The propulsion group of TU Wien Space Team is working on liquid fueled rocket engines, designed to be used in future rocket projects.
Our division started working on project “Thor”, our first try at building our own rocket engine, in 2017. More on that at the end of this page. Since then a lot has changed. Work on LE-01 during project “Thor” was abandoned in favor of building a new engine. We needed a modular system for testing a variety of injector designs. A design was proposed, which marked the birth of Proteus. With this new engine, we’re able to test multiple common injector designs. Currently, we’re set for showerhead, impingement, pintle and pre-mixing swirler configurations. This flexible design approach allows for side-by-side comparisons across a wide range of possible configurations, especially considering the already huge number of variable parameters in every single injector design itself. If you then take the use of different pressure settings and resulting flow rates into consideration, the amount of resulting testing sessions becomes enormous. This testing effort and the integration of upgrades on our small test stand kept us quite busy. Upgrades included additional and improved pressure sensors, thrust vector measuring, flame protection shield for fuel tank and electronics, among other things.
Because of safety reasons and concerns regarding infrastructure, our decision on the subject of fuel fell on a combination of ethanol and pressurized air. The reduced percentage of oxygen in the air, compared to N2O, improves safety, especially during the first tests. The oxidizer was subsequently exchanged for Nitrox (pressurized air with higher oxygen ratio) performance-wise this combination is similar to that of ethanol and N2O.
Injector and Thrust Chamber
As already mentioned the injector is highly modular in order to enable the testing of a wide range of concepts and configurations. The oxidizer enters the assembly from the top through a long tube. In the showerhead, pintle and impingement configuration this tube ends in six small holes acting as the oxidizer injection ports. In the swirler configuration this tube holds the swirler and the fuel is injected at the lower end through three ports in the wall. In the other configurations the fuel is injected directly into the chamber after entering through a line on the side of the injector and being distributed to all injection ports by a ring shaped channel. To regulate the mass flow we are using venturi nozzles placed in the propellant lines that operate at critical condition in case of the gaseous oxidizer and at cavitating condition in case of the liquid fuel. This measure decouples the mass flow rate from the chamber pressure making it easily controllable through the tank pressure.
For the first couple of tests a simple stainless steel pipe was used as a combustion chamber, with a machined block of steel acting as a nozzle.
Ignition and Controls
Ignition and sequencing of the test run are initiated and controlled digitally from a safe distance. The electronic control valves as well as the self-built arc or pyro ignition are controlled via Ethernet and follow a previously defined test sequence. In case of an electronics failure, both oxidizer and fuel flow can be interrupted manually.
The currently installed sensors include pressure sensors and a thrust vector measurement system. Currently, three piezoelectric pressure sensors are installed, measuring fuel, oxidizer and combustion chamber pressure. The thrust vector measurement system consists of three load cells sitting between the engine and the top frame of the test stand, measuring not only the total thrust of the engine but the direction as well.
Our hardware-based development makes access to a secure testing environment essential. The test stand is suitable for small engines with up to 200 N thrust, both cold-flow and hot-fire tests can be performed. The setup has already been used on previous missions to test key components such as electrical valves, piping and various ignition systems for use in a large engine. The test stand is fully automated and can be operated from a safe distance. Under certain conditions, the auto-abort function automatically interrupts the test run; manual abort by hand valves is always possible. The test setup is regularly updated and improved, and will play an important role in component testing and design evaluation for future missions. Team members can also be introduced to the system before working on larger test rigs in the future. In addition, the small test stand can also be made accessible to the public in the future.
The cold-flow tests preceding the first firing serve the purpose of ensuring the operational readiness, an extensive check of the system and the sequences as well as the calibration of the control settings. Test scenarios and various series of measurements are carried out; first with water and later with ethanol. Within the framework of cold flow tests, a transparent counter-pressure-chamber allows the flow characteristics to be observed and documented under realistic conditions. This allows the determination of the injection behavior of the various injector configurations.
The first ignitions take place without combustion chamber and nozzle. This allows a direct observation of the flame and first statements about its stability and the quality of the mixture of fuel and oxidizer.
The next step in the direction of working propulsion consists of firings in the test chamber. For hot fire tests a modular mount allows the use of different test chambers. Here the properties of the flame, a reasonable mixing proportion to the position of the igniter, as well as the optimal igniter sequence.
While we obviously couldn’t test every single configuration, we eventually isolated a group of configurations which meet our requirements. With more tweaks to our settings, we achieved our first stable combustions in late summer of 2019, which are further described in our Hot Fire Testing blog post. For these tests we’re still using 36/64 Nitrox. We’re expecting even better results using higher percentage Nitrox.
Our teststand remained a compact, non-stationary unit, with most of the components directly attached to it. With all the upgrades included, room for installing additional upgrades is becoming a growing issue.
Before and after every testing session the teststand is moved out and in to storage, solving the problem of missing stationary testing grounds. While this may seem like a huge hassle to outsiders, and it sure is from time to time, it is a perfect example for our adaptive approach. You’ve got to work with what you have.
The precursor project: the propulsion project “Thor”
This project is the result of the endeavour of the TU Wien Space Team to create its own rocket motor for future experimental rockets within the team. Currently we are using commercially available solid rocket motors. The dream of our own rocket engine resulted in the founding of this work group, which is working on its own rocket engine.
First tests in December of 2017
On December 17th the propulsion team completed tests on fuel atomization, as the effectiveness of the atomization has a big effect on the combustion process and is therefor an important criteria for the efficiency of rocket propulsions.
Water was used as a less volatile replacement for fuel. Using two nozzles and compressed air it was shot through a nozzle and nebulized only a few millimeters after exiting the nozzle. The angle between the two nozzles, as well as their distance was varied. Using a camera–setup. During the tests the size of the water drops was captured with a camera, while angles and distances between the nozzles were varied and the pressure was held constant at 10 bar. Two months before a similar experiment had shown that an angle of 130-140 degrees was the optimum in terms of effectiveness and feasibility.
Summary of Progress in 2018
The advances in the Propulsion group of the TU Wien Space Team are hard to miss. In the year 2018, the construction of the first rocket engine test stand of the TU Wien Space Team was implemented. The first engine prototype LE-01 was manufactured and mounted on the test stand.
After almost three months of planning and construction, the first cold-flow test was carried out on March 25, 2018, with water being used instead of the fuel for safety reasons. Although some “teething troubles” were noticeable, these were mostly quite easy to fix and the test series was a total success.
After having prepared accordingly, first Hotfire tests were tackled. The first Hotfire test was conducted on April 7, using an ethanol / compressed air mixture. Later, the compressed air was substituted by Nitrox. It showed that both the ignition and the holding of a stable flame are anything but trivial. The Hotfire tests were initially performed only with the injector and later with an open combustion chamber. Thanks to numerous optimisations, the results could be continuously improved, but no successful ignition test with attached exhaust nozzle succeeded.
The insights gained from the tests and optimization with Nitrox now serve as the basis for the further course of the project.