ATHEAt – high-speed atmospheric flight of reusable space transportation systems

ATHEAt (Advanced Technologies for High Energetic Atmospheric Flight of Launcher Stages) is a DLR space research project focusing on the development of re-entry technologies for reusable, cost-effective and reliable space transportation systems.

Re-entering Earth's atmosphere is a highly complex and challenging process. Spacecraft are exposed to extreme conditions, including very high speeds of up to 28,000 kilometres per hour, or Mach 25 – 25 times the speed of sound. Above Mach 5, speeds are referred to as 'hypersonic' and are characterised by particular high-temperature phenomena such as chemical reactions and plasma effects. Re-entry results in very high surface temperatures of more than 2000 degrees Celsius, as strong shock waves around the spacecraft convert high kinetic energy into thermal energy (heat). In the process, a wide range of aerodynamic forces also act on materials and structures.

Closing a global technology gap – a milestone for research and industry

As part of the ATHEAt project, we are researching and testing technologies for reusable spacecraft. The focus is on the re-entry phase at high Mach numbers – a particularly critical stage. To this end, we are developing design tools at DLR and validating them using real-world experimental data. In this way, we are making an important contribution to the design and construction of future spacecraft that are even more capable and reliable. With ATHEAt, we are aiming to close a technology gap in research and industry in the field of reusable, cost-effective and reliable space transportation systems.

A key element of the ATHEAt project is the implementation of two flight experiments with complementary objectives. The first flight experiment was successfully carried out on 6 October 2025 using a dedicated sounding rocket. During this experiment, ATHEAt flew at high Mach numbers for an extended period. In a second flight experiment, we will test a novel single-stage hybrid rocket with controllable thrust.

Overview of the objectives and key activities of ATHEAt:

• Improved reliability of critical high-temperature structures for reusable launch vehicles
• Reduced safety margins in the design of such structures to improve cost-efficiency
• Development of technologies for high-energy atmospheric flight of future space transport systems and their qualification through ground tests
• Successful performance of a long-duration flight experiment at high Mach numbers using a new-generation sounding rocket
• Further development of the DLR VISERION hybrid engine and demonstration of its performance in the single-stage ALDUINA flight experiment
• Development and validation of experimental and numerical methods for application in industrial space projects
• Expansion of DLR's expertise in systems analysis and data management for launch vehicles and related technologies

Combining modelling, ground tests and flight experiments with true-to-scale components

Ground test facilities can only partially replicate the extreme conditions of flight at high Mach numbers. Numerical design tools are also not yet sophisticated enough to allow such flights to be comprehensively modelled on a computer. Flight experiments are therefore important and necessary to close existing technology gaps. At the same time, flights with full-scale components are rare and expensive. At DLR, we rely on flight experiments developed and built in-house, using components relevant to real-world applications, launching them on sounding rockets to altitudes of up to several hundred kilometres.

During tests of the flight experiment components on the ground and in flight, our researchers collected comprehensive and reliable data. Existing numerical design tools are also used and can be improved using the flight test data. Numerical design tools are software tools used to model and simulate physical or technical problems. The combination of flight experiments, simulations and modelling is a core element of DLR's space research and is embedded in our space transport strategy.

ATHEAt flight experiment successfully completed

In October 2025, our ATHEAt hypersonic flight experiment successfully launched on board a sounding rocket from Andøya Space in northern Norway. Mounted at the top of the rocket, the experiment flew for approximately four minutes, two of which were at speeds exceeding Mach 9. During the flight, the research rocket reached a maximum altitude of more than 30 kilometres. The scientific payload, together with the service modules, had a mass of approximately 244 kilograms. Compared with our other successfully flown hypersonic flight experiments – SHEFEX I, SHEFEX II, ROTEX-T, ATEK and STORT – ATHEAt was able to fly at high Mach numbers for considerably longer.

The front section of the payload consisted of a fibre-reinforced ceramic composite specially developed and manufactured by DLR. Behind the rocket nose cone were ogival segments, followed by an octagonal segment fitted with four flaps. Two active cooling experiments were also on board.

Several modules were attached to the rocket's forebody, housing experiments for monitoring the structural integrity of the vehicle. In addition, more than 300 sensors were used, including specially developed miniaturised non-invasive sensors, two infrared cameras, radiation thermometers and laser scanners. During the flight, the collected data was transmitted by radio to the receiving station. Despite the extreme conditions, comprehensive measurement data was successfully transmitted to the ground stations.

ALDUINA flight experiment – development and testing of DLR's VISERION hybrid engine

In hybrid propulsion systems, the oxidiser is stored in liquid form and the fuel in solid form. This enables controlled operation and multiple ignitions of the engine, thereby combining the advantages of solid and liquid propulsion. In a liquid-fuel rocket, liquid propellants – the oxidiser and the fuel itself – are stored in separate tanks and injected into the engine's combustion chamber, where they are ignited. There, a chemical reaction takes place: the fuel reacts with the oxidiser, and the combustion increases the pressure and temperature in the combustion chamber. The resulting hot gas, containing the reaction products, flows out of a nozzle, generating thrust in the opposite direction. Because the oxidiser is carried on board the rocket, the fuel can burn without oxygen from the atmosphere – in space, for example. Such propulsion systems have high thrust efficiency, but the propellants are difficult to store. In solid-fuel rocket motors, the oxidiser and fuel are premixed in solid form and only need to be ignited. As a result, their design is simpler and they provide high thrust. However, this thrust cannot be controlled during flight.