Alexis J. Pabst and Dr. Paul Eastman, Mechanical Engineering

Introduction

The Unity IV rocket is a twenty-foot tall rocket with a 16.5-inch diameter. It is a super-sonic, hybrid rocket that uses a solid propellant and liquid oxidizer. The rocket has been designed and built by students from three Utah universities. It is intended to take an instrumentation package super-sonically to an altitude of 130,000 ft and return it safely to earth.

This paper will focus on the design, analysis, manufacture, and testing of the composite body section of the fuselage for the Unity IV rocket. The upper body section contains the avionics equipment and the recovery parachute. The middle and lower sections house the oxidizer tank and rocket motor.

The body sections are designed to withstand aerodynamic forces at mach speeds as well as the snatch forces of the parachute. Weight is a concern in aerospace because it takes more fuel to propel a heavy object. Strength is important to make the rocket safe and able to withstand the aerodynamic forces generated when an object reaches intense velocities. The budget for this project was limited and traditional manufacturing practices of composite materials are expensive. These were all factors to be considered in design.

Theoretical and Experimental Approach and Results

In flight, the rocket will experience forces in tension and compression along its axis as it goes through positive and negative accelerations. Other forces such as shear and torsion may also be present. Delamination occurs when the fibers are not bonded well to each other due to poor layup procedures or voids in the resin. Under stress these layers will peel apart and the strength characteristics are greatly diminished.

In order to determine how many layers were needed for the rocket, tensile tests were done on samples of several layers. This determined the strength of each layer of carbon fiber and allowed for calculations that would decrease material and therefore weight. The samples failed on average with an ultimate strength of 3900 lbs/in. With about 51 inches in circumference, the airframe is thus predicted to hold up to 194,000 lbs. of tensile force. This is less than typical carbon fiber should be able to hold because the material used in the rocket was donated from old stock and the manufacturing procedures were not perfect because of lack of funding. The rocket therefore has a six-layer lay-up pattern of [0/90, 0/90, +30/60, -30/60, 0/90, 0/90] and weighs only 700 lbs. With a max acceleration of 7g’s the rocket will provide a dynamic safety factor of 3.3 and a superior design to all others tested.

Due to the thin walls of a carbon fiber tube without core material, buckling became an important factor to consider in the design. A compression test was done on a small cylindrical section of the body. Placing a sample of the final lay-up in a compression machine and pressing down axially with a uniform force around the cross-section did this. Stiffness is proportional to the cube of the thickness and therefore an increase of 0.25 inches in thickness will make the body 140 times stiffer in bending. In order to conserve weight the thickness needed to be increase by a lightweight material. Strength of the core material is not a factor and the material needed to withstand the high temperatures necessary for curing the composite. Nomex honeycomb ¾ inch thick was used with three layers of carbon fabric on each side.

To determine the delamination danger and void content in the material, some of the broken tensile samples were viewed under scanning electron microscope. This made it possible to examine the material very closely and look for inconsistencies in the resin. Results showed excellent consolidation of fibers. The resin attached well to the fibers and there was no sign of delamination.  To eliminate delamination at the edges of the tubes where the honeycomb was expose the ends of the tubes were sealed with carbon fiber tow and resin.

Fabrication

Carbon fiber must be formed on a mandrel and then cured for the resin to harden and set the material in shape. For the rocket a cardboard mandrel was chosen. Sono tubes used for concrete forming are inexpensive and can be dissolved to release the part when finished. The cardboard can only take small amounts of radial stress before buckling and so filament winding could not be used. Filament winding induces high stress on the mandrel as it winds the carbon fiber similar to line on a fishing reel.

Roll wrapping is a process where the composite fabric is rolled around a mandrel in sheets. Material is placed on a flat surface and the mandrel is then rolled over the top of it in order to wrap the material. Careful attention was paid to keeping delaminating bubbles out of the material and keeping the material in the correct directions.

A water jet was used to cut the ends off the tubes after it had cured. The size of the water jet bed was a limiting factor on the length of the tubes. In order to make a clean cut the tube was place on a bar and rotated in the bed.

Conclusions and Recommendations

This paper has discussed the analytical and experimental methods used to design a supersonic, hybrid rocket. It has also discussed manufacturing issues and challenges for the composite materials. The driving factor for the design was manufacturing ability and structural integrity. Design of the structure through analysis, testing that design, and then careful manufacturing has produced a superior airframe that meets and exceeds its requirements. The rocket is prepared and ready for launch with great expectations.