Space Grant Consortium - University of Wisconsin - Green Bay

of the basis for fin designs came from the Handbook of Model Rocketry [1]. It suggests that the
most aerodynamic fin shape is the “clipped delta.” The ideal dimensions of this being a root cord
length of 2D, a tip cord length of D, a span of 2D, a leading edge cord of D, and a thickness of
0.2D (where D is the diameter of the body of the rocket). The ideal profile would be a
teardrop/airfoil. The fins should be located as far back on the rocket as possible, as to provide the
most CP benefit. Obviously, a minimum of three fins should be used.
Booster. There were three constraints that had to be considered when designing the
booster stage. The first and most important was the four inch diameter that was required by the
competition. Another constraint was the length of the motor. The booster had to be long enough
to fit the entire motor in it. The last concern was the transition area. The booster had to have the
ability to accept a tail cone from the dart stage. The Haack series determines the shape that gives
the least amount of drag for the transition cone is to be a raindrop [5]. Thus, this would be the
ideal shape of the cone. The value of C used in the Haack to find the ideal length of the cone is 0
[5]. A length to diameter ratio of roughly 5:1 was found to be the most ideal. However, RockSim
simulations showed that the additional weight of the cone added reduced the positive benefits of
the cone. Thus, a 1:1 ratio was used for the transition cone.
The length of the body section should be kept to a minimum. This section is only increasing drag
by taking away from the tear drop shape. The tail cone shape came from trying to maintain the
tear drop shape as much as possible. Ideally, the cone should be as long as possible, but it was
limited to the length of the motor as to not add unnecessary weight. Because of the motor that
needed to be placed in the tail cone, it would have to end flat instead of coming to a point, as
would be ideal. The design of the booster fins was nearly identical to those of the dart, with a
couple of differences. The location of the fins was chose such that they extended past the end the
rocket. Thinking ahead, this would help keep the CP towards the tail end of the rocket. However,
this then required that small triangles be cut from the inside corners to prevent the motors hot
gasses form damaging he fins. Lastly, the fin size was also scaled as to provide appropriate CP
assistance. The air flow inside of the four inch diameter is very turbulent [1], hence it is
suggested to have fins that extend past this area. For this rocket, the fins extend a whole body
diameter past the perimeter of the four inch diameter body.
Structure
Dart. The ideal mass of the dart was quite difficult to calculate as described earlier. A
known booster stage mass had to be used and was plugged into the methods described later on.
The small diameter of the body limited the usable shapes for the cone. The nose cone was
purchased through Public Missiles Ltd., as a solid composite cone. However, the material was
carefully drilled out, so that a large brass weight could be placed in the cone. Brass was chosen
as the weight, because it is more dense than steel and there was easy, free access to it. This extra
mass was used to keep the CM closer to the nose of the rocket. Phenolic tubing was used for the
body of the dart stage. This kind of tubing had the lowest density for a reasonable price.
Additionally, the strength likely exceeds the needs for the rocket.
A custom wooden bulkhead was used to separate the electronics bay from the parachute bay and
to give the parachute a solid attachment point. The bulkhead was kept in place by using a dowel
that was placed perpendicularly through the body and bulkhead. It was friction fit in place.
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