HYBRID Progress Report
Tobias Knop, Stefan Powell, Ralph Huijsman, Rob Hermsen, Jeroen Wink, Robert Werner, Johnnes Ehlen, Felix Lindemann, Kapeel Samarawickrama.
A BRIEF HISTORY
Project DAWN was started by a group of students from the TU Delft in 2010. The initial objective was to investigate the concepts of a hybrid propulsion system. For this purpose a simple test setup was designed and built (see figure 1). The oxidizer and fuel combination used are nitrous oxide and sorbitol respectively. After some successful tests, several components of the test motor were revised to see how certain changes would improve the performance/workings of the motor. A lot of the changes made in the design of the motor were carried on to later stages of the project.
Figure 1. Early test setup firing
DAWN ACADEMIC MINOR
DARE initiated the setup of a minor in the academic year 2011/2012 where almost all of the DAWN team members participated. This minor focused on improving the test setup and aimed at developing a flight ready motor. A new test bench was constructed which would simulate a flight motor more accurately (see figure 2). While in the old test bench the motor was orientated horizontally, in the new test bench the motor is positioned in a vertical ‘flight’ orientation.
Another big improvement was the addition of a self made oxidizer tank. This meant that next to the motor, also the tank contents could be controlled. Also the environment in the oxidizer tank during the burn could be measured more accurately. Eventually the idea was to use the same tank design in an actual flight motor. The self made oxidizer tanks were tested and verified in the tank testing facilities at ALE (Advanced Lightweight Engineering) in Delft.
A heating system was implemented around the tank to further improve the controllability of the environment in the oxidizer tank. Since the pressure of nitrous oxide is related to its temperature, the pressure in the tank can be controlled by controlling the temperature in the tank. This way the tank pressure can be kept constant during the beginning of every test, and as such improve the consistency of every test.
To accurately predict outcomes of tests a mathematical computer model was constructed. This model allowed for further refinements of the motor design and its performance. After a lot of tests it was found that a fuel mixture of sorbitol and paraffin gives a higher performance while keeping approximately the same fuel density.
Figure 2. Test firing on the new test bench
FLIGHT MOTOR
At the end of the minor, the DAWN team was ready to take the next step and start working on an actual flight motor. The DARE minor proved very important in the development process of the final Stratos II motor design. It also provided the DAWN team with an advanced test bench on which a lot of new concepts were tested.
In the months after the DARE minor, the DAWN team started focusing on launching an actual hybrid rocket to prove that also the last part of the design process was achievable. Most of the motor had already been designed during the minor. The only thing that was missing was to integrate all the parts into one rocket.
Figure 3. Successful launch of the small hybrid rocket
The final design of the flight motor was kept conservative. Only well proven concepts were chosen for the actual flight motor. This meant that although different experimental fuel combinations were tested, the fuel for flight motor was to be plain sorbitol.
On the 19th of May 2012 the small hybrid rocket was successfully launched at the RJD launch days in Germany (see figure 3) as a technology demonstrator. The rocket reached a maximum altitude of approximately 1000 meters and had a maximum thrust of approximately 220 Newton’s.
DESIGNING THE STRATOS II MOTOR
The last step of fulfilling the Stratos II objectives is designing and building the full scale Stratos II motor. The design of the Stratos II motor is largely based on the flight motor as previously discussed. The main design philosophy was to keep things simple and cheap. As a result many of the designed parts are also optimized for simplicity.
Rough first estimate calculations showed that in order to reach the target altitude of 50 kilometers a more efficient fuel was needed. Many fuel tests were performed to see which fuel combination would be best suitable for the Stratos II motor. It was found that a fuel combination consisting of 80% sorbitol, 10% paraffin and 10% aluminium gave the desired performance.
A downside which affected all of the fuels was that they all had a relatively low regression rate (in the order of 1 to 2 mm/s). Having a low regression rate means that in order to burn the same amount of fuel, a longer combustion chamber is needed. In the case of the Stratos II motor it is more beneficial to have a short combustion chamber. A shorter combustion chamber will mean that the available diameter will be used more efficiently. A way to increase the regression rate was to use swirl injection instead of the previously used straight injection. This change resulted in an increase in the regression rate of more than 50%.
On the 12th of September the DAWN team successfully conducted several tests with the new fuel combination. The goal of these tests was to verify the consistency of the burns. The results from these tests can be seen in figure 4.
