BMS School Rockets, 29mm and Cluster

[HOMA]

First off my Thanks to @Alan R and @waltr for their contributions in Alan's build thread. I have several sims attached and for the 3" School Rocket I used their sims as my starting point. The sims attached with the -stock label are for anyone else that needs them. I have adjusted all the weights per the parts I received. I also included a sim of the original LPR School Rocket

I am building two versions of the BMS 3" School Rocket. The first and primary project, will be a 3FNC version with the 34in body tube and payload bay. The second project is a Cluster of 24mm motors using the original 17" body tube.

29mm - Purpose of this build is to construct a rocket that can stay in the mid-power motor range, work on new building techniques, and launch capabilities. As I will be using the stock 4 fins on the cluster project, I will need to make my own fins. I haven't decided on a design yet and I could use some advice on that. I have read that the fin height should be a least one caliber of your body tube. I'm also well aware that it needs at least 1 caliber of stability. I've attached two sims using the fin design from some kits that I like. An advice or suggestions? The ply stock I have, is slightly lighter than the OpenRocket default weight for birch ply. I am going out tomorrow to some local hardware stores to see if they have any 1/8in ply in stock of better quality then what I picked up from HobbyLobby.

Payload bay/Recovery- this rocket will utilize motor ejection/single deploy. I was gifted an Estes Altimeter which I plan to make use of, but I don't currently have any other electronics. I plan to friction fit the nose cone with a backup Kevlar line as the nose cone will be shared between this project and the cluster. I have a 24" parachute from Top Flight. I will probably need to pick up the 30", but I will decide once I have a final build weight. Landing should be on a field or farm land that is usually not too hard. I have plenty of kevlar line from emma kites to make a harness. What is the preferred layout? Could I go from the booster to the payload bay and have another line from the booster to the parachute or should it be all one line with the parachute 1/3 of the way or more down the line to the booster?

Cluster- My plan was to just use the original 17" body tube and the nose cone. In OpenRocket this configuration with motors resulted in a rocket with less than 1 caliber of stability. If I use the payload bay as well then stability is over 1 caliber. While I wanted the build to be more of a stubby, I don't see how I can just use the 17" body tube without adding nose weight. Adding nose weight isn't an option right now as I am sharing the nose cone with the 29mm build.

 

[Alan R]

Cluster- My plan was to just use the original 17" body tube and the nose cone. In OpenRocket this configuration with motors resulted in a rocket with less than 1 caliber of stability.

Stubbys don't need a full caliber. With just the 17" tube on the cluster your sim says you have about a .5 cal which is plenty good enough.

For comparison, my Tiki rocket (definitely a stubby) sims at ~.38 caliber and flies great.
Looking at your parts picture, I don't see a cluster CR. wondering why you didn't buy one from BMS.

I love what you're doing.

 

[HOMA]

Stubbys don't need a full caliber. With just the 17" tube on the cluster your sim says you have about a .5 cal which is plenty good enough.

For comparison, my Tiki rocket (definitely a stubby) sims at ~.38 caliber and flies great.
Looking at your parts picture, I don't see a cluster CR. wondering why you didn't buy one from BMS.

I love what you're doing.

Awesome! I just pulled up some sims of other stubbys and you are correct, they have calibers of less than 1. I should have done more research on that.

I didn't post this picture yesterday but, I did pick up centering rings, motor tubes, and blocks from BMS for the cluster. I forgot to buy engine hooks. I had two on hand and picked up another one when I bought the Aerotech retainer for the 29mm project. The fin tabs will have to be reduced to accommodate the cluster mount. I'll re-weigh them and update the sim once that is complete.



Thanks for the reply!

 

[Alan R]

I forgot to buy engine hooks.

Just go to the trash bin outside the auto parts store. Should be lots of old wipers this time of year.
If you use the Qualman design, you don't even need the blocks:
http://www.qualmanrocketry.com/qualman_clips.html
http://modelrocketbuilding.blogspot.com/2010/07/make-your-own-engine-hooks-part-1.html

 

[HOMA]

Got a little bit of work done. Tube spirals have been filled with Bondo spot putty, but not sanded yet. I extended the 2" fin slots to 4". I decided on a fin design and cut them out. They need a some more sanding and then I will paper them. I also cut the motor mount tube down to 23".



I bought a 3D Printer. Once I get it working I will print out a fin alignment jig. Setting it up/dialing it in has taken up most of my free time this past week.

Thanks for the replies @Alan R !

 

[Alan R]

I bought a 3D Printer. Once I get it working I will print out a fin alignment jig. Setting it up/dialing it in has taken up most of my free time this past week.

Looking good.
I think I print out a custom fin alignment jig for almost every rocket, because they're all a different size. I did design a quasi-universal one for 18/24mm that I get to re-use once in a while.
After you get your PLA dialed in... you can then spend another month trying to set up PETG.
You can make rocket parts with PETG, but PLA doesn't really work for most things except fin guides and toys.
Thingiverse is your friend

and here's one of my favorites (PETG): 29mm Motor Retainer