Old man new builder

[Skipper_52]

As I said in the intro area I am building a rocket powered with (1) Estes F15-4. I am using open rocket to help with the design and have viewed many videos searching for answers. I need some help.

#1 I found that I can throw out all the designing I did as a kid and am replacing it with safe rocketry but I believe the goal is still the same, the higher the better.

#2. I learned about CG & CP. There are many opinions as to the difference between the two but it is still based on tube diameter. I am using a 2" tube. Some videos (most of them) say the factors should be >1 and <2 while a few say >1 and <3. With my 2" tube I am assuming I need to be somewhere near a 3" difference. Please correct me at anytime and what is the best number to shoot for.

#3. Is there a rule of thumb for fin shape and size based on tube diameter and length? By playing with Open Rocket, 3 fins flies higher than 4 fins of the same shape and size. But I think 4 fins will be more stable.

#4 In Open Rocket in the upper right corner of the design window is Stability: 1.56 cal / 7.89%. Is there a number I should be shooting for as in 7.89% or is this number just a result of CG-CP and I shouldn't worry about it

#5. Right now my tube is at 32". The only way to adjust this length is to increase nose weight to shorten the tube or move the fins lower or a combo of the two.. Does the tube length affect the size/area that the fins should be and given that the rocket will still weigh the same, which is better, longer or shorter tube?

#6. I've looked for a best placement for the launch lug. Mine is 2" long 3/8" i.d. x .0625" wall thickness. My launch rod is 1/4" stainless rod x 72" long. When I created it in Open Rocket it placed it near the CG. Is this the best place to put it?

Any help with the above will be greatly appreciated.

 

[Azamiryou]

#1) I don't see a question

#2) You understand correctly

#3) I don't think there's a rule of thumb, use the sim to check your design. It's probably easiest to go higher (your goal from #1) with three fins.

#4) the percentage is another way to look at stability, based on rocket length instead of tube diameter, with the goal 10-20%. For a "typical" 1:10 rocket, this is the same as 1-2 calibers. This might give better results than calibers for rockets that are extra long or short and stubby. (Your rocket seems pretty long.)

#5) everything affects everything . If a shorter tube pushes the CG back too far, you can add nose weight to move it forward, or change the fins to move the CP aft.

#6) there are lots of ideas about lug position, but anything not too extreme is probably fine. For a long rocket, two lugs might be a good idea.

 

[dhbarr]

This is a very rockety-shaped rocket, should scoot pretty well. 4 fins that are a little taller than the airframe outer diameter, overall length 10-12x airframe OD. Watch your weight (incl. motor) as you glue/paint/assemble to be sure you don't get into that 21oz max takeoff weight territory.

 

[Skipper_52]

For those of you interested in 3d printing PETG weighs .003 ounces per in of material. I use this calc to estimate the weight of a piece in Prusa slicer before printing.