Sustainability – the Fuel and Power Plant Options

Of the many challenges that faced Horizon in the beginning, sustainability was certainly one of the most difficult ones. The easy route to take was to employ Batteries. This is the route taken by all of the first generation of vertical take-off urban transports. But the practical impact of their high weight per unit of power on the performance of the helicopter was something that would have resulted in the Autocopter being confined to what is a niche, low range, low speed market, reliant on a complex and slow re-charging infrastructure which has yet to be built. Horizon recognised that, to be a commercial success, the Autocopter would need to be aimed at a much wider market and to address the shortfalls of the first generation of air vehicles. To enable that, the Autocopter would have to offer a flexible approach to its power sources by embracing and taking advantage of the ever-changing fuel and battery technology which is driving the “Green” agenda. This approach also allows the Autocopter to address a much wider market. The Autocopter would also need to be future-proof and our approach to the design of its hybrid, multi-fuel power system addresses this. There may, however,  be some circumstances under which, in future, only battery-powered machines are acceptable. High-density city environments with a developed support infrastructure may be a case. Horizon could readily comply with this because the aircraft is designed from the start to have an electric final drive system, and one day batteries may be light and efficient enough and be rapidly re-chargeable so that this makes commercial sense.

The better solution in Horizon’s view was to start by offering a gas turbine powered aircraft fuelled by either Synthetic Kerosene or high bio-content Sustainable Aircraft Fuel (SAF). The former could in time get to carbon-net-zero and SAF probably to 85-90% of that. The combustion system of the Autocopter’s gas turbine engine can actually handle Diesel Fuel, Gasoline and Ethanol as well as kerosene, but there are practical difficulties with these in terms of aircraft use. This unique turbine generator will also have further attractive commercial applications in the automotive and other industries.

Horizon has also looked at Hydrogen as a possible fuel. This has 5 times as much energy per unit of weight in it than kerosene, but the weight of storing and using it in a Fuel Cell is almost on a par with battery power. So it’s not a practical helicopter fuel, at least in Horizon’s eyes.

The first use of Hydrogen may then be a Gas Turbine fuel and the numbers here are much closer to being practical. Horizon has looked at a fuselage-integrated hydrogen store, either engineered into the floor of the helicopter or vertically in front of the engine bay. Done properly, this could overcome the storage weight issue. Airworthiness issues may preclude its integration into the floor of the aircraft, Why? Because today, the certification authorities like to see the bottom of the aircraft being a crushable structure (VTOL aircraft must be able to sustain a drop onto the ground from 50ft) so that in the event of a power failure at near ground level those in the aircraft would have a reasonable chance of surviving. As a hydrogen store would be an immensely strong structure, able to store hydrogen at 700 bar, it would most likely not be crushable.

The Hydrogen Economy has, however, become of great interest to Horizon. In the UK, areas where windpower is abundant (virtually all coastal areas with the exception of the South Coast) there is a need for some means of storing the energy produced by the wind farms. The conversion of electricity into hydrogen (and oxygen, sold to users of this) is looked upon as a serious and practical solution. Some of the coastal areas are then turning themselves into ‘hydrogen economies’ where power and heating all come from the use of hydrogen, created by a variety of machines called Electrolysers. These come in many shapes and sizes, from household size to city size. The same could happen in countries where Solar Power is abundant.

Clearly, such an economy, were it large enough, could also support a synthetic kerosene plant (perhaps supplied by Shell).  In fact, Horizon did investigate the possibility of persuading investors to build one in southern Spain, using solar power produced in Morocco and exported to Spain over the DC link that connects Morocco to Spain. The plan, however, was to produce diesel fuel as well as kerosene (very closely related) and offer this to fuelling stations in southern Spain as a carbon-net-zero option for motorists, in much the same way as both diesel and gasoline are offered in different forms or blends. Diesel cars could then be sold well into the future, much to the delight of manufacturers and owners. In one step, this would overcome all the problems raised by battery power and the storage and use of hydrogen on board cars. It’s such a blindingly obvious solution.