Liquid Fly-back Booster (LFBB) was a German Aerospace Center's (DLR's) project concept to develop a liquid rocket booster capable of reuse for Ariane 5 in order to significantly reduce the high cost of space transportation and increase environmental friendliness. LFBB would replace the existing solid rocket boosters, which provided the majority of thrust from liftoff to separation. Once separated, the two winged boosters would perform an atmospheric entry, go back autonomously to the French Guiana, and land horizontally on the airport like an aeroplane.

Additionally a family of derivative launch vehicles was proposed in order to take an advantage of economies of scale, further reducing launch costs. These derivatives include:

German Aerospace Center studied Liquid Fly-back Boosters as a part of future launcher research programme from 1999 to 2004. After the cancellation of the project, publications at DLR continued until 2009.^{[citation needed]}

The German Aerospace Center (DLR) studied potential future launch vehicles of the European Union under the Ausgewählte Systeme und Technologien für Raumtransport (ASTRA; English: Systems and Technologies for Space Transportation Applications) programme from 1999 to 2005, with additional studies continuing until 2009. The LFBB design was one of two projects within the ASTRA program, the other being Phoenix RLV. During development, scale models were constructed for testing various configurations in DLR's supersonic Trisonische Messstrecke Köln (TMK; English: Trisonic measuring section at Cologne) and in their Hyperschallwindkanal 2 Köln (H2K; English: Hypersonic wind canal at Cologne) wind tunnels. The preliminary mechanical design of other major elements was done by the companies ESA and NASA.

The advantages of reusable boosters include simplicity from using only one type of fuel, environmental friendliness, and lower reoccurring costs. Studies concluded that reusable fly-back boosters would be the most affordable and the least risky way for European space launch systems to start becoming reusable. These fly-back boosters had the potential to reduce launch costs. However, when other projects, such as Space Shuttle or VentureStar, undertook this objective, they failed to meet their goals. Supporting technologies needed for LFBB construction can be developed within 10 years, and additional launchers can be developed based on fly-back boosters to minimise costs and provide maintenance synergy across multiple classes of launch vehicles.

Eventually, the hardware grew too large and the LFBB project was scrapped, with one member of the French space agency (CNES) remarking:

The thing that shocked me was that at the beginning, this reusable flyback booster was just a cylinder with engines and little wings, just a turbo fan in the back. And three years later these were complete Airbuses in terms of size with four engines in each of them.

— Christophe Bonnal, CNES launcher directorate