I did 3 quick Open Rocket designs using 3 different diameter cones. As I described earlier, the designs are all foam core CRs, skinned in card stick, with foam core fins. For the most part, I used materials sitting around the house or available from Hobby Lobby. I have a tube from a roll of foil that fits a 29mm motor to use as a motor mount, and I have some BT80 for a parachute bay, so all the designs are only as long as can be made with these two tubes. The card stock is 22" x 28", so I kept the BT length to 22" on all the designs, for comparison purposes. The 22" BT gives some overlap of the motor tube and parachute bay tube.
The small 4.875 cone design ended up weighing 9.8 oz, with 1.01 cal stability (unloaded). That weight is just about right for an F15, comparable to the PSII E2X kits. With the motor loaded, the stability drops to .37 cal! The rocket is just so light, that the heavy motor radically affects stability. The heavier composites are unstable without nose weight, which sort of defeats the purpose of what I'm trying to do. I'm sure there are some tweaks to the length and fin design that could improve this rocket and make a great rocket for F15 motors, but this was just quick and dirty to see if it would work at all. As it is, with my quick and dirty design, An F15-4 puts this rocket at mere 463' apogee, which is about what I'm looking for.
The 7.5" inch cone resulted in rockets that were a bit heavy for an Estes F15 --- about 17 oz, unloaded, and only 32 fps off the rod. Also, the only F15 that would work was the F15-2, which is a motor in the Open Rocket list, but I'm not sure it really exists in that short delay. It's too bad this is probably not viable, because otherwise this might be a really amazing thing to launch in the schoolyard to 240 feet!
The flights of the 7.5" rocket on the low-thrust composites were pretty good. The rocket put in flights under 1,000 feet even on the H motors. I had to sweep the fins back to get decent stability when loaded, which unfortunately seems like it would be a fragile design with foam core fins. The stability was pretty good though --- .95 cal unloaded, and about .5 cal with the composite motors, which should be fine given the high speeds off the rod.
I thought the 9.75" cone produced some interesting designs. Even though the cone itself is still very light for a 9.75" cone, it is significantly heavier than the other cones and is much heavier than the other construction materials, so it has a lot more nose weight, and it doesn't lose much stability compared to the 7.5", even though it is much fatter but just as short. The weight means the G33, is probably out due to low speed off the rail. I added in some other G motors to see if there would be other options for flying at my club's low power launches. Unfortunately, some of them have a minimum delay that is probably too long for such a slow rocket --- the long-burn motors have delays that can be trimmed very short, but most of the other propellants have minimums of 4 or 5 seconds. Looks like the G57 Classic would work, but the G68 white probably would not. The idea of launching a 10" rocket on a G motor at my club's LPR launch causes me to laugh maniacally!
I don't have much experience with analyzing simulation plots (usually just use sims to predict delays), but look at this plot below for the 9.75" rocket on the H53 Mellow Yellow. The motor burns for 4.4 seconds, but the acceleration drops below zero around 2.5. At that point, the motor still burns for almost another 2 seconds, but the rocket is actually slowing down. My interpretation is that the drag forces on the rocket at its peak speed are greater than the motor thrust. Presumably, if the motor continued to burn for an even longer period of time at constant thrust, the forces would balance at a certain speed, and the rocket would just maintain that speed until burnout. That would be fun to see. As it is, I think this rocket would be very fun to fly on that H53 mellow, with 4.4 seconds of burn in under 900 feet!
If I have time, I'm going to build one of these. Which one should I try first?
