Every year since 2018 we at Propulse NTNU have been designing and building rockets to compete at international rocket competitions. So far we have competed at Spaceport America Cup (SA Cup) three times, and the European Rocketry Challenge (EuRoC) once. This June we competed in SA Cup 2022 with Birkeland, and came 2nd in our class and 3rd place out of all 152 teams!
Team 2022's rocket was called Birkeland, named after the Norwegian scientist Kristian Birkeland, the "father" of Aurora Borealis. Approximately 10 months after we started development in September of 2021, we competed in Spaceport America Cup 2022, where we came 2nd in our class, 30'K COTS, and 3rd overall in the competition.
The fundamental task of the outer structure is to provide structural integrity, and to ensure a stable flight. Birkelands outer structure comprises two fiber-composite airframe sections, a nose tip, coupling tube, boat tail, and four symmetric, wedged trapezoidal fins. It is through the boat tail and aft airframe that the motor force is transferred to the rest of the rocket.
The forward airframe is made out of radio transparent fiberglass, whereas the aft airframe is made out of carbon fiber. Both sections are tapered, following a von Kármán ogive to minimize drag. The four fins are made of carbon fiber, and mounted to the bottom of the aft airframe with aluminum fin brackets. The nose tip functions as an aerodynamic continuation of the upper airframe, and is made out of aluminium to be able to withstand the high temperatures of super sonic flight.
Stetind was the name of team 2021's rocket. In June 2021 the team competed in the digital Spaceport America Cup 30'K COTS class and won first place in its category, and 2nd place overall among all the 75 participating teams. 4 months later, in October, the team launched Stetind at the European Rocketry Challenge and won the 9000 meter SOLID class.
Team 2021 started the project with a new structure of the techical systems, it being divided into; Inner structure, Outer structure, Recovery and Avionics. Each of these systems have their role in the rocket which you can read more about below.
The team started conceptualizing the design of Stetind in September of 2020. Design continued to April of 2021 when after 3 design reviews, many iterations, tests and protypes, the final design was complete. Production of many of the components had started as early as February of 2021, and continued throughout the summer, with a full rocket reveal taking place mid August.
The rockets airframe consists of highly optimized parts for supersonic flights up to Mach 1.7. To minimize drag, a von Kármán shaped nosecone and aft section have been selected. The trapezoidal fins will ensure stable flight conditions during the ascent phase and increased ground clearance during impact from the recovery phase. The fins double wedge angle reduces drag from the shockwaves as well as higher lift performance in the supersonic regime, leading to better stability. The boat tail made of aluminium will lower the overall drag by reducing the wake generation downstream. Forward airframe is made of glass fiber to make radio communication with the ground station during the whole flight possible. Aft airframe, as well as the fins are made of carbon fiber due to the materials high strength to weight ratio.
These technical design decision have been made based on highly complicated numerical analysis using both empirical methods and software such as computational fluid dynamics. Detailed knowledge of both the flight and aerodynamic characteristics, such as pressure, energy and temperature, have been used in an iterative process to ensure that the rocket has the highest possible performance during flight.
Both forward and aft airframe have been produced using a method called winding, using a mandrel and a robot-arm, whereas the fins is CNC milled out of pre-produced carbon fiber plates. The latter production technique is also used for the fin brackets and the boat tail.
Team 2020 of Propulse were to compete in the SRAD 30K class of Spaceport America Cup 2020.
The goal was to integrate the ongoing hybrid-project into the competing rocket, called Mjølner. This required a complete restructuring of the organization to develop the rocket, which would stand around 5 meters tall and 18cm wide when finished. The rocket was designed to reach supersonic speeds with 5000N of thrust over 8 seconds, utilizing nitrous oxide and paraffin wax as fuel. Mjølner featured many SRAD components, such as a five layered PCB-stack. All components and subsystems were attached to a modular load bearing skeleton structure of aluminium, allowing for quick parallel development of components and subsystems.
The payload was a reaction wheel experiment, investigating the development of a control system that would manipulate the orientation of a free standing body. This type of system is often used in satellites.
Due to delayed development of the SRAD engine, the team changed to compete in the 30K COTS class with an Cesaroni Pro150 engine. This downscaled the size of the rocket substantially. Due to the COVID-19 pandemic, the rocket was never finished - despite many of the parts being produced.
Sleipner participated in Spaceport America Cup's 10k COTS category in 2019, and was the first rocket built and developed by Propulse NTNU. Sleipner was also the first Nordic rocket to launch at Spaceport America. The 3U CubeSat payload was a collaborative project with Orbit NTNU, where 1 unit consisted of an information gathering and processing system developed by Propulse NTNU, and the remaining 2 units were developed by Orbit NTNU. Orbit NTNU’s unit contained several components of their SelfieSat satellite to be flight and stress tested during Sleipner’s flight, with emphasis on vibration and temperature endurance.
Sleipner had a length of 2.4 m, a diameter of 18.5 cm, a liftoff weight of 18.8 kg and a maximum velocity around 1000 km/h. The propulsion system for project Sleipner was a COTS Cesaroni Pro-98 4G motor. The airframe sections were made of carbon fiber, the nose cone was made of fiberglass, the couplers and boat tail were aluminium, and the fins were constructed by aluminium, carbon fiber and foam core. An air brake system was placed in the coupler section between the upper and lower airframe section to control the apogee of the rocket. The recovery system employed a dual deployment system consisting of a drogue and main parachute, located inside and directly below the nose cone of the rocket. The drogue was deployed using Hawk CO2 systems, and the main chute was deployed with a 3-ring release system. Sleipner had an SRAD flight computer to tie all the different electronic systems together. A custom PCB was developed, featuring a 32 bit processor, IMU and barometer sensors, GPS, Bluetooth, and SD card and a radio transmitter. Ground station software was designed to receive and display real-time data during flight.
Sleipner was launched successfully and had a stable flight until approximately 1500 m altitude. Instabilities occurring near the maximum velocity of the rocket, likely caused by fin fluttering, caused the nose cone to be ejected prematurely, and the rocket to quickly decelerate. The rapid ejection of the recovery system at such a high velocity caused the chords of the parachute systems to fail, and the rocket fell back to the ground without a parachute system. Sleipner was retrieved from the launch site afterwards, and video files and flight data could be assessed to further analyse the flight.
Out of the 121 teams participating at Spaceport America Cup 2019, Propulse was one of 20 teams to be selected for the poster sessions, where the airbrake control system was presented.
Sleipner received 188.5/200 points for the project technical report and 210/240 points for design implementation.