Sunday, May 31, 2015

Weight is everything...

As I begin construction of my first Geezer TARC rocket, I find myself confronted by a sobering aspect of this year's competition... Weight.

The TARC rules specify that the rocket may weigh no more than 650 grams fully loaded on the pad. This means that you have to pack 2 eggs, the protection for those eggs, altimeter, parachute, wadding (or dog barf or baffle), shock cord, and the motor(s) into a rocket that will loft those eggs to 850 feet and bring them safely to the ground at a descent rate of around 21 feet per second. At first, I thought "no problemo"; after all, I build light, and I am usually well below - sometimes by 50% - the weight limit. But as the designs, both simple single motor and complex cluster, developed, I found that weights under 600 grams were difficult to achieve - and that was with little egg protection. Why is this?

1) The rocket has to withstand hitting the ground at 21-22 feet per second. Doesn't sound terribly fast, but this is a 15 mile per hour impact speed. You are going to have to build robustly if you want the model to last more than one flight, especially if the ground is not nice soft grass. This means basswood/plywood fins and thick body tubes - all of which are heavier than standard body tubes and balsa fins.

2) The eggs have to survive the impact with nary a crack. They are going to have to be protected well, which means a decent amount of weight devoted to padding. This is the big unknown in my designs - I do not yet have a feel for how much this will weigh. I do know that it cannot be much more than 50 grams, or my rockets will bust the weight cap. Time for some research and experimentation. But this brings up a good point - TARC teams and mentors are going to have to pay careful attention to weight in this year's competition, as it will be very easy to exceed the 650 gram limit.

As I ponder egg protection, I have made good progress on my first entry - a 4 engine cluster I call "Nemesis", after the Greek goddess of revenge. Hopefully, she will not end up too fat to fly.

Open Rocket simulation of Nemesis under power.

1 comment:

  1. I haven't done that much egg lofting, but this post made me curious. I have a 15X18 inch piece of foam rubber, about 2 inches thick. I weighed it, and it came out at about 128 grams. Since it's a much bigger piece than you'd need for an egg lofter, it seems like that might be a good, lightweight candidate, well under 50 grams.

    The one time I did egg lofting, I just used a bit of dog barf wadding. I have no idea how much it weighed, but it did the trick.