A modular TARC rocket?

Now that the season has begun I have been thinking quite a bit about this year's TARC. The big change this year is that the 100 teams participating at Finals will fly not to their qualifying 800 foot altitude, but to 775 and 825 feet, 25 feet higher or lower (order to be determined by a coin toss at Finals). The traditional way of dealing with this scenario is to add or take away ballast weight from the model, but another way has occurred to me, one that adds or reduces a consistent amount of weight and drag (so less weight needed). And it came to me when I glanced at my fleet the other day.

The Estes Converter was an almost-ready-to-fly (ARTF) offered by the Big E from 2007 to 2009. It featured a plastic fin unit and 4 BT-56 body tube segments with screw type couplers, enabling the owner to vary the rocket's length from 24 to 40 inches. I am one of these owners, and the sight of it a few days ago got me wondering if one could design and build a TARC rocket that could reach the three altitude marks simply by screwing on different lengths of body tube. After pondering the matter for a couple of days, I decided to look into the concept further.

Estes Converter (Click to enlarge).

A shorter Converter
(Click to enlarge).

The first step was to determine if similar screw-type couplers could be had from the rocket vendors. Such things could probably be 3D printed, but I don't have easy access to this technology, so I need to be able to purchase them from somewhere. Fortunately eRockets offers them in various diameters, including BT-70. This obstacle now eliminated, I fired up Open Rocket and set to work.

eRocket's BT-70 screw type coupler (Click to enlarge).

I did not want to start from scratch, so I selected a TARC rocket I had built for the 2015 challenge, which was quite similar to this year's goal (1 egg to 800 feet). The Bullpup 1 is a 4 finned, BT-70 based cluster named after my Jr. High School mascot, and it was a simple matter to use this design to check out my modular hypothesis. I replaced the cluster configuration with a single 24 mm motor mount, shortened the main tube, and changed the number of fins from 4 to 3, taking care to scale them up to preserve the stability margin. Then I added the eRockets coupler at the top of the sustainer and inserted various lengths of body tube between it and the payload section, watching the effects on altitude.

The Bullpup 1(Click to enlarge).

What I found was that a 5 inch length of painted BT-70 added to the 11 inch sustainer body tube would send the rocket to 830 feet on an E12-6. Adding an 8 inch tube would drop the altitude to 804 feet, and an 11.5 inch tube would result in an apogee at  777 feet. So you could envision a "basic" Bullpup with an 11 inch main body and 3 tubes of 5, 8 and 11.5 inches, chosen according to the desired altitude. This set of simulations are, of course, highly idealized, as you generally need to aim 50-100 feet higher in the sims to compensate for rail friction, imperfect finish, and so forth. However, they do show the concept is viable, and I do think Open Rocket can be used to determine the short and long tube lengths once the medium tube length has been established by flight testing.

Open Rocket simulations in order of decreasing altitude (Click to enlarge).

I wonder if any team out there will use this approach or something similar?