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SS1 Reduced Scale Test Vehicle: Flight 1
The first test flight of the miniature SS1 took place on Saturday, May 21 at a construction site on the north side of Glendale, AZ. The photos are taken by Billy Dahlberg.
Getting ready for launch
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Installing the ignitor
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Telephoto lense shrinks the range.
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F50-4T roars to life
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Up and away!
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The 90º turn at approximately 30 feet.
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The SS1 flies off horizontally
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and gently rolls until it hits the ground.
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Part of the debris field
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The honeycomb Nomex survived nicely
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The worst repair will be the crumpled tube
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It is said that "calm seas make inexperienced sailors." So this flight did exactly what is should have done -- showed us what we need to avoid.
Analysis
I had several motors available for the first flight; an F50-4, a G80-4, a G64-4, and a G64-7. I chose the smallest motor (the F50) since it allowed for the least amount of nose weight and because we were flying on a relatively small field.
I did a number of swing tests on the model before lighting the motor. We repeated the experiment on four different axes:
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swinging rope perpendicular to the wing plane, top facing out
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swinging rope perpendicular to the wing plane, top facing in
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swinging rope parallel to the wing plane, top facing up
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swinging rope parallel to the wing plane, top facing down.
The rocket trued up instantly with minimal nose weight in orientations 1 through 3. Interestingly, the rocket did not true up quickly in orientation 4. We added more nose weight, and the rocket trued up more quickly than before, but not instantly.
It was on this axis that the rocket did its 90º to 100º turn.
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RTV Member-at-Large Burl Finkelstein once sent me an excerpt from an aerodynamics paper which stated that a marginally stable design will exhibit the tendency to reverse directions when under thrust. I believe that is the cause of what happened with this flight. We had this discussion a lot when we were analyzing the first flight of the Blowfish Avenger, which experienced the same sudden 90º shifts in angle of attack.
All the bulbous rockets I've been building have unconventional airframes that make it difficult to accurately predict the Center of Pressure (CP), which in turn makes it difficult to accurately determine the margin of stability for the design. Also, since the CP location is not static during flight, the risk in a design with marginal stability is that the CP can shift forward of the Center of Gravity (CG), making the flight unstable. I believe this was the primary factor in this flight, as well as the flight of the Blowfish.
I borrowed the drawings on the left from article on predicting stability in Issue #18 of the Apogee Rocketry E-Zine. I liked them because they graphically depict in a simple way what happened during flight.
At liftoff, the CP was roughly even with the CG on the rocket, which is the pivot point (top illustration). As velocity increased, the CP shifted forward, causing the nose of the rocket to pitch down (center illustration). The angle of attack is suddenly perpendicular to the airflow (bottom panel) which stops the forward momentum of the rocket. Since the rocket is still under thrust, it jets off in a new direction.
My brother Rick has advanced the hypothesis that the problem was caused by, among a number of other factors, excessive lift generated by the airfoil. He adds "[and] because lift distribution is not constant accross the wing, there is also pitching movement (torque) caused by the wing. Also because the wing is off the centerline, there is assymetrical drag. this momentum rotates the craft in same direction, nose up around the centroid of the craft."
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This is an interesting theory, and I'm certain that these are all factors that will contibute to the SS1's flight characteristics; but I don't think these were the major factors resutling in the 90º turn during the first test flight. At worst, this would case the SS1 to have a looping flight trajectory, not a right-angled one.
Lift is caused by the airflow velocity differential across the upper and lower airfoil surfaces -- because the air has to travel further across the upper surface, the pressure above the wing becomes lower than the pressure beneath the wing. This generates lift.
My theory may be flawed, but it has been my hope that the airfoil would affect the SS1 flight trajectory as depicted in the illustration to the right. The lift would have drawn the SS1 along a diagonal path, but the angle of attack would remain unchanged and the vertical axis would remain aligned along its original vector. As long as the elevons are properly aligned along the vertical axis, the rocket would not loop or dive.
I don't see anything about the airfoil dynamics that would have caused a sudden 90º to 100º change in the angle of attack. But I will be the first to admit that airfoils and wing generated lift are both areas where I have no expertise. Maybe someone else can lend some insight here.
So until I am convinced otherwise, I will assert that the problem was marginal stability along the wing plane, and that the issue can be resolved by moving the CG forward with additional nose weight (just as it can with the Estes version of the SpaceShipOne).
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There remains a delicate trade-off, though -- I still prefer to keep the liftoff weight low enough to allow the 4-inch version of the SS1 to fly on a G motor.
The dry rocket without any nose ballast weighs 30 ounces, and a G80 weighs just over 4 ounces, for a total of 34 oz. A safe 5:1 thrust to weight ratio for a G80 puts the combined rocket weight at ~52 ounces. Therefore I can add as much as 18 ounces - over a pound - of nose ballast before the thrust to weight ratio starts to become marginal.
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Although this doesn't deal specifically with the airfoil concerns that my brother Rick raised, it may positively address those issues as well. If the CG (pivot point) can be moved far enough forward of the airfoil CP, the lift generated by the wing will (theoretically) generate less torque on the rocket.
We'll know for sure after the second test flight.
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