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Building an electrifying defence

Posted 4 December 2018 · Add Comment

Benjamin Stafford, materials science expert at materials search engine Matmatch, explores materials and technologies that could make electrification possible for transportation in aerospace and defence.

The growing public concern for greenhouse gas emissions is just one reason for the shift towards electrification. Transport makes up 14% of global emissions, however, this isn’t confined solely to road vehicles. Global electric car sales are surging but the big question now is, can we also electrify other modes of transport?

Looking beyond the road, military activity is the most oil-exhaustive activity on the planet and the US air force (USAF) is the single largest consumer of jet fuel in the world.

The B-52 Stratofortress, for example, is one of the USAF’s strategic bomber planes. This particular plane has eight turbojet engines and guzzles 500 gallons of fuel per minute. To put this into perspective, this means that just ten minutes of flight uses as much fuel as the average driver does on the road annually.

It hasn’t always been this way, of course. In 1940, the US military consumed just one per cent of the country’s total energy usage but, by the end of World War II, this significantly increased to 29 per cent.

Now, mechanical and design engineers are looking to make a fundamental change in the way we power the various transportation methods around us. Electrification offers a more sustainable means of powering a system but is it powerful enough to operate a naval vessel or an aviation system?

First, the requirements for electrical motors in aircrafts or seagoing vessels compared to electric vehicles significantly differ. These modes of transportation must withstand extreme conditions and require a greater power to weight ratio from electric motors. While conventional electric motors — which are traditionally composed of copper, iron and permanent magnets — may be suffice for electric cars, they will not be enough to get an aircraft off the runway.

High-temperature superconductors
Instead, design engineers need to develop electric motors, which generate greater thrust per unit weight. For this, high-temperature superconductors (HTS) are a key enabling technology. HTS materials lose their electrical resistance below a superconducting transition temperature and are making high power, low weight motors possible as a result.

For conventional superconductors, the transition temperature is usually so low that the superconductor needs to be cooled using liquid helium at around 4 Kelvin (-269 degrees Celsius). HTS, however, can operate at comparatively high temperatures and be cooled by the cheap and abundant coolant, liquid nitrogen, which boils at 77 Kelvin (-169 degrees Celsius).

It is for this reason that companies are working to further decrease the cost of manufacturing HTS, with the aim of making the price comparable to copper.

At sea
Being able to electrify naval vessels could mean new shipping routes that allow for expansion into one of the least explored parts of the Earth; the Arctic. To do this, however, engineers need to ensure that naval vessels have efficient subsystems and propulsion systems capable of travelling long distances.

So, how can HTS help achieve this? While electric ship propulsion has been around since the 19th century, it has traditionally been limited to small vessels. However, HTS wire can conduct the same current as a copper cable in about one tenth of the cross section. So, when used to replace copper windings and permanent magnets, HTS wire can provide a huge weight reduction and can create a higher, more intense magnetic field. This allows for much more compact, higher power electrical motors.

For naval vessels, HTS are also being implemented in degaussing coil systems. The system encircles the vessel with superconducting cables that can neutralise the ship’s magnetic signature and reduce the chances of the vessel being detected by enemy ships, submarines or magnetically activated mines.

For these types of electronic and magnetic applications, companies tend to use copper or silver. C110, for example, which features 99.9%of copper content and is the purest grade of oxygen-free copper, offers the highest electrical and thermal conductivity values available from a commercially available copper alloy and is not susceptible to embrittlement when hardened. C110 also has high ductility and is readily cold worked, making it suitable for electronics in naval vessels and other marine applications.

Replacing copper in motors and cables in seagoing vessels with HTS can help significantly increase the distance that ships can travel, reduce the journey time, reduce risk of detection, minimise the amount of energy used by the marine sector and allow for travelling longer distances. Similarly, the aviation industry can greatly benefit from the electrification of aircrafts.

In the sky
One of the biggest challenges for aerospace engineers is being able to lighten the overall load of the aircraft without jeopardising its output. To tackle this issues, engineers can replace the copper cables that are traditionally used in the planes motor with HTS cables.

In fact, in NASA’s NX-3 aircraft, the plane is designed to use several superconducting electric motors to drive the planes distributed fans, which in turn lowers the fuel burn and emissions. The aircraft also makes use of HTS cables to distribute power from a superconducting generator to the motors.

In addition to offering greater benefits for the environment, electrical propulsion means quieter aircraft. During an operation, air forces can hold an advantage over the enemy by reducing the probability of detection.

The future
For the defence sector, design engineers should consider the value of integrating HTS into their systems. Whether it’s electric propulsion or power distribution in an aircraft or sea vessel, HTS can offer greater power per kilogram, lower noise and reduced emissions compared to conventional electric motors.

Despite the advantages, the adoption of HTS in propulsion has many hurdles to overcome. This is down to the complexities of the technology and the associated developmental costs, yet we will undoubtedly see the next generation of cleaner and quieter military aircrafts and vessels using this technology in the years to come. This will become increasingly more popular as the price of manufacturing HTS becomes comparable to copper, making electrification a viable option for aerospace and defence companies.


 

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