Re-entry and landing technologies

Reusable space transport

Topic in focus: Reusable space transport
DLR has been conducting research into reusable launch vehicles and space transport applications for many years, with the aim of developing climate-compatible and cost-effective space transport for Europe.
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Model reusable rocket stage travels seven times the speed of sound in hypersonic wind tunnel
This picture shows a reusable rocket stage model re-entering Earth's atmosphere at seven times the speed of sound, in the hypersonic wind tunnel in Cologne (H2K). The engine fires against the direction of travel in order to reduce its speed and in-so-doing minimise the high stresses experienced during re-entry. As part of the ESA RETPRO project, funded by the Future Launchers Preparatory Programme (FLPP), a study was carried out on the influence of firing a varying number of engines on the overall aerodynamics of the rocket stage. The flow is visualised using the schlieren method, in which differences in density lead to the deflection of light guided through the measurement segment. This makes the flow structures clearly visible, especially when it comes to supersonic flows. In the upper image with one engine active, flows are reproduced comparatively sharply, which indicates little dynamic behaviour in the flow field. In the lower image with three active thrusters, the mutual influence of the thruster jets and a clear increase in dynamics can be seen. Such results in the future consideration of how dynamic flow influences the behaviour of the structures and control of spacecraft.
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DLR, ESA, Future Launchers Preparatory Programme (FLPP)

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Video: Technologiedemonstrator ReFEx
The Reusability Flight Experiment (ReFEx) is a technology demonstrator for future winged, reusable stages. This animation shows ReFEx in its re-entry configuration after it has been launched from its carrier rocket. Instead of a vertical landing, the test focuses on the horizontal landing of a stage, with autonomous navigation and controlled flight during each phase of the vehicle's mission. The vehicle is stabilised using an active aerodynamic control system, which transforms inputs from the navigation system and the guidance into control commands for the individual actuators. A demonstration flight is planned for 2025 from Koonibba Test Range, Australia.
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In the history of space exploration, rockets have traditionally been disposable product. The first step towards making these space transport systems reusable was NASA's Space Shuttle programme. The shuttle, equipped with wings, did not need to reignite its engines to re-enter the atmosphere or to land; it returned to Earth with a horizontal glide. NASA ended the programme in 2011 after 135 missions. Currently, the Falcon 9 launch vehicle from Space X is the exception amongst other disposable stages – so far, only Space X regularly flies into space and lands the first stage.

The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) has been conducting research into reusable launch vehicles and space transportation applications for many years. The aim is a future space transport system for Europe that is climate-compatible and cost-effective. Researchers from various DLR institutes and departments contribute their extensive knowledge to projects at national, European and international levels.

Many research questions remain unanswered. For instance, the highly complex physics of flight have still not yet been conclusively investigated. Ideas and designs to return rockets through horizontal and vertical, reverse landing methods are being investigated, as both approaches offer unique advantages and disadvantages. What are these, and how do these technological methods function? Why is restarting a rocket engine more challenging than restarting an aircraft engine? How can artificial intelligence (AI) support research and development in this area? What capabilities should a modern test bed for new prototypes offer? And what is the best way to test various technological concepts?

This 'topic in focus' provides an overview of the extensive and complex research field of reusable space transport, highlighting current missions, projects and key DLR activities in this area.

Background articles

Horizontal vs vertical landing – a comparison

For years, DLR has researched various concepts aimed at making rockets reusable. Instead of treating rockets as disposable, at least parts of the rocket, such as the first stage, should be able to land and be re-flown multiple times. This shift is crucial for making space transport more resource-efficient, climate-compatible and cost-effective.

CALLISTO – Launcher innovation for vertical landing

To ensure the competitiveness of the European space industry, it is essential to reduce the costs of transporting payloads into space. A key factor in achieving this is the reusability of rocket stages, a challenge being addressed by the German-Japanese-French project CALLISTO.

ReFEx – Vertical take-off, horizontal landing

With ReFEx, DLR is developing a technology demonstrator for the vertical take-off and horizontal landing (VTHL) of a rocket stage. The launcher will demonstrate various key technologies to ensure an optimal flight path, and a status monitoring system employing the latest sensor technology.

LUMEN – AI-assisted reusable rocket engines

DLR is taking a significant step in the development of reusable space transportation systems with its innovative LUMEN project. The focus of LUMEN is an engine fuelled by a combination of liquid oxygen and methane – considered to be a promising option for future rocket engines.

FALCon project – catching rockets with DLR in-air innovation

When it comes to horizontally landing a rocket, there are two main approaches: with or without a dedicated propulsion system for a winged rocket stage. The FALCon project focused on a solution for return flights without propulsion, and one innovative approach is 'in-air capture'.

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Video: CALLISTO

With CALLISTO, DLR engineers, in collaboration with the French space agency CNES and the Japanese space agency JAXA, are designing a demonstrator that can launch vertically and then land in the same alignment (this is referred to as vertical take-off, vertical landing (VTLT)). CALLISTO is guided using aerodynamic control surfaces during a non-powered phase, featuring a transition from supersonic to subsonic flow conditions. The engine is then reignited to decelerate the CALLISTO Vehicle. The landing system therefore absorbs the residual kinetic energy, enabling CALLISTO to perform a safe and stable landing.

Video: DLR researchers successfully test the interaction of individual components of the LUMEN demonstrator

As part of the LUMEN project, DLR researchers are developing a pumped liquid oxygen and methane propulsion system. The focus is on investigating the behaviour of an entire engine and its individual components in a representative test environment. The tests are being conducted on the European Research and Technology Test Bed P8.3 at the DLR site in Lampoldshausen.

Simulation of a rocket in reverse flight

The Spacecraft Department in Göttingen is conducting research into the aerodynamics of reusable launch vehicles. This image shows the thermal load on a rocket's exterior during a braking manoeuvre in reverse flight, when three of its nine engines are running. Normally, the rocket would heat up considerably during re-entry into Earth's atmosphere. The exhaust plume created by the three central engines shields the underside of the rocket from the hot air flow and prevents this.

Video: technology demonstrator ReFEx

The Reusability Flight Experiment (ReFEx) is a technology demonstrator for future winged, reusable stages. This animation shows ReFEx in its re-entry configuration after it has been launched from its carrier rocket. Instead of a vertical landing, the test focuses on the horizontal landing of a stage, with autonomous navigation and controlled flight during each phase of the vehicle's mission. The vehicle is stabilised using an active aerodynamic control system, which transforms inputs from the navigation system and the guidance into control commands for the individual actuators. A demonstration flight is planned for 2025 from Koonibba Test Range, Australia.

Downloads on the topic

Whitepaper

Sustainable space technology – Strategies toward a predictive aerothermal design of re-useable space transportation systems

This paper presents a review of current aerothermal design and analysis methodologies for spacecraft. It briefly introduces the most important system architectures, including rockets, gliders, and capsule-based configurations, and gives an overview of the specific aerothermal and thermo-chemical effects that are encountered during their different flight phases and trajectories.

Whitepaper

System Aspects of European Reusable Staged-Combustion Rocket Engine SLME

The Paper published in 2024 provides an overview of the reusable cryogenic staged combustion cycle engines investigated in Europe and the associated applications for launch vehicles. Originally defined as the basic propulsion system for the SpaceLiner reusable rocket-based high-speed intercontinental passenger transport concept, the SLME is now also being used as a reference for closed-cycle LOx-LH2 propellants in several studies for future European spacecraft.

Whitepaper

A viable and sustainable European path into space – for cargo and astronauts

The Paper published in 2021 summarises the results of preliminary technical studies from Germany - both for the development of Ariane 6 and for the next generation of (partially) reusable concepts. For the return, options for vertical landing and technologies with wings for horizontal landing are considered. The overall shape and aerodynamic configuration, the propulsion system, the architecture and arrangement of the stages and various return concept solutions will be analysed and compared.

Contact

Philipp Burtscheidt

Senior editor DLR media relations
German Aerospace Center (DLR)
Corporate Communications
Linder Höhe, 51147 Cologne
Tel: +49 2203 601-2323