Carbon composites and the Lilium Jet

• Our Chief Program Officer, Yves Yemsi, on rocket nozzles, wing boxes and the Lilium Jet

 

Carbon fiber composites: A brief history

Since the very earliest days of aerospace, innovators have been engaged in the pursuit of improved performance. How do we go faster? How do we make the aircraft lighter? How can we improve efficiency? All the while putting safety at the heart of everything, not just for those flying the aircraft, but for passengers and the public too.

The introduction of carbon fiber composites provided a watershed moment in aircraft design, and an answer to some of those questions, offering two to five times as much rigidity than aluminum or steel, weight for weight, while still being cost effective.

I first began studying carbon fiber composites in the 1990s, as part of a Masters in Aeronautical Engineering, then as a visiting scholar at UCLA. My focus back then was on space launcher booster nozzles, where we would combine carbon fibers with a ceramic matrix to achieve high-temperature resistance, but more recently, as Head of Composite Materials Procurement at Airbus, I had the chance to support the selection of composite materials for the Airbus A350XWB program.

 

Yves Yemsi, Chief Program Officer, Lilium

 

Both Airbus and Boeing have a long history of using carbon composite materials within aircraft, which began with the introduction of Carbon Fiber-Reinforced Plastics (CFRP) into secondary structures such as flaps and ailerons in the 1980s, before progressing to bigger moveables such as horizontal and vertical tailplanes (HTP and VTP).

By the time we were looking at materials for the A350, we were able to produce not just the fuselage and wings of the aircraft using CFRP, but even the center wing box, which is one of the most heavily loaded structures on an aircraft. The Boeing 787 Dreamliner of course makes great use of composite materials too, with nearly half of the airframe composed of CFRP or other composites.

 

The first Boeing 787 Dreamliner. Its airframe is more than 50% carbon composite. ("All Nippon Airways launched the 787 program with an order for 50 aircraft in 2004" by Masahiro TAKAGI is licensed under CC BY 2.0)

Carbon Composites and the Lilium Jet

Fast forward to 2015 and I was based in Toulouse with a new role, as VP for Program Quality on the A350. Around the same time as the first commercial flight of the A350 took place, the four Lilium co-founders were coming together to design their own aircraft. One which would rely heavily on advances made in carbon composite materials.


The Lilium Jet is designed to deliver regional air mobility. That means reducing energy consumption to achieve longer range, while still meeting the stringent safety requirements set out by EASA and the FAA. Carbon fiber is one of the keys to unlocking this sort of performance, offering stiffness and strength at low density, helping to reduce the weight of the aircraft.

The fibers form a thread which is very thin yet incredibly strong, and this thread can be woven and complemented with other materials, such as plastics, dependent on the specific end-use. The versatility of the material is one of its most important features, with both the type of fiber (high strength, high modulus) and the type of resin (epoxy, thermoplastic) able to be specified.

 

A roll of Toray carbon fiber material

While carbon fiber naturally offers high strength, resistance to fatigue and fabrication flexibility, an epoxy resin offers extraordinary adhesion capabilities, high durability, strength and resistance to corrosives as well as low shrinkage - an ideal combination ideal for the Lilium Jet.

Working with a global leader

The Lilium Jet will make the most of the latest advances in carbon composites, with the aircraft's primary structures, such as the fuselage, wings and flaps all being made from carbon fiber, and the overall number of parts required in the aircraft being reduced thanks to the fabrication flexibility of the material.

 

The Lilium Jet's primary structures, such as the fuselage, wings and flaps will all be made from carbon fiber.

 

We've led an in-depth selection process to find the right supplier for us and are privileged to be working with Toray Industries who are a global leader in this field, as evidenced in their role as the sole supplier of carbon fiber materials to the Boeing 787 Dreamliner and Boeing 777X program.

In the first instance they will provide material directly to us so that it can be used in the production of further technology demonstrator aircraft, but in the longer term they will be delivering directly to suppliers who will manufacture parts not just for our prototypes but also for the serial aircraft itself.

Photograph by Gregg Segal
Caption: A Toray Industries technician working with carbon fiber strands destined for the Boeing 787 Dreamliner

Looking ahead

As Chief Program Officer at Lilium I have a responsibility for ensuring we have the procedures and policies in place to deliver an aircraft that meets the requirements for certification, and qualifying all our materials, including carbon composites, is a key part of that process and a focus for us at the moment, as is building our wider supply chain.

But we aren't satisfied with the status quo. We'll be looking ahead to how future materials might enable high manufacturing rates, or enable better recycling. Or how short-cycle, high-volume manufacturing can be achieved. Or how thermoplastic materials might be incorporated into future designs. Just like those who pioneered the use of carbon composites in the first place.