John Cipolla's high power rocket construction techniques

High Power Rocket
Construction Techniques back
CHEETAH Advanced Model Rocket
The AeroTech Cheetah Advanced Model Rocket is designed to use G engines having a Total Impulse (I) of 120 Newton-sec and a Specific Impulse (Isp) of about 190 sec. The construction techniques required for advanced and high power rockets are stronger than those for model rockets due to higher speeds and accelerations experienced during the thrusting phase of flight. As illustrated, fins are attached through slots in the body tube using Through-The-Wall (TTW) mounting onto the motor tube. Epoxy is used to glue all joints rather than white or yellow glue. This design employs a labyrinth ejection gas cooling system instead of recovery wadding. The motor tube assembly contains a mesh which cools the ejection gases but still pressurizes the airframe to activate the recovery system. Notice the eye hook at the end of the motor tube which secures the shock chord and parachute assembly. Fins are also stronger than ordinary model rockets and are typically made from plywood, fiberglass, phenolic or waferglass.

CHEETAH Motor Mount
Engine mount rings and other motor adaptor structures are made from 1/8" or thicker aircraft plywood, fiberglass or phenolic sheet instead of paper or balsa. Finally, a sturdy motor hook is a necessity to prevent the rocket motor from kicking out during recovery system ejection. However, some high power rockets such as the LOC Precision EZI-65 uses a slightly different method to secure the rocket motor. For example, the EZI-65 rocket requires a retainer plate to keep the motor secure during activation of the recovery system ejection charge. A motor-block is not required to prevent the motor from pushing forward because the AeroTech rocket motor design has a rear closure with a diameter larger than the motor mount tube.

Altitude and Velocity Analysis
The equations of motion for vertical flight were integrated using a finite difference technique to predict altitude as a function of time. A MathCAD finite difference analysis indicated that maximum rocket altitude is achieved 12 seconds after engine ignition. A single G64-10W is specified to have a burn time of 2 seconds and a time delay of 10 seconds. Therefore, this engine as verified by the manufacturer, will maximize altitude performance because the rocket will be at the apex of its trajectory when the parachute is deployed. From this analysis a theoretical maximum altitude of 3,200 feet is predicted. In addition, this analysis also predicted that during the thrusting phase of flight the maximum velocity is Mach 0.68. Finally, an analysis was conducted to determine the static stability of this rocket by quantifying the distance between the center of pressure (Cp) and the center of gravity (Cg). The static margin was computed to be 1.1, which is considered satisfactory for model and high power rockets.

**CHEETAH Analysis Input Data**
Drag Coefficient (Cd)	0.50 (Theoretical)
Total Impulse (I)	27.0 lbf-sec
Average Thrust (Favg)	14.4 lbf
Cross-Sectional Area (S)	0.019689 ft^2
Propellant Weight (Wp)	0.1375 lbf
Initial Rocket Weight (Wi)	0.956 lbf
Time Delay (Tc)	10 sec
Engine Burn Time (Tb)	2.0 sec

**CHEETAH Flight Analysis Results**
Maximum Altitude (Z)	3,200 ft
Maximum Velocity (Mn)	Mach 0.68
Time to Apogee	12 seconds

Altitude vs. Time Plot

MathCAD Flight Simulation used before AeroDRAG & Flight Simulation

High Power Rocket Propulsion
High power rocket motors, defined as having a total impulse greater than 160 newton-seconds, require certification by the rocketeer through the NAR or Tripoli to legally purchase and fly. The following drawing of an AeroTech RMS 38/240-480 rocket motor was used to propel an EZI-65 rocket using an I161-10w RMS-38 reload kit. The I161-10w RMS-38 rocket motor uses ammonium perchlorate, a high energy composite propellant.

High Power Rocket Motor