Periodic Reporting for period 3 - DISCOVERER (DISCOVERER – DISruptive teChnOlogies for VERy low Earth oRbit platforms)
Reporting period: 2019-10-01 to 2021-03-31
However, at reduced orbital altitude the residual atmosphere produces drag which decreases the orbital lifetime, and aerodynamic perturbations challenge the ability of a satellite to remain stable affecting image quality. DISCOVERER envisions a radical redesign of Earth observation platforms for commercially viable, sustained operation at significantly lower altitudes than the current state-of-the-art and overcomes these challenges.
The project addresses the following research questions:
1. Are there materials which reduce drag on satellites?
2. Are there propulsion methods for drag compensation which use the residual atmospheric gas as a propellant?
3. Can we use orbital aerodynamics to control a satellite’s pointing and orbit?
DISCOVERER is also developing commercial and economic models of Earth observation systems which include these newly identified technologies, allowing the optimum satellite designs to be identified.
1. Prospective aerodynamic material coatings, those which are expected to scatter away the flow specularly, have been developed and are being tested in a number of ways:
a. DISCOVERER’s Rarefied Orbital Aerodynamics Research (ROAR) facility is currently in the final stages of commissioning. The ground-based facility will reproduce the atmospheric flow around satellites VLEO, at the correct density, speed and predominant composition, and measure the flow as it scatters off material samples. This will be used to characterise the aerodynamic properties of the new materials.
b. An additional test to determine the survivability of the materials in the VLEO environment has been completed. Materials samples were launched to the International Space Station (ISS) on 2 November 2019, and were exposed on the outside the ISS for nearly 12 months before being returned to Earth for analysis.
c. Our Satellite for Orbital Aerodynamics Research (SOAR) has now been developed, tested and prepared for flight. SOAR will launch to the ISS in June 2021, and will be released from the ISS later that month. SOAR is a small test satellite to validate the aerodynamic performance of the materials in the space environment, and demonstrate aerodynamic control of a satellite.
2. Aerodynamic control methods have been developed to help control satellites in VLEO despite the variations in atmospheric density and high wind speeds they will encounter. Some of these control methods have been implemented on SOAR which will demonstrate their performance.
3. An advanced prototype Atmosphere-Breathing Electric Propulsion system (ABEP) is being developed. The system collects the residual atmosphere in VLEO by means of an atmospheric intake and uses it as propellant in an electric plasma thruster to compensate atmospheric drag. The design approach enables maximum flexibility for the propellant in terms of density and composition, the thruster can use an inherent acceleration mode based on polarization, additionally to the magnetic nozzle effect, the plasma jet is quasi-neutral such that no neutralizer is needed.
a. Different atmospheric intake designs have been optimised to achieve high collection efficiency and, currently the three designs based on DSMC simulations are available providing up to 95% collection efficiency.
b. The RF Helicon-based Plasma Thruster has been designed, simulated, built, and tested. It is based on radio frequency and utilizes an innovative birdcage antenna, that maximizes electrical efficiency. Its successful ignition has been achieved in March 2020, its electrical properties verified, and its operation on atmospheric propellant such as O2 and N2 has been proven requiring P ~60 W for operation.
In addition, studies of business models and an analysis of stakeholders in remote sensing from VLEO have been carried out. Combined with studies of spacecraft system models, designs of future VLEO satellites, using the technologies that have been developed, can now be produced.
The launch of SOAR in June 2021 will be a major milestone for the project. It will allow the aerodynamic performance of materials to be assessed directly in VLEO, demonstrate novel aerodynamic satellite attitude control strategies, directly measure the composition, density and velocity of the atmospheric flow, and help determine the speed of the atmospheric winds in the thermosphere.
In addition, the completion of the commissioning of ROAR will allow DISCOVERER to assess the scattering of atomic oxygen at orbital speeds from materials samples, characterising their aerodynamic properties.
Samples of the developed materials have been exposed to the space environment on the exterior of the International Space Station to test their long-term survivability. Data about the effect of the space environment on the materials, alongside data from SOAR and ROAR, will be used to develop a complete picture of the performance of the materials for use on VLEO satellites to reduce atmospheric drag in improve aerodynamic control performance.
Based on the designs of the atmospheric intakes for the ABEP system, sub-scaled intakes have been designed specifically for testing in ROAR. The tests will provide a verification of the DSMC code and feedback into atmospheric intake design.
Helicon waves are key for high efficiency of plasma production. A B-dot magnetic inductive probe has been designed and commissioned as a plasma diagnostic tool to verify the presence of helicon waves within the plasma plume of the thruster, and will be used to characterise the thruster’s operational envelope.
A baffle plate is in the early design phase and will be used to measure thrust, and derive specific impulse, and thruster efficiency. These are needed to support system and mission analyses, and to optimise the design of a vacuum version of the thruster, reducing required power, mass, and volume.