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

Abstract

Abstract. 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. 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 1 N thrust for a baseline construction with 2000 km total tether length.

Electric solar wind sail mass budget model

“…The first one is that the thrust is proportional to the total tether length times the tether's electron sheath width. For a fixed voltage, the latter is proportional to the solar wind plasma Debye length, which, in turn, is proportional to 1/ √ n. As a result (Janhunen, 2009), the thrust is approximately linearly proportional to the tether voltage V , but it has only a square root dependence on the solar wind dynamic pressure P dyn = m p nv 2 , where v is the solar wind speed and m p is the proton mass. The second effect is that, because the tether current is proportional to n √ V , V must be varied as n −2/3 in order to maintain P eg constant.…”

“…If the main tethers are sufficiently long such that the electric potential structure overlapping between them is negligible, the propulsive thrust varies as 1/r, where r is the Sun-spacecraft distance (Janhunen, 2009). Note, for comparison, that in the classical photonic solar sail (Wright, 1992;McInnes, 1999) the propulsive thrust decreases more rapidly (that is, as 1/r 2 ) with the solar distance.…”

“…At 1 AU the potential structure radius is ∼ 100 m under average solar wind conditions (Janhunen, 2009). In all E-sail models considered in this paper the distance between the tether tips is 2π × 20 km/100 = 1257 m. Thus, under usual conditions, overlapping affects about 200 m/1257 m = 16 % of the tether length.…”

“…The electric solar wind sail (E-sail) is an innovative deep space propulsion concept that uses the solar wind dynamic pressure for generating thrust without the need of reaction mass (Janhunen, 2006(Janhunen, , 2009Janhunen et al, 2010). The Esail spacecraft is spun around its symmetry axis and uses the centrifugal force to deploy and stretch out a number of thin, long and conducting tethers, which are kept in a high positive potential by an onboard electron gun pumping out the negative charge from the system .…”

Boltzmann electron PIC simulation of the E-sail effect

“…In order to engineer E-sail devices in detail, one should predict the magnitude of the Coulomb drag that the flowing solar wind exerts on the charged tether. Thus far this problem has been studied using particle-in-cell (PIC) simulations (Janhunen and Sandroos, 2007;Janhunen, 2009aJanhunen, , 2012, Vlasov simulations (Sánchez-Arriaga and Pastor-Moreno, 2014) and other methods (Sanmartín et al, 2008;Sanchez-Torres, 2014). However, a fully satisfactory way of estimating E-sail thrust has not yet emerged, except for negative polarity tethers (Janhunen, 2014a), which are however more suitable to use in low Earth orbit (LEO) as a deorbiting plasma brake device (Janhunen, 2010) than in the solar wind (Janhunen, 2009b).…”