While many eVTOL developers say they will be able to offer a practical aircraft using battery power only, some manufacturers are looking to hybrid solutions to extend their range and endurance. Turbotech, a French company based at Toussus-Le-Noble, not far from Paris, is offering range extenders based on a regenerative turbine solution. The company has two product lines: the TP-R90 turboprop and TG-R55/TG-R90 turbogenerator.
I spoke to Turbotech founder and CEO Damien Fauvet and sales director Marie-Claire Camin about their engines and how they see turbogenerators shaping the eVTOL space in the coming years.
Alex Scerri: Damien, can you give me a brief background on yourself and the founding of Turbotech?
Damien Fauvet: Aviation was always my passion as it is for the other co-founders of the company. I worked at Safran group for 20 years, mainly on large commercial aircraft turbine engines such as the CFM56 in the after-sales department. I started by building and flying RC aircraft and I also assembled small, homebuilt turbine engines.
When I had my first aerobatic aircraft, I wanted to retrofit it with a turbine engine, but the problem was fuel consumption because when you reduce the size from a 1,000-horsepower to a 100-hp turbine engine, scaling leads to very poor fuel efficiency. Several attempts to install surplus military APUs [auxiliary power units] on light aircraft have been made. These units are relatively light, reliable and have good performance, but the fuel consumption was about 60 liters per hour.
I knew that a more efficient engine could be commercialized as there is a lot of interest and demand, so I started working in my free time, mainly on weekends and evenings, to develop a regenerative turbine. The principle is the use of a heat exchanger to recover heat that would otherwise be lost in the exhaust. This greatly improves fuel consumption, so I decided to invest more time and personal funds into a prototype.
While discussing the project around the coffee machine at work, we formed a team of four guys with the same passion and vision. We continued working together on weekends to improve the engine and when we thought we had something that was working well, we decided to go for external funding. Three years ago, we came to speak to our employer, Safran, and some equity fund companies. Safran provided part of the funds and that is how Turbotech started.
Alex Scerri: How does your engine differentiate itself from other similar regenerative turbine concepts?
Damien Fauvet: We did not invent the regenerative turbine as it has been around for about 50 years. They are already quite common in industry such as in power generation plants. Even the M1 Abrams battle tank uses a regenerative turbine engine, as well as some large ships.
However, although there were many attempts to make regenerative turbines for aircraft and helicopters, they were never deployed mainly because the heat exchanger, made up of thousands of microtubes, was technically very challenging and costly to produce. At Turbotech we managed to develop a patented heat exchanger which can be manufactured at a reasonable cost and [has] a practical service life. We are not aware that there are any other comparably advanced projects.
Alex Scerri: Under which certification specification will your product be certified?
Damien Fauvet: It’s a good question. We analyzed the market, and since we come from the world of large turbines, we are very familiar with certification process and the associated costs of engine development. Our market analysis suggested that we should start with a non-certified engine because there is an order of magnitude greater need for engines for experimental and ultralight aircraft in the 100- to 200-hp range, than for certified engines.
In the process, we will be developing and maturing the technology in the next three years, but in parallel we are preparing the certification under CS-E. We are doing preparatory work with EASA [European Union Aviation Safety Agency] so that in the future we can produce a 260- to 300-hp fully certified engine. Our strategy is to prepare our organization with all the necessary certification steps such as the Part 21 Design Organization Approval etc., which takes time, and that will be partly funded by the revenue from the non-certified engines. We also believe that in the initial stages most eVTOLs will be operated under a Permit to Fly because the certification process is still evolving.
Marie-Claire Camin: In fact, we are developing two different types of engines, the turboprop and the turbogenerator. As Damien said, we have the option of certifying the first under CS-E while the turbogenerator has other possible pathways. This could be certified under CS-APU or even through the recently released EASA SC E-19 that could allow us to install our non-certified engines to be incorporated with the global aircraft certification. This would be an ideal solution for us as we can gather more in-service data to help the transition from non-certified to certified engines, even more so because both our turboprops and turbogenerators share the same core components.
Alex Scerri: Being in Europe your main certification authority is EASA. What is your action plan with the FAA [U.S. Federal Aviation Administration]?
Damien Fauvet: In general, the FAA recognizes EASA certification and vice versa. Of course, we will work with the U.S. aviation authorities wherever necessary because for sure Turbotech is a worldwide project.
Alex Scerri: One of the advantages that you state is that your engines have is multi-fuel capability including Jet-A1, diesel, avgas, and biofuel. Will hydrogen be also one of the possible sources of energy?
Damien Fauvet: We want to use cryogenic hydrogen, rather than high-pressure hydrogen. You can store four to five times more energy with cryogenic hydrogen when you factor in the weight of the storage system. Our two engine types share a common yet quite innovative spray fuel injection system so practically any liquid fuel can be used without power loss. Naturally, there will be a change in the rate of fuel consumption linked to the calorific value of the specific source. Our main work in this subject is to fine-tune the fuel injection system to make it compatible with hydrogen and [we] are targeting to have a working demonstrator by the end of next year.
Alex Scerri: A hybrid system could give you the flexibility of using a less energy dense [battery] cell, but which then could have other advantages such as a safer chemistry, better cycle life, and a longer service history that would be helpful in certification. What are your thoughts?
Damien Fauvet: In aviation we always need to balance several factors. If you only focus on a single parameter, you will not be successful. For aviation-standard batteries we have several parameters. The main one is of course safety. Secondly it must be cost-effective. It is easy to spend a million euros in a prototype battery pack, but you will not be able to commercialize this.
Besides energy density you must also look at power density as even a four- to five-seater eVTOL aircraft will need 400 to 500 kW during take-off and landing where the battery will be at the end of its discharge cycle. In a pure electric VTOL, you will be demanding a lot of power from a large, heavy battery and possibly having deep discharge cycles which are detrimental to battery life. A hybrid system would mitigate this and allow a much longer life. There are some overly optimistic energy density predictions that are being published, but we know that market leaders forecast that we are decades away from a significant technology step at the battery level.
Alex Scerri: What are the latest fuel consumption figures you are seeing in your development for Jet A1?
Damien Fauvet: Efficiency is slightly better than expected. For the TG-R55 model, at cruise power we expect a 15- to 22-liter per hour consumption of Jet A1, comparable to a mainstream reciprocating engine. For the TG-R90, also at cruise power, i.e. between 60 and 70 kW produced, we will have 20- to 22-l/hr consumption of Jet A1 for one-third of the weight of a reciprocating engine.
Alex Scerri: What is the estimated cost of the TG-R55 and the TG-R90 range extenders?
Marie-Claire Camin: The price for a TG-R55 will be will be around 80,000 euros and the TG-R90 will be in the region of 140,000 euros. This includes the engine, the generator, the generator controller, and the FADEC [full authority digital engine control].
For the first integration there is a one-time 60,000-euro fee. Each hybrid architecture is unique, and we need to provide an extensive integration support study. Some of the new eVTOL manufacturers are new to the aviation business and we will be there to provide the necessary support for all the ancillary systems related to the hybrid unit.Our total cost will be comparable to reciprocating engines powering a generator.
Damien Fauvet: While the cost will be about the same, our solution is more than 60% lighter and does not need the complex cooling system of the reciprocating engine, so that is where we see our advantage.
Alex Scerri: Do you have orders or letters of intent already?
Damien Fauvet: We are working with about 10 companies at different stages of collaboration. With some we are still in the study phase, while with others we have firm delivery dates with the aircraft flying next year.
Marie-Claire Camin: What we are seeing today is that many eVTOL companies are starting out as pure electric projects. However, when they start flying the prototype or a demonstrator, they start to see the range and endurance that they achieve in a real-world application. This is the point where some of them are coming to us as a second step in their development process as they need to show a commercially feasible range.
Damien Fauvet: The full electric architecture is simpler, and many components are found as COTS [commercial-off-the-shelf]. But we believe that few are achieving usable flight times with a practical payload with this approach. Another big factor on cost will be the battery life as operators cannot afford to change battery packs every few hundred flights. Although many eVTOL projects are still publicly stating that they are committed to a full electric solution, we are sure that there is a lot of work in the background where we will see a wider user of hybrid solutions.
Alex Scerri: If I order an engine now, when can I have it delivered?
Marie-Claire Camin: At the moment we can provide you a pre-series engine within about eight months, which you can use for prototype testing and system integration. Of course, when the first series engines come off the line, we would exchange your pre-series engine with one of these. Once we are in full production, we expect a lead time of around six months from order to delivery and this should be starting sometime next year.
Alex Scerri: How do you see the market for turbogenerators developing in relation to eVTOLs?
Damien Fauvet: I think initially we will have a stronger demand for the turboprop version for conventional aircraft. As for the turbogenerators destined for the eVTOL market, the next two to three years will see a gradual ramp up as airframers continue to develop their aircraft. However, we anticipate an uptick in demand in about five years for this type of hybrid solution. At that point we can see a requirement of about 200 to 300 turbogenerators per year which would be in line with our expectations. As the market develops further towards the end of the decade, we can potentially see a surge in the required number of units.
Marie-Claire Camin: As of today our facility has the potential to produce 500 engines per year so we will be well positioned to meet this demand. If we need to do more than that we have the flexibility to increase this and we also have a lot of support from Region Île-de-France.
Alex Scerri: Hybrid applications will need batteries and other ancillary components. Will you supply this integrated solution by yourself or a preferred partner?
Damien Fauvet: We are speaking with several partners. Turbotech will not be a battery manufacturer and our core business will remain as engine manufacturers. However, we will still be able to offer a complete hybrid solution to eVTOL airframers with partners that are specialized in this field.
Alex Scerri: You describe the engine as ultraquiet. Can you give us some quantitative description of this?
Damien Fauvet: Our engine is quieter than most piston engines. We measured a noise level of 68 dB at 10 meters, from a bare engine without any cover/nacelle etc., so the engine will be very low-noise.
Alex Scerri: Marie-Claire and Damien, thank you for your time and we will be looking forward to following the progress of these engines in the eVTOL space.
This interview has been edited and condensed.