8" Fincan

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cryoscum
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8" Fincan

Postby cryoscum » Tue Jun 21, 2016 6:10 pm

Hi All

I've been asked to make a fincan for some of our American friends for use on a large rocket early next year. From what I understand it'll fly on a P7,500-ish (4 grain 8" motor). The fincan design and manufacture they left pretty much up to me with the only criteria that it had to be strong, durable and do the trick for the purpose.

I have yet to start the actual manufacture and I'll keep you posted on the progress, but I have done the initial analysis and design. I thought I would share as it may be of interest to some of you, so here goes...

They gave me some dims and weights for the airframe (mostly existing) and I did up a basic sim to work with. The motor file I created from the BurnSim file they had for the specific motor. It's not perfect, but close enough and they could weigh the motor hardware.
1.jpg


They didn't want to change the airframe design much, so the basic numbers came down to about 30k' and Mach 1.6. This is fine by me and I started by looking at the fin shape.

-Design Speed is approx. Mach 1.6, so calculate the optimal leading edge angle, i.e. what is the Mach Angle for Mach 1.6?
2.jpg
2.jpg (31.09 KiB) Viewed 3024 times

- Optimal leading edge angle is 38.7 deg or less (exact is best, but a lower angle number is also OK, just not as efficient), so let’s use an angle of 35 deg. It’ll give you a bit of tolerance.
- The Trailing edge angle is simply a function of how much stability you need for the rocket and how worried you are about flutter (more about this later). Let’s just use an angle of 75 deg swept forward for now, given that it is somewhat out of the way when the rocket lands on the motor end. Something like this:
3.jpg
3.jpg (21.95 KiB) Viewed 3024 times

- Air foiled edges and construction:
4.jpg
4.jpg (33.54 KiB) Viewed 3024 times


Let’s look what this does with the sim:

Stability margin is high, but clearly OK
5.png


CP shift at Max V is not a problem:
6.png


And overall flight profile is like this:
7.png

About 30k’ AGL and Mach 1.6

So looking at the fin’s structural design, using some numbers for my specific composite material:
8.png
8.png (59.57 KiB) Viewed 3024 times

The design is fine in terms of flutter and the divergence speed.

And it also looks like it could handle fairly extreme angles of attack without any issue, primarily due to the high sectional modulus of the fin itself, i.e. the fact that it is as thick as it is (even though the real structure is only in the outer skins):
9.png
9.png (34.18 KiB) Viewed 3024 times


So from the above you can see that the fin can will be:

- Larger fin area than required
- Provide more stability than required
- Is stronger than required
- Will cost more than required.
- None of the above matters, as they want it overkill :)

More to follow...
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Total impulse for 2016: 32,458 Ns (thus far)
Total impulse for 2015: 84,231 Ns
Total impulse for 2014: 40,757 Ns
Total impulse for 2013: 62,927 Ns

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Re: 8" Fincan

Postby Viking » Tue Jun 21, 2016 6:28 pm

Veeeery interesting!
Thanks so much for sharing, I might have to pick your brain about fin angles sometime in future.
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Re: 8" Fincan

Postby SpaceManMat » Tue Jun 21, 2016 8:20 pm

Sounds like a great project, interested to see where this leads.
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Re: 8" Fincan

Postby CATO » Tue Jun 21, 2016 8:26 pm

Subscribed... 8)

Just awesome Nic, there is always something new to learn on this forum, thanks for sharing.
"In thrust we trust"

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TRA 07459 L3

Impulse:
2019: 7,898 Ns (54% M)
Ns 18: 14,767; Ns 17: 5,973; 16: 34,558; 15: 35,955; 14: 6,016; 13: 10,208
PB - Gorilla N2717WC, H: 10,260', S: M1.14

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Re: 8" Fincan

Postby Sumo310 » Tue Jun 21, 2016 9:14 pm

Where did 2.jpg and the optimal LE angle of 38.7 degrees come from? The finshape is quite similar to the fins on the rocket what David G. and I kicked your butt with at WWW years ago, a shape Im planning to duplicate on my next rocket.
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Re: 8" Fincan

Postby vance2loud » Tue Jun 21, 2016 10:18 pm

If I am reading it correctly,
The distance along the horizontal is the distance covered at M1.6 with each point representing a single second.
Then at each point a circle with a radius of the speed of sound multiplied by the seconds.
A line from the 0,0 point should then intersect the side of each of the circles.
The Angle is the then calculated between that line and the horizontal.

Cryoscum, Can you share the formula for working out that angle? (It's been a long time since I did any trigonometry)(It is Trigonometry isn't it?)

I would love to learn more about the AeroFinSim program as that went over my head a bit, do you have suggestions on where to start?
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Re: 8" Fincan

Postby cryoscum » Tue Jun 21, 2016 10:43 pm

Sumo310 wrote:Where did 2.jpg and the optimal LE angle of 38.7 degrees come from? The finshape is quite similar to the fins on the rocket what David G. and I kicked your butt with at WWW years ago, a shape Im planning to duplicate on my next rocket.


The image comes from my CAD file and the concept comes from a NACA white paper I read some time ago... No doubt it could have some corrolation with successful flights in the past? Happy to share again, if I can find the doc again...

I too have not forgotten that fateful day a few years ago and am still plotting the ultimate revenge, you scoundrel!! Doing Mach 2.64 on a J was pretty decent though... :)
AMRS L3 | NAR L3 | QRS 089 | MDRA 224
AMRS Technical Advisory Group

Total impulse for 2016: 32,458 Ns (thus far)
Total impulse for 2015: 84,231 Ns
Total impulse for 2014: 40,757 Ns
Total impulse for 2013: 62,927 Ns

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Re: 8" Fincan

Postby cryoscum » Tue Jun 21, 2016 10:56 pm

vance2loud wrote:If I am reading it correctly,
The distance along the horizontal is the distance covered at M1.6 with each point representing a single second.
Then at each point a circle with a radius of the speed of sound multiplied by the seconds.
A line from the 0,0 point should then intersect the side of each of the circles.
The Angle is the then calculated between that line and the horizontal.


Spot-on, Vance. Simple geometry, not even trig.
As for FinSim, that is harder to explain. I might do an AMRS talk on it at some point soon, as I have gotten a fair few questions on it recently (noting that I am no expert in it, or rocketry in general for that matter!)
AMRS L3 | NAR L3 | QRS 089 | MDRA 224
AMRS Technical Advisory Group

Total impulse for 2016: 32,458 Ns (thus far)
Total impulse for 2015: 84,231 Ns
Total impulse for 2014: 40,757 Ns
Total impulse for 2013: 62,927 Ns

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Re: 8" Fincan

Postby martymonsta » Tue Jun 21, 2016 11:12 pm

vance2loud wrote: Can you share the formula for working out that angle? (It's been a long time since I did any trigonometry)(It is Trigonometry isn't it?)


As Nic said it is geometry however you can use Trig to work it out. For a quick look at the info I understand the equation to be,

Angle = Sin-1 (1/x)

Where x is Max V in Mach.

Looking good Nic. Subscribed to this one.
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Re: 8" Fincan

Postby cryoscum » Tue Jun 21, 2016 11:21 pm

100% Marty! Gets you the exact right number too. Simply a matter of personal preference how you get there...
AMRS L3 | NAR L3 | QRS 089 | MDRA 224
AMRS Technical Advisory Group

Total impulse for 2016: 32,458 Ns (thus far)
Total impulse for 2015: 84,231 Ns
Total impulse for 2014: 40,757 Ns
Total impulse for 2013: 62,927 Ns

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Re: 8" Fincan

Postby vance2loud » Tue Jun 21, 2016 11:27 pm

cryoscum wrote:Spot-on, Vance. Simple geometry, not even trig.
As for FinSim, that is harder to explain. I might do an AMRS talk on it at some point soon, as I have gotten a fair few questions on it recently (noting that I am no expert in it, or rocketry in general for that matter!)


Now that you say that I realize that the intersection on the circles creates a right angle making it a lot easier to figure out,
This thread is certainly helping me and I would think others understand a bit more about optimal fin design, You might have to share a video of the talk as I doubt I could make it but would definitely be interested.
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Re: 8" Fincan

Postby Scoop1261 » Wed Jun 22, 2016 8:46 am

vance2loud wrote: You might have to share a video of the talk as I doubt I could make it but would definitely be interested.



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Re: 8" Fincan

Postby CATO » Wed Jun 22, 2016 3:05 pm

cryoscum wrote:100% Marty! Gets you the exact right number too. Simply a matter of personal preference how you get there...


He's a smart lad that Marty...









































    Speed in MM x 1/xASINDegrees(s)
    111.57079632790
    1.10.9090909091.14109666165.38002267
    1.20.8333333330.98511078356.44269024
    1.30.7692307690.87763641950.28486277
    1.40.7142857140.79560295345.5846914
    1.50.6666666670.72972765641.8103149
    1.60.6250.67513153338.68218745
    1.70.5882352940.62887492536.03187907
    1.80.5555555560.5890309733.7489886
    1.90.5263157890.55426183431.75686386
    20.50.52359877630
    2.10.4761904760.49631736228.43689015
    2.20.4545454550.47186183727.03569179
    2.30.4347826090.4497968625.77146174
    2.40.4166666670.42977543124.62431835
    2.50.40.41151684623.57817848
    2.60.3846153850.3947911222.61986495
    2.70.370370370.37940771521.73846079
    2.80.3571428570.36520722120.92483243
    2.90.3448275860.35205509920.17127135
    30.3333333330.33983690919.47122063
    3.10.3225806450.32845461818.81906337
    3.20.31250.31782370418.20995686
    3.30.3030303030.30787086817.63970139
    3.40.2941176470.29853220117.10463518
    3.50.2857142860.28975170116.6015496
    3.60.2777777780.28148007316.12762021
    3.70.270270270.27367373815.68035013
    3.80.2631578950.26629401715.25752329
    3.90.2564102560.25930645614.85716554
    40.250.25268025514.47751219
    4.10.2439024390.24638779114.11698057
    4.20.2380952380.24040421713.774147
    4.30.232558140.23470711413.44772704
    4.40.2272727270.22927620313.13655879
    4.50.2222222220.22409309212.83958841
    4.60.2173913040.21914105912.5558578
    4.70.2127659570.21440486512.28449386
    4.80.2083333330.20987059212.02469918
    4.90.2040816330.20552550411.77574399
    50.20.20135792111.53695903

"In thrust we trust"

AMRS 21 L3 RSO
TRA 07459 L3

Impulse:
2019: 7,898 Ns (54% M)
Ns 18: 14,767; Ns 17: 5,973; 16: 34,558; 15: 35,955; 14: 6,016; 13: 10,208
PB - Gorilla N2717WC, H: 10,260', S: M1.14

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Re: 8" Fincan

Postby OverTheTop » Wed Jun 22, 2016 3:24 pm

If you want to read some really good stuff on shock waves (or aerodynamics in general) check out the book I recommended here:
viewtopic.php?f=52&t=5432

There is a hint there on how to find a free PDF of it.

Even if only some of it sticks, it is well worth the read. Alternatively, read it from front to back for a good understanding of aerodynamics from the ground up (pun intended). Very well written.

It is amazing what happens in a little isentropic layer a few microns thick :)
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Re: 8" Fincan

Postby drew » Wed Jun 22, 2016 4:46 pm

Now there's a useful table! Cheers for the share CATO.
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Max Alt AGL - 26,850ft - L935 - THUNDA 2019
Max V - 3,004 ft/s, ~Mach 2.67 - L935 - THUNDA 2019


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