CAUTION, HOT: How a rocket holds up in hypersonic flight

The exterior of a rocket is exposed to extremely high temperatures during hypersonic flight. But how exactly does the surface structure change under varying air resistance and with respect to heat flow and acceleration? Scientists and engineers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; 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 Center near Kiruna in northern Sweden. The payload reached an altitude of around 182 kilometres, returned to the ground after a flight of approximately seven minutes, and was recovered by a helicopter.

"The 180-kilogram 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.

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 182 kilometres 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 15 kilometres and decelerated by tumbling. It finally landed in an uninhabited part of northern Sweden at a speed of approximately 95 metres per second. The modular data acquisition system with data acquisition rates of one, 10 and 2000 kilohertz has survived the landing, and the data has been saved.

"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.