ASTRA develops a hybrid rocket using Kistler measurement technology for rocket engine testing

Back in 2021, young engineers from the University of Bremen (Germany) joined forces to design and build a recoverable hybrid rocket that can make it all the way into space. Virtually from day one, the student team – named ASTRA – were supported by Kistler measurement technology including sensors, DAQ and software, e.g. for rocket engine testing. And now, in the run-up to the EuRoC 2024 rocketry competition, ASTRA is about to test its second prototype: Karma 2.0.

“You can trip over the stones in your way, or you can use them to make something beautiful.” (Goethe) A similar thought must have crossed the mind of Jesaiah Coy when he came up with the idea of building a hybrid rocket during the pandemic lockdown back in 2021, while the universities were closed for a long time. What started out with brainstorming sessions among Jesaiah's fellow engineers and space enthusiasts has now evolved into ASTRA: the Association for Space Technology and Research Applications. Over 50 people are involved in the organization (most of them part-timers): they include project management teams taking care of legal aspects, HR, funding and event organization, as well as technical teams focusing on aerodynamics, avionics, rocket engine testing and propulsion, recovery, ground systems, and structure. 'Karma' is the name chosen for ASTRA's rockets, and 'Karma 2.0' is the 2024 prototype. ASTRA is located in the north German city of Bremen where most of its participants are studying, but the association has already built up partnerships with rocket testing facilities in Delft (Netherlands), Baden-Baden (Germany) and at the German Aerospace Center (DLR) site in Lampoldshausen.

Reaching for the Karman line – with a hybrid rocket

Jesaiah Coy, ASTRA's founder and Chief Technical Officer (CTO), heads up most of the test campaigns and other activities such as events and competitions. He takes up the story: “Our main goals are to learn, and to bridge the gap between theory and practice. It's incredibly instructive and rewarding when you try out the things you learn in the classroom in actual practice. And there's so much more to it than just building a rocket: logistics, partnerships, legal aspects and, above all, budget planning.” ASTRA's long-term goal: to build a rocket that can reach the Karman line, 100 kilometers above sea level. Named for the Hungarian-American mathematician, aerospace engineer and physicist Theodore von Kármán (1881-1963), this line is widely considered as the approximate boundary between earth's atmosphere and space.

To achieve their goal, the team pursues a hybrid rocket approach: this is less complex than a liquid-only rocket, and it avoids the regulations on solid rockets that are particularly strict in Germany. “There is only one liquid involved – nitrous oxide – which is self-pressurizing,” Coy explains. First steps on the journey to the Karman line included launches to altitudes of three and ten kilometers. “In 2023 we participated in the EuRoC competition for the first time. We'd successfully completed all the testing and preparatory procedures, but then we ran out of time and we weren't able to launch the rocket in the end. So naturally, we want to go one step further this year.”

For rocket development and rocket engine testing, ASTRA relies on measurement technology from Kistler, among other providers. The two partners made initial contact at the Space Tech Expo Europe (also in Bremen) in November 2022, when ASTRA presented its ideas to the public for the first time. From then on, Kistler has supplied sensors and measuring chains for the young engineers as well as valuable assistance – even including onsite support. Coy again: “Frank Busch was very interested in our project from the beginning, and he’s maintained his interest ever since then. It’s really a huge advantage for us to be able to apply proven measurement technology from Kistler, which is well-known throughout the space industry.”

Rocket engine testing and thrust measurement with Kistler

At the time of writing (late February 2024), ASTRA's Karma 2.0 hybrid rocket is in phase B of the construction process, so rocket engine testing and wind tunnel testing will soon get under way. With a height of about 4.5 meters, the latest rocket should be 2.5 times more powerful than the first ASTRA prototype, with a somewhat lower weight. Four 9021A piezoelectric force sensors from Kistler are used to measure thrust: on the test stand, these are installed between a plate and the rocket's bulkhead. Coy explains: “The sensors work very well in this setup. In combination with the data from the Kistler pressure sensors – which measure the pressure inside and outside the combustion chamber, as well as upstream of one of the injectors – we're able to calculate the specific and total impulses of our hybrid rocket. This data is highly relevant for performance optimization.” 

“These measurements are far from simple and, fortunately, we were able to help the ASTRA team improve the sensor integration and measurement chain,” according to Frank Busch, Sales Engineer at Kistler. “Their data quality was not so good at the start, and they weren't able to get the desired results – so I went along to the DLR site with two colleagues to support the student team.” The setup was changed and a different type of sensor was used, with a switch from piezoelectric to piezoresistive pressure sensors to reduce the influence of thermal shock – and after that, everything worked out very well. Coy again: “In the end, even the onsite test engineers were very impressed, not only by our data quality but also because no less than three engineers from Kistler came in person to support a student initiative such as ASTRA.”

Complete measurement technology – from sensor to software 

All the sensors are connected to the KiDAQ data acquisition system, which can easily be linked to a standard laptop running the KiStudio Lab software for parameterization and configuration. To complete the measuring chain, ASTRA also has access to Kistler's jBEAM software for data visualization and analysis. Shancia Andrew has been working at ASTRA for almost a year as a member of the propulsion department. She reports: “Working with the sensors, KiDAQ and the software is really easy: I can do almost everything from my laptop. The data quality is very good and it's really convenient and efficient to perform analyses with the jBEAM software.”

ASTRA aims to have the propulsion system ready by May 2024 so the focus can then switch to wind tunnel and vibration testing. Although Kistler's acceleration sensors have not yet been used in the project, they could well play a useful part at that point. “We learned so much about measurement technology and testing during the last year that we're now in a much better position to optimize our hybrid rocket and fulfil the requirements for EuRoC,” Coy points out. In the course of this testing phase, ASTRA will also have an opportunity to test the new 4011A piezoresistive pressure sensor from Kistler, which offers advantages in handling and accuracy thanks to its integrated digital temperature compensation.

Drop tests and mock launches are planned during summer before the second attempt at EuRoC in October 2024. A success in this competition would be a very important milestone on the journey to the Karman line, which the team hopes to reach in 2025 with a rocket that is more than eight meters high. “One major challenge still facing us is to find and pay for a space port for such a launch. Obviously that isn't possible in Germany – for safety reasons, you need to be closer to the open ocean,” Jesaiah Coy explains. If they succeed in reaching the Karman line, the ASTRA team will be only the second student initiative ever to achieve such an ambitious goal – and the first to do so with a hybrid rocket design.