Mark Ayton provides an overview of the Prismatic Phasa 35 solar-powered unmanned air vehicle

An artist’s impression of a Phasa 35 aloft showing the air vehicle’s configuration which is most noticeable for its 35m wingspan. All images BAE Systems

No surprise then, that 35 is a significant number for the UK’s latest unmanned air vehicle. Designed with a 35m (114ft 10in) wingspan, the whole purpose of the Phasa 35 is to build an aircraft that is so light and efficient, it can be powered by the sun during daylight, and by battery power overnight.

More significantly, it can remain airborne at high altitude in the stratosphere, above the wind and weather, to give persistent surveillance and communications capability for up to one year.

Cheaper to operate than today’s conventional unmanned air vehicles, Phasa 35 is set to provide persistent surveillance on station all of the time.

Explaining the rationale for doing that, Prismatic CEO, Paul Brooks said: “A satellite flies over the earth at 7,000 metres per second, passing over an area of interest for just a few seconds and does not come back for another three days. A conventional aircraft requires an operating base, takes time to get to an area of interest, and after its maximum loiter time needs to be refuelled back at base, all taking a long time. A solar-powered aircraft equipped with batteries does not have to be refuelled. The trick in designing such an air vehicle lies in the ability to store energy in batteries and to make the air vehicle very efficient given the amount of energy that can be stored in a battery is about 50 times less than in aviation fuel.

So is there a market for this type of air vehicle yet? Evidently yes otherwise Prismatic would not have built two production-standard prototypes. According to Paul Brooks, for the defence and security sectors, persistent surveillance and communications are two applications that have driven development of Phasa 35. He said: “For defence and security applications Phasa 35 can continually monitor what is happening on a nation’s borders, and provide quick, very low latency communications to people in the field. As sensor technology gets ever more capable, the amount of data in a battle space is increasing vastly such that satellite communications can’t cope, and antennas on the ground can’t see far enough. The civilian sector has equivalent applications in remote sensing, agriculture, monitoring for forest fires and deforestation over large areas of land, neither of which can be accomplished by a small UAV or a conventional aircraft. However, the highest profile business will be in communications to enable internet access where fibre and towers are non-existent.”


As mentioned above, Phasa 35 is designed to remain aloft for up to a year; a timeframe that is limited by the life-cycle of the batteries. Like all rechargeable batteries those fitted to the air vehicle also degrade, but very gradually such that at least 400 cycles are possible before the air vehicle must return to earth.

Key to everything about the aircraft is its design. The engineering team has made the air vehicle light weight and efficient, for example, the bespoke electric motor gives over 90% efficiency. Similarly, the propeller also gives great efficiency, designed specifically for the stratosphere’s flight environment.

Reliability is vital, so the aircraft is designed with very few mechanisms. There is no gearbox, the motor drives the propeller directly, and the tail and the rudder are the only control surfaces. Phasa 35 has a low operator control requirement, automatically navigating to GPS coordinates and the ability to remain at a position for as long as required.

Full automated control was in part driven by the conditions of the stratosphere where the air is very benign with little turbulence. Nobody has to actively fly the air vehicle very much at all, but simply set it on its path to the next waypoint.

Getting up to the stratosphere requires sufficient power to climb through and above the Jetstream by using the solar and battery technology effectively. Phasa 35 requires less than one kilowatt of power to fly, which is an impressive statistic in itself but more so when you consider that it’s approximately the same size as a Boeing 737 aircraft. To do that, the air vehicle has to fly very slowly at altitudes between 65,000 and 70,000ft [19,812 and 21,336m].

Operating in an atmosphere with an air thickness just 10% of air at sea level powered by batteries that hold 50 times less energy than conventional aviation fuel, shows the challenges the design team have to make work and the diffculties of flying an aircraft in the stratosphere.

The air vehicle only requires a (small or short) runway for take-off, leaving the undercarriage behind to reduce weight and climbs like any other aircraft, the difference being it enters and climbs through the troposphere on its ascent to the stratosphere. Given the expected low frequency of take off s and recoveries to and from mother earth, and no need to launch and recover close to the area of operation, Phasa 35 can be launched from locations closer to the equator where conditions are generally more benign avoiding bad weather on its ascent to the stratosphere. This concept of operation avoids designing the air vehicle to handle heavy rain, gusting and thunderstorms.

Phasa 35 is a most unusual type of air vehicle as seen in this shot of the first prototype in a hangar somewhere in the UK, probably arton, Lancashire. Its wings and fuselage are supported by steel stands and the undercarriage, which for weight saving purposes releases after take-off.

Prismatic has designed the Phasa 35 with a direct communication link and satellite links so messages, such as GPS location updates, can be received from a ground station. There is no dependency on a large communication infrastructure to get a message to the air vehicle.

Discussing the design requirements of the aircraft, Paul Brooks explained that the operating environment is of paramount consideration in everything aspect of the air vehicle’s design and configuration. He said: “The air vehicle will operate in an environment with a very high UV [ultraviolet] content which causes some materials to embrittle so we have to consider that and surface temperatures may dip as low as -80°C at night and climb to +80°C during the day. Prismatic has its roots in the design of satellites, so the kinds of environmental conditions to be encountered by Phasa 35 are not challenging for us to work to because we’ve worked with such design requirements throughout our careers.

Systems integrated onto the air vehicle need to be optimised for operating at altitudes around 65,000ft (20,000m), some 30 times closer to Earth than a satellite operating at 197,000ft (600,000m), so for something like an optical system, which goes with the square of the distance, it is 900 times easier for us to do something on the air vehicle than on a satellite. So if we use a 1,000kg payload on a satellite [a spacecraft] we can achieve the same capability with a 1kg payload on a Phasa 35.

The air vehicle is designed to carry a 15kg maximum payload throughout the day at the time of the winter solstice which is the worst time for flying because it requires 300W of power.

Several kilowatts of power are available to the payload during the day in the summer, the time when demand for most payloads is greatest, especially communications. Phasa 35’s ability to provide sufficient power to payloads throughout a year fits the markets very well.

Active payloads demand the most power, those such as broadband communication and synthetic aperture radar.

Phasa 35’s form of construction is modular so different payloads can be installed as required mission by mission without the need to reconfigure or change the whole aircraft to suit. The payload is housed in an ecosystem in the nose of the aircraft with a very simple interface. Designed by BAE Systems, the ecosystem provides safe thermo protection with suitable power conditioning in all environments.


Phasa 35 offers two distinct advantages to an operator in comparison to other high-altitude long-endurance air vehicles such as the Reaper and Global Hawk. Firstly, in a military scenario the idea of persistence is to remain on station all of the time, so an enemy can never assume they are not being watched. Phasa 35 can achieve that objective without having to return to base to be refuelled and replaced by a second expensive Reaper or Global Hawk.

Second, are the through life costs of operations. Any operator would like to have persistence and continual video feeds, but if the cost of providing that does not meet a market need then there is no point in doing it. Phasa 35’s advantages are not limited to its size, weight and cost compared to a Global Hawk, but its low through life costs of operations, which are considerable for operating a constellation of such aircraft with a single team.

Discussing quality of the products downlinked from Phasa in comparison with other current systems, Paul Brooks replied: “The data products delivered by a Phasa have to be compatible with both legacy air vehicles and their integrated systems, but of course Phasa does offer a completely new dimension because of its persistence. Suppose, as an example, that a Phasa 35 is gathering real-time HD video of an area, how will the operator use the video? Certainly not with operators looking at that video all the time, so capabilities such as cueing systems, automatic detection and change detection are necessary. BAE Systems can provide such capabilities to utilise the persistent capability much more effectively.”

This top down artist’s impression shows solar panels atop the gargantuan wing. Each panel is produced by American company MicroLink, is wafer thin, and helps to generate up to 12GW of power to keep Phasa 35 aloft and run its on board sensors.
Phase 8, a quarter scale model of Phasa 35, completed its maiden flight in 2017.

Both full scale prototypes were built using carbon moulds in 8m (26ft 3in) sections, each mould can be used many times over; 500, 1000, who knows, and the entire parts count is in the hundreds. The first two were built by a group of small British companies, and system integration by BAE Systems. A location for production will depend BAE Systems’ customers, but can be moved to suit customer requirements.

Phasa 35 has a monocoque structure, which means it has a solid skin of carbon fibre comprising very thin materials to minimise weight but also provide the strength and stiffness of a conventional aircraft to enable flight in some awkward conditions.

The wing was the part the engineering team spent the most time to design and test to ensure strength and stiffness requirements were met with a given weight. In short, the fuselage simply holds the tail to the wing. Prismatic built many full-scale test articles, loading each one to test for strength. Brooks reckons a one metre section held at each end can support 660kg (1,455lb) in its centre without breaking, and one person could pick the section up with one hand.

BAE Systems has contracted Prismatic to design the air vehicle and has taken a stake in the company to secure the relationship which will enable further prototyping to be undertaken for other aerospace systems. BAE Systems has exclusive rights to exploit and produce Phasa 35 air vehicles for whatever markets open up. Brooks reckons one of the great advantages of the collaboration is that Prismatic is really good at prototyping and BAE Systems is accomplished at aircraft production and associated services. Marrying the two together without interfering in how each other works is a key factor in the success of the collaboration. In terms of future production and operation of air vehicles, we have done everything right to make it the market leader. At the moment, only one other aircraft, the Airbus Zephyr, has successfully flown demonstrations. Phasa 35 is designed to be the first such production standard air vehicle.

BAE Systems

According to BAE Systems Business Executive for Phasa 35, Phil Varty, the collaboration between Prismatic and BAE Systems is based on two important aspects. He said: “Phasa 35 brings together a range of state-of-the-art, but on the shelf, technologies that give the platform a unique capability, and our partners and customers are interested in its capabilities. It’s a product we could get into market very quickly, and the technologies have lots of applications in other BAE Systems’ projects.

For instance, the state-of-the-art panels give about 31% efficiency [very high], are paper thin and bendable, so could we wrap them around other types of equipment to get additional power into that piece of equipment whether that be a tank or a truck? Everything is power hungry. The aircraft is made from very thin carbon fibre, a material used in the commercial space sector, so maybe we can apply that to other products, and we are working with Prismatic to understand those technologies, not just for application to Phasa 35.”

BAE Systems is interested in the aircraft’s mission application and what can it do for customers; this is dependent on its payload. Use as an intelligence and communications node are the big money earners for this aircraft.

OFF -the-shelf technology similar to the broadband systems used in domestic houses can provide communication over fairly wide areas beneath the air vehicle; a useful capability to both military and commercial customers for filling in the gaps of dead zones. Mobility is a big advantage. A Phasa 35 can easily be moved to another area to suit the requirements of either a military or a commercial operator. Integration of a radar, electro-optical and infrared sensors enables a remote sensing capability for sensitive military purposes. The air vehicle is designed to carry a suite of such sensors weighing roughly 10% of its gross weight. That’s no massive challenge to Prismatic given the size of current communication and radars used in the commercial space industry.

The Phasa 35 design team plans to provide 4GW to power the 150kg prototype when the air vehicle makes its maiden flight in February 2020. A feat of engineering indeed.

Discussing a need to re-engineer such sensors to cope with being airborne for up to one year, and operation in the stratosphere environment, Phil Varty said the team is working through a programme to test the payload and ecosystem using facilities owned by Prismatic. He said: “The prototype’s ecosystem includes thermal management that keeps the sensors cool when they need to be kept cool, and warm when they need to be kept warm. This will be further developed to meet customer needs.”

Phil reckons the Phasa 35 flight test programme will be undertaken in a similar way to those of commercial and military aircraft. He said: “The first flight will be a test of the solar panels charging the batteries and the batteries discharging overnight to keep the aircraft aloft at a certain altitude.

Once the sun has risen again, the solar panels should once again recharge the batteries ready for overnight; a process repeated on a daily basis. We will probably need a few cycles of that process because we need to prove to all interested parties that we can recharge the batteries each day. This will also prove the aerodynamics are right to get the performance from the aircraft. A few days of flight testing should be sufficient to give us the confidence to move forward.

Once that’s complete, we will evaluate what needs to be done to productionise the avionics, features within the flight control system, and how the payload’s communication and sensor systems perform at altitude. First flight is planned for February 2020, and if it performs how we expect, then the air vehicle will be operating at a standard that will allow us to conduct some operational tests on longer missions.

Discussing the likelihood that a Phasa 35 might be flown in UK airspace, Varty outlined some details about certification challenges with the authorities. He said: “A lot of work has been undertaken with various bodies including the CAA [Civil Aviation Administration] and the ICAO [International Civil Aviation Organization] about how and where UAVs in general fit into airspace. As a business, BAE Systems is involved in a lot of those discussions about certification of unmanned air vehicles, and what kind of sensors are required on board the air vehicle to provide collision avoidance and de-confliction. We would use an airspace corridor clear of other air vehicles, and climb to altitude in a spiral ascent over the course of half a day, and take it on its way to the required operating location. That process might only be required once or twice per year so interference with the air Traffic control system would be limited to the one day during which the Phasa 35 air vehicle takes-off on a mission, and again on its return to base.

Operating air vehicles at stratospheric altitudes creates a need for additional regulations, and a number of working bodies are in discussions with regulatory authorities about a concept of operation.

According to Phil Varty, industry-led control seems to be the direction of travel for that right now. He said: “There is not a solution today, defence will start that ball rolling and over time the commercial side will take over as companies demand 5G communications with lots of air vehicles aloft. It’s a fascinating new concept of operation and regulation.

Phasa 35 is made from moulds in a very efficient and non-complex way. It is made out of 12 major parts which are bolted together, an easy process. Production of each air vehicle is largely manual, but eventually BAE Systems will use automated techniques to produce some of the 12 major parts. In entirety, much of the production effort is currently dedicated to manufacturing the solar panels, which are made by American company MicroLink Devices Inc. According to Phil Varty, the cells are wafer thin and very expensive to produce because they are currently, all but individually made. He said: “In order up-scale to series production rates, an automated solar panel production process is key. We are in discussions with MicroLink about how up-scaling can be implemented. That needs to have a productionised process applied, so concepts from BAE Systems’ factory of the future might be useful setting that line up, more so than for the air vehicle itself.”

The solar panel configuration fitted to the Phasa 35 prototype is designed to generate 12GW of power, but currently uses just 4GW. AI