Ion propulsion, the genesis of emerging research

Can scientific curiosity, human encounters and chance be decisive factors in exploring a line of research up to then discarded by the scientific community?

The work currently carried out by Nicolas Binder in the aerodynamics and propulsion department (DAEP) would seem to validate this hypothesis.

A matter of encounters

As a specialist in fluid dynamics and aeronautical propulsion, Nicolas Binder’s scientific curiosity initially drew him to take a closer look at the research work carried out by student engineer, Nicolas Monrolin (E2015), at IMFT.

“We were not working at the classic engine incremental evolution, we were beyond that and I was curious to see how far we could go” Nicolas said. Then he kept in touch, because they got on well together, and because this research work focused on ground-breaking propulsion systems.

Then, in 2018, the researcher attended the brilliant thesis defense of his former student at IMFT. One week after this, Franck Plouraboué, the thesis supervisor, called ISAE-SUPAERO in search of a high-speed wind tunnel. After coming up against some dead-ends, by chance he met Nicolas Binder, the person in the lab interested in this cutting edge work.

“This thesis is the starting point for the creation of a genuine electro-hydro-dynamic multi-physics unit in the DAEP department. For me it was, at first, a real subject of scientific curiosity. Then we started to work together with Franck, then with the P’ Institute in Poitiers. Eventually, we joined the European IPROP consortium on ionic propulsion with the École Polytechnique, the Institut van Karman in Belgium, the University of Milan, which was leading the project, and other Italian universities, to develop a comprehensive line of research, and that’s when things really started to take off,” Nicolas Binder explained. An unprecedented scientific adventure was on the move.

The Institute as a reference in conventional propulsion

Mastery of conventional propulsion is one of the strong points of the Turbomachine and Propulsion research group at DAEP. This expertise provides a benchmark against which the performance of alternative propulsion is assessed.

The principle of ion propulsion is to create a plasma, around two electrodes of different sizes, between which a high voltage is applied. Ions escape from this plasma, and are accelerated by the intense electric field towards the second electrode. These ions undergo a large number of collisions with neutral air molecules, which slow them down, inducing an ion cloud between the two electrodes. This electrically charged area is the origin of the thrust applied to the electrodes. The purpose of this research is to understand all the parameters that make this process ineffective for the time being. At present the system generates a drag greater than the thrust it induces, for modest flight speeds. There is room to work on this subject to increase the thrust density at low speed, and especially to see if it can be made to operate favorably at high speeds, as the equations would seem to indicate.

“There is a chance that the thrust process is activated by the presence of the external flow at high speed, but we still need time to develop the measuring instrument, because this is really exploratory research. Trying to measure this thrust at high speed has never been done before, so this setup at ISAE-SUPAERO is quite unique,” confirmed the scientist.

IPROP, a European project to go even further

The IPROP project is part of the Horizon Europe Pathfinder program and aims to raise the level of maturity of atmospheric ion thrusters for aeronautics. Ion thrusters, with no moving parts, guarantee robust and silent propulsion. This is an emerging field of investigation in the context of aircraft electrification and low-carbon air transport.

Within the European consortium, ISAE-SUPAERO is responsible for the high-speed experimental part and is working closely with IMFT and Franck Plouraboué, a specialist in digital simulation. The Ecole Polytechnique is also involved and is working on a new electrode device capable of sending more ions into the flow to try to increase thrust density. These electrodes will be tested on the ISAE-SUPAERO wind tunnel to carry out ambitious electric field measurements and laser flow measurements.

The French team is in charge of the external velocity coupling part of the flow and the development of new electrodes. Each party is working in its own specific area then grouping their results in the ISAE-SUPAERO lab. Indications are beginning to emerge, trends are encouraging, but they are not yet precise enough to be published.

The other universities are working on the design of a small-scale demonstrator incorporating these optimized propulsion systems. The ultimate aim of this project is to design a full-scale, highly autonomous stratospheric airship using solar-powered ion thrusters. Airships can perform many satellite functions, such as telecommunications or remote sensing, at a lower cost and with the added advantages of being recoverable and having a low environmental impact.

“If the first results are encouraging, it will then be possible to launch research on co-propulsion. The ion system could be coupled to conventional propulsion, switched off on take-off and activated at altitude in cruising conditions when thrust requirements are less important. It could cut back on recourse to internal combustion engines that have significant room for improvement in terms of efficiencies. But there are certainly many applications that have not yet been considered and that could be considered at high speed and at high altitude, Nicolas Binder concluded.

The study of this technology opens up a wide scientific field, covering fundamental research into ion production, how it is coupled with airflow, electrode optimization, aircraft integration and even more if chance comes into play!