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GAP

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Hello everyone,

I'd thought I would post my build thread here for my Level 3 certification rocket. This is a little bit different in that this is after the fact, but I want to post this here to help others and for advice and comments on what I could improve on for builds in the future. That said I successfully flew this rocket on 10/30/2021 and obtained my L3 certification.

I'll start from the beginning. I chose to buy a kit instead of custom because it was cheaper and quicker as I was running on a tight time schedule with the flying season in Minnesota running out. For this I turned to Wildman as I've built some of their kits before and I have a club membership. With initial simulations I was predicting a weight of around 25 lbs fully loaded which was reaching an altitude of 12k ft and going around mach 1.1 with an Aerotech M1350W DMS motor. I originally chose the DMS motor because it's easiest to assemble and use. However, after a couple of weeks I realized the M1350 would not arrive in time and chose a CTI M1101WH instead from my local vendor which leads to the exact same flight profile. With this information I ran the fin geometry through AeroFinSim and found that the fins were predicted to flutter off at a speed of around mach 0.75. After a couple weeks I realized it woudn't arrive in time and instead bought a CTI M1101WH through my local vendor which was the exact same flight profile. Thus, I knew tip-to-tip layups of the fins would be necessary. Of this I chose to do a traditional 1/3rd, 2/3rds/ and full tip-to-tip layer with 3k bidirectional carbon fiber. I put the first and third layers straight up and down with the middle layer titled 45 degrees to try and get as isotropic of a material as possible. Carbon fiber was used as stiffness was needed most which carbon fiber has a lot of.

I planned to use Rocketpoxy and JB Weld throughout the build for fillets and adhering things together, while Aeropoxy was used for the fin layups.

I started off by giving all the parts a soapy bath. Then I started by epoxy the centering rings and shock cord attachment to the MMT and the retainer as well. I also epoxied the bulkhead into the coupler for the nosecone. In hindsight I definitely should have put this bulkhead further in to give me more room in the main compartment.

I can't upload everything tonight, but will fill in this thread pretty quickly with the build.
 

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FredA

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Great to see college team members (leaders in this case) get their advanced certs!
Kudos

But I do question your fin-flutter analysis -- really doubt that kit needs anything special for an M.
 

GAP

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Great to see college team members (leaders in this case) get their advanced certs!
Kudos

But I do question your fin-flutter analysis -- really doubt that kit needs anything special for an M.

Keep in mind the Intimidator 4 has some big fins.
The inputs and results from FinSim are shown below. The steps outlined on the website for supersonic flight were used.
1636089746210.png

1636089833946.png

1636089919788.png
Now the divergence velocity is slightly below what I was expecting on this flight, but it is always good to have a healthy safety margin and I wanted to build the rocket in such a way that I could fly it on larger Ms later to the Mach 1.5-1.8 range. Granted the software could have been inaccurate, but not something I really wanted to test on my L3.

P.S. The UMN Rocket Team actually tested the results from AeroFinSim and we found that the fins actually broke off early than predicted. However, I did not run the simulations in that case and can not verify that they were correctly done. Probably something I should check on tbh....
 

FredA

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Keep in mind the Intimidator 4 has some big fins.

Yep -- fly mine on 90% M's all the time and fly our Intimidator-5's on N4200's without fin augmentation.
So No, not done the analysis. But have seem plenty of flights.
That's why I said I question your analysis.
 

GAP

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Yep -- fly mine on 90% M's all the time and fly our Intimidator-5's on N4200's without fin augmentation.
So No, not done the analysis. But have seem plenty of flights.
That's why I said I question your analysis.


Knowing that I of course would have done things differently. However, it doesn't hurt to do the tip-to-tip. Maybe someone with more experience with the software can point out something I missed.
 

GAP

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From here I tacked the fins on with JB Weld on the root and injected internal fillets with Rocketpoxy and did external fillets with Rocketpoxy. The interal fillets were are hard to see in the picture, but they worked very well.

After this I did the tip-to-tip carbon fiber layups. I did these layups wet and with no vacuum bagging and no peel ply. The first two sets of layups I did very wet with lots of epoxy. I have now seen a few things that say this is bad as well and the third layup I used a squeegee on to reduce the amount of epoxy in the layup. I originally saw no disadvantage to this as the epoxy would just run down and form larger fillets. As far as the peel ply was concerned I had read a couple of things that maybe suggested that peeling off the peel ply creates micro tears in the structure of the composite and I didn't want to risk it. Overall, I'm pretty happy with how these went.
 

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GAP

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I was slightly worried about de-lamination so added a small amount of JB Weld on the leading edges to help prevent such issues. I do not have any pictures of this.

With the fincan complete I turned my attention to the av-bay. My av-bays are very simple, messy, and ugly. But they work! The altimeters I wanted to use were going to be ripped off of my Punisher 3 to save money. These were a MissileWorks RRC3 and an Altus Metrum Easy Mini, both excellent altimeters. With the altitude this flight was reaching I was required to actively track my rocket by the club I fly at. So I put two radio beepers taped onto the shock cord, one in the drogue compartment, and another in the main compartment. However, I wanted something with GPS on it. So I grabbed an Entacore AIM XTRA that the team has in the lab to use for GPS tracking only. This was only used for GPS tracking because I didn't trust it to fire deployment charges as it had been uh.... "slightly crashed".

The RRC3 was used as the primary avionic with the drogue charge firing at apogee and the main charge set for 800 ft AGL. The Easy Mini was set to fire drogue at 1 second after apogee and main at 725 ft AGL. I chose 725 ft because the RRC3 dip switch only allows changing the settings by 100 ft. So if some reason I messed up the programming on the RRC3 there would be no way both charges would fire at the same time.

As far as the actual structure of the av-bay I epoxied two 5/16" threaded rods directly to the sled. Additionally, the coupler and top bulkhead were epoxied directly into the upper body tube. This makes it a pain to get in and out, but I was running on a tight schedule and just wanted something that would work without really having to think how many bolts to use to retain it ect. I used 1/2" forged eyebolts with a rated strength greater than 2000 lbs. These were offset to be lower than the sled so the nut would not interfere with the sled. A hole was drilled in both bulkheads to run the ejection charge wires directly through. I wasn't particularly concerned with the wires being pulled out and again this is the easiest and quickest solution. These holes were than sealed with rubber sealant tape (which works like magic btw). The ejection charge wires on both sides were color coded so I wouldn't mix up which altimeter was which. The picture posted show the approximate positions of where everything sits in the av-bay, it is not a picture of the completed assembly of the av-bay. Another 9V was slated to go on the bottom left side of the sled.
 

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GAP

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At this point a simple and quick task I wanted to complete was measuring the shock cord needed for drogue and main. I used 1/2" tubular kevlar shock cord rated for 7800 lbs of tensile strength. Given this strength rating the typical rule of thumb of 3-5 times the rocket length doesn't really apply. Even so, since I had the room in the drogue compartment I chose to make it slightly longer than 3 times the length. The main compartment was much more pressed for space. I ended up having to basically use as little shock cord as possible such that parts don't hit on the way down to get everything to fit. Pictures of the various positions during descent are shown. The picture showing the shock cord for the main parachute are before I cut it down even more, again to the absolute bare minimum.
 

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GAP

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I wanted strong parachutes in case of off-nominal deployment. Skyangle's cert-3 parachutes fit the bill. With the weight I was expecting I would have a descent rate of around 85 fps under the Cert-3 drogue, a little on the slow side, but I'm not the biggest fan of the drogueless as I like making sure everything has an orderly descent. In fact, there are flights of Intimidator 4s on youtube that fly drogueless, but the main parachute compartment is so heavy that it actually starts to drag the booster section down a bit. This increases the chances of the booster falling on the main during main deployment. In the future I will likely switch to a smaller drogue.

The main parachute was a large Cert-3 chute which results in a descent rate of 20 fps, plenty safe enough for some nice soft Minnesota sod. These parachutes were both wrapped in Nomex burritos to save them from the ejection charges.

Ejection charge was conducted the night before. 3 shear pins was used for the booster section with 4 for the upper section as I was worried the upper section would come out due to the compressive force required to fit the parachute in (it was very tight). This resulted in apogee charges of 4g and 5g respectively of FFFFg black powder. The main used 2.25g and 3g respectively. I wanted to make sure that this thing separated no matter what, BLOW IT OUT OR BLOW IT UP!
 

GAP

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This pretty much wraps up the build. I had a couple last minute changes the night before. The biggest one was with the switches used for my altimeters. I was using screw switches for all the altimeters. I broke the one for the AIM the night before and switched that one to twist and tuck. This is not a danger concern as no charges are attached to this flight computer.

The day of the flight started out very disappointing. Cloud cover was 100% with cloud ceilings at 1000 ft. These did not let up for until around 5 hours later at which point are started assembling my motor, an M1101WH. This was terrible to assemble. The liner needed to be sanded and I had problems screwing the forward retainer completely closed. Eventually I got this forward retainer to only protrude out 1/16" making it within spec with the help of like 6 people. After this I was able to put it on the pad and let her rip. The final loaded weight was 31 lbs with a stability around 1.5 and predicted altitude of 13,000 ft and Mach 1.

The flight was a total success. The rocket landed around 1.3 miles away and was easily found thanks to GPS coordinates.
 

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aerorocket

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Knowing that I of course would have done things differently. However, it doesn't hurt to do the tip-to-tip. Maybe someone with more experience with the software can point out something I missed.

GAP, congratulations on your successful Level 3 flight. From what I see you performed your flutter analysis correctly. I just have a few suggestions. First, fillets at the base of the fins reduces effective semi-span to maybe 5.5 inches from the value (6.0 inches) you used initially. A smaller effective semi-span stiffens the fins and is a valid assumption if fillet material shear modulus is approximately equal to the strength of the fin material (G-10 fiberglass) itself. Second, from flying rockets in Minnesota I believe the launch altitude is approximately 400 feet above sea level so if your rocket's maximum velocity is 2000 feet above ground level your flutter analysis for proper air density is approximately 2400 feet above sea level. Applying these two effects in FinSim, I attached a screen shot illustrating that your rocket's flutter velocity at 2400 feet is approximately UF = 1.1 Mach. The attached analysis also shows flutter velocity varies from UF = 1.06 Mach at launch to UF = 1.34 Mach at your peak altitude of 13,000 feet. These same results should be achieved using your earlier version of FinSim.

Thanks, John Cipolla
 

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