On a cold and wintry morning in November 2019, Airbus Defence and Space escorted a group of journalists into the anonymous-looking Building 179 at Manching, Germany. Here, in an anechoic test chamber, the company revealed its once highly classified Low Observable Unmanned Testbed (LOUT). Jon Lake was there for Air International.
Seemingly under the protection of a full-size Eurofighter Typhoon radar cross-section (RCS) model, the clandestine shape of a hitherto unseen design was there to behold. The LOUT project, explained Airbus Future Combat Air System (FCAS) programme manager Mario Hertzog, is a non-flying model designed to allow accurate radar signature measurement and to demonstrate German national ca such, the aircraft is similar to the BAE Systems’ Replica stealth model that was eventually revealed in 2003.
The LOUT demonstrator is the result of a 12-year classified research and development eff ort for the German Bundesministerium der Verteidigung (BMVg, defence ministry). It followed extensive work on stealth by Airbus Germany and its predecessors MBB and EADS, including MBB’s Lampyridae low-observable fighter.The heavily faceted diamond-shaped Lampyridae bore some resemblance to Lockheed’s Have Blue stealth demonstrator (precursor to the F-117), but it was cancelled in 1987 – reportedly as a consequence of US political pressure, perhaps because it represented a potential competitor to the F-117, or possibly because it could have compromised the Nighthawk’s unique configuration and characteristics.
Since then, Airbus has honed its LO capabilities by working on signature reduction measures for non-stealthy aircraft, including the Panavia Tornado and Eurofighter Typhoon.
The company renewed its studies of dedicated stealthy configurations in 2007, leading to RCS simulation of the planned outer shape in 2009. The LOUT testbed itself was manufactured from 2010 onwards in Manching and Bremen following what Airbus called a “Skunk Works’ approach” This followed a contract award from the BMVg to refine configuration and material choices. LOUT was intended as a very low observable (VLO) ground test article for the demonstration of wideband signature reduction technologies and to support the integration of future VLO technologies for potential operational platform configurations, bringing together simulation and measurement. Airbus followed an integrated design process, from operational requirement through to system design.
The process of refining the final shape began with 2D planform evaluations, followed by 3D layouts and then full 3D configurations.
About 11 planforms were evaluated, including three innovative diamond configurations, one of which resembled the Northrop Grumman X-47A Pegasus, as well as six arrowhead shapes that were reminiscent of the Boeing X-45C or the Lockheed Martin RQ-170 Sentinel, the Dassault nEUROn, the Lockheed Martin Polecat and the Northrop Grumman B-21. Two more planforms were cranked-deltas.
Five configurations were studied further as 3D layouts, including two kite-like diamonds (one with twin engine exhaust ‘slots’), two BAE Systems Taranis-like flying wings and a more graceful design with higher aspect ratio wings that bore some similarity to the RQ-170.
The three most promising designs (the two diamonds and a Taranis-like arrowhead) were then built in scale-model 3D form and were tested in the wind tunnel. Aerodynamic properties were then assessed before the final configuration was selected.
Final Configuration
The basic LOUT configuration was developed between early 2014 and early 2016, leading to the production of the unveiled demonstrator. A period of iterative ‘camouflage optimisation’ followed from early 2016, which saw the LOUT placed in an anechoic chamber for a rigorous series of signature measurements to establish a reference.
Next, some modular elements and different coatings and surface treatments were replaced to address signature ‘hot spots’ or to test and evaluate new technologies or alternative design approaches.
The ‘camouflage optimisation’ phase is now complete, bringing the LOUT programme to a close, for now at least, and enabling Airbus to reveal the demonstrator’s existence.
Hertzog refused to discuss whether any additional LOUT models had been produced or whether scale models had been flown, although Airbus did fly the superficially similar Sagitta UAV on July 5, 2017 from South Africa’s Overberg test range.
This smaller, 3-metre (9. 8-ft) span, twin-engined UAV had a remarkably similar diamond-shaped planform (albeit with two vestigial vertical fins) and it was also launched in 2010, when the contract award for LOUT was placed.
The Sagitta was developed in conjunction with the Deutsches Zentrum für Luft- und Raumfahrt (DLR, aerospace centre) and universities in Chemnitz, Munich and Ingolstadt.
The Sagitta was described as a one-quarter-scale model of a UAV demonstrator, and it may have been used to further validate the flying properties of the final LOUT design.
The twin-engined, subsonic, unmanned LOUT was optimised for multispectral stealth, using an innovative design to support radar, infrared, visual and acoustic measures. The craft was designed to explore the limits of LO, particularly against the new generation of emerging wideband and lowfrequency ‘counter-stealth’ radars and advanced infrared detection systems. Approximately 12m (39. 4ft) long, and with a wingspan of roughly 12m, the LOUT testbed weighs in at four tonnes, though this represents the weight of the bare, engineless, unequipped shell, and it is representative of a 21-tonne class aircraft.
The blended-wing/body incorporates an ultra-wideband wing leading edge concept (with an unusual S-bend)and integrated apertures. LO coatings provide surface wave attenuation, while steps and gaps are avoided or sealed where possible.
The aircraft includes very low RCS diverterless engine inlets incorporating ducts constructed from radar absorbing structure and using a broadband LO leading edge design, with provisions to suppress infrared radiation. It uses an integrated flat exhaust nozzle on the upper surface, shielded from underneath, making it well optimised for use against ground-based air-defence systems. Various LO compatible Thrust Vector Control (TVC) options have been investigated and Airbus has looked at options for the cooling of structural parts.
Particular attention has been paid to the shielding of transparencies, including electrically conducting and optically transparent cockpit canopies for use in any manned stealth derivative.
The LOUT takes an holistic approach towards survivability, encompassing signature reduction in terms of radio frequency (VHF to Ka-Band), acoustic, thermal, visual and infrared signatures as well as electronic countermeasures and emission control and management. Complementary projects have included work on linking a ‘self awareness’ of its own signature to the mission management system, passive sensing, cross-platform electronic countermeasures, electromagnetic emission control, covert communications and cyber security.
Though reporters were allowed to view the LOUT from the front, side and three-quarter rear, the lighting was very carefully controlled, making some areas of the aircraft (especially on the upper surfaces) difficult to discern.
The future
The German government backed the LOUT development effort to support earlier iterations of FCAS about 10 years ago. Hertzog said the LOUT will feed into the ongoing Franco-German-Spanish FCAS development programme and confirmed that findings from the LOUT study had been shared with French partners.
The LOUT was conceived as a non-flying test bed, making it unlikely that it will ever form the basis of a full-up combat aircraft, nor was it ever intended to do so. It was created to help develop and understand future LO technologies that might be integrated in future systems.
Airbus believes that stealth is and will remain an enabler for survivability, according to Hertzog. The LOUT has tested stealth technologies that will form part of the FCAS programme, which may be used on the Next Generation Fighter, and in any other manned or unmanned elements of FCAS. Hertzog claims that Airbus is “at the cutting edge of low observability” adding: “We proved a significant reduction in radio frequency, acoustic and infrared signature. He is, he says, “convinced we can make a considerable contribution to the development of a future fighter or a remote carrier”
Some technologies demonstrated on the LOUT may form part of the Eurofighter Typhoon programme’s Long Term Evolution (LTE) roadmap.The design could also conceivably serve as a basis for the development of a LO combat air vehicle in the future, able to operate in the highly contested environments that are expected in the 2040s and beyond.
Like the British Replica demonstrator, the LOUT programme is providing a showcase for Germany’s formidable national ability in this area. Some have even claimed that the LOUT shows that Germany has joined the UK as one of the two leading European nations in the field of next-generation stealth technology. This status may be used by Airbus to gain greater leverage in the FCAS programme, leadership of which was originally awarded to Dassault. (The UK’s claim to be at the forefront of this technology is based on the experience gained in the Replica, Taranis and F-35 programmes).
There is already some tension within the FCAS programme with conflicting demands and expectations surrounding national workshare and design leadership in particular areas.The unveiling of the LOUT has allowed Germany (and Airbus) to demonstrate a hitherto unexpected degree of expertise and experience in LO design, challenging the previously assumed advantage enjoyed by France as a result of Dassault’s participation in and leadership of the nEUROn UCAV programme.
