Motor Type
It is my intention to use a Hypertek M1010FX hybrid motor in the Zephyr for its maiden flight. My local rocketry club, the Superstition Spacemodeling Society owns the motor hardware and ground support equipment, and this is really the only reason I thought about building a Level Three rocket. Hybrid launches are much cheaper than composite reloads, so the availability of the club's equipment suddenly made the prospect of a Level Three certification financially attractive.
The M1000 is also a viable motor. I opted to go with the M1010 grain because it offers slightly more punch off the pad. I was cautioned about some suspicions that the M1010FX carries a higher probability of a CATO, which I ultimately dismissed as folklore. Time will tell if I was wise or a fool.
The following is excerpted from www.thrustcurve.com:
HyperTek M1010 (4630/98-RG-M-FX)
Motor data from the manufacturer:
Launch Weight
I do have an early concern regarding the total liftoff weight. The M1010 doesn't have a strong peak thrust at ignition, so a heavy rocket will be a problem. Following the rule-of-thumb that average thrust should equal three times (3f) the liftoff weight, then my fully loaded rocket can't exceed 75 pounds. Here is the formula:
1012Ns ÷ 4.448222 Ns/lbf ÷ 3f = 75.84 lbf.
The fully fueled M1010 weighs 19 pounds, so using this value as the upper limit would leave only 56 available pounds for the dry weight of the rocket. As it turns out, the dry weight of the Zephyr is 60 pounds, so this leaves no margin for error.
Another way to look at the maximum liftoff weight is to calculate the average thrust over the initial part of the motor burn -- and this certainly seems appropriate for the M1010, as the vast majority of the thrust is generated in the first five seconds after ignition.
The average thrust for the first four seconds is 1393Ns. With this value inserted into the equation, we get the following result:
1393Ns ÷ 4.448222 Ns/lbf ÷ 3f = 104.38lbs.
The 104.38lbs. estimate seems to be a more appropriate value to use in calculating the headroom available for maximum launch weight. The target liftoff weight will still be around 80 to 85 pounds, which gives me a weight-to-thrust ratio of 3.8 to 3.9 at liftoff - an acceptable estimate.
Estimated Drag Coefficient
As I did in the compution of the Center of Pressure, I rely on the AeroCFD computational fluid dynamics software to estimate the Drag Force Coefficient (Cd). AeroCFD predict the Cd on the Zephyr to be 0.468. As a point of comparison the "anti-zephyr" (a fictional rocket with identical length,diameter, and fin configuration, but a tubular rather hat cigar shaped design) has a Cd of 0.771 -- proving there is significantly more drag in a traditional design.
Screen shots from AeroCFD are shown below:
