Poll: Speed or Altitude?
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Total 3 vote(s) 100%
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29mm Flying Case
#1
After last saturdays weather grounded my Frenzy XL, a friend and I got the genius (or incredibly stupid) idea of building a 29mm flying case. Now this all came about while talking about our Red Glare plans for this year and oddity of the CTI 29mm tapered closure. For those who don't know, the tapered closure doesn't have a thrust ring, making it flush with the case. After a few minutes, I decided to build a flying case with one.

But how to do it? Maybe I could glue a few fins to the case so I can keep a consistent diameter. Or better yet (or possibly the worst idea ever), weld a few really thin machined aluminum fins to the case. The case of course would be a 6XL. Go big or go home, right?

But I'm still trying to decide on a final design. The shape of the fins would be consistent with the rest of my MD builds. The general idea of "no empty space" would also be used. But should I stick with three fins, or go with four slightly smaller fins? The simulations are pretty close either way, so I may go with four to experiment.

The next question I have is should I build for speed or altitude? I've always been an altitude guy, and the thought of breaking 16,000' on an I motor (over 1,000' higher than the sims for MD-38 SkyPunch, a rocket specifically designed to go as high as possible on an I motor) entices me. But the opportunity to break Mach 2 with a 29mm motor is just way to good to pass up.

Here are the simulations as they stand right now:
Four fin design:
[Image: attachment.php?aid=1031]
-With optimal mass (designed for altitude):
     -I224 Classic:
           -Altitude: 16,279'
           -Speed: Mach 1.97
     -I243 White:
           -Altitude: 16,252'
           -Speed: Mach 1.95
-With minimum mass (designed for speed):
     -I224 Classic:
           -Altitude: 16,000'
           -Speed: Mach 2.39
     -I243 White:
           -Altitude: 16,027'
           -Speed: Mach 2.35

Three fin design:
[Image: attachment.php?aid=1032]
-With optimal mass:
      -I224 Classic:
            -Altitude: 16,577'
            -Speed: Mach 1.98
      -I243 White:
            -Altitude: 16,531'
            -Speed: Mach 1.96
-With minimum mass:
      -I224 Classic:
            -Altitude: 16,522'
            -Speed: Mach 2.2
      -I243 White:
            -Altitude: 16,502'
            -Speed: Mach 2.17


So as you can see with the sim results above, the four fin design, with minimal mass is ideal for max speed, topping out at Mach 2.39 (2,643 FPS, 1,802 MPH). However, the three fin design with optimal mass is ideal for max altitude, with an apogee of 16,577'.

Now for the explanation behind each design and why one is better for speed and the other for altitude:
Three fin: This design is better for altitude because there is less drag created by the fins. This allows the rocket to maintain its speed longer and achieve higher altitudes.
Four fin: This design is better for speed because it allows the rocket to stay stable even while removing mass form the nose of the rocket. And while the fourth fin does add mass, the gained stability allows you to remove more mass from the nose to offset the weight dramatically.

Disclaimer: Unless you know what your doing, and you are very experienced at welding aluminum, DO NOT EVER attempt to weld things onto your motor casings. Doing so will void the warranty of the casing and reload, and should you have a CATO, the motor/reload manufacturer will NOT refund you. This is also not a certified way to use your motor casing. If this is allowed to fly at all anywhere, it would be at a Tripoli research launch, or MDRA event. That being said, I can speak for neither organization and do not know if either will allow such a configuration at a launch event.


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NAR# 98194
Level 1: CTI I-216, 3,043'
Level 2: CTI K-740, 5,999'

Personal altitude record: 12,400' CTI L395
2014 total impulse: 9,018.2 Ns (76% M)
2015 total impulse: 7,171.7 Ns (40% M)
2016 total impulse: 18,664.2 Ns (91% N)
2017 total impulse: 8,281.1 Ns (80% M)
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#2
The biggest problem with welding the fins directly to the casing is that you lose the temper of the aluminum in the heat effected zone. If you're using 6061-T6, the weld and the area around the weld become 6061-T0. That'll drop the ultimate tensile strength of the aluminum from 45,000 psi to 18,000 psi. I've actually seen this cato some pretty large motors. If you know the motor pressure you can work around this strength loss with a really wide margin (I've considered it, but it's not worth the mass increase. I'd just JB weld some fins right to the case.

Alex
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#3
Not my cup of tea, but good luck!
John S.
NAR #96911
TRA #15253
MDRA
Level 1, 2014-Mar-15 -- Aerotech Sumo, H133BS
Level 2, 2014-Jun-21 -- Giant Leap Vertical Assault, J240RL
Level 3, 2016-03-12 -- MAC Performance Radial Flyer, M1101WH, 13,028 feet
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#4
(01-17-2016, 07:32 PM)Aksrockets Wrote: The biggest problem with welding the fins directly to the casing is that you lose the temper of the aluminum in the heat effected zone. If you're using 6061-T6, the weld and the area around the weld become 6061-T0. That'll drop the ultimate tensile strength of the aluminum from 45,000 psi to 18,000 psi. I've actually seen this cato some pretty large motors. If you know the motor pressure you can work around this strength loss with a really wide margin (I've considered it, but it's not worth the mass increase. I'd just JB weld some fins right to the case.

Alex

Thanks for the insight Alex. This definitely brings up a concern that I hadn't considered. I'll see if I can get the data from Jeroen. That would answer the whether or not it would work. If the answer is unsatisfactory to me, I'll probably use JB weld or other epoxies I have in my basement. But I was really trying to avoid that attachment method because of the extreme speeds it sims to.

(01-18-2016, 11:30 AM)Bat-mite Wrote: Not my cup of tea, but good luck!

Thanks. If I get this built I'll be flying it at red glare in April. Just try and keep your eye on it.

NAR# 98194
Level 1: CTI I-216, 3,043'
Level 2: CTI K-740, 5,999'

Personal altitude record: 12,400' CTI L395
2014 total impulse: 9,018.2 Ns (76% M)
2015 total impulse: 7,171.7 Ns (40% M)
2016 total impulse: 18,664.2 Ns (91% N)
2017 total impulse: 8,281.1 Ns (80% M)
Reply


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