Current aircraft use kerosene for propulsion, a carbon-based fuel which is stored in the wings of the aircraft. When you would want to fly on hydrogen, storage in liquid form within the wings is not possible. To be able to store the hydrogen onboard, you would need to find space for that in the fuselage of the aircraft, thereby changing the shape, drag and other specifics of the aircraft. With the APPU project, the researchers try to design an aircraft that can carry and fly on both, some amount of hydrogen and some amount of kerosene. Such an aircraft is fit for the transition as not every airport around the world would have hydrogen available at the same time. 
In the new design, the aircraft has a different configuration – including a fuselage tail mounted propeller - with a third engine that can burn the different types of fuels in its dual-fuel combustion chamber. This new type of engine and propeller configuration enhances the efficiency of the aircraft and also allows for steeper take offs and descends. 

The current configuration can store approximately 15 to 20% of the total energy in form of hydrogen, thereby reducing the aircraft emissions by around 25%. So for the next configuration, the researchers hope to bring increase the H2 % gradually to around 50 %, using the multi fuel combustor technology.