A recently launched research project involving researchers at Bristol University and UK aerospace company Cobham is attempting to solve the problem of entirely autonomous aerial refuelling.
The project is actually one facet of the wider ASTRAEA (Autonomous Systems Technology Related Airborne Evaluation and Assessment) programme, which aims to open up non-segregated airspace for UAS and, thus, a wider range of applications, many of them non-military.
One of the key quoted advantages of UAS is that they can remain in the air for longer periods, unburdened by the physiological limitations of human pilots, who tire and by law must be relieved by a fresh crew after a period. However, unlike their piloted counterparts, UAS cannot, at the moment, refuel mid-air and so will inherently be limited by the amount of fuel they can carry.
’If unmanned aircraft systems are going to fulfil their complete potential, there’s a good chance they’ll have to refuel,’ said Richard Bourne, a former naval officer and now programme manager of research and technology at Cobham Mission Equipment.
’That may seem bizarre in the non-military sense but it’s where refuelling started – Cobham’s initial concept was to support Imperial Airways, enabling its flying boats to get across the world without having to land and refuel regularly.’
Indeed, conventional air-to-air refuelling was originally explored in a civilian setting, with the US interested in achieving trans-Atlantic flights for a faster postal service to Europe, while Britain wanted to service the Empire with its flying boats.
However, the cost of the equipment and the complexity and skill involved in actually executing the procedure meant that aerial refuelling to this day has been confined to military operations. So, ironically, part of the ASTRAEA programme is to bring the technology full-circle.
’Obviously the challenges of removing the human from the process of refuelling is considerable. It’s only when you start picking the problem apart that you realise just how smart humans are and how they are able to absorb information, translate that into action and react accordingly,’ Bourne said.
For cost and practicality purposes, the project will work on the assumption that UAS will use existing refuelling tankers employing the ’drogue-and-probe’ method. Most of the development will therefore go into the autonomous receiver aircraft, which will make positional decisions.
’You’ve got to emulate the eyes, arms and brain of the pilot to sense where the tanker is, where the drogue is, how they relate to each other, where you are, then how to move in. Once you’ve connected with the drogue it becomes part of the receiver, so you have to maintain formation with the tanker,’ Bourne said.
So the first aspect of the project is to understand what actually happens during a successful air-to-air refuelling manoeuvre – the various physical forces at play and the decisions that the pilot makes.
As one of the early pioneers of aerial – refuelling technology, Cobham enlisted the help of Bristol and Cranfield universities to assist it with wake, atmospheric and aerodynamic modelling, and some of the UAS’ existing control systems.
Having established a ’synthetic environment’ incorporating all of these aspects, the team needed an arena to test out possible hardware and software solutions.
As every engineer knows, computer simulation is no substitute for the real thing, but clearly it wouldn’t be practical to test out new solutions in real UAS.
Cobham therefore commissioned a unique robotics system to be installed at Bristol University as a surrogate for the real thing. It comprises an exact copy of the probe that any recipient unmanned aircraft would have, mounted on moveable head, which in turn is placed on a 10m-long rail rack – replicating that vital distance where the aircraft come together. The other robot is an exact replica of the shuttlecock-shaped drogue found on most tankers, which is also able to move as if being trailed on a hose.
Researchers at Cranfield University are also developing bespoke sensors to aid the recipient UAS in locating its spatial position in real time. Versions will be fitted to the robotics platform at Bristol, then tested and honed as part of the overall hardware product.
’The first task is to get the end of those robots to mirror as closely as possible what the synthetic environment is doing – that interface to the real world, which brings very difficult technical problems,’ said Bristol’s Dr. Tom Richardson, who is leading the research effort.
’You need to get those right, so the answers you get out of the flight-simulation control and hardware and loop testing are going to be right.’
Having received the system back in February, the team is still verifying the synthetic environment, running numerous simulation scenarios that UAS might encounter up at real altitude.
But there are still many problems that may not have definite solution. Cobham found that pilots can often ’anticipate’ the behaviour of the drogue and adapt based on experience. This is something very difficult to imbue a machine with.
For example, occasionally when the recipient aircraft approaches a tanker, it creates a bow wave with enough force to shift the drogue. Richardson will be supervising a PhD project specifically to see if bow-wave effects can be predicted, although it’s certainly not a given.
Eventually Richardson hopes to have some sort of product, most likely a combination of hardware and software, which he will hand to Cobham to be fitted on a candidate unmanned aircraft.
’We are doing this constantly, bearing in mind that we have to work with existing aircraft. It’s highly likely that whatever aircraft this will go on will have existing hardware and software that we need to interact with. We can’t design a fantastic solution, but we can say: “Okay, we want full control of the aircraft”.’ Richardson said.
Source: The Engineer