Tufts SEDS Rocketry Team
CARM - Spaceport America Cup 2024
The mechanical subteam of the CubeSAT-Adorned Rocketry Masterpiece (CARM) project was responsible for designing and manufacturing the rocket’s airframe and all internal structural components. Throughout the year the team increased in numbers and learned how to improve upon design through simulation and test flight data.
The team chose to make the primary airframe components out of filament-wound G12 fiberglass. High strength-to-weight ratio needs and competition regulations limited our choices to composite materials; the commercial availability, lower cost, and RF transparency of these fiberglass tubes made them an excellent option. The 6-inch inner airframe diameter was the smallest diameter that could accommodate our competition-mandated payload (which had a 10cm x 10cm footprint) and its mounting adapter. This fiberglass airframe allowed CARM to easily survive flight loads (including the motor’s high thrust and parachute deployment forces), and potential landing impacts. The low density of fiberglass also helped keep the total vehicle mass at a manageable level within thrust-to-weight ratio requirements.
There are three main sections of the airframe: the booster section, the avionics bay, and the forward airframe section. All three of these were based on the same 6-inch diameter fiberglass tubes, referred to as “body tubes.” For all sections, we cut the body tubes to size with a bandsaw and hand tools before preparing the mating surfaces and drilling the mounting holes for bulkheads and section-to-section attachment. We took great care to square off the ends of the tubes as best we could, and to match the ends of each successive airframe segment to ensure the flight loads would be evenly distributed throughout the entire diameter of the airframe joints. These joints were designed to separate in-flight to deploy the parachutes.
The booster section consists of the fin assembly, thrust plate, and aft body tube section. The booster is the primary contributor to the rocket’s stability and overall aerodynamic performance, and was therefore a critical area of focus in CARM’s mechanical design The fins were mounted in a through-the-wall configuration to maximize strength and rigidity, and to improve alignment during manufacturing. On the outside of the fins, we added tip-to-tip layups in a 0o- 45o- 0o angle orientation. These layups were intended to increase fin stiffness and strength to reduce the risks of fin flutter and landing shock, respectively. Since this was our first time building a rocket of this size, we decided to take extra precautions and construct two booster sections.
The first booster section would have fins made from 3/16” G10 fiberglass and have extra thick epoxy fillets to maximize strength. The second booster section would be used for the competition, and have thinner 1/8” G10 fiberglass fins. This second booster section was intended to be lighter with a lower factor of safety to maximize performance. G10 fiberglass was chosen for the fins due to its stiffness and weight. In each booster section, we cut out the fins from a G10 fiberglass composite sheet using a dremel, and then aligned them using a set of 3 internal centering rings to ensure precise 90o angles between each of the fins. This assembly was bonded with epoxy to a 98mm fiberglass motor mount tube (MMT), sized for the competition motor we planned on using. An alignment ring was added to the forward end of the MMT, helping the tube remain aligned in the rocket. This made the assembly process easier, and minimized the potential for any bending moments induced by an off-center thrust. We cut out fin slots in the aft airframe using a dremel over a ventilation table to collect hazardous dust. The fin assembly was aligned within the airframe and bonded with epoxy root fillets between the airframe and fins for structural support. The fillets were applied carefully and later sanded, to provide a good bonding surface for our tip-to-tip layups. Booster 1’s layups consisted of 4 layers of 7.5 oz fiberglass fabric, in an alternating 0o-45o-0o-45o orientation, each coated in FibreGlast 2000 epoxy. Peel-ply was applied to the final layer for a smoother finish to reduce drag. The final finish had a fair amount of imperfections, localized voids (especially in the more curved regions along the fillets), and a very rough surface finish caused by an insufficient amount of laminating epoxy for the final layer, which allowed us to learn from our mistakes for the second booster.
When it came time to construct Booster 2, we used 4 layers of fabric again, however each layer contained less epoxy and a 5th cosmetic layer of 2 oz fiberglass fabric was applied. Our layups for Booster 2 were much more efficient and our process was improved from the experience we had gained working on Booster 1. The end of each booster section contained an aluminum thrust plate, intended to absorb the force from the motor throughout the flight. The thrust plate was manufactured on a lathe and then fin slots were cut precisely with a Bridgeport (manual) Mill. We went with a grooved inner wall design to increase effective surface area for adhesion to the airframe.
CARM’s recovery system consisted of a parachute dual deployment system, housed within the airframe and avionics bay. The drogue chute was stored in the aft airframe, main chute in forward airframe, and the electronics housed in the avionics bay. Each parachute was linked to a U-bolt mounted on a 3/8” thick aluminum bulkhead, each screwed into their respective airframe section. The other ends were attached to the avionics bay, which contained the ejection charges necessary to separate the rocket at apogee and during descent. Each parachute was held in a deployment bag, to allow for an easy removal from the airframe tube while minimizing tangling. The size of each parachute compartment in the airframe was designed to maximize space efficiency, effectiveness of the black powder charge for separation, and the physical constraints of the parachutes themselves.
Competition Launch
We put CARM through 3 test flights to collect flight data and improve design between each launch. Such examples include improvements in wire management and labeling, parachute deployment analysis, and validation of simulation. As we got experience launching CARM, we built up a thorough pre-launch checklist, streamlining our process and ensuring the rocket was properly checked before being put on the launch pad. Our continuous design improvements through test launches, construction of two booster sections, and precise engineering decisions allowed us to maximize our success for the final competition launch and land a respectable placement as a first time team.
