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November 06, 2009

Electric Solar Wind Sail Could Have Five Times Higher Thrust


An electric solar wind sail is a recently introduced propellantless space propulsion method whose technical development has also started. The electric sail consists of a set of long, thin, centrifugally stretched and conducting tethers which are charged positively and kept in a high positive potential of order 20 kV by an onboard electron gun. The positively charged tethers deflect solar wind protons, thus tapping momentum from the solar wind stream and producing thrust. The amount of obtained propulsive thrust depends on how many electrons are trapped by the potential structures of the tethers, because the trapped electrons tend to shield the charged tether and reduce its effect on the solar wind.

A new research paper shows that if trapped electrons can be removed that thrust can increase five times from 500 nN/m [at 1AU for average solar wind conditions and
for reasonable values of the driving voltage] to 2500 nN/meter, which means 1 Newton of thrust for 2000 kilometers of total tethers. From the picture above you could have 50 tethers (wires) that were each 40 kilometers long.

There is a project to launch a prototype solar electric sail in 2012 with an Estonian satellite

The ESTCube-1 is a 1 kg nanosatellite and Estonia's first satellite, with planned launch in 2012. It will open a 10 meter tether made of very thin metal wire and charge it to 200 V with a miniature onboard electron gun. As the satellite flies in its orbital path through the ionospheric plasma, the speed difference between the satellite and the plasma induces a small force on the tether which can be measured. The measurement is used to validate and calibrate existing plasma physical theory of the electric sail effect.

Later, production-scale electric sails will use much longer tethers and will fly in the solar wind, utilising the much larger speed difference between the satellite and the fast-moving solar wind. According to estimates, electric sails can be orders of magnitude more efficient than existing methods (chemical rockets and ion engines) for many transport tasks in the solar system. Scientifically, they could revolutionize solar system science by enabling fast missions out of the heliosphere and affordable sample return missions from planetary, moon and asteroid targets. Commercially, electric sail could enable the economic utilization of asteroid resources for e.g. orbital rocket propellant production or orbital manufacturing of structural parts.



Picture caption: A more advanced siamese twin deployment of two sets of slar electric sail tethers

Increased electric sail thrust through removal of trapped shielding electrons by orbit chaotisation due to spacecraft body

Here we present physical arguments and test particle calculations indicating that in a realistic three-dimensional electric sail spacecraft there exist a natural mechanism which tends to remove the trapped electrons by chaotising their orbits and causing them to eventually collide with the conducting tethers. We present calculations which indicate that if these mechanisms were able to remove trapped electrons nearly completely, the electric sail performance could be about five times higher than previously estimated, about 500 nN/m, corresponding to 1N thrust for a baseline construction with 2000 km total tether length.




2000 km total length of tether (for example, 50 tethers 40 km long each) could weigh 50–100 kg (frame, solar panels, high-voltage power source, electron gun, motorised tether reels, various sensors and control processor), of which the tether mass is 10 kg. According to the new results, such a device could produce 1N thrust and produce a specific acceleration of 10–20 mm/s2. If used to move a 500 kg payload, for example, the device would produce a 30 km/s velocity change over six months.

The theoretical results presented here call for experimental verification. The verification could come from a measurement of electrosphere size, thrust force or both in a space or laboratory experiment. Two-dimensional particlein-cell or Vlasov plasma simulations might give a better estimate of the thrust force than the rough analytical calculations presented in this paper. The 2-D simulations would need to be equipped with some kind of trapped electron removal scheme. Because the electron temperature 12 eV is several thousand times smaller than the depth of the potential well, extra care should be taken into the simulations to avoid spurious trapping by numerical errors.

Although the electric sail plasma physical problem is simple in the sense that only electrostatic forces are involved, the problem spans a wide range in parameter space. The range in energy goes from 12 eV electron temperature to 20 kV tether potential. The spatial scale is from 10μm radius wires to 100 m wide potential structure and to 20–100 km long tethers, which gives 7 to 10 orders of magnitude in space. Finally, the timescales start from 0.1 ps needed for an electron to move across a 10μm wire width to several minutes needed to remove the trapped electrons (15–16 orders of magnitude). It is evident from this range of scales that a brute-force simulation approach is not fruitful. Thus, while theory is essential and simulations helpful, experimental studies are crucial in designing the electric sail.

Finally, it is worth remarking that if the electric sail thrust is indeed as large as the estimates presented in this paper indicate, the potential of the electric sail for space transportation in the solar system is enormous. Exploring the potential scientific and commerical applications and implications is, however, outside the scope of this theoretical study.


FURTHER READING
Electric sail site

A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions "see" the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or "flying" the resulting spacecraft electrically.

The main limitation of the electric sail is that since it uses the solar wind, it cannot produce much thrust inside a magnetosphere where there is no solar wind. Although the direction of the thrust is basically away from the Sun, the direction can be varied within some limits by inclining the sail. Tacking towards the Sun is therefore also possible.


Negativly charged solar electric sail (9 page pdf)
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