Air-breathing propulsion to explore the yet unexplored, to exploit a yet unexploited Earth orbit
The AETHER project aim was the early design of a ram-EP thruster, from the intake to the ionisation stage, to the acceleration stage which would counteract atmospheric drag and keep the spacecraft in very Low Earth Orbit
The concept was supposed to be demonstrated with a ram-EP prototype operated in a relevant environment, with a particle flow generator able to re-produce the residual atmosphere at about 200 km, to test the system capability in producing a net positive thrust against drag.
The ultimate goal of AETHER was to prepare the ram-EP technology for a future In orbit Demonstration mission, and to advance Europe at the forefront of the still unexplored air-breathing electric propulsion scenario. Both goals have been achieved.
Summary of AETHER results
Nowadays, Low Earth Orbits (LEO) are the most crowded orbits in space due to easy accessibility, very short lag in communication and frequent revisit times. Very Low Earth Orbits (VLEO) offer the opportunity to open new mission scenarios for space applications. Located between 160 and 250 km, VLEO could further strengthen the benefits of LEO thanks to its closer proximity to Earth’s surface.
One of the most demanding constraints of VLEO is the need to continuously counteract the drag generated by the interaction of the spacecraft with the surrounding atmosphere. The mission lifetime will then be determined by the amount of propellant stored on-board to produce thrust.
The RAM-EP concept combines the advantages of electric propulsion with the possibility to harvest the particles available in the upper atmosphere and use them as propellant. Once demonstrated that the thrust produced can counteract the atmospheric drag that typically causes rapid orbital decay, the use of VLEO and a significant extension of the mission lifetime would be enabled.
In recent years, the full RAM-EP concept has become more realistic, as several theoretical and experimental investigations are being undertaken in Europe, Japan, US and Russia to develop a feasible prototype. The most advanced result in air-breathing electric propulsion was achieved in 2017, when SITAEL obtained the first operation of a full air-breathing system in representative conditions. The ram-EP thruster was successfully ignited with a generated flow simulating the atmospheric propellant: even though successful, that test campaign highlighted the need for further development to achieve full drag compensation.
Starting from the heritage of SITAEL and the test campaign performed in 2017, the AETHER project (Air-breathing Electric THrustER), has aimed at developing the first propulsion engine able to maintain a spacecraft at very-low altitudes for an extended period. The main objective of the project is to demonstrate, in a representative on-ground environment, the critical functions of an air-breathing electric propulsion system, and its effectiveness in compensating atmospheric drag.
The technical work carried out during the project has reached the following results:
- Characterization of specific application cases for the RAM-EP technology and definition of system requirements. Based on the preliminary assessment of relevant market opportunities and on potential Earth orbit payloads, the project team has identified and characterized the potential mission scenarios of interest for S/C operating with an air-breathing electric propulsion system (EPS). The characteristics of the solar system atmospheres, i.e. of inner and of outer planets and natural satellites atmospheres, have been investigated and taken into account.A set of specific tools have been developed for the evaluation of key design drivers and EPS requirements derived in turn. Parallel activities have been carried out in order to derive the requirements at different levels (mission, platform, and subsystems).
- On-ground experimental simulation of the environmental conditions relevant to the identified mission cases. A particle flow generator (PFG) has been designed to reproduce on-ground representative conditions of the target application environment. Extensive analyses have been performed to analyze the expected properties of the plume flow and its evolution and to identify the necessary modifications of an existing Hall thruster to allow generating a representative flow. Finally, the prototype of the PFG has been manufactured and successfully tested in representative vacuum conditions.
- Development of the critical technologies for the collection, ionization and acceleration of rarefied atmospheric flows. The durability of materials that must operate in a harsh oxidative and corrosive environment is the most relevant life limiting factor for the air-breathing technology. For this reason, significant effort has been put on the development of material science knowledge beyond the current state-of-the-art. Specific modelling and experimental investigations have been performed to define suitable materials for extended contact with ionized corrosive atmospheric gases. A thruster performance model has been set up to provide the sizing of the thruster and the main operational parameters (thrust, power, etc.) and allow EPS design. The specific subsystems have been designed, based on the specific requirements derived from the EPS. Most subsystems have been manufactured and successfully tested in standalone configuration, showing a behavior in line with expectations.
- Assessment the performance of the RAM-EP system. In order to characterize the engine operations in terms of integral parameters (thrust, drag, power) as well as local properties of the accelerated particle beam (composition, electron temperature, velocity), invasive and non-invasive diagnostic systems have been designed and assembled, on purpose for the specific test needs. Intensive co-engineering activities have been performed to define the optimal end-to-end test sequence and test set-up for the RAM-EP system verification, as well as to timely highlight and prevent potential criticalities of such a complex test campaign. Dedicated tests have been performed at sub-system level with atmospheric propellant, i.e. PFG; acceleration stage; RF neutraliser; intake (reduced scale); hollow cathode. Such tests have prepared the ground for a future end-to-end test campaign, that was not possible to perform within the Project perimeter for programmatic reasons.
Even if the Project has not succeeded in completing the end-to-end test campaign and demonstrating the target TRL 5, significant advancement has been achieved by the Consoirtum in the development of the air-breathing technology: this overall achievement can effectively pave the way for further development to in-orbit demonstration of the technology.
When compared to traditional electric propulsion systems, the availability of an air-breathing engine would allow for enabling a whole spectrum of missions never thought possible before. The attractiveness of in-situ resource utilization for the propellant is associated with the removal of the main mission duration limiting factors, that is propellant. This will, in-turn, change completely the paradigm of in-space propulsion for LEO, VLEO and low orbit interplanetary missions.
The AETHER project builds on the technological advantage of Europe in air-breathing propulsion, moving to a more advanced development stage compared to the international competition. This has the potential to secure the market leadership on air-breathing, xenon-independent electric propulsion. Air-breathing electric propulsion can allow Europe to achieve a prime role in the exploitation of VLEO. The AETHER project has settled and tested a fully European supply chain, including large companies, SMEs, excellent research centers and academia, and suppliers, pushing the limit of what is currently achievable in the space industry.
If you wish to effectively go in very Low Earth Orbit
you must get rid of onboard propellant
and exploit what you have around you: atmosphere!
Very Low Earth Orbits (VLEO, from 160 to 250 km from Earth) are extremely interesting for Earth Observation and null-lag telecommunication missions, but impose severe limitations to spacecrafts since at these heights atmospheric drag is so high it has to be continuously compensated with positive thrust.
This means the spacecraft has to always turn on its propulsion system, which in turn means a lot of propellant onboard.
You finish the propellant, you deorbit: it’s a matter of days.
AETHER wants to exploit the atmosphere by collecting the molecules and accelerating them to counteract drag, so to allow a VLEO mission to last months if not years. Without any tank onboard!