Building a Better Rocket with the Help of Piezoelectric Sensors

The moment of truth: Students from the EPFL Rocket Team watch their liquid rocket launch. (Image: Kistler)

Airplane turbines and rocket engines are very powerful, hot and noisy and yet in need of extremely sensitive measurement technology. And they have another thing in common: They are most efficient when they run on a constant and even flame. Specialized measurement technology helps aerospace engineers improve combustion chambers and fuel injectors. In Switzerland, two ambitious student organizations have been using iterative pressure measurements to develop and build a significantly more efficient next generation of rocket engines.

On their test stand, ARIS hybrid engine testing team aimed at perfectly balancing the oxidizer injection. The measurements of dynamic pressure gave them the necessary insights to improve the combustion process. (Image: Kistler)

Every successful space program is the result of several years of development and countless tests. The acquired data enables an in-depth understanding of the physical events and allows for significant improvement of an aircraft’s or spacecraft’s efficiency and functionality. Engines are one of the areas where detailed insights make a significant difference in research and development. Inside the combustion and ignition chambers, it is common to find instabilities causing oscillations in thrust output — an undesired effect that may lead to inefficiencies. Optimizing ignition chambers to prevent these instabilities also holds a lot of potential to reduce emissions. Piezoelectric pressure sensors with high temperature resistance make it possible to monitor these instabilities in both applications extremely closely.

Based in Switzerland, two organizations of ambitious student teams show how it is done. ARIS from the ETH Zurich (Swiss Federal Institute of Technology Zurich) and the EPFL Rocket Team (École Polytechnique Fédérale de Lausanne) compete in a literal race to the stars with rockets they develop and build themselves. Keeping their goal of reaching orbit in mind, they showcase their progress in yearly international competitions. Throughout the year, these passionate engineers in the making invest many hours and even their weekends for testing and evaluating data to improve their rocket engines. Specialized measuring equipment plays an integral part in these tests. As an expert for dynamic measurements, Kistler supports both teams with expert knowledge, high-end piezoelectric sensors, and equipment such as cables, data acquisition systems, and software for analysis.

The sensors used for rocket engine testing must withstand extreme conditions. Kistler provided ARIS with temperature-resistant piezoelectric pressure sensors 601CAA. (Image: Kistler)

Measuring Dynamic Pressure: ARIS Tests Hybrid Rocket

ARIS is currently working on two different kinds of rockets — the bi-liquid rocket Prometheus and a hybrid rocket, ASTREA. In both rockets, the students use pressure sensors to monitor and reduce combustion instabilities. In the combustion chamber of ASTREA, solid polystyrene reacts with a liquid oxidizer, which is injected as a fine mist. The pressure of this injection is a crucial parameter for ensuring a constant burning speed of the solid fuel. The more the young engineers understand about the relation between the oxidizer injector and the combustion, the better they can control it.

The high data acquisition frequency of the piezoelectric sensors allows EPFL to gain a deep understanding of the rocket engine dynamics. (Image: Kistler)

“We placed one of the pressure sensors in the injector manifold chamber where the oxidizer enters the combustion chamber. Another one was placed on a test nozzle and set up to measure pressure in the combustion chamber. Because of the position of the sensors, the equipment needs to withstand very high temperatures,” Mathieu Sandoz, Hardware and Software Engineer of the ASTREA hybrid rocket engine team, explained.

Kistler provided the team with temperature-resistant pressure sensors 601CAA, which are specifically designed for dynamic pressure measurements with high thermal shocks. Still, the student team took every precaution to make sure the valuable measurement equipment would take no harm. “To avoid direct contact with the hot gases, we installed the sensors via passage mounting. Also, we created an acoustic model of the passage to determine the natural frequency and evaluate the influence on the measurements as suggested by Kistler,” said Sandoz describing the precautions. This step is important because, in this case, the natural frequency of the passage has an influence on the measurements in the combustion chamber.

Piezoelectric sensors are particularly well suited for dynamic and quasi-static pressure measurements in harsh environments with very high temperatures. Besides their robustness, they also feature an especially high sampling frequency, which allowed a detailed analysis of the acoustics in the combustion chamber and gave the students valuable insights into the combustion stability.

“Thanks to the data we were able to detect three different combustion instabilities during the whole duration of our test campaign. For instance, we diagnosed instabilities in the pre-combustion chamber and with the help of the acquired data, we were able to pinpoint these as the reason for an engine failure that we had experienced. After we had found the cause of the problem a solution was quickly implemented. This allowed us to proceed with our firing campaign without too much delay,” said Sandoz.

A Balancing Act: EPFL Tests Bi-Liquid Rocket Engines

In contrast to the hybrid and solid fuel engines, bi-liquid engines use both a liquid fuel and a liquid oxidizer. Since this kind of engine is more efficient, it is also the engine type used in most commercial rockets. The development of a bi-liquid rocket however is even more challenging in some respects: The pressure of the fuel delivery system must be balanced perfectly to achieve a constant and even flame.

The EPFL Rocket Team from Lausanne tests its rocket engines again and again on the institute’s test bench — the heart of which consists of 10 temperature-resistant piezoelectric sensors from Kistler. Three pressure sensors measure the pressure in the combustion chamber, two more monitor the pressure in the ethanol and the oxidizer injectors. Three load cells measure forces behind the thrust plate and additional pressure sensors and load cells monitor the two tanks.

It is not surprising that, at full power, the test bench generates 87 MB of test data per second. The KiStudio Lab software package, which includes the jBEAM post-processing software, records and analyzes the data. So far, the team has performed more than 250 tests with bi-liquid engines on the test bench.

Florent Gaspoz, Team Leader, Propulsion at the EPFL team, described why the measurements are key to the engineering process: “The high data acquisition frequency of the piezoelectric sensors allows us to gain a deep understanding of the dynamics of the engines. This summer, for instance, we struggled with igniting the engine and could not figure out where this problem came from. That’s when we zoomed in on the data and took a real close look. We found out that the oscillations in the combustion chamber preceded the oscillations in the injector minimally. We knew immediately how to solve this issue but without the measurements we might not have found the reason for the failed ignition.”

A few months later, the problem came up again. Without hesitation, the team checked the data and found that this time the oxidizer was at fault. “It is an amazing experience to be able to look at data this detailed and be able to draw conclusions from it,” Florent Gaspoz added.

Reinhard Bosshard, Sales expert at Kistler, supported the students with technical background on the correct installation of measuring chains. “I am amazed at the students’ enthusiasm for innovation. They will learn a lot from developing test strategies themselves, interpreting the test data and drawing conclusions — these are important skills not only in the space industry, but also in many other fields of engineering. The same kind of tests, for instance, leads to very promising results in airplane turbine tests.” In the end, it’s the passion for aerospace engineering that counts for the students — and the experience of designing increasingly efficient research rockets to achieve the ultimate dream: reaching the stars.

This article was written Martin Marinak, Marketing Manager for BU Test & Measurement and E-Business, Kistler (Winterthur, Switzerland). For more information, visit here .