Supersonic Model Rocket Fin Alignment Questions

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diyaerospace

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Hi,
I recently launched a Mach 1 24mm rocket on an F44W Aerotech motor. I shredded on ascent and I found the cause of this to be bad fin alignment which caused high speed oscillations that caused the rocket to turn 90 degrees at the speed of sound which shredded the body tube.
I am now working on a new rocket with the same intention of launching it to the speed of sound. My main questions is how good is good enough for supersonic model rockets in terms of fin alignment? I found a fin alignment jig on the internet and printed it out on my 3d printer. The finished print does allow the fins to wiggle a little and I am wondering if this will be a problem at Mach? My previous attempt only had a tiny bit of misalignment that I could detect and I think another potential culprit was asymmetry in the fillets of my model rocket. Do you think the fillets could have caused the previous failure, and will slightly misaligned fins be okay for supersonic flight?
Thanks for you help on this

Images of previous failed model
IMG_1100.jpg
IMG_1101.jpg
IMG_1102.jpg
IMG_1103.jpg
 

smstachwick

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Hmmmm….how did you come to the conclusion that it was an inaccurate fin alignment? There are a lot of factors that can cause a rocket to shred at that speed.
 

diyaerospace

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Based on slowed down footage I could see the rocket occulting before it turned 90 degrees, I think a 90 degree turn at supersonic speeds would shred a cardboard airframe. If you have any other suggestions on the source of the oscillations I would be happy to hear them! My conclusion was mainly that I couldn't see any other reason for the oscillations other than bad fin alignment and asymmetrical fillets.
 

smstachwick

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Having absolutely no familiarity with your design, I’d also suggest looking into the rocket’s dynamic stability. Not too long ago I was running a 29mm Estes Pro Series II Star Orbiter (the one I have as my avatar) through RockSim 10 with some hideously overpowered motors like the RATT I90 hybrid, just for ***** and giggles.

The heavy weight and small fins combined to produce dynamic instability, despite the rocket being statically stable. Basically, the rocket was unable to recover from an upset because the small fins didn’t produce enough restoring force to overcome the greater inertia, even though they put CP safely aft of CG. Most simulations in these configurations had the rocket coming off the launcher alright but then tumbling violently at high speed during boosted flight, certainly with enough energy to self-destruct.

That result sounds a bit like what happened here, but without having any details on your rocket’s design, I can’t positively diagnose that as the problem. That F44 isn’t as heavy as an I90 with the whole oxidizer tank assembly but I’d call it plausible enough to at least look into.
 

Mike Haberer

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I doubt it was fin alignment, they don't look that off. Use this spreadsheet to calculate fin flutter. It's very conservative (more conservative than FinSim). If this says the fins will shred at your clocked speed, then it's your material, not the alignment. In that case you would need either thicker fins, a stronger material (FG or glassed plywood) or a different fin shape (fin shape MATTERS!), or some combination. Fin design for Mach+ is an engineering challenge and an art form...
 

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Alan15578

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I think your fins are too blunt, and your rocket is too ugly to post pictures of here. Your fins and fillets seem stout enough, However, fin flutter may be because of root bending due to tube flexure. Make sure the motor is fitted tightly in the tube under the fin area.
 

waltr

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My thoughts:

I would go with slotted BT and TTW fins. The slots can be cut very accurate and ensure longitudinal alignment of fins to BT.
Get better at making fillets. Go to the Badass web site and watch the video on doing fillets using a Fondant tool.

Stability:
When Mach 1 speed is approached the CP moves forward. If the stability Cal is too small then the rocket becomes unstable as the CP goes in front of the CG. This may be the reason your rocket made a sharp turn. The turn then caused higher drag which slowed the rocket and the CP moved rearward and stability returned.

Use Open Rocket to simulate stabilty at speed. Show the 'cal factor' in the plot to check.
 

Back_at_it

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Hi,
I recently launched a Mach 1 24mm rocket on an F44W Aerotech motor. I shredded on ascent and I found the cause of this to be bad fin alignment which caused high speed oscillations that caused the rocket to turn 90 degrees at the speed of sound which shredded the body tube.
I am now working on a new rocket with the same intention of launching it to the speed of sound. My main questions is how good is good enough for supersonic model rockets in terms of fin alignment? I found a fin alignment jig on the internet and printed it out on my 3d printer. The finished print does allow the fins to wiggle a little and I am wondering if this will be a problem at Mach? My previous attempt only had a tiny bit of misalignment that I could detect and I think another potential culprit was asymmetry in the fillets of my model rocket. Do you think the fillets could have caused the previous failure, and will slightly misaligned fins be okay for supersonic flight?
Thanks for you help on this

Images of previous failed model
View attachment 482788View attachment 482789View attachment 482790View attachment 482791
Your fins appear to be more than straight enough. Oscillation doesn't generally occur from misaligned fins unless they are grossly off. Oscillation is generally a result of flexing materials like fins, tubes or general instability as speed increases.

One observation. You mentioned that the rocket turned 90 degrees after oscillations. Generally sharp turns during flight indicates a structural failure of some component. Assuming the body tube didn't collapse, my guess is the sharp turn was the result, not the cause of the fins shredding.

As for the oscillations. These could be caused by a few things. The body tube could have been deforming under load. I've seen some rockets like the the Mean Machine and Equinox actually bend during flight on strong motors. A stiffer body tube would correct this. Another very possible cause is the fin design. They seem to be very small. While this might help with performance, it doesn't help with stability and once the oscillation begins the loads on the rocket begin tearing things apart. You want a nice stable flight.

Another thing you need to consider is the surface finish. The body tube needs to be smooth and the fins should be rounded or airfoiled. Air flowing over the rough surface is only going to slow you down.

If you really want to go supersonic cheaply, there is an easy way to do it but good luck getting it back. I did something like this but we never got it back as it landed in a pond. Mine was a BT50H heavy wall tube 10" long. Fins were trapezoid made from 3/32" hobby plywood and papered for additional strength. Nose Cone was an off the shelf Alpha piece from Estes. Added a motor block, shock cord and streamer. Rocket required .25oz of nose weight to be stable. I still have the SIM file somewhere if you want it.
 

watheyak

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The thickness of the fins relative to their size makes them more like bumps rather than fins, especially at high speeds. And the massive, chunky fillets are reducing the effective area of an already way undersized fin.

I disagree with Back at It. I think the 90 sudden turn was due to the "fins" suddenly becoming completely ineffective when it went transonic. The resulting high angle of attack increased the load to the point that structural failure occurred.
 

diyaerospace

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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
 

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smstachwick

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I can only assume that the higher Mach numbers made it statically unstable. 1.73 seems to be adequate for subsonic flight but may not be adequate for following in the footsteps of Chuck Yeager.

A shame that the failure didn't occur while the rocket was in frame. I'm afraid the video isn't likely to be a viable diagnostic tool.
 

diyaerospace

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Yes, I need to work on my tracking video!
Do you know what a safe stability margin would be for supersonic flight?
 

boatgeek

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Yes, I need to work on my tracking video!
Do you know what a safe stability margin would be for supersonic flight?
Most people recommend (a) 2 calibers of static stability and (b) fin span (fin tip to body tube) at least 1 caliber so that the fins are in clear air and not in the boundary layer.
 

diyaerospace

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Thanks for the suggestions!
I will change my design to have 2 or more stability calipers and make the fins larger!
I had another question about electronics to measure speed and altitude for supersonic flights. When looking at commercially available altimeters I don't think any of them will be able to measure data with my rockets flight profile. Many of them use barometric sensors to measure altitude, this will not work for supersonic flights because as you go transonic then supersonic the air build up causes inaccurate air pressure readings. So the only option is a 3 axis gyro and accelerometer to calculate speed and altitude. The problem with this is that all of the altimeters available only read accurate values up to 24 to 40 gees of acceleration. Unfortunately my rocket will experience 70 gees which will render any accelerometer reading invalid. Do you know a way to bypass this problem or other altimeters with high gee sensors?
Thanks.
 

MJW

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There are GPS based solutions but at those velocities and altitude you're looking at export controlled parts that are hard to get your hands on and presumably very expensive. It might be cool to do it with downward pointing laser range finder. Probably not practical either though.
 

watheyak

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I've run into this
Thanks for the suggestions!
I will change my design to have 2 or more stability calipers and make the fins larger!
I had another question about electronics to measure speed and altitude for supersonic flights. When looking at commercially available altimeters I don't think any of them will be able to measure data with my rockets flight profile. Many of them use barometric sensors to measure altitude, this will not work for supersonic flights because as you go transonic then supersonic the air build up causes inaccurate air pressure readings. So the only option is a 3 axis gyro and accelerometer to calculate speed and altitude. The problem with this is that all of the altimeters available only read accurate values up to 24 to 40 gees of acceleration. Unfortunately my rocket will experience 70 gees which will render any accelerometer reading invalid. Do you know a way to bypass this problem or other altimeters with high gee sensors?
Thanks.
All of the commercially available Altimeters work just fine past supersonic. Even the baro-only ones.

Most these days are combo baro and accelerometer. The last rocket I flew pulled 92 Gs and went M3.5 with a Featherweight Raven altimeter. Worked awesome. Always does.
 

MJW

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But do they log accurately in transonic and supersonic flight phases? They'll give you apogee height for sure.
 

MJW

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Yes. They do.
I didn't know that. I'd have thought the pressure wave at mach would create a negative error in altitude until going back to sub sonic. Do you need larger orifices to equalize pressure in the eBay in real-time at those assent rates?
 

watheyak

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I didn't know that. I'd have thought the pressure wave at mach would create a negative error in altitude until going back to sub sonic. Do you need larger orifices to equalize pressure in the eBay in real-time at those assent rates?
I'm assuming it's due to a Kalman filter or something similar.

Baro Altimeters used to be Mach immune because of simple "Mach lock-out" but I think things have become a little more sophisticated since then.

The point being is that it's rare that a high power rocket doesn't break the sound barrier, and most Altimeters and flight controllers made in the last 20 years handle supersonic flights without breaking a sweat.
 

GrouchoDuke

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Supersonic balsa and paper? Did you do any fin flutter calcs on that? Also, those fins look like their mean chord line is swept forward - generally that's not a great thing for a structure going fast.
 

diyaerospace

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I am still working on the fin flutter calcs but had another question in the meantime.
Rocksim gives me the option on each part I place it to have a gloss or polished finish. After looking at many other forums and youtube videos I was unable to find what the difference between the two is and how to get each finish respectively. Does anyone know anything about the difference between gloss and polished finish?
Thanks.
 

boatgeek

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But do they log accurately in transonic and supersonic flight phases? They'll give you apogee height for sure.
In my (somewhat limited) experience with supersonic flights, I've seen blips in the altitude data when the rocket went supersonic and later back subsonic. The blips were noticeable but easily ignored. If you wanted to get an accurate altitude in the blips, you could easily fit a curve through the data. If you're just trying to document how high/fast the rocket went, the data you'll get from any modern recording altimeter will be fine.

On the GPS side, my GPS units always kept lock through the supersonic phase. I've seen reports that GPS units sometimes lose lock, though that's more likely at higher speeds (say >M2). Regardless, you'll get data when the rocket slows down at apogee. On the way up, the GPS data is a little suspect anyway because the rocket is moving so fast. You'll probably want GPS tracking on these, since it'll be awfully hard to find an Aspire-like rocket that goes to several thousand feet.

I'm not sure if this has been mentioned before, but you'll also want to put some effort into airfoiling your fins, or at the bare minimum rounding the leading edges and tapering the trailing edges. That will make it more likely that you make it supersonic.

[Edit] Re: gloss vs. polished: Gloss is smooth glossy paint with minimal or no bumps and ripples. Polished requires a lot of time wet sanding through many grits and then polishing with rubbing compounds. If you aren't going to spend 8+ hours finishing the rocket, the best you'll get is gloss. Based on the pictures at the top, I'd actually go a step lower than that for a more accurate simulation.
 

Budro0

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I think your fins are too blunt, and your rocket is too ugly to post pictures of here.
That may be the best insult I've ever heard. Up there with calling my mother a hamster.
 

RocketScientistAustralia

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Having absolutely no familiarity with your design, I’d also suggest looking into the rocket’s dynamic stability. Not too long ago I was running a 29mm Estes Pro Series II Star Orbiter (the one I have as my avatar) through RockSim 10 with some hideously overpowered motors like the RATT I90 hybrid, just for ***** and giggles.

The heavy weight and small fins combined to produce dynamic instability, despite the rocket being statically stable. Basically, the rocket was unable to recover from an upset because the small fins didn’t produce enough restoring force to overcome the greater inertia, even though they put CP safely aft of CG. Most simulations in these configurations had the rocket coming off the launcher alright but then tumbling violently at high speed during boosted flight, certainly with enough energy to self-destruct.

That result sounds a bit like what happened here, but without having any details on your rocket’s design, I can’t positively diagnose that as the problem. That F44 isn’t as heavy as an I90 with the whole oxidizer tank assembly but I’d call it plausible enough to at least look into.
The RattWorks I 90 is a long motor. The front of it will undoubtedly be in front of your CG. So as your N2O tank empties, until it gets to the now moving CG point in emptiness, the CG will move backward making the CG CP situation worse. For hybrid simulation I create the motor with an empty tank and then add distributed static masses in Open Rocket. Then remove them to make sure my fins are big enough to give stability at all variations of tank emptiness. I don't think OR takes hybrids into account and the way they change CG as they empty. For solid burn motors the calculation is easy. It's just the change in mass as per the burn rate at what was the CG. It doesn't really change. For a solid motor, they generally start behind the CG and as they burn the back gets lighter so they become more stable. CG moves further forward of CP = Stable, mostly.
 

RocketScientistAustralia

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Hi,
I recently launched a Mach 1 24mm rocket on an F44W Aerotech motor. I shredded on ascent and I found the cause of this to be bad fin alignment which caused high speed oscillations that caused the rocket to turn 90 degrees at the speed of sound which shredded the body tube.
I am now working on a new rocket with the same intention of launching it to the speed of sound. My main questions is how good is good enough for supersonic model rockets in terms of fin alignment? I found a fin alignment jig on the internet and printed it out on my 3d printer. The finished print does allow the fins to wiggle a little and I am wondering if this will be a problem at Mach? My previous attempt only had a tiny bit of misalignment that I could detect and I think another potential culprit was asymmetry in the fillets of my model rocket. Do you think the fillets could have caused the previous failure, and will slightly misaligned fins be okay for supersonic flight?
Thanks for you help on this

Images of previous failed model
View attachment 482788View attachment 482789View attachment 482790View attachment 482791
From your photos, the first thing that stands out is the quality of build doesn't seem to me to meet what any RSO would consider to be reasonable for a rocket going into a Mach speed range. The fins are stubby and forward of the rear. This makes your CP further forward than it needs to be. My gut feeling is that as you approached mach your cp moved forward. Latest version of RAS Aero gives a CP change at mach of up to 2 calibres forward. Of course there are still some non believers out there who think your CP doesn't move and the earth is flat.
 

bjphoenix

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Based on slowed down footage
How did you get footage of any of the flight? Years ago I did essentially the same thing- I bought a kit the size of the Estes Alpha and put in an Aerotech F72, essentially a minimum diameter rocket. When the button was pushed on the launch controller the rocket didn't launch, it teleported into another dimension. One moment it was on the pad, the next moment there was no trace that it ever existed, no smoke trail or anything.
Back to your rocket- if you try this again I would suggest glassing the fins in place. Lay on a piece of fiberglass long enough to go from one fin tip down to the fin root, across the body tube, up the next fin to its fin tip. Do this for both sides of every fin.
 

diyaerospace

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Thanks to everyone that replied!
I am now working on a new rocket and will take on board some of recommendations you gave!
One more question I had is about a "Matt" finish. Rocksim gives me the option for "Matt" finish. I did research on how to get this surface finish and came back empty handed. Does anyone here know what a "Matt" finish is and how to get one?
Thanks a lot!
 

David_Stack

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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).
 
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