Rocket Science


Whether it is an aerodynamic body, a turbofan or a rocket engine, measuring test results needs a complex arrangement of sensors to acquire that all-important data. The German Aerospace Center’s (DLR) Institute of Space Propulsion has been working on upgrading its data acquisition equipment at its rocket engine test site in Lampoldshausen in southern Germany.

“Our goal is a concept based on the Industry 4.0 requirements,” DLR researcher Wolfgang Stuchlik tells Aerospace Testing International. The Industry 4.0 (see sidebar, page 72) network needs intelligent sensors, control elements and communication channels via a local area network or the internet. The quantity of data collected is vast. The current test bench data acquisition system that exists will archive five million samples per second. A sample is a string of analog data from the sensor, which would be converted to a digital format for computer analysis.

The acquisition system has both analog and digital data feeds from the sensors. “For the analog inputs, we have an archiving rate from two up to 500 points per second,” explains Stuchlik. “For all channels, we have a constant scanning rate of 1,000 samples per second.” This rate can also be described as a bandwidth of 20kHz.

The signals sent to control the test bench and engine, and the returning sensor data, will arrive within 1ms of being sent. There is a total of 1,600 analog and digital channels for data being sent to open and close the rocket engine test bench’s valves and to receive signals from pressure, temperature, vibration, flow and force sensors.

Of the 1,600 channels, 500 are command channels for automatic valves operating at 4-20mA. A further 1,000 are digital feedback channels for the valves. Then there are 16 analog output channels for control valves, also operating at 4-20mA.

Stuchlik has been working on improving data acquisition for what the institute calls its ‘test benches’. “The next step will be to find solutions regarding Industry 4.0, for example intelligent sensors and amplifiers, data exchange by data clouds, and providing data from tests for customers with data security,” says Stuchlik, referring to a recommendation in his 2016 technical paper, Evolution of real-time computer systems for test benches.

Otherwise known as P5, P4.1 and P4.2, these three benches were the focus of Stuchlik’s work, although DLR has other test stands. The benches, P5, P4.1 and P4.2, are used to test and qualify the rocket engines Vulcain 2, Vinci and Aestus. Vulcain 2 powers the first stage of Airbus Safran Launchers’ (ASL) Ariane 5 rocket and Aestus propels the launcher’s upper stage. The Vinci engine is also designed for an upper stage, but will actually contribute to propulsion advances for ASL’s in-development Ariane 6 launch vehicle. In May, ASL announced that it would be changing its name to ArianeGroup from the start of the Paris Air Show in mid-June.

Wired or wireless?

One technology that will not be used for Stuchlik’s bench improvement, but would be synonymous with Industry 4.0 in the eyes of many, is wireless data transmission. “Wireless signal transmission is not an option, because the data from the tests is not available for all persons and companies. The data and details are for the customer only,” says Stuchlik, citing security as the reason for only using wired data transmission.

He adds, “The alternative to long cables is the installation of the computer front ends – the real-time system, the data archiving system, the command and control system, the amplifier and the digital interface on the test bench – about 30m from the test cell in a specially protected room.” The cable between this protected room and the server where the data is stored could be an optical fiber.

The current 1,600 channels are not enough and Stuchlik wants more high- and low-frequency data channels with a high sample rate. He also wants to be able to halt incoming data when the load on the computers’ central processing unit becomes to great. He describes it as “on demand” subsystem connect and disconnect.

Stuchlik is under no illusion that this is a task that can be completed relatively quickly. He expects the technical specification will take two years alone, with all the internal consultation that has to be carried out. Calling for industry bids and selecting a supplier will take another year, he says. And this is only the first stage. “After the critical design review, we start the realization of the system, the hardware and the software,” he explains. “After factory acceptance we have to install it on the test bench. Finally, after the installation, the system needs to be accepted.”

Data acquisition improvement has been included in a number of DLR projects. The DLR’s Institute of Space Propulsion Status Report 2011-2017 was published in February this year and sets out the various projects the rocket testing site has undertaken to overhaul its data acquisition. Rocket testing installations are capital intensive and will not undergo major refurbishment and technology improvement without reinvestment approved at the highest level of management.

Two projects described in the report have improved methods of data acquisition as one of their goals. They are Antriebstechnologien und Komponenten für Trägersysteme (ATEK), which means drive technologies and components for carrier systems, and Flexible Structures. Flexible Structures ended last year after seven years of work, while ATEK, which started in 2015, will continue until 2018. Flexible Structures and ATEK’s respective primary goals are to examine rocket nozzle deformation and develop reusable engine components.

The results from Flexible Structures, a €60,000 (US$67,300) European Space Agency-financed project, are being presented at the Joint Propulsion Conference in Atlanta, Georgia, USA, in July, according to its Lampoldshausen supervisor Joerg Riccius.

Based on the results of the work that has been done for the project, Riccius explains that the Lampoldshausen fluid flow team is to start work in mid-July on selecting suitable measurement equipment for further nozzle experimentation. One of Flexible Structures’ goals was to put forward recommendations for the test setup and data acquisition rates for the nozzle experiments. The recommendations have not yet been finalized.

In camera

ATEK deployed a camera with a large memory as its improved data acquisition technology. ATEK is primarily about the fundamental analysis of hybrid rocket combustion, especially for liquefying paraffin based fuels.

The experiments were carried out with a basic hybrid rocket combustion chamber with windows for data collection by cameras. The researchers analyze the boundary layer combustion of the hybrid rocket using high-speed video imaging and Schlieren videos. Schlieren imaging shows the effect of a medium – in this case the boundary layer – on the passage of light, manifesting itself in dark streaks across a picture where the light was blocked by the medium.

Led by the DLR Institute of Hypersonics in Cologne, Mario Kobald oversaw the ATEK-related work at Lampoldshausen. “We used two high-speed cameras,” notes Kobald. “A Photron Fastcam 1024 PCI was used for the initial tests. Later an improved new model, the Photron SA1.1, was bought and used, mainly with higher memory for higher frame rates and longer acquisition times.”

The Photron Fastcam uses a light-sensitive 10bit analog-to-digital converter sensor with large 17µm square pixels. The camera operates from 60-1,000fps at full 1024×1024 pixel resolution, and at reduced resolution it has a maximum frame rate of 109,500fps. “The improvements only concern the internal camera storage, which was increased. This enables the acquisition of larger amounts of test data,” explains Kobald. “The increased memory enables higher frame rates, which improves the quality and resolution of both POD [proper orthogonal decomposition]and ICA [independent component analysis]techniques.”

POD produces the raw data from, in this case, the hybrid rocket combustion boundary layer that is being imaged by the camera, but this is not the video data. The ICA technique will find relationships between the POD data points that will describe and resemble the physical process, the boundary layer, being imaged.

“These techniques are novel and unique in the working field of hybrid rocket propulsion, especially with these high frame rates and details – 10,000fps and a maximum resolution of 1024×1024 pixels,” adds Kobald.

A great deal of work is yet to be done by DLR researchers on improving their data acquisition; they are still in the process of finding suppliers for the Industry 4.0 test bench.

However, what is not in doubt is that advances in modern computers, LAN capacity, internet protocols and sensors will realize test benches with faster acquisition, greater integrity and ultimately better analyses.

Rob Coppinger is an engineer turned journalist, who has been writing about aerospace technology for approaching 20 years. 

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