Matt McAlonis is Engineering Fellow, Aerospace at TE Connectivity
What are the main technical challenges to developing connectivity and electrical systems for electric vertical take-off and landing (eVTOL) aircraft?
Safety is above all else the key element. The second main challenge is weight. Unlike in traditional aerospace systems, where fuel burns and the aircraft becomes lighter and more efficient as you fly, the battery is as heavy at the end of the flight as it is at the start. You can’t just drop batteries as you exhaust them. So how do you design the network of components to take account of the shape, size, weight, and power requirements of an eVTOL?
Let’s start with charging. In the automotive world you plug in and charge your battery which stays on board the vehicle. That requires extra cables going from the charger inlet to the battery; so thermal management is important here because you want to charge quickly and that means things will get hot. If your batteries are getting hot, are they still safe? Electrical fires are very different from fossil fuel fires; they burn very hot are very hard to extinguish.
So if you are charging an eVTOL battery what types of thermal management do you need? There are companies looking at active cooling chargers using liquid cooled systems. But these are heavy. So, should you instead exchange the battery? In that case you will need a battery interconnect system.
Then you have the power distribution system. The propulsion system probably requires around 800 to 1000 volts, which is very high voltage relative to normal aircraft. In a car you have a 12-volt battery system while most aircraft today have systems operating at 200-something volts. So now you’re looking at five times that voltage. Does that mean you will need to segregate the different electric systems?
You don’t want a mechanic cutting into a 1000-volt cable so you need to look at cabling very carefully, making sure each cable is properly identified and protected with metal shielding. So how do you make that lightweight?
Most aircraft electrical cables are designed to operate at 30,000 ft plus so you can start to look at thinner insulation materials as eVTOL vehicles will operate at lower altitudes; most of these cables are round in cross-section but you can now start looking at maybe using different optimized geometries. Tighter spaces mean you can maybe flatten a cable.
Another area is articulation.
In traditional aviation an engine is a static mechanical device, with power generating system connecting to the aircraft with a straight run of cables. Articulation is a new phenomenon, with cables being constantly flexed during flight. In our medical division we design ultrasound cables with a high flex-life endurance, but this is new requirement for aviation power cables, and we will have to design these for thousands of flex cycles which will be subject to new inspection and maintenance procedures.
So TE does not just operate in the aerospace sector?
We operate in many different markets and we can leverage this diverse experience to make better eVTOL systems. We’ve been working with electric cars and other electric ground vehicles and data centers for many years and autonomous vehicles are often, essentially, a flying data center. So, we have a great deal of experience in designing high speed connectivity systems and knowledge of rugged reliability, high-speed data performance, power-switching products. We can use the experience we have in markets that have already scaled up – such as automotive – and bring that to aerospace. Everyone wants to be connected and TE is a key part of connectivity; that’s what we do.
How do you see this technology evolving over the next few years?
NASA has developed a notional roadmap suggesting smaller aircraft like eVTOLs will probably need to be 1000-volt DC battery operated but as aircraft increase in size the power required would then change by orders of magnitude.
I’m looking at this from a systems architecture viewpoint. When I talk to customers I try to separate the knowns from the unknowns. The knowns will be issues such as how are you charging? What kind of system is it? What kind of plug do you use and what does your receptacle look like? Are you leveraging any existing standards? Do you need to reinvent that for aerospace? Aerospace products have to operate at altitude whereas most electrical charging standards are based on ground vehicles.
Then we have to look at battery types: cell, prismatic, and pouch. And what is the interconnect between them? Many customers today design their own battery, and electrical aircraft manufacturers will have different power and shape sizes for their aircraft. Some put batteries in the wings while others in the fuselage. Then there is the power distribution center and the battery interconnect system requiring different kinds of cables. Some of the cables will need different flexibility capabilities while others will need to be designed for tight confined spaces.
Do we have all the standards and regulations in place for this?
They are being developed. SAE International has set up a series of electrical propulsion groups divided into several subcommittees studying this, such as the Electrical Wiring Interconnection Systems and Energy Charging Energy Storage Energy Switching workgroups.
How do you know what levels of redundancy will be needed for eVTOL operations?
This concerns safety levels. If you have an emergency landing or a bird strike on a propellor that knocks the motor out, there will be more strain and stress on everything. In a traditional aircraft you might need 30% more fuel than the flight requires but with eVTOLs these requirements are still being defined.
Batteries entail a different redundancy challenge. When a battery is fully charged the charge and energy quality is very different when the charge is at the bottom end of the scale. Temperatures can make a difference. The Middle East can be a very harsh environment: very hot, very dusty, very dirty. In an environment like that you need to keep things clean so you will have to design for that. Altitude is another factor that affects the voltage requirements.
So, in terms of the maintenance challenge, every year you may have to change the cables to take account of the flexing, strains and mechanical fatigue. It is similar to changing the spark plugs and respective electrical systems in cars.
How does the challenge of designing these new systems impact the relationships between suppliers and eVTOL OEMs?
We wouldn’t make a wire to exist by itself; each is intended to work on a certain platform and in this industry everything gets certified, from where we make it, to how we test it and how and where the customer uses it. So, customers can be sure that the product has been designed for the function intended: flight profiles, the engine, the demands on the product such as temperature, current carrying, voltage and potential thermal exposure. If it is inside the cabin it has to meet a flame, smoke and toxicity test.
That’s why partnerships are so important – we need to work closely with the customer to understand exactly how and where the product is to be used.
Many eVTOL manufacturers want to use electrical system designs which have been developed for the car industry. How can we ensure which of these will be a good fit for the aerospace sector?
At TE we test systems to failure in all areas.
In mechanical configurations we test how many times the component can be mated and unmated; how it withstands shock, vibration, and crashes. We look at end-of-life challenges. When systems and structures operate at high temperature, plastics start to degrade, springs stress relax and with connectors, resistances go up.
The big challenge with electrical failure is thermal runaway. As things age, they lose their efficiencies and with power systems that means that things get very hot. The connector can melt away, short circuit and cause a fire. So you need to mitigate that.
There’s an extra feature we put into these systems called High Voltage Interlock – a separate circuit so the connector has some level of intelligence in case it becomes unmated and breaks the live electrical circuit.
The maintenance requirements for product replacement are just like a normal aircraft. The engine is evaluated for its integrity after so many hours of flight and the same should apply with electrical systems.
What opportunities for new systems design do eVTOLs bring?
The aerospace industry likes to leverage proven technologies. There is a tendency to prefer traditional materials for plating conductors – a cable, a connector contact or something in the power switching. In military this is often in gold even though high-power silver has a lower interface resistivity than gold.
So as we look at the optimization of materials I think we can innovate and not just rely on 50-year-old technology.
Many of MIL-spec crimps have been around for decades and while they’re adequate they’re not optimized. So, in the crimping of these cables, we’re getting into weight reduction by using aluminum conductors which are not new but can be used in new ways. High performance polymers sometimes have the strength of aluminum but they’re even lighter weight, although not yet aerospace-certified.
We want to use artificial intelligence to see how we can use these new materials and even more esoteric materials such as graphene, a two-dimensional structure which can be formed into a three-dimensional structure as a carbon-nanotube.
For more information
TE Connectivity: Interconnect Solutions for eVTOL, Urban Air Mobility and AAM
