Image copyright BAE Systems

Taranis, named after the Celtic god of thunder, is easily BAE Systems' spookiest-looking aircraft.

Sleek, with swept-back wings, the grey, wedge-shaped Unmanned Combat Air Vehicle (UCAV), or drone, is "the most advanced aircraft ever produced in the UK," says the firm.

Taranis is designed to be the prototype for a combat drone - capable of carrying out long-range strikes in hostile territory.

With the UK's latest jet fighter, the Typhoon, scheduled to need a replacement by 2030, the success of this project will help the Royal Air Force make decisions on the future numbers of manned and unmanned combat aircraft.

Taranis's exact details are secret, but it is roughly the size of the RAF's current jet trainer, the Hawk - the plane used by the Red Arrows display team.

Hide the engine

Designed to have a low radar and infrared signature, it has a complex exhaust system to minimise any tell-tale heat trail from its engines, making it hard to detect or shoot down.

"What we had to do was fully embed and hide the gas turbine in the body of the aircraft," says Conrad Banks, Rolls-Royce's chief engineer for research and development - Rolls-Royce being one of the firms working with BAE on Taranis.

At this week's Farnborough Airshow, BAE was keen to highlight the news from the latest flight tests at "an undisclosed location" where Taranis demonstrated its advanced stealth capabilities.

"The Taranis project is a tremendous example of how the UK government and industry can work together," says BAE's Chris Garside, engineering director of future combat systems.

Image copyright BAE Systems Image caption BAE's advanced Taranis drone - test-flying at an "undisclosed location"

Just don't ask what kind of weapons such a drone might carry. "That is a classified topic area," comes the short response.

Franco-British collaboration

Taranis is not Europe's only combat drone in development. Just across the Channel France's Dassault Aviation is testing a similar aircraft, called Neuron.

This is about finding cost-effective solutions to enable decisions regarding whether a plane will fly with or without a pilot Hakan Bushke, Saab chief executive

Neither will see active service - both are designed to flight-test technologies for use in future drones.

At Farnborough, France and the UK announced a two-year £120m ($205m) study on a potential Future Combat Air System, which would combine the lessons learnt from Taranis and Neuron.

Such European collaboration is badly needed, say some industry insiders, if Europe is ever to produce its own combat drones on an economic basis, rather than rely on off-the-shelf models from Israel or the United States, which currently dominate the market.

Last year, even France opted to buy US Reaper drones as a money-saving measure.

Individually, Europe's nations are too small to be an effective market for combat drones. Only if they can develop a multinational drone would such a system be economical.

Image copyright Dassault Aviation Image caption Preparing Neuron for its maiden flight

Yet it is important to put these combat drones in perspective. There are almost 80 firms with Unmanned Air Vehicles (UAVs) of all shapes and sizes at Farnborough, but most of these are not designed to carry weapons.

The vast majority are designed for information gathering and surveillance. The US armed forces, the world's biggest users of drones, may have 8,000 of them, but only 1% of them are armed.

It is an economic fact of life that building and developing combat planes is expensive, whether those aircraft are manned or not. Taranis itself has already cost £185m. Yet, despite their rising costs, drones will continue to play an increasingly vital role in air forces around the world.

Future combat missions

It's easy to see why - as they are good at doing boring, complicated things quickly. Yet what they are not doing is taking nuanced judgements, so for practical, as well as moral, reasons we are not going to see them replace manned fighters any time soon.

In the combat of the future, mixed flights of manned and unmanned strike aircraft are likely to be common, but the challenges of operating such very different systems in close proximity to each other is only just being worked out.

Earlier this year, Dassault's Neuron achieved an important first for combat drones by flying in formation with other aircraft, including France's latest Rafale fighter jet.

Image copyright Dassault Aviation Image caption In future, manned and unmanned fighters may fly together

But unmanned combat aircraft might not look like drones such as Taranis and Neuron - they could equally be unmanned versions of existing, currently manned, fighter jets.

In Sweden, Saab is considering developing an unmanned version of its Gripen multirole fighter. Saab's reasoning is that it would be cheaper for an air force to operate one type of airframe rather than a conventional jet fighter and a combat drone.

"This is about finding cost-effective solutions to enable decisions regarding whether a plane will fly with or without a pilot on various missions," says Saab's chief executive Hakan Buskhe.

And it is not just the Swedes - Boeing is also considering a similar role for its existing QF-16 target drones, used in training by the US air force.

Image copyright AP Image caption Fighter jets like Saab's Gripen could be flown unmanned, in future

Despite the challenges facing Europe's policymakers when it comes to military drones, the global UAV market has been largely immune from the tightening of military budgets in many countries in recent years.

The development of multinational programmes is politically challenging Derrick Maple, UAV analyst, IHS

The market is currently growing at some 5% a year, and this is a growth rate that is likely to continue for the next 10 years, according to defence and aerospace specialists IHS.

Overall, the market from 2014 to 2023 will total almost $90bn (£52bn).

Russian and Chinese aims

Crucially, much of this growth will no longer be driven by the huge US defence market which is set to slow in the short term.

Indeed outside the US, the UAV market's annual growth rate over the next decade is set to be 10% or more.

"Much of this is coming from Russia and China, which are both developing their industries," says Derrick Maple, UAV analyst at IHS.

Image copyright Getty Images Image caption Russia has revamped its armed forces in recent years - and is now investing in drones

"Their initial requirement is home-based, but China especially has aspirations to export in the longer term."

He says Russia aims "to spend $10bn over the next 10 years" and "they are going to develop unmanned combat air vehicles".

Europe 'needs its own capability'

Despite the success of Taranis, it is not yet clear whether European countries will, in the end, plan for their own combat drones to stand alongside manned fighters, sometime after 2030.

"The development of multinational programmes is politically challenging," says Mr Maple.

"But in my view, overriding this is a need for Europe to develop its own capability - otherwise it will continue to be dependent on the US and Israel."