The Liquid Fly-back Booster (LFBB) was a reusable booster concept developed by the German Aerospace Center (DLR) between 1999 and 2004, aimed at revolutionizing the way Europe launches spacecraft. Specifically designed as an upgrade for the Ariane 5 rocket system, the LFBB was envisioned as a cost-effective and environmentally friendly alternative to expendable boosters.
Unlike traditional rocket boosters, the LFBB would provide main thrust during liftoff and then detach, reenter the atmosphere, and autonomously fly back to its launch site—much like an aircraft landing on a runway. This reusability was intended to drastically reduce the cost per launch and minimize the environmental footprint of each mission. Though it never progressed beyond the conceptual and testing phases, including wind tunnel trials, the LFBB laid important groundwork for future reusable launch technologies and reflected growing interest in sustainability within the space sector. Even after cancellation, research and publications related to the project continued until 2009.
Design and Construction
The LFBB was conceptualized as a pair of winged, reusable liquid-fueled boosters attached to either side of the Ariane 5 core. These boosters would use kerosene and liquid oxygen (LOX) as propellants—chosen for their relatively low cost, storability, and high thrust capabilities. The design incorporated aerodynamic surfaces, including wings and vertical stabilizers, allowing the boosters to perform controlled reentry and autonomous return flights back to the Guiana Space Centre in French Guiana.
Construction materials for the LFBB were expected to include lightweight composites and aerospace-grade aluminum alloys to reduce structural weight while maintaining strength and heat resistance. Thermal protection systems were a key focus of the design, as the booster would undergo significant thermal stress during atmospheric reentry. This called for advanced heat shielding and precise flight control algorithms to maintain structural integrity and navigational accuracy.
One of the major engineering challenges was balancing the demands of a rocket booster with those of an autonomous aircraft. This required developing a robust guidance, navigation, and control (GNC) system capable of handling two drastically different flight environments—space launch and atmospheric flight. The system had to manage complex flight transitions, including stage separation, reentry orientation, aerodynamic deceleration, and powered flight to a safe landing. Despite the challenges, scale models were tested in wind tunnels, and simulations demonstrated the concept’s feasibility. Ultimately, technical complexity and funding constraints led to the project’s cancellation before a full-scale prototype was built.
Mission Objectives
The primary mission objective of the LFBB was to reduce the cost and environmental impact of space launches by introducing reusability to the Ariane 5 launch system. Traditional expendable boosters are discarded after a single use, which not only increases the cost per launch but also contributes to environmental concerns and logistical waste. The LFBB aimed to solve this by returning to the launch site after each mission for refurbishment and reuse, similar to how aircraft are operated.
Secondary objectives included increasing launch reliability and operational flexibility. Reusable boosters could be maintained and inspected between flights, potentially reducing the risk of in-flight failures. Additionally, DLR proposed that the LFBB could serve as the basis for a whole family of derivative launch vehicles, with modular configurations tailored to different payload sizes and mission profiles. This would take advantage of economies of scale, further lowering costs and streamlining production.
The project also sought to contribute to European leadership in reusable launch systems, positioning the continent alongside major players like the United States in developing advanced space transportation technologies. By demonstrating key capabilities such as autonomous flight, precision landing, and booster recovery, the LFBB was meant to pave the way for future reusable vehicles within the European space sector.
Launches and Deployment
The LFBB never reached the stage of a full-scale launch or deployment; however, its theoretical integration with the Ariane 5 system was carefully modeled and simulated. In its intended use, the LFBB boosters would have launched from the Guiana Space Centre alongside the main Ariane 5 core. Once the upper stage ignited and the boosters were no longer needed for thrust, they would detach, reorient, and begin a guided descent back through Earth’s atmosphere.
Upon reentry, the boosters would activate their aerodynamic control surfaces and deploy air-breathing jet engines, allowing them to transition into controlled, powered flight. They would then autonomously navigate back to a designated airstrip near the launch site and land horizontally like an aircraft. This process would require precise coordination between rocket and aircraft systems, ensuring safety and reliability throughout.
During the development period from 1999 to 2004, the LFBB underwent extensive wind tunnel testing and computer modeling. These simulations helped refine the design and demonstrated that autonomous fly-back was technically achievable. However, challenges including cost, complexity, and competing priorities within the European Space Agency (ESA) ultimately halted further development. Although no actual missions took place, the LFBB became a reference point in studies of reusable launch systems and contributed valuable knowledge to the field.
V2 Technical Specifications
Length: Approx. 30 meters (estimated based on Ariane 5 booster dimensions)
Diameter: Approx. 3 meters
Launch Mass: ~200 metric tons (each booster, estimated)
Payload Contribution: Designed to replace standard Ariane 5 boosters
Propulsion: Liquid rocket engine using LOX/RP-1 (kerosene) during ascent; turbofan or jet engines for return flight
Thrust: Comparable to Ariane 5 solid boosters (~6,300 kN per booster during liftoff)
Reentry System: Heat-resistant aerodynamic structure with thermal protection
Flight Control: Advanced Guidance, Navigation, and Control (GNC) system for transition between space and atmospheric flight
Recovery Mode: Horizontal landing on runway using autonomous systems
Power Source: Battery systems for electronics; kerosene/jet fuel for return propulsion
Instruments and Equipment: Onboard avionics for navigation, telemetry, thermal sensors, aerodynamic control actuators, and communication systems
