DLR STERN Project

The STERN (STudentische ExperimentalRaketeN) program of the DLR (German Aerospace Center) is a program which gives student groups from German universities the possibility to participate in a real space project. Student groups have the mission to develop an own rocket within three years, which shall reach at least 3km altitude and the speed of sound. All rockets are launched from the Esrange Launch Site near Kiruna, Sweden.

STERN finances the student projects with funds from the BMWi (Federal Ministry of Economics and Energy). During the STERN program, several German student groups participated with solid, liquid and hybrid rockets which were successfully launched from ESRANGE. During the program, each team is controlled and supported by reviewers from industry and DLR during the development of their rocket and undergoes the space-typical review process. The main goals of the STERN program are to train students on real space projects, gather teamwork experience (which is inevitable for space projects) and give the students the possibility to develop new technologies and inventions.

The following picture shows the schematic structure of the Compass rocket. The structure of N₂ORTH will be similar.

Both rockets will feature a lightweight rocket engine with a CFRP combustion chamber hull. To evaluate the newly developed fuel, evaluation tests with a smaller engine called HyFIVE are done in advance. Later, an optimized version of HyFIVE will be used as propulsion unit of the demonstrator rocket. The experience gained with HyFIVE will be used to build the larger HyLIGHT engine.

	HyFIVE for first Compass Launch	HyFIVE max. Performance	HyLIGHT max. Performance
Nominal Thrust	800N	800N	10,000N
CC Pressure	25 bar	25 bar	25 bar
Operation Time	6s	10s	25s (TBD)
Specific Impulse	220s*	220s*	>210s (TBD)
Fuel Mass	650g**	900g**	21.5kg (estimate)
Dry Mass	950g	950g	16kg (estimate)

*: Measurement on test bench; **: Includes additional fuel (safety factor); TBD: to be determined; Estimate: based on current CAD design

In-house developed Fuel

The propulsion team has developed a new, polymer-based fuel for its hybrid rocket engines. This fuel is non-toxic and castable at room temperature. In addition to excellent fuel properties, this will enable HyEnD to produce the fuel grains with high reliability and low risk.

Lightweight Engine Design

For our project, we decided to use a filament winded combustion chamber design. All casings are produced within our facilities with our new filament winding machine.

Testing our Engines

All tests are conducted at the DLR Institute of Space Propulsion Lampoldshausen at Test Bench M11.5.

The rockets will feature a lightweight oxidizer tank that is part of the hull. Therefore, the structure team develops a concept of a wound tank mostly out of CFRP. With calculation methods like the classical laminate theory and tests it will be assured that it is capable of safely carrying high-pressured oxidizer and strong enough to be part of the load-bearing rocket hull.

The shape of the nose cone is based on the HAACK series and will be realised with aramid fibre. To achieve a perfect nose cone the team manufactures prototypes using our winding machine and a positive mold.

The team is also in charge of embedding all the systems inside the rocket and link them to other components or the outside if necessary. The CAD model of the whole rocket is of great help to keep an overview and get an idea for the system integration.

With simulations and calculations the rocket’s performance is getting optimized while also minimizing its structural mass. This includes FEM and CFD analyses to verify the airframe and a trajectory analysis showing the aerodynamic performance and the flight path.

The fluid system takes care of the piping inside the rocket. The main tasks consist of the refueling interface to the GSE, securing the pressurized system and connecting the tank to the engine. The focus is on the development of a lightweight, compact and reliable system.

The fluid system of the N₂ORTH and demonstrator rocket consists of the refueling interface, overpressure protection, release valve, main valve and the piping.  The release valve and the main valve are in-house developments and the rest are purchased parts.

At the moment, prototypes are being manufactured and already tested. Preparations are also being made for overall system tests.

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The recovery system is designed to ensure a safe landing of the entire rocket. For this purpose a two-stage parachute system was chosen. This consists of a drogue parachute and a main parachute. The drogue is ejected by a mortar like ejection mechanism at apogee.
After sufficient deceleration the main chute is pulled out by the drogue chute. To ensure that the drogue parachute is triggered and can open safely, numerous tests are carried out with our specially developed mechanism.

The entire recovery bay is located in the upper part of the rocket. The following picture shows a schematic of the structure and the lines.

The avionics need to fulfil multiple tasks, mainly activating the pyro lines firing the recovery system as well as collecting as much data as possible to allow further analysis after the flight.

It consists of many different modules, each one dedicated to a specific task e.g. gathering trajectory data. The systems can communicate between each other to allow the different modules to send their measurement data to a central microcontroller. The collected data will then be transmitted to the ground station.

In order to obtain pressure data of the engine and tank with a high sample rate, a custom module is designed to read and store the measurements with a sample rate of up to 10 kHz. This enables a detailed frequency analysis after the flight, making it easier to compare the behavior of the rocket in flight with the previous tests and simulations.

Another part of the avionics is dedicated to the powertrain. A stable and redundant power supply is extremely important. A loss of electricity will render the whole mission unsuccessful. The power train is customly designed to prevent a variety of common problems, like overvoltage and overcurrent as well as being able to switch between two sets of batteries and voltage regulators.

The parachute deployment is done via two parallel Telemegas. They will detect the apogee and trigger the recovery mechanism. Both will send additional data regarding the trajectory.

The ground support team is responsible for the infrastructure on ground. We provide a safe launch platform. We want the system to be modular. Therefore, it is easy to assemble and to transport. The open-end design provides a platform that can be adjusted and repurposed for future projects.

The GSE control system has four main tasks:

1.            Controlling the fueling process through valves and sensors

2.            Communicating with the rocket on the ground via a CAN-Bus

3.            Lighting pyrotechnic devices during the ignition sequence

4.            Controlling mechanical functions of the launch platform

To accomplish these tasks, the GSE control system is split into three different custom PCBs controlled by a Raspberry Pi 4. This allows for the use of higher quality shielded connectors while still maintaining high density. Furthermore, the pyrotechnic section can be galvanically separated from the rest of the system to prevent any risk of damaging the  delicate electronics with high currents.

The Raspberry Pi is running C Code and communicates with the control computer over Ethernet. It can run completely independently which allows for automated actions and fail safe in case of communication issues.

The deployment of the oxidizer will be realized by our oxidizer supply system. It is crucial to deliver the right amount to ensure that the engine will operate within the defined parameters. Therefore, the nitrogen oxide used as oxidizer will be stored in an intermediate gas tank. The weight of the tank will be measured. Combined with the level indication inside the rocket tank the amount of oxidizer shall be determined.

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