During the pandemic in 2020-21 it became clear that in order to get a new design fin bracket machined I was getting too dependent on external factors. Hence the decision was made to become self-sufficient with the conversion of a manual HBM BF 25 / G0704 mill/drill into a full blown DIY CNC milling machine. However that endeavor is actually an entire different chapter which I may or may not write a page about. Rest assured it took many hours of learning and a year long journey for machining a fin bracket with a perfect 43mm radius.
There are some dynamics at play during the supersonic speeds of the GIGA sustainer.
- The center of gravity moves forward due to burning of the propellant making the aft end of the motor ‘lighter’ thus shifting Cg forward actually improving stability.
- However, the center of pressure dramatically moves forward due to the reduced the effectiveness of the fins at supersonic Mach numbers.
- The forward Cp shift is more pronounced than the forward Cg shift of the depleting motor. The Cp is effectively overtaking the Cg and thereby causing the rocket to become unstable. Some good examples can be found online with 3FNC type rockets (3 fins and a nose cone) becoming unstable at the end of their burn around Mach 3.
- To keep a rocket stable a minimum stability margin of 1 cal throughout the entire flight of the rocket is required. Preferable 1.25cal but for GIGA and the uncertainty of supersonic speeds in the OpenRocket program I will use a minimum of 1.5cal of stability.
For the 86mm diameter rocket fins I’m using 100x3mm extruded flat profile which limits my fin height to 100mm. Reason for using extruded flat profile over a free-from fin cut from a 3mm plate is that the material is readily available, holds tight tolerances and is quite stiff. The fins are cut to size (4 stacked fin blanks) using an industrial metal cutting band saw with the swivel head set at 30-60°. I did not have good experiences with using a sheet metal shear as the 60° cut would twist the fin.
From the above picture some of the considerations can be recognized for improving the GIGA IV fin design:
- Longer fin root.
- The previously used fin brackets were suited for relatively short root fins. OK for slender rockets and < Mach 2 flights. Because of the higher speeds, shorter & lighter GIGA IV- V rocket and short fin root I was forced to make a delicate parallelogram fin design to get to the required min calibers of stability. However this resulted in some loss of fin rigidity. Furthermore the pointy fin tip at the trailing edge was prone to damage being the most aft part of the rocketm (see above red circle).
- 30° leading edge of fin bracket.
- The original fin bracket had a 45° degree leading edge. Good for trapezoidal fins but this always looked strange in combination with a 30° leading edge from the later preferred swept clipped delta fin shape.
- 90° trailing edge of fin bracket.
- Similar to the leading edge the original design had a ± 45° degree trailing edge. To make the bracket more robust for the swept clipped delta fin shape a simple 90° degree trailing edge would suffice.
- Mounting holes of the fins and fin bracket now at off set from each other.
- Driven by the 30° leading edge this made more sense when modelling but it was never a firm design criteria.
- Only bevel the leading edge.
- The leading edge has a 7,5° bevel while the trailing edge is left at 90°. It also removes 1 extra machining step.