Accomplishing one of my goals (dual deployment) on the very first day of the year has me in search of a replacement. Something a bit more challenging. Looking to later in the year when I hope to make my Level 3 certification flight, I realized that flight will result in a number of firsts for me, if I am successful. It will be the highest, probably just shy of 13,000′, and the fastest, Mach 1.5. However, that is all being done by sheer brute force of a 75mm M3100 motor. Certainly there must be a way to finesse some of these. I started playing around with some simple designs and simulations with the quick realization that I could go a lot faster with far less motor. So my goal is to do Mach 1.8 with a 38mm motor. This is actually fairly easy as well so, I added a little twist. I want to do this with a tube-finned rocket.
I spent yesterday evening searching for good information on the specifications of various materials used to make the airframes. There really is a bit of a void here and I think I see an opportunity to make a contribution. I’m going to make several, at least 4, identical rockets that differ only in the materials used and see how they perform. There is quite a range of specifications too which should make this interesting – perhaps even exciting if material limitations are surpassed in flight testing. A tube-finned rocket is ideal in this respect for several reasons. The tube fins create a lot of drag and will be put to the test, structurally, when the rocket is pushed through Mach 1. I plan to choose a fin geometry to accentuate that. The geometry of tube-finned rockets spreads the loads in such a way that my ability to achieve uniform glue properties and proper bonding of the fin to the main body tube should not be the limiting factor – at least for the weaker materials.
Right now my list of tubes includes: LOC/Precision tube, Blue Tube, G12 fiberglass epoxy, and Carbon Fiber epoxy. I may add Quantum tubing after a little more study to see if it warrants inclusion. The initial consensus is that CF is the obvious choice – lighter weight than fiberglass and stronger. G12 will certainly work but is the heaviest. G12 is a safe bet. LOC tube is standard thick-walled kraft paper tubing. It is the lightest and also the weakest. Most people expect it to fail at high speed. Blue Tube will be interesting. It is strong and light, lighter than CF epoxy, but begins to yield much earlier that CF. All of these tubes have the same wall thickness so, this should be a fair test.
I plan to use motors in the Vmax series from Cesaroni (although the White Thunder and RedLine series have interesting thrust curves). These Vmax motors burn for less than one second with very high thrust. In these minimum-diameter, high-speed rockets powered by high thrust motors, weight is of the essence. The lighter it is, the faster it will go with a given motor and rocket geometry. Lighter rockets will not go as high as the heavier rockets, however, because more of the energy put into the system ends up as momentum in the heavy rocket rather than being “wasted” pushing the air out of the way with the lighter, faster rockets. I could have decided to use mid-thrust longer burning motors but, that would be mainly a test of the drag differences between the rockets as much as it was about structural stability. I could select altitude as the metric but, that would favor heavier rockets. The proper metric must be chosen for this test and I think it will be top speed – fastest rocket wins. I want to see which material is the lightest that will survive. You can think of this another way. The lighter the rocket, the greater the fraction of the total thrust that must be dissipated as drag (i.e., stress to the structure). The lighter materials will be tested harder than the heavier ones.
This should be fun!