Keep in mind the E6 and F10 that burn for 7-8 seconds and the G12 reload which also burns about 8 seconds.
Those are perfect for a gimbaled engine project.
I did a gimbaled engine project in 1989, using F10 power. The drawback to the system was partly the use of Sunguidance, which often caused liftoff over-reaction. One flight literally took off, veered one way to try to point at the sun, overshot the sun, and tried to correct back the other way, but the ground got in the way before it could complete it’s attempt to pitch back the other way. If it had used a gyro system or horizon sensor system, so it could have lifted off vertically and tried to keep that vertical flight attitude, it would have been a lot better off.
Two other problems I had. One, the servos turned out to be too slow. They were Canon super-micro servos. If they had been faster servos such as today, it would have worked better.
And the final problem was it had too much movement of the motor mount gimbal. It was over-controlled. It had at least twice as much angle movement than it should have. But the project ended before I tried to make adjustments, as again the sunguidance system just made the whole thing too flaky.
One issue that I considered was that as the rocket gets moving faster, the stability provided by the fins would cause the gimbaled thrust to have less and less of an effect, possibly to the point the rocket would go off course. The ideal way to solve that would be to have the guidance system set in a manner to provide more and more corrective angle (as needed) to the gimbaled mount as the model flew faster (probably just by programming it to do so over time, though using an air data probe for real-world feedback of airspeed would have been interesting. Or, making use of acceleration sensors in the pitch and yaw axes).
I tested out a far cruder method. I arranged for some fins to be glued to the engine mount itself, so the moving mount also produced aerodynamic control that became more effective with more airspeed, to balance out the reduced effectiveness of the thrust vectoring. An idea sort of “borrowed” from VonBraun, the Redstone Missile had air rudders that were driven by the same mechanisms that controlled the steering vanes in the rocket exhaust.
BTW - it is not a good idea to try to force an unstable rocket to fly straight by using vectored thrust. A model has such a small moment of inertia that it can respond way too fast, compared to the real things. Think for example of how easy it can be to balance a broom in the palm of your hand. Then think of trying to do that using a pencil. Then think of trying to do that with a toothpick. Our models are like trying to balance a toothpick in your palm compared to a broom stick (real launch vehicles), as far as moments of inertia are concerned. I won’t say “cannot be done”, but I am saying it is a very very difficult task that requires great precision and design to achieve compared to “just” trying to make a stable rocket fly slowly on gimbaled thrust. So a better goal would be to do it first with a stable finned rocket, work out the bugs, then maybe move on to finless.
John Pursley worked out a great merger of model plane gyros and model plane horizon sensors, for gimbaled engine scale models. He arranged it so that gyros kept it straight at lift-off and then at maybe 1 second in to the flight the horizon sensors took over (or took priority). The horizon sensors do not work too well for the first 50-100 feet.
I have to cut this short, too much other stuff I need to get to. At any rate, look at the F10 expendable and the G12 (32mm “R/C” reload).
- George Gassaway