page_title: Liquid fly-back booster
section_title: Reusable first stage
hierarchical_section_title: Liquid fly-back booster / Derivatives / Reusable first stage
context_page_description: Liquid Fly-back Booster was a German Aerospace Center's project concept to develop a liquid rocket booster capable of reusing 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, providing main thrust during the liftoff. Once separated, two winged boosters would perform an atmospheric entry, fly 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:
  A reusable booster in a class of small and medium-lift launch vehicles like Vega and Arianespace Soyuz.
  A super-heavy-lift launch vehicle capable of lifting nearly 70 tonnes to the orbit.
  A two-stage-to-orbit system operating a dedicated reusable orbiter.
  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.
context_section_description: The LFBB was studied with the three upper stage composites, to attain a Reusable First Stage (RFS) configuration. The first was a Vega derivative, with a Zefiro 23 second stage, a Zefiro 9 third stage and an AVUM upper stage. With the LFBB replacing the P80 stage, the payload to sun-synchronous orbit (SSO) would increase to 1,882 kg (4,149 lb), compared to the 1,450 kg (3,200 lb) of the Vega. The second was an Ariane 4 derivative called H-25. It was  based on an H10 upper stage with a Vinci rocket engine and 25 tonnes (28 short tons) of cryogenic fuel. Depending on the method of deceleration, the payload to SSO is between 1,481 and 2,788 kg (3,265 and 6,146 lb). The third was a large cryogenic upper stage, called H-185, based on an alternative, yet-to-be-developed Ariane 5 main stage with 185 tonnes (204 short tons) of cryogenic fuel. Its payload to SSO is 5,000 kg (11,000 lb).
  Two of the lighter configurations (the Zefiro 23 and the H-25) use upper stages mounted on top of the booster. Due to the lower weight, it might have been necessary to lower the amount of fuel in a booster to ensure that the separation velocity, the flight path, and the reentry do not exceed design bounds. In the case of H-25, it might be necessary to accelerate the fly-back boosters to above 2 km/s (1.2 mi/s) to help the upper stage achieve its desired orbit. Consequently, two solutions were proposed to decelerate the boosters after separation. The first option was to actively decelerate them using 10 tonnes (11 short tons) of fuel and reduce the velocity by 300 m/s (980 ft/s). However, launch performance would drop below that of the Vega derivative. Another option is to use aerodynamic forces to decelerate. However, a hypersonic parachute was deemed too expensive and too complex. As a result, an alternative ballute was proposed. Flight dynamics simulation revealed that a ballute with a cross-section of 45 m² (480 sq ft) offered the best compromise between loads on the booster and deceleration by aerodynamic forces. In this configuration, a launch performance of up to 2,788 kg (6,146 lb) could be achieved, partly thanks to a higher separation velocity.
  The heaviest configuration uses a single booster with an asymmetrically mounted, large, expendable cryogenic stage designated H-185. It was proposed as a future variant of the Ariane 5 core stage (H158), eventually meant to phase out the main stage in a standard launch configuration with LFBB. H-185 would use a new Vulcain 3 main engine, with increased vacuum thrust. When launched with a single booster, both stages would be operated in parallel, and be delivered to a 180 by 800 km (110 by 500 mi) orbit before separation. The remaining upper stage composite would weigh 7,360 kg (16,230 lb), with a 5,000 kg (11,000 lb) payload performance to SSO. When launching to Low Earth orbit, payload mass can be increased to over 10,000 kg (22,000 lb).
caption: Top view of RFS configurations: Vega and Ariane 5 derivatives (top), large cryogenic upper stage (bottom) with the LFBB shown in blue