air foiling and rounding fins. Rocksim tells me that this will only increase the speed by 1 mile an hour at most. Personally I don't find air foiling the fins worth the extra mile an hour. Is there some other reason that air foiling or rounding the fins is helpful?

I wouldn't trust anything Rocksim says for anything supersonic. It's for model rockets.

Even though OpenRocket gives you a warning about supersonic speeds it's much more accurate for higher and faster flights. . But RASAero is the gold standard for high performance rockets.

 

[waltr]

The closer to Mach velocity the great the effect of fin drag (and total drag). Therefore giving the leading and trailing edges a bevel does help to get past Mach 1. Re-read the Aspire super-sonic build doc, first thing is sanding leading and trailing edges.
Did a Sim of the Aspire in Open Rocket with a G77R motor- Max velocity with square fins is 347m/s and 374m/s with Airfoiled fins. This is a pretty big difference and could make the difference between sub-sonic and super-sonic flight.
If the top velocity is much lower then airfoil fins do not make much difference.

A stability margin of 1.7 Cal is probably not enough if going over Mach 1. The Aspire's is 2.6 Calibers of margin. I would go with 2.5 to 3 calibers.

 

[diyaerospace]

You mention that rocksim is not as accurate as open rocket when the simulated rockets go mach. After looking online I have heard conflicting sources on the topic, what are your thoughts?
I have also noticed some of my epoxy clay fillets peeling off when I was aggressively sanding. I coated the fillets in CA to try and minimize this. Do you think this will be a problem at 750 miles an hour? And how would you suggest fixing and or minimizing this in the future?

 

[smstachwick]

You mention that rocksim is not as accurate as open rocket when the simulated rockets go mach. After looking online I have heard conflicting sources on the topic, what are your thoughts?

RockSim’s method is similar to the simplest of the equations derived by James Barrowman back in the 1960s, the ones most rocketeers are familiar with and can learn to do by hand with a little bit of practice and maybe somebody to walk them through it. RockSim just adds a few features from Barrowman’s more complex equations to make the software more versatile when working with unusual shapes that are difficult for the simplified Barrowman equations to deal with accurately.

But the equations are most applicable to flight below the transonic regime. Ergo, so are the numbers spit out by RockSim.

I can’t attest to the accuracy of the results predicted by other simulators but I’m almost certain that they’d be more accurate for anything flying Mach 0.8 or faster.

 

[Alan15578]

I have been hearing a lot of people mentioning air foiling and rounding fins. Rocksim tells me that this will only increase the speed by 1 mile an hour at most. Personally I don't find air foiling the fins worth the extra mile an hour. Is there some other reason that air foiling or rounding the fins is helpful?

That may be true, but using an appropriate airfoil may also reduce mass. Your initial question was about the fins and stability. Your fins will likely have a detached bow shock, and that may result in a worse and less predictable Cn-alpha of the fins, which are marginal to begin with. There is relationship between speed, static stability, and body stiffness required. But I'm betting a sense of the sort of person you are, and I would suggest that you "just do it" and find what works by trial and error.

 

[tsmith1315]

@diyaerospace, I think these are pretty good questions given your age.

Airfoiling balsa doesn't take long, and helps you get a feel for sanding and shaping it. The thicker your fins are, the more influence it will have. How much influence? Well, build two identical rockets except for airfoils and test it! You could even build one rocket with two interchangeable fin cans...