The major technical challenges for In-Space Propulsion Systems Technology Area (ISPSTA) were identified and prioritized through team consensus based on perceived mission need or potential impact on future in-space transportation systems. These challenges were then categorized into
near- (present to 2016),
mid- (2017–2022), and
far-term (2023–2028) time frames,
representing the point at which TRL 6 is expected to be achieved. It is likely that support of these technologies would need to begin well before the listed time horizon.
Over 240 xenon Hall thrusters have flown in space since 1971 with a 100% success rate. Commercially developed flight Hall thrusters operate between 0.2 and 4.5 kW with 50% efficiency, thrust densities of 1 mN/cm2, and Isp of 1200–2000 secs.
Hall thrusters have been demonstrated from 0.1 to 100 kW with efficiencies of 50-70%. Recent research has demonstrated operation with alternative propellants and Isp increases to 3000–8000 secs. A major challenge is to capitalize on recent
breakthroughs on reducing wall erosion rates to realize very long life and throughput (over 1000 kg) and increase Isp. Life validation of high-power, long-life thrusters requires development of physics-based models of the plasma & erosion processes.
Magnetoplasmadynamic (MPD) thrusters
Magnetoplasmadynamic (MPD) thrusters employ the interaction of high currents with either applied magnetic fields or the self-induced magnetic field to accelerate ionized propellant. MPD thrusters offer high efficiency and very high power processing capability in a small volume, and have been demonstrated at steady state power levels up to 1 MWe. There are three variants on the MPD thruster: steady state self-field engines, steady state applied-field engines, and quasi-steady thrusters. MPD thrusters show that they can achieve efficiencies over 50% at Isp’s greater than 10,000 secs and thruster power levels of multi-MWe. State-of-theart laboratory model (TRL 3-4) lithium applied field thrusters have demonstrated efficiencies greater than 50% at 4,000 secs in a 200 kWe device and modeling indicates they can achieve over 60% efficiency at power levels of 250 kWe and above. Challenges are component lifetime, thermal management, and performance limitations due to the “onset” phenomenon.
Solar Sail Propulsion
Solar sails are large, lightweight reflective structures that produce thrust by reflecting solar photons and thus transferring much of their momentum to the sail. The state-of-the-art solar sails were produced for the NASA In Space Propulsion 20 meter Ground System Demonstrations (GSD) in 2005. The JAXA funded Interplanetary Kite-craft Accelerated by Radiation Of the Sun (IKAROS), launched in May, 2010, deployed its sail in June and has since demonstrated both photon acceleration and attitude control. IKAROS has a square sail that is approximately 14 meters long on each side, 7.5 micrometers thick and used a spin deployment method to deploy its sail.
System level integration and test of the component technologies for over 1,000-m2 sail using existing materials and technologies are needed.
Solar Thermal Propulsion (STP) heats the propellant with concentrated sunlight inside an absorber cavity and provides a very high specific impulse (~500–1200 seconds). The L’Garde flight experiment in 1996 demonstrated the deployment of a large inflatable concentrator (TRL6). The 30-day LH2 storage with controlled boil-off was demonstrated in 1998-1999 (TRL5).Various engine concepts have been made and fabricated from Rhenium, Tungsten/Rhenium, and Rhenium coated graphite (TRL4). A new Ebeam manufacturing process has been demonstrated to fabricate complex STP engine designs. In addition, a new ultra-high temperature material (tri-carbide) has the potential to allow greater Isp than 1000 seconds.
Electric sail propulsion was in the advanced propulsion section of the NASA report.
The electric sail (ESAIL), invented by Dr. Pekka Janhunen at the Finnish Kumpula Space Centre in 2006, produces propulsion power for a spacecraft by utilizing the solar wind. The sail features electrically charged long and thin metal tethers that interact with the solar wind. Using ultrasonic welding, the Electronics Research Laboratory at the University of Helsinki successfully produced a 1 km long ESAIL tether. Four years ago, global experts in ultrasonic welding considered it impossible to weld together such thin wires. The produced tether proves that manufacturing full size ESAIL tethers is possible. The theoretically predicted electric sail force will be measured in space during 2013.
An electric solar wind sail, a.k.a electric sail, consists of long, thin (25 to 50 micron) electrically conductive tethers manufactured from aluminium wires. A full-scale sail can include up to 100 tethers, each 20 kilometres long.
The E-Sail mass is expected to only weigh in the range of hundreds of kilograms, hence the E-Sail is 100 – 1000 times more efficient than traditional techniques. To produce the same total impulse one would need 100 tons of chemical fuel (specific impulse 300 s) or 10 tons of ion engine propellant (specific impulse 3000 s). Instead of a 13 ton launch of one solar electric propulsion system, one could launch fifty or one hundred of the E-Sails which could combine towing to provide 50 Newtons of towing capacity. The E-Sails would be able to capture 20 to 40 times the mass of asteroids for equivalent launches. Also, the E-Sails can be used repeatedly if there is a long term power source for the electron gun, they would not have other consumables and could keep capturing the solar wind.
The ESAIL EU FP7 project (2011-2013) develops laboratory prototypes (TRL 4-5) of the key components of the E-sail. The project involves five countries, nine institutes and has a budget of about 1.7 million Euros.
* Deploy and confirm the deployment of a 10 m conductive Hoytether from a 1U CubeSat
* Test of a 100 m tether deployment on Aalto-1 3U CubeSat (2014)
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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