IREC 2018 was the second SSI IREC team to participate in the Intercollegiate Rocketry Engineering Competition, placing second in the 30k commercial off-the-shelf motor category. The rocket was named Redshift and featured an avionics bay with a long-distance radio system, a reduced-diameter recovery system, a fiberglass airframe with a carbon fiber fin lay-up, a powered decoupling mechanism, and a software-defined GPS payload.
Overview
At four inches in diameter and 128 inches long, the rocket design used a minimum diameter airframe to house the Cessaroni Technology N2900 motor. With a wet mass of 58 pounds (dry mass of 27.8 lbs + 1-7 lbs lead shot ballast), the rocket was built to fly to 30,000 feet.
Goals and Requirements
The goal of the rocket design was to be capable of delivering an 8.8 pound payload to an altitude of 30,000 feet while employing as much student innovation as possible.
System Design
Avionics
The avionics system consisted of multiple custom printed circuit board assemblies (PCBAs), which used direct board-to-board interconnects to eliminate the use of wires.
The main boards in the avionics system were:
- Skybass, an altimeter developed by John Dean
- Motherboard, which was the main interface between all other boards and contained the power distribution, arming systems, and e-match firing pathways.
- Daughtership, a board for mounting the StratoLogger and Raven COTS altimeters.
Recovery
At apogee, the recovery system deployed a small drogue parachute for a swift, controlled descent that minimized horizontal drift. The deployment of the drogue was triggered by a CO2 canister deployment mechanism.
When the rocket reached an apogee of 1,500 feet, the main chute, which was retained in the recovered tubing by the Tender Retention System, was deployed for a soft landing.
The recover tube was selected to have a reduced diameter, smaller than that of the rocket airframe. This reduced diameter was chosen so that the parachutes would be able to deploy through the restriction in the airframe size caused by the staging ring.
Structures
The structure of the rocket was constructed using COTS fiberglass tubing, with a student-built carbon-fiber fin lay-up. The structures team attempted to build a custom airframe using the X-Winder filament winder, but was unable to produce a usable airframe in time using this method.
Payload
The payload flown on the rocket was a software-define GPS experiment. The goal was to use a COTS software defined radio USB dongle (RTL-SDR), connected to a Raspberry Pi Zero, to capture raw samples from the GPS L1 spectrum. One the rocket is recovered, the raw samples could be downloaded from the Raspberry Pi's SD card and run through tracking algorithms to solve for the rocket's position throughout the flight. The advantages of such a system were specifically for rocketry applications, namely that such a system could be more resistant to losing lock with satellites during high accelerations during flight due to high rates of doppler shift, and that it could provide a way around the GPS COCOM restrictions. The COCOM restrictions limit consumer GPS operation under high velocities and altitudes, which poses an issue for SSI's Spaceshot project.
Staging
The staging project was a decoupling mechanism developed for high-speed decoupling. A stepper motor drives a lead screw, on which a nut rides, lifting and lowering a set of lever arms that turn the vertical motion of the nut into horizontal motion. This horizontal motion actuates a set three clamps which affix two metal rings together. These rings are each mounted on one half of the airframe. By backing off the clamps, the rings, and consequently the airframes, are free to separate.
Issues with the staging system arose when it was near to its completion. The greatest problem was that the stepper motor would have to provide a constant holding torque in order to keep the rocket rigid during the period of time between integration and when separation was required during launch. This application of holding torque required a significant amount of power, and the original battery size resulted in a 20 minute maximum lifetime. Given the lack of adequate space to add enough batteries for a multi-hour lifetime, required in the event of launch delays, and uncertainty about the speed with which the system would drain the battery, it was decided before test launch three that the project should be cut from the final rocket design.
Launch Operations
Launch Operations handled all launch logistics, including food at the launch, lodging for multi-day launches, checklist organization, pre-launch packing, and transportation.