July 27, 2018 | Tina Stäbler is flying high with extremely durable heat shields

Beating the heat

Whether on the climbing wall or in the laboratory, extremes are Tina Stäbler’s thing. She enjoys conquering an apparently unscalable precipice. At the DLR Institute of Structures and Design in Stuttgart, the doctoral student is developing a structural health monitoring system for ceramic matrix composite (CMC) heat shields. This system makes it possible to identify damage to materials on space vehicles during atmospheric re-entry. The scientist was recently rewarded for her work: She was awarded the prestigious Amelia Earhart Fellowship.

Tina Stäbler is flying high with extremely durable heat shields

Flight experiments, stabilisation fins, rocket motors – Tina Stäbler walks past numerous aerospace structures as she makes her way across the Technology Hall. On this particular day, she is working at the Indutherm, a facility that enables the mechanical testing of materials at temperatures exceeding 1000 degrees Celsius. A porthole provides a good view of the interior. Clamped inside is a rectangular panel made of fibre ceramics, glowing with a light orange colour. Stäbler’s eyes also light up when she talks about her research work. At the DLR Institute of Structures and Design in Stuttgart, the 32-year-old scientist is investigating the behaviour of ceramic matrix composite structures upon re-entry into Earth’s atmosphere and the detection of damage to the material in the process.

A monitoring system could save lives in extreme cases

"Atmospheric re-entry is one of the most critical phases of a space mission. With intense heat and high mechanical stress, spacecraft are subjected to extreme conditions. An intact heat shield is essential for a safe re-entry,” Stäbler explains. The importance of identifying damage in time was tragically demonstrated in the Columbia space shuttle accident of 2003. A damaged heat shield caused the United States space shuttle to break apart while re-entering Earth’s atmosphere. It burned up and all seven crewmembers perished.

Stäbler is devoting her doctorate to developing a method for monitoring the condition of heat shields to be able to replace critical components of future heat shields before failure, or to prevent further damage through adjustment of the flight path. The doctoral candidate has been awarded with an Amelia Earhart Fellowship. These are given in memory of the eponymous female aviation pioneer and are granted each year to as many as 30 female scientists worldwide who are undertaking doctorates in the aerospace field.

In her studies, the engineer is primarily examining the electrical properties of ceramic matrix composites in detail, measuring the electrical resistance from which conclusions about changes in the material can be drawn. In this way, she can detect mechanical stress that can lead to dangerous cracks. The glowing sample, visible through the window of the thermionic test facility, consists of CMC. This class of material is highly resilient to stress and is characterised by a high resistance to cracks; resistance to corrosion, abrasion and oxidation; as well as a high tolerance for temperature and damage. It is also electrically conductive.

The panel, which is the size of a small chopping board, has 12 contacts on each side to measure electrical resistance. Stäbler explains how it works: “All contacts of the sample are connected to a test stand. Two contacts on opposite sides are always activated and impressed with current. The electrical resistance between the contacts on the one side and those on the other side is measured one after another. With 12 contacts on each side, one normal sample has 12 times 12 – that is 144 – measurements per cycle. The current test stand makes it possible to connect up to 32 contacts per side, resulting in 1024 individual measurements per cycle. Since resistance and voltage are always measured, the measurements per cycle double to 288 and 2048, respectively.” The network of measurement paths with 32 contacts per side is more finely meshed, and the accuracy significantly increased.

The sample is heated in a vacuum to 1100 degrees Celsius and in other experiments up to 1750 degrees Celsius. As in a spacecraft, the contacts are in a less hot area. Cyclic tensile tests are carried out while the sample cools down. This means: strain, release, wait. What happens with the first strain? How does the sample behave with the xth strain? “In this way, I can determine whether a critical load condition has occurred. If that is the case, the tile must be replaced.” The advantages of the new test procedure are that it prevents structures from failing unexpectedly and lengthens the maintenance intervals of the heat shields.

“When measuring, we have to be fast,” Stäbler explains. “It is important that damage is detected swiftly.” At present, the data must be evaluated after each experiment. In future, they are to be examined in real time using a special algorithm, and the critical changes are to be reported. Stäbler aims to have her measuring procedure eventually used in actual flight operation: “You could also use it in aviation.” The prerequisite is that the material to be monitored is electrically conductive.

Spaceflight – a childhood dream

Spaceflight is Stäbler’s passion. Ever since she was a small child, it has wielded a particular fascination. It might be a coincidence that the scientist’s middle name is Ariane. “Ariane – like the European launcher,” she laughs. The name says it all. “The question of whether there is life beyond Earth has intrigued me for a long time.” As a schoolgirl, Stäbler took part in the Berkeley University SETI@home project, which deals with the search for extraterrestrial intelligent life. For this purpose, a telescope scans the heavens for radio signals; the processing load is distributed to PCs worldwide for the evaluation of the large amounts of data. It also went to the computer in the Stäbler household. The prerequisite was simply an Internet connection and the installation of free SETI@home client software. “The computer ran the whole day trawling through the data packets. I found it exciting to browse through the statistics,” Stäbler recounts. Does she still believe in extraterrestrials today? – “I think that there is life somewhere. But will we ever catch sight of it?” However, why roam in faraway galaxies when there are also interesting things to discover so ‘close’ to home. “A crewed mission to Mars would naturally be terrific!” Has Stäbler, with all her enthusiasm for space travel, thought about becoming an astronaut? – “Definitely! But the next objective is first and foremost the dissertation,” she says with a wink. So, although there is no going to space just now, she can certainly climb higher: Stäbler can be regularly seen in the climbing gym and at the cliff face. And when she is not working on her doctorate, she is very involved with young scientific talent. For several years, she has actively participated in Girls’ Day to pass on her enthusiasm. “The book and film ‘Hidden Figures’, has raised awareness of women’s significant contribution to spaceflight. That should remain so and will hopefully continue to increase."

Doctorate in a Helmholtz Young Investigators Group

When Stäbler joined DLR in May 2014, she had just earned her degree in aerospace engineering. “What convinced me about DLR was the opportunity to work on my project from start to finish – from the first thoughts on how the system should work, through the tests, and up to the prototypes of the test stand. The production of samples, the evaluation of data, the development and improvement, and right up to the functioning system. At DLR, you do not just see a small part of your project – you see the whole, and that is truly fantastic!”

She is still enrolled at Stuttgart University – now as a doctoral candidate. What is the challenge with getting a doctorate? “If you get your doctorate, you become the greatest expert in your field at some point. No one has devoted themselves to this more than you yourself. So beyond a certain depth, the points of contact are few and far between.” Such a focal point for Stäbler is Hannah Böhrk, the deputy head of the department ‘Space System Integration’. Böhrk also leads the Helmholtz Young Investigators Group ‘High temperature management in hypersonic flight’, where she investigates modern heat shields in simulations and experiments, with the help of Stäbler and other young scientists. They wish to better understand the thermal behaviour of structures during re-entry and hypersonic flight. “In the Helmholtz junior research group, we research a comprehensive matter intensively in a small team,” Böhrk says. “As a result, we are in constant communication with each other.” Stäbler will be the first graduate of the group and is now really inspired to complete her work and then to further develop her topic in new directions. In addition to the high-temperature applications from spaceflight, she sees the detection and localisation of damage in aircraft structures as an area of application. “It is fun to engage in excellent research with such focused doctoral students!”

With so much hard work, it is good to take things easy once in a while. Almost excitedly, she reports on her latest achievement, called ‘Terraforming Mars’ – a board game that involves transforming Mars into an Earth-like planet. Who knows, perhaps Stäbler will one day fly to the Red Planet… You need intact heat shields for this, too.

  • Women in astronautics

    Until 14 October 2018, the Universum® Bremen will be showing the photo exhibition ‘Space Girls Space Women’ in which women from aerospace are portrayed. For more information visit www.universum-bremen.de/space-girls-space-women/

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Nicole Waibel

German Aerospace Center (DLR)
Institute of Structures and Design
Center for Lightweight Production Technology (ZLP)
Linder Höhe, 51147 Cologne