Are we on the brink of new era of flying machines able to stay airborne for weeks, months or even years at a time? RICHARD GARDNER looks at ultra long endurance UAVs.

This is an excerpt from an article published in Aerospace International: November 2010
[caption id="attachment_3322" align="alignnone" width="198" caption="QinetiQ's Zephyr UAV - aloft for a fortnight (QinetiQ)"][/caption] The intensity of operations in Afghanistan has placed enormous reliance on space-based military assets to enable air and ground forces to maintain a technological edge. Satellites have provided secure, reliable, surveillance and communications capabilities that have transformed the way the war is being waged, even though they remain unseen and attract little media attention. GPS is now an essential tool for both navigation and precision attack. Without these capabilities it would be difficult for local commanders to maintain a dominant position on the ground without needing many more troops than are in theatre at present. Regardless of the end result of this particular conflict, the pattern of operating, with reliance on space assets and air power domination, is likely to remain a key feature of warfighting for the rest of the century. But it is becoming colossally expensive, even for the US, and the availability to NATO of global positioning and space surveillance space assets, largely provided by the US taxpayer, is now very much taken for granted. The UK has its own highly successful Skynet military communications network, but it is completely dependent on the US for GPS and space surveillance. Unless it starts to invest in its own ‘Son of Topsat’ small surveillance satellites, this dependency will remain. Closer to earth, the RAF’s current armed UAV capability provided by General Atomics Reaper drones, is equally dependent on US-supplied infrastructure, with No 39 Squadron based at Creech AFB, Nevada. If military space assets and UAVs are growing in importance to the UK, and the RAF in particular, as frequently espoused by the Chief of the Air Staff, ACM Sir Stephen Dalton, then how is the future requirement for persistent surveillance to be met — and kept affordable? Ultra high altitude long endurance (HALE) unmanned air vehicles (or remotely piloted vehicles) could be a part of the answer.

UK World record beater

[caption id="attachment_3323" align="alignnone" width="300" caption="QinetiQ's Zephr 7 UAV taking off (QinetiQ)"][/caption] The UK defence technology company QinetiQ has developed an ultra-lightweight solar-powered UAV, the Zephyr which, last July broke the world endurance record for an unmanned air vehicle, remaining airborne for 336 hours and 22 minutes (over two weeks). It also broke the absolute altitude record for a UAV at 70,740ft. This flight, conducted over Arizona, USA, beat the previous UAV endurance record by more than a factor of five, and was only terminated by ground control after it had proven all the test parameters that had been set for it. The Zephyr programme was aimed at designing a HALE UAV that could fly for months at a time, with a year round performance in mid-latitudes. The concept centred on an ultra-light and cheap solar electric aircraft with solar charged batteries providing power during the night and with an operating ceiling of between 50,000-70,000ft. The work on Zephyr dates back to an earlier QinetiQ project for a high altitude manned balloon attempt on the world altitude record in 2003. A 12m span proof-of-concept prototype tethered UAV was built, and was intended to be launched from the balloon at above 140,000ft. Although the balloon record attempt failed, the Zephyr UAV concept showed considerable promise and two years later Zephyr 4 was launched from a balloon over the Woomera test range in Australia. This led to further ground-launched test flights of Zephyr 5, this time conducted at the White Sands Missile Range in New Mexico. Two variants were flown — one powered by primary battery power only, and the other by both primary battery and solar power. Between 2005-2006, Zephyr 5 flew with the first solar powered propulsion system for a maximum of 18hours, with a 1kg payload and at an altitude of 36,000ft. From 2007-2009, Zephyr 6 acted as a technology proving test platform for the concept of multi-day missions at an altitude of 62,000ft. The Zephyr 5’s wingspan of 15m was extended to 18m for the Zephyr 6, which achieved an endurance of 82 hours, 37 minutes. In 2010, the latest Zephyr 7, with a 23m wingspan, a 2.5kg payload and a 70,000ft altitude capability, proved that multi-weeks, or months could be flown non-stop, paving the way for the development of a vehicle with an operational capability. The Zephyr 7 proved its improved reliability over Zephyr 6, with increased performance in flight duration and overnight altitude capability. The in-flight performance data will be used for future system improvement.
Aerospace International Contents - November 2010 News Roundup - p4 News Focus - p13 Five priorities for the post-SDSR environment Sizing up the flyweights - p14 Phenom 100 and Mustang battle it out Making security work - p21 New trends in airport passenger screening The shape of aerospace to come - p 24 Report on ICAS 2010 conference Patent predictions can open doors  - p28 Monitoring aerospace technology trends Deal or no deal - p30 Focus on EU merger and alliances rules The Last Word- p34 Keith Hayward on airport competition
Zephyr has become a remarkable low-profile breakthrough air vehicle. By no means the first solar-powered aircraft, it has nevertheless taken a very simple, but desirable, aim and turned this into a viable vehicle with real potential for growing into a transformational technology. The key to success to date has been the achievement of low cruise power at high altitude. The design required low mass, low wing loading and low drag, with high propulsion system efficiency. The onboard avionics package required minimum power requirements with high energy density power storage, with a good cycle life and efficient power generation. Although at first sight the Zephyr air vehicle resembles a giant model aircraft, its design has involved extremely sophisticated computational analysis and development of every aspect of the structure, propulsion and avionics systems, the batteries and solar array. The carbon composite airframe features a very low-drag wing section with efficient propellers. The brushless direct drive electric motors have high efficiency speed controllers and the avionics systems are light weight and low power. SION Power has developed the secondary batteries with lightweight insulation and low power heating system used to maintain battery temperature. United Solar Ovonic thin film silicon array covers most of the wing area and forms the flying surface. Zephyr is intended to be capable of carrying a useful 2.5kg payload and this can be carried either in a nose pod or within the wing. There is rich potential for developing the platform for communications relay use as from an altitude of around 65,000ft the line of sight diameter is 1,000km. Payloads can include cross band VHF-UHF repeaters and dual channel SINCGARS VHF relay. An ISTAR payload can include visible real-time video pan and tilt camera and far IR real-time video. Demonstrated ISTAR performance has shown large field of regard for pan and tilt visible payload and a zoom capability for wide field of view for detection, and narrow for identification. QinetiQ has also built low level test aircraft (LLTA) based on scaled versions of Zephyr to be used for aero-proving and avionics/ground control station integration. Each is a test bed for systems demonstrations, payload investigations and crew training. The current Z7 ground station is a container-based transportable unit with all the air-ground data links. The July flight trials of Zephyr 7 which set the recognised absolute world records for UAV endurance and altitude (beating the Global Hawk’s altitude record by 5,000ft and flying ten times longer non-stop) proved that the concept works and that it can be developed into a reliable air platform.

Five years aloft

[caption id="attachment_3324" align="alignnone" width="300" caption="Solar Eagle (Boeing)"][/caption] The thought of an aircraft being able to remain aloft continuously for up to five years is a sobering one but that is the aim of Boeing’s Solar Eagle. The US military Defence Advanced Research Projects Agency (Darpa) has awarded Boeing an $89m contract to build and fly a Solar Eagle demonstrator UAV by 2014. This news emerged in September and follows extensive work with the UK’s QinetiQ on the Zephyr programme. The Darpa programme was competitive between Lockheed Martin’s Skunk Works and Boeing’s Phantom Works. Dr Tony Tether, Darpa Director, said: “A single Vulture aircraft could support traditional ISTAR functions over country-sized areas, while at the same time providing an unblinking eye over a critical target, monitoring by day or night and providing unprecedented high-value intelligence.” It could lead to a military platform that is relatively low cost but which might remain at very high altitudes (above 60,000ft) for years on end, well away from ground-based opposition and free from manning constraints and ground servicing requirements. It would have a modest but useful payload of around 1,000lb, with a 5kW power output, capable of operating onboard sensors and with the ability to distribute surveillance and intelligence data to ground stations by day or night. The Boeing HALE programme manager, Pat O’Neil commented: “ Such a pseudo-satellite system could provide compelling operational advantages in terms of persistent intelligence,surveillance, reconnaissance and communications. Our Vulture programme will develop an aircraft capable of remaining on-station for over five years, pushing technology and design so that the system will not need refuelling or maintenance.” Freed from the orbital constraints of satellites, which require several vehicles to maintain a persistent presence, a HALE air platform could fly autonomously relying on solar energy alone, collected during daylight and which will be stored in fuel cells for night flying. Dr Tether commented, “Vulture aircraft would be able to provide communications capability available today only from geostationary satellites — offering opportunities for new, more flexible, expandable and relocatable communications architectures at a fraction of the cost of dedicated satellite capabilities.” The super-efficient engines would be electrically powered and fitted to a 400ft long wing. The skeletal structure will be very light in weight but will have to be strong enough to withstand bending and twisting movements in strong winds. There will be some stiff challenges to meet if the demonstrator vehicle is to contend with extreme cold temperatures as well as strong winds. There will be no humans to interfere or repair the vehicle once on station, so reliability under all extreme conditions will be crucial to turning this into a viable operational tool. Solar cell reliability and energy storage capacity will need to be developed further so that the vehicle can operate continuously, unrefuelled, for over 44,000 hours.

Other HALE contenders

[caption id="attachment_3325" align="alignnone" width="300" caption="Boeing unveils its Phantom Eye UAV (Boeing)"][/caption] Few UAVs can currently remain airborne for more than a day or two but the military potential for air platforms that can ‘stay up’ for extended periods is well recognised. At this year’s Farnborough International air show, there was much interest in a model of Boeing’s Phantom Eye hydrogen-powered UAV. Work has started on building the prototype aircraft. It will be the first HALE UAV to feature liquid-hydrogen as its fuel. Some five years of technology development activity has already gone into the project and now the company is deploying rapid prototyping to create a UAV that will be ready for a first flight early in 2011. A product of Boeing’s Phantom Works, the project is intended to lead to a practical design that can lift a payload of 450lb up to 65,000ft and remain on station for over four days. The key to Phantom Eye’s performance lies in its radical propulsion system, comprising liquid hydrogen fuelled 2.3L truck engines, turbochargers and integrated engine controls. Eighty-hour test chamber running at a simulated altitude of 65,000ft has already been completed and work on the prototype build is now underway. The UAV will have a 150ft wingspan and two engines and will maintain a persistent presence in the stratosphere over a selected area performing missions that include intelligence, reconnaissance, surveillance and communications. The bulbous fuselage contains a couple of 8ft fuel tanks containing the liquid hydrogen. The technical challenges are considerable, but if all goes well with the demonstrator next year, Boeing will have alternative offerings to take over some ISTAR tasks from satellites — one powered by solar cells and one by hydrogen. Dave Koopersmith, Advanced Boeing Military Aircraft vp, said: “These innovative demonstrators reduce technology risks and set the stage for meeting both military and commercial customers’ future needs.” Phantom Eye is a co-operative programme and Boeing is working with Aurora Flight Sciences, the Ford Motor Co and Mahle Powertrain to develop the demonstrator. [caption id="attachment_3326" align="alignnone" width="300" caption="AeroVironment Global Observer (AeroVironment)"][/caption] AeroVironment’s Global Observer is designed to fly for seven days and is aimed at the same extended endurance requirement as the Phantom Eye. Also hydrogen powered, it is supported by US Special Operations Command and the US Coast Guard. The initial prototype, to fly at the end of this year, is much smaller than Phantom Eye and would carry a 180kg payload. Hydrogen fuel is a long- discussed subject in aviation but, in the past, its adoption has been defeated by the complexities of storage and handling. It remains to be seen if this can be overcome in an active theatre of operations working environment to enable the promise to become a practical UAV system. [caption id="attachment_3327" align="alignnone" width="204" caption="Aurora Flight Sciences Orion UAV"][/caption] Although a MALE (medium altitude) rather than a genuine HALE design, Aurora Flight Sciences Orion (carrying a 455kg payload at 15,000ft) slips into the category of ultra long endurance UAVs as it is intended to remain on station for up to five days. It is just the latest in a growing portfolio of UAVs that seem certain to change forever the way nations carry out ISTAR missions. As the electomagnetic pulse (EMP) threat to satellites becomes more widely recognised as a high defence priority for the future, the availability of cheap HALE air platforms might offer a more affordable way of replacing vulnerable orbital space assets. While not exactly disposable in use, they are certainly far cheaper than hardened military satellites, and can be launched and in use in a fraction of the time to replace a damaged or destroyed space platform. Aerospace International would like to thank Paul Stevens from the QinetiQ Zephyr team for his assistance in the preparation of this article.

Royal Aeronautical Society
5 November 2010