Longer body vs. adding weight to nose cone.

[Senior Space Cadet]

It was becoming obvious to me, even before you guys spelled it out for me, that it would be possible to build a stable rocket without adding weight to the nose cone, but the idea, that you needed to add weight to the nose cone, was so ingrained in me that I had a hard time accepting it.
But, here's the thing, just because you can do it doesn't mean you should do it.
Let's suppose you are designing a rocket on Open Rocket, and you've got CG and CP the distance apart that you want but you have a pretty long rocket.
If I understand things correctly, you have three options.
Option one, don't change anything.
Option two, shorten the body and use larger fins (Let's assume the fins are already swept back).
Option three, shorten the body and add weight to the nose cone.
So why might you go with option three?
1-I would think that a longer body and larger fins would both add drag.
2-A slightly heavier rocket might actually fly higher because it will travel farther after engine burnout.
There probably is no right or wrong answer, but if your goal is maximum apogee, seems like there might be a good argument for option three.
If I'm wrong, set me straight.

 

[Funkworks]

IMO, we're in a territory where specific numbers are needed. Other factors to take into account are nose cone shape, fin shape, surface friction (finish quality), weight distribution, selected motor (thrust profile), weather (wind speed in particular), launch angle, and probably more.

I think whether you're right or wrong on option 3 could depend on these other parameters as well. Are you starting with a very long rocket? or one that's already really short?. Longer rockets tend to fly straighter because of longitudinal stability, so that's good. If surface quality is perfect, the additional drag due to being longer is negligible. If surface quality is poor, then the additional length can indeed add some drag.

Overall weight of rocket also matters. If it's a very light and short one, winds will affect its flight path and apogee more than if it's heavier.

 

[neil_w]

Option two will rarely be your best option, due to drag.

Between options one and three, OpenRocket can do the analysis for you. Make sure you have a mass object for nose weight in the nose cone. Go to Tools -> Rocket Optimization:


In the upper right, under "Available parameters", you can choose the mass of the mass object, and the length of the body tube, which I have done and are now listed in the "Parameters to optimize" in the upper left.

I've specified a stability margin between 1.25 and 3, and the minimum and maximum values of nose mass and tube length. I chose to optimize for the C6-7 (optimization will be different for each motor!) Then click "Start optimization" and it'll find your answer.

For this very light and simple 3FNC (this is the first example rocket under File -> Open Example, but I added a nose weight) , it got best results with a shorter body and *no* nose weight. Exact results will differ depending on the details of the rocket.

If you like the results, click "Apply optimization".

 

[neil_w]

Also note: For true drag minimization, you also want to:
1) minimize fin thickness, and/or airfoil the fins.
2) Get the slickest, glossiest finish on the rocket you can muster (although there's a tradeoff there between paint finish and weight of paint).
3) Add a tail cone. This will tend to push the CP *forward* however, and you'll need to compensate accordingly.