Electric Sail Frequency Asked Questions
Created: April 25, 2008
- Why is the electron gun shooting away from the Sun in the pictures? Wouldn't it contribute to the thrust if it would shoot sunward?
Since electrons are 1800 times lighter than protons, the extra thrust provided by the electron beam is negligible. Hence the electrons can be shot in any direction in principle and its placement can be selected by technical considerations.
- How can one "tack" towards the Sun?
By inclining the sail one can produce a thrust component which either brakes or accelerates the spacecraft in its orbit around the Sun, depending on which way one inclines it. If braking, one spirals inwards, if accelerating, one spirals outwards. This works as long as the sail's thrust is not so high that the radial component overcomes Sun's gravity. In practice, in the inner solar system one can then travel both inward and outward with the electric sail with traveltimes that range from months to some years, i.e. not faster than with traditional methods, but using no fuel.
- Why does inclining the sail alter the thrust direction? Aren't the tethers always pushed along the solar wind?
Consider a tether spinplane inclined for spiralling in or out. For
each tether, only the component of the solar wind flow which is
perpendicular to the tether produces thrust (and the thrust is
perpendicular to the tether). When a tether is "vertical", it is
perpendicular to the solar wind and is pushed radially outward. When
a tether is "horizontal" (i.e. in the ecliptic plane), however, it
is pushed partly sideways perpendicular to itself. Averaged over all
tethers, the thrust vector then has an angle which is approximately
halfway between purely radial (solar wind) direction and the normal
direction of the spin plane. Hence, if one inclines the sail e.g. 60
degrees, the thrust vector gets turned about 30 degrees off radial.
- What about interstellar flight?
When accelerating, our theoretical limit is the solar wind speed
400-800 km/s which is still only about 0.1% of speed of light; not
enough for going to another solar system. But it might be possible
to use an electric sail as a brake for an interstellar probe which
has been accelerated to high speed by some other method such as the
laser sail. This hasn't been analysed in detail yet by anyone, though.
- I saw a mention about solar power satellites in a newspaper story. How come that may relate to the electric sail?
The electric sail can be used to eject a small probe at high speed out
of the Solar System, but the same device can also be used to transport
larger payloads e.g. from asteroids to Earth orbit. The transport is
economical because the payload fraction is high: the electric sail
weighs less than 100 kg but the payload can be several tons if several
years transfer time is allowed. One could use this capability e.g. for
mining water ice from asteroids, bringing it to Earth orbit and using
it as rocket fuel there for reusable orbital transfer vehicles. Such a
system would make launching e.g. geostationary satellites less
expensive because the required orbital transfer fuel would be obtained
from an extraterrestrial source (asteroids) so that one does not have
to lift it from Earth. Taking large solar power satellites to orbit
would be one major application for this kind of space transportation
infrastructure. Ultimately, predicting the economicality of this
approach boils down to estimating how much it costs to produce
electric sailers and what is their operational lifetime, versus how
much it costs to lift a payload from Earth. If icy asteroids exist
nearby Earth, there are also other economical means to transport their
materials to Earth orbit, such as electrolysis rocket or steam rocket.
- What will be ultimately the production cost of an electric sailer?
Naturally, it is too early to give any firm numbers, just some general
remarks. The electric sailer is a relatively simple electromechanical
device containing no intrinsically expensive, poisonous, radioactive
or dangerous elements, substances or manufacturing methods. The most
nontrivial components are the long multiline tethers which contain
hundreds of millions of wire-wire bondings in a full-scale
mission. Attention must be paid to making the throughput of the
reel-to-reel automatic bonding as high as possible. The electronics
industry is able to make wire-platform bondings economically in a very
large scale nowadays. Our process is similar, the main difference
being that the bondings are wire-to-wire rather than wire-to-platform.
- I've heard that Sun has become less active and that the trend
may continue. Is this a problem for the electric sail?
It is true that in recent years, the solar wind density has been
smaller than the 40-year average. The electric sail's thrust is,
however, only weakly dependent on the solar wind density, because of
two compensating effects. The first effect is that the effective
electric width of the tether potential structures scales as the
inverse square root of the solar wind density. The second effect is
that for fixed tether potential, the electron current gathered by the
tethers is proportional to the solar wind density. Thus the electron
gun needs to produce less current, which allows one to increase the
tether voltage while keeping the gun's power consumption the same. For
fixed power consumption, the usable voltage is proportional to the
solar wind density to power -2/3, while the thrust is proportional
roughly to the square root of the voltage. All in all, approximately
the thrust is then proportional to the solar wind density raised to
power -1/6 which is a quite weak dependence. There are some additional
details and tradeoffs related to the optimum length and number of
tethers and the operating distance from the Sun, but the answer to the
question is that even if the solar wind's density would become
permanently weaker in the future, electric sailing would not be in
Latest updated: March 20, 2009
For more information, see the technical papers provided near the bottom of the main electric sailing page.