The Rocket - "Dottie"
Dottie is an 84-inch rocket with a 72-inch airframe made from LTM 45 carbon fiber. The rocket will be built in 6 sections and it will house all electronics within 3 bays. The centering rings will be machined from aluminum and designed to be removable to allow access to the bracket system as well as aide in the wiring of the strain gauges. All four brackets and grid fins will be bolted in to place on the booster section.
Dottie has been divided into six subassemblies for simplicity of construction. Each subassembly will be built individually, with common materials and hardware to ensure the rocket will fit together easily. Extra precautions will be taken during fit testing to ensure that each connection can be made, allowing the Rocket Girls to have a successful build. A flowchart of the rocket construction has been created to ensure that each subassembly is constructed properly and to limit waste of materials. Following the flowchart are descriptions of the subassemblies, its components, and interfaces.
The Rocket Girls will design and test grid fins of different aspect ratios and will use the one that has the closest coefficient of drag (CD) to the average of the previous two rockets. Grid fins are fins perpendicular to the airflow with a lattice of struts to create a grid within the fin. They are used on several different missiles such as the MOAB (Massive Ordinance Air Blast) and have been tested on the Orion Launch Abort Vehicle (LAV). The purpose of the experiment is to obtain data from strain gauges placed on the fin attachment to further analyze the aerodynamic forces on the grid fins.
Changes to Design since Preliminary Design Report
Several changes have been made to Dottie since the PDR. Due to concerns of the large kinetic energy at landing, the entire rocket was redesigned to redistribute the weight across more sections. This allows each section of the rocket to comply within the restrictions set forth by NASA USLI. Major changes that have occurred from the redesign include the following: the rocket is now 7 feet in height, and it will be launched on either a CTI K570, K660, L730 motor. The recovery system parachutes have been enlarged, and the Rocket Girls will now use a drogue parachute instead of a streamer. The decision to enlarge the parachutes was made to lower the velocity at landing.
Another key change involves the transition from a typical motor mount system to a removable centering ring system. This change allows easier access to the bracket and wires, as well as eliminating the requirement of a wire tunnel constructed of foam. Additionally, it reduces the weight of the booster section and reduces the kinetic energy at landing. In order to have removable centering rings, the material changed from birch plywood to aluminum. Another material change that has occurred is all bulk plates will now be made of G10 fiberglass.
After the redesign of the full-scale rocket, it was necessary to redesign the sub-scale rocket. The sub-scale, Thor, remained an exact half-scale of Dottie, making the new height 3.5 feet. This was done because the Rocket Girls felt it was important the sub-scale be as similar as possible to the full scale to allow for the flight data to be anchored to the flight simulations in RockSim. Consequently, the payload has a direct impact on the flight of the rocket and could have an unforeseen effect on the flight that is not demonstrated in the simulations. Another design change to the sub-scale was that the parachute was no longer deployed from the nosecone, and the recovery system of rocket only had a single deployment.