Liquid fly-back booster

Liquid Fly-back Booster (LFBB)

Company type	Concept/Project
Industry	Aerospace
Founded	1999
Defunct	2004
Fate	Project discontinued due to hardware complexity
Parent	German Aerospace Center (DLR)

Liquid Fly-back Booster (LFBB) was a concept developed by the German Aerospace Center (DLR) aimed at creating a liquid rocket booster with the capability of reusability for the Ariane 5 launcher. The primary goal was to lower space transportation expenses while enhancing environmental sustainability. This design intended to substitute the current Solid rocket boosters, which generate the majority of liftoff thrust. Upon separation, these winged boosters would autonomously navigate back to French Guiana, executing a horizontal landing similar to that of conventional aircraft.

A series of derivative launch vehicle variants was proposed to capitalize on economies of scale, thereby further diminishing launch costs. These proposals encompassed:

• Reusable boosters tailored for small-, medium-, and heavy-lift categories, comparable in capability to Vega and SLS.
• A super-heavy-lift launch vehicle designed to deliver payloads approaching to orbit.
• A two-stage-to-orbit configuration featuring a dedicated reusable orbiter.

Between 1999 and 2004, DLR carried out LFBB research as part of its future launcher development program. Although the project was eventually discontinued, DLR continued disseminating findings up until 2009.

Development

From 1999 through 2005, the German Aerospace Center (DLR) investigated potential European launch systems within the framework of the Ausgewählte Systeme und Technologien für Raumtransport (ASTRA) initiative, with study activities extending to 2009. The LFBB project was one of two ASTRA undertakings, alongside the Phoenix RLV. Scale model testing occurred in DLR’s supersonic Trisonische Messstrecke Köln (TMK) and hypersonic Hyperschallwindkanal 2 Köln (H2K) wind tunnels. Key structural design components involved collaboration with ESA and NASA.

The advantages of reusable boosters include operational simplicity through single-fuel usage, ecological improvements, and reduced recurring expenses. Analyses concluded that fly-back boosters represented the most cost-effective and lowest-risk option for Europe's reusable launch system ambitions, offering substantial potential for cost savings. Nevertheless, comparable initiatives such as the Space Shuttle and VentureStar programs did not achieve their intended outcomes. The enabling technologies for LFBB were deemed achievable within approximately ten years, opening opportunities for additional fly-back booster-based launchers optimized for cost efficiency and maintenance across varying vehicle categories.

Ultimately, escalating complexity in hardware design led to the termination of the project. A representative from CNES remarked:

Description

The LFBB initiative sought to substitute Ariane 5’s solid rocket boosters (EAP P241) with reusable liquid-fueled boosters, while preserving the existing core and upper stages along with the payload fairings. These liquid boosters generate the primary thrust during liftoff and subsequently return to the Guiana Space Centre, executing a vertical takeoff and horizontal landing (VTHL) maneuver. This method maintains Ariane 5’s original ascent trajectory largely unchanged and enhances the payload capacity of the Cryogenic Evolution type-A (ECA) variant from to .

Each LFBB is equipped with three Vulcain engines featuring a lower expansion ratio, arranged circularly at the rear, complemented by three turbofan air-breathing engines positioned in the nose section for powered fly-back. The booster’s fuselage measures in length and has an external tank diameter of , designed to match the Ariane 5 core stage dimensions to reduce production expenses. It employs a low-wing V-tail canard layout with an approximate wingspan of and a wing surface area of . Its gross lift-off weight (GLOW) stands at , compared to for the EAP P241.

The booster incorporates four distinct propulsion systems: the primary rocket propulsion consisting of three gimbaled Vulcain engines fueled by propellant; Eurojet EJ200 turbofan engines utilizing hydrogen to reduce fuel weight; ten thrusters each generating 2 kN for reaction control; and solid rocket motors facilitating separation from the core stage. These solid motors are scaled-up versions of those on current EAP boosters, installed within the attachment ring and wing framework.

A standard mission profile commences with ignition of both main stage and boosters, accelerating to before booster separation at an altitude of . The core stage continues its trajectory to orbit while the boosters follow a ballistic arc reaching altitude. Upon reentry into the atmosphere at low energy, boosters execute a banking turn toward their designated airfield, glide until reaching optimal altitude, then activate turbofan engines for cruise flight approximately downrange over the Atlantic Ocean. This return flight consumes roughly hydrogen fuel and lasts over two hours. The boosters extend their landing gear and perform autonomous landings, with flight paths carefully planned to eliminate any collision risk post-separation.

Derivatives

The LFBB development targets increased manufacturing output and economies of scale by enabling three supplementary space transportation systems. At DLR, the LFBB project aimed to lower Ariane 5’s operational expenses and foster future derivatives such as a reusable first stage for small-to-medium launchers, a super-heavy lift vehicle capable of delivering payloads to Low Earth orbit, and a reusable two-stage-to-orbit system. Initially, LFBBs would be integrated solely with Ariane 5 but could eventually replace Arianespace Soyuz and Vega launchers over time.

Reusable first stage

Studies examined combining LFBBs with three different upper stage assemblies to create a Reusable First Stage (RFS) configuration. The first option was a derivative of Vega, incorporating a Zefiro 23 second stage, Zefiro 9 third stage, and AVUM upper stage. Substituting the P80 stage with LFBB would increase payload capacity to Sun-synchronous orbit (SSO) from up to . The second configuration was an Ariane 4 variant called H-25, utilizing an H10 upper stage equipped with a Vinci engine and carrying of cryogenic propellant, capable of delivering to SSO depending on deceleration technique. The third was a sizable cryogenic upper stage named H-185, based on a prospective Ariane 5 main stage containing of cryogenic fuel, able to transport payloads to SSO.

The two lighter setups (Zefiro 23 and H-25) mount their upper stages atop the booster. To comply with constraints on separation velocity, flight path, and reentry conditions, reductions in propellant load might be necessary. For H-25, boosters may require acceleration beyond for orbital insertion. Two deceleration strategies were evaluated: active deceleration involving consumption of fuel to reduce velocity by , which diminishes launch performance below that of Vega; or aerodynamic deceleration employing a ballute instead of expensive hypersonic parachutes. Simulations indicated that a ballute with a cross-sectional area effectively balances aerodynamic drag and structural loads on the booster, enabling launch performance up to , partly due to increased separation velocity.

The heaviest configuration consists of a single booster paired asymmetrically with an expendable cryogenic upper stage H-185, proposed as an Ariane 5 core stage successor (H158). This setup is intended to replace the standard main stage when used alongside LFBBs. It would feature a new Vulcain 3 engine delivering improved vacuum thrust. Both stages operate simultaneously to achieve a orbit before separation occurs. The combined upper stage assembly weighs , providing payload capacities of to SSO and exceeding to Low Earth orbit.

Super-Heavy Lift Launcher (SHLL)

The Super-Heavy Lift Launcher (SHLL) incorporates a novel cryogenic main stage, five liquid fly-back boosters (LFBB), and a restart-capable injection stage. It is designed to support intricate missions such as manned lunar and Martian expeditions, as well as deploying large solar-powered satellites.

The central stage stands tall and has a diameter of , providing of LOX/LH₂ propellant to power three Vulcain 3 engines. Its expanded diameter allows attachment of five LFBBs, each fitted with wings that can retract or alter their geometry. The upper stage, adapted from the Ariane 5 ESC-B, is enlarged to in size and structurally strengthened to endure greater stresses, utilizing the reliable Vinci engine for orbital insertion. Payloads are enclosed within an fairing. Overall, the launcher reaches a height of and has a gross lift-off mass of , capable of placing up to into low Earth orbit (LEO).

At liftoff targeting a orbit, the LFBBs detach at an altitude of traveling at . To manage booster separation timing, techniques such as cross-feed fuel systems or engine throttling might be applied. Returning the boosters requires roughly of fuel, which includes a 30% safety margin.

Two-stage-to-orbit

Approximately fifteen years following the integration of LFBBs with Ariane 5, a reusable two-stage-to-orbit (TSTO) LFBB concept was proposed, though it remained at the preliminary analysis stage. This design envisioned two boosters featuring retractable wings connected to an external fuel tank, combined with a reusable orbiter possessing fixed wings that would carry payloads atop. An expandable upper stage was intended to facilitate missions to geostationary transfer orbit (GTO).

The external tank core measures in diameter and in height, storing of propellant. The orbiter itself is tall with a diameter of , carrying of propellant. The payload fairing situated on top of the orbiter measures . For missions to LEO, the total vehicle height reaches , with a gross lift-off mass of , supporting payloads up to for LEO and as much as to GTO when employing the expandable upper stage.

See also

• Baikal (rocket booster)
• Falcon Heavy
• RETALT
• Reusable Booster System

External links

• ASTR LFBB – technical details