Wednesday, May 30, 2018

My first Geezer TARC rocket for 2018...

I was suffering from rocket burnout last year at the time of the release of the TARC rules, so I did not get around to building my Geezer TARC rocket until the latter part of Summer. Normally I build two to maximize my chances, but I started so late that I could only complete one before the launch. I have determined I shall not repeat last year, so I got to work designing as soon as the rules for TARC 2019 came out earlier this month. Four completed designs are now stored on my computer, but I am only satisfied that one - christened the Reliant - is sufficiently well thought out to construct. Another will make the grade soon enough, but it'll take a little more work with Open Rocket.

Open Rocket visualization of the Reliant - still trying to decide decor (Click to enlarge).
So what am I looking for in my Geezer TARC entrees?

1) Simple and relatively easy to build - hard to cut straight with my eye condition, so I need fins with simple geometries, like swept trapezoids or deltas. No elliptical shapes, even though those are less draggy.

2) BT-70 based - BT-80 tubes have too much area, hence too much drag (remember we are trying to get to 856 feet this year, 56 feet higher than last year's mark). Centuri ST-18 tubes are "minimum diameter"; a regulation size egg will barely slip into them, but there is no room to pad the egg sides. OK if you land nose down, but bad if you hit on the side of the capsule, especially with 3 eggs inside. A BT-70 is a good compromise between the two. Another plus is that Apogee makes some relatively lightweight egg protection for this tube diameter.

3) A weight around 520-525 grams fully loaded without motor - The fact that this year's rockets have to be painted means added weight. Throw in the additional egg and higher altitude and you may find it extremely hard to make altitude with a BT-80 based bird. Too much drag, too much weight.

4) Assuming the weight range above and a typical rocket drag coefficient of about 0.75, a F motor should get me to about 850 feet. Open Rocket has the model going about 150 feet higher, but it always over estimates altitude.

1st-order altitude prediction for the Reliant (Click to enlarge).
The Reliant meets all these criteria. As you can see from the Open Rocket screen capture, it is very classic, very simple. The fins are balsa laminated with paper to keep weight low and increase durability, and the payload (capsule) section can be loaded from both the front and back to help in getting the eggs in and out - hopefully without breaking. The balsa coupler at the base of the capsule is augmented by a JT70 coupler wood glued to the front to help facilitate insertion of the Apogee 2 egg foam protector, and to add a little strength back there. A hardwood dowel will be glued into the back of the coupler, and the screw eye will be inserted into the dowel. That point - to which the dual parachutes are attached - will take a lot of stress at ejection, and I do not want the screw eye pulling loose (which would make me a prime candidate for the Flying Pig trophy). The rocket is powered by a single 24 mm composite, which is retained by the Estes plastic 24 mm screw-on retainer (available in 3 packs online).

Open Rocket design of the Reliant (Click to enlarge).
As I stated near the beginning of this post, I am pretty confident in this design - so I had to build it. The construction went quickly, and here she is, ready for primer when the stormy weather leaves. Getting started this early gives me the time to work up and build another design, probably a cluster of some sort. I am ahead of the game!

Reliant ready for primer (Click to enlarge).
And I am going to win this year... Mark these words!


  1. I like the BT-70. I don't know if people use it that often, but it's a nice size for a mid power rocket, and perfect for this year's TARC rockets.

  2. I have one of Jim Flis' canted dual mounts for a BT-70. At one point I was thinking of upscaling the Andromeda. I've since come to my senses.

  3. My lift off mass estimate for this year is sitting at 621 grams. Be afraid Bill, be very afraid...