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How a rocket holds up in hypersonic flight

25 July 2016

The exterior of a rocket is exposed to high temperatures during hypersonic flight. How does the surface structure change under varying air resistance and with respect to heat flow and acceleration?

Image courtesy of DLR

Scientists and engineers from the German Aerospace Centre (DLR) and students from RWTH Aachen University are aiming to investigate these and similar questions in the ROTEX-T (ROcket Technology EXperimenT) flight experiment.

ROTEX-T successfully took off on 19 July 2016 at 06:05 CEST from the Esrange Space Centre near Kiruna in northern Sweden. The payload reached an altitude of around 182km, returned to the ground after a flight of approximately seven minutes, and was recovered by a helicopter.

"The 180kg scientific payload consists of more than 100 sensors and measures the aerothermal loads and structural behaviour throughout the flight. In doing so, temperature, pressure, heat flow and acceleration sensors record the flight parameters. Measurement of the heat load as a function of the velocity, density and turbulence level of the flow is one of the focal points of the research in this flight experiment," explains Ali Gülhan from the DLR Institute of Aerodynamics and Flow Technology in Cologne.

The DLR researchers from the department of super- and hypersonic technologies collaborated with engineers from the Mobile Rocket Base (MORABA) at DLR Space Operations in Oberpfaffenhofen to design and build a two-stage rocket for the hypersonic flight experiment. The two rocket engines were provided by Swedish partner and Esrange operator Swedish Space Corporation (SSC). "The key element of the research rocket is the service module, which ensures data transmission and the timing of the scientific experiments, and contains all the necessary sensors for measuring acceleration, rotation rate and position," explains Andreas Stamminger from MORABA.

The aerospace students from RWTH Aachen University have strongly supported the project in the design and instrumentation and will play an important role in the assessment. "We wanted to demonstrate that our students could develop and implement such a project with DLR," says Andreas Henze, from the Institute of Aerodynamics at RWTH Aachen University. The DLR Space Administration in Bonn financed the students' participation.

Image courtesy of DLR

The ignition of the first rocket stage accelerated the entire system to 2.3 times the speed of sound in five seconds. After a short glide phase through the thicker layers of Earth's atmosphere, the second stage was ignited and ROTEX-T reached a velocity more than five times the speed of sound. The scientific payload then flew on without propulsion to an altitude of around 182km before returning and re-entering Earth's atmosphere. No parachute was provided for the landing. Instead, the payload separated from the rocket stage at an altitude of 15km and decelerated by tumbling. It finally landed in an uninhabited part of northern Sweden at a speed of approximately 95m per second.

"We were able to use a newly developed method to quickly and precisely measure the temperature distribution along the rocket motor casing," says Gülhan. In addition, researchers were able to use strain gauges and temperature sensors to simultaneously measure the deformation and temperature of the fins. "This will enable us to improve the quality of future flight experiments," adds Gülhan. In addition, numerous compact video cameras were used to document the flight. The windows and housing of these cameras also needed to be carefully constructed to withstand the hot environment of hypersonic flight.

A modular data system captures the sensor data at different rates. This means that a globally unique system with a data rate of 2000kHz was developed for ROTEX-T to capture the data from the ultrafast pressure sensors as well. The storage unit for this system was designed for the high impact velocity of the payload and 'survived' the hard landing. The students at RWTH Aachen University and the DLR scientists intend to use the flight data to validate and improve their analytical tools for aerodynamics, thermal analysis, flight mechanics and structural dynamics. Another goal of the experiment is to compare the flight data and experimental results of the DLR ground facilities with those of RWTH Aachen University.

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