SCROTON by Chris Eilbeck, MARS Flight Crew chris@mars.org.uk

Introduction Scroton is a three bladed rocket powered helicopter designed to fly on black powder motors. It was conceived as something novel to fly at the International Rocket Week 2002 at Largs in Scotland and based on the Roton concept for cheap access to space.	click image to enlarge
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Materials 3 off 18mm motor mount tube 3 off 18mm engine blocks if not using masking tape thrust rings 3 off 1/4"x1.5"x18" lengths of balsa 6 off 1" wide 25-30 degree balsa wedge 2 off 3" plywood bulkplate 6" length of 3/16" ID brass tube light glass to laminate blades heavier glass to attach motor mounts laminating epoxy 5 minute epoxy filler

Construction

Using a sanding block and 80 grit paper sand the three blades to have an approximately aerofoil cross section except for a 1-2" section at one end which will be used to bond the blade to the centre plates. Laminate the three blades using light glass and good epoxy. One or two wraps is enough. Fill and sand smooth.

Sand/cut the six wedges at approximately 25-30 degrees. The blades need more than 11 degrees AoA to ensure autorotation to the ground. Attach the balsa wedges to the ends of the blades using 5 minute epoxy to set the angle of attach for the aerofoil.

Bond the three blades between the bulkplates at 120 degrees apart. Drill through the centre of the two bulkplates and bond in the brass launch lug with approx 4" below the lower bulkplate.

Bond the motor mount tubes on the ends of the blades using 5 minute epoxy then laminate on using heavier glass (4 oz or so). Fair the tubes in using filler. Laminate the blades onto the bulkplates using heavier glass too.

Make a crude prop-balancer using either a couple of steel rulers held over the edge of a desk or a length of thin, stiff rod. Add filler or sand existing filler off until the scroton balances with motors installed.

Ignition

The three motors can be lit using three igniters or using quickmatch and a single igniter. I suggest the latter. An Estes pad can be used but very little of the rod length is required, 9" should be sufficient. A heavier pad is recommended depending on how well balanced the model is.

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Flight Test

The model flies very well on three C6 motors to approximately 100ft and is definitely a crowd pleaser. It should auto-rotate for a large proportion of its descent and soft land requiring no additional recovery devices. It would be very interesting to fly on longer burn motors and an investigation into the effect of changing the angle of attack would also be good.