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§ 3.3.3 Electric Propulsion for Inter-Orbital
Vehicles
Contents
Overview
There are various ways of using electricity to thrust
propellants, rather than using chemical explosion as in launch
rockets.
Electric thrusting of propellants is useful only for interorbital
transportation, not launch from the Earth's or Moon's surface.
There are two electric propulsion techniques used in space today:
- Some American interorbital satellites today use electric
"ion drive" for stationkeeping. Ion drive is a simple and
fairly mature technology. Ion drive can just as easily be used as
the primary means of interorbital propulsion for delivering
cargoes, and has been projected in future scenarios for space
industrialization as a competitive kind of reusable interorbital
vehicle. The Deep Space 1 probe, launched on October 24, 1998, was
the first vehicle to depend upon electric ion drive for all of its
propulsion needs, to perform a close flyby of an asteroid.
- The Russians have for at least 10 years been extensively using
an electric propulsion technique called a "plasma thruster"
which they have begun to market overseas. This thruster has been
used on close to a hundred Russian military satellites, but is
relatively unknown in the West.
The main advantages of electric propulsion are:
- It uses much less propellant than chemical rocketry (or, from
another perspective, it gets much more mileage out of a given
quantity of propellant)
- It could use 100% lunar or asteroid derived propellant
- It may promise better reliability and simplicity than chemical
rocketry
Its main disadvantages are:
- It requires an electric power plant
- It offers only low thrust propulsion, which means a longer
time to deliver the cargo
For an analogy, chemical rockets are like express delivery via
powerful and fast airplanes, whereas ion drive vehicles in
interorbital space are like the big tankers on the oceans which
deliver their cargo slowly but cheaply and safely via surface
transport and more mundane technology.
For missions to asteroids, it is actually advantageous to use a
continuous, low thrust vehicle as this greatly expands the "launch
window" period in which it is economically feasible to go to these
objects, as compared to chemical rockets which impart short blasts
of acceleration and deceleration. (This is also called a "thrust
profile".)
Whereas chemical rocketry uses a chemical reaction and controlled
explosion for thrust, electric propulsion uses electricity to
accelerate the propellant out of the thrust chamber. Unlike chemical
rocketry, there is no chance for an explosion with ion drive. Ion
drive is much safer and a simpler technology, and has no relation to
chemical rocketry at all.
Since electric propulsion vehicles use electricity, the vehicle
must produce that electricity, typically by solar cell panels. In
other words, ion drive vehicles are solar powered vehicles. The DS1
ion drive thruster is a 2.5 kilowatt device powered by a solar cell
array.
Many studies into future large scale space industrialization base
all interorbital propulsion on reusable electric propulsion
vehicles, with chemical rockets being used only to launch material
from Earth's surface to low Earth orbit, and from the lunar surface
to lunar orbit, where the cargo is transferred to an electric
propulsion vehicle for interorbital transport. However, some very
conservative "immediate term" studies assume all interorbital
propulsion is based on today's chemical rockets, e.g., with oxygen
and hydrogen extracted from asteroids or the Moon being used to
refill the fuel tanks of the chemical rockets, which means no use of
electric propulsion. That's because lunar and asteroidal materials
can be used to refuel today's hydrogen-oxygen chemical rockets with
no design modifications at all, whereas large scale electric
propulsion vehicles would need development of vehicle designs.
For a comparison between electric propulsion and chemical
rocketry in the mainstream studies: Chemical rockets consume 8 TIMES
as much fuel propellant than electric propulsion for the same
service. The cargo-to-propellant ratio from low Earth orbit to high
Earth orbit is around 4:1, round trip, with ion drive vehicles
refueling in high orbit where lunar or asteroid derived propellants
are tanked. In comparison, for today's chemical rocketry used for
transporting satellites from low to high orbit, it's a 1:2 cargo to
payload radio (not 2:1, but 1:2), and the vehicle goes on a one-way
trip, being discarded afterward. Ion drive ejects its propellant at
a speed about 15 times that of chemical rockets, hence imparting 15
times more momentum per unit mass of propellant. However, the ion
drive vehicle is heavier than a chemical rocket vehicle due to the
electric power plant so that the performance comes down to about
800% better than chemical rocketry. Yes, 800%.
(Notably, in a few studies, the propellant for ion drive vehicles
is stated to be pure oxygen from the Moon or asteroidal material.
This is a highly questionable assumption. Oxygen presents several
problems for ion drive, as discussed later. However, other lunar and
asteroid derived propellants should work fine. Also, oxygen should
work in the Russian plasma thruster.)
One reason why only chemical rockets have been used in space for
propulsion to date (except by the Russian military, addressed later)
instead of ion drive is the continued lack of basic infrastructure
in Earth orbit -- there is no reusable interorbital vehicle service
in space at all. To date, the interorbital vehicle has always been
launched with the payload and thrown away after it delivers its
payload. To date, ion drive has been used only for stationkeeping
propulsion once the satellite is delivered and its solar panels
deployed. Hughes Space and Communications Company is starting to
market an ion drive system as an interorbital upper stage, as
covered at the end of this article. Nobody is yet marketing a
reusable interorbital vehicle, whereby we just launch up fuel tanks
and dock for fuel and payload transfer.
It is hoped that the DS1 mission will stimulate more interest in
ion drive and electric propulsion in general, perhaps leading to a
commercial venture to offer interorbital services via a reusable
interorbital vehicle. These services could include hauling
satellites from low orbit to geosynchronous orbit, moving old
satellites to new orbits, and refueling and maintenance of
satellites in orbit. Future satellites could be designed for
interaction with such interorbital infrastructure, though this is a
chicken-and-egg situation that must be overcome by broad industry
recognition.
The Russian plasma thruster is used primarily on military
satellites (e.g., space based radars), and their commercial space
program has struggled to get onto its feet after the massive Cold
War subsidation of their military program waned. The Russian plasma
thruster is a well developed and very efficient engine, and is seen
as one of the potentially most valuable exports in their space
program. The Russian plasma thruster is seen by many in the Western
space program as an unusual technology, but its efficiency and
reliability in space for more than 10 years on nearly 100 military
satellites makes it a very well proven piece of flight hardware.
DS1 is a step in the right direction to gaining acceptance of
electric propulsion in the west, by using ion drive to get from
Earth to an asteroid. A big part of the DS1 mission is simply to
demonstrate and analyze the ion drive propulsion system in space.
DS1 will carry a set of sensitive instruments to analyze the effects
of the ion drive's propellant on the spacecraft and its local
environment, and how well the drive is working as an engine. It's
expected that when this mission is over, its "NSTAR" ion drive
engine will be a proven and well understood piece of hardware which
can be used by anyone for a primary means of interorbital
propulsion.
Lunar and asteroidal
propellants in electric propulsion
The Russian plasma thruster can use most any propellant in the
thrust chamber, including oxygen.
However, some studies propose using oxygen as a source of
propellant for ion drive as well. This is questionable. The main
issue is the effects of ionized oxygen with the thruster materials
(cathode, neutralizer, filament grids) of the ion drive engine. The
thruster would need to either be made of different materials than
today's ion drive engines and/or have easily replacable parts,
neither problem being easily solved. A thruster design such as the
German RIT10, which uses RF energy instead of a cathode to ionize
the gas, may be the best present day design to start to consider for
oxygen use. A second issue is that oxygen is not easily ionized,
which means lower electrical efficiency. Thirdly, ion drive works
best with heavier elements. Overall, oxygen does not look attractive
for ion drive interorbital propulsion. (For satellite
stationkeeping, the effects of ionized oxygen on the satellite would
preclude its use.)
Xenon is the most popular propellant for ion drive today since it
is a heavy gas (high atomic mass) that is easily ionized. Argon and
mercury have also been used. Xenon and argon are inert gases which
are not expected to be recoverable in useful quantities from the
Moon and asteroids. The best candidate fuel for ion drive using
lunar or asteroidal materials may be sodium, which is fairly
abundant in some nonterrestrial materials, extractable without very
much effort, easy to store and handle, and would work well with ion
drive -- easily ionized, not damaging to the engine materials, and a
relatively heavy element.
The Russian plasma thruster is different, and probably could use
oxygen. It uses a separate, external cathode discharge and has no
grid, so you can run just about anything you want through the thrust
chamber, and use about 5% of, say, sodium, xenon, or argon in the
cathode discharge chamber. JPL has done some preliminary work in the
area and hasn't found any show-stoppers.
The next two sections are technical.
How ion drive works
Ion drive thrusters use an electric field to accelerate charged
atoms or molecules (e.g., oxygen) to a high velocity as they are
expelled out the thrust chamber, thus accelerating the spacecraft.
Ion thrusters generally use a cathode (a negatively charged grid
similar to that found in a tv set) to generate a stream of
electrons, which form an electric circuit with a positively charged
ring - the anode. This stream of electrons is used to ionise the
propellant. A small magnetic field is used to aid this process
(electrons spiral around the magnetic field lines, increasing the
chance of electron-atom collisions). The magnetic field may derive
from either a permanent magnet or an electromagnet.
The ionised gas drifts towards an extraction grid system (two or
three plates with many small holes in them, held at high voltage)
where they are accelerated out of the thruster, so producing thrust.
A neutraliser similar to the cathode is used to generate free
electrons and balance the overall space charge in the outgoing beam
so that the spacecraft doesn't charge itself up.
The electric power comes from a solar cell array. Of course, in
orbital space, there is no air drag or weather forces, so the solar
cell array doesn't need to be aerodynamic at all. Since the ion
drive vehicle is relatively low thrust, the structural strength and
mass can be low as well. For example, in a General Dynamics report:
"The solar array performance was conservatively assumed for sizing
purposes to be 150 watts/kilogram" based on a very conservative
assumption of solar cells having only 7% efficiency. The assumed
efficiency of ion drive at converting electrical energy into beam
kinetic energy was 63%, though some ion thrusters today produce
efficiencies between 70% and 90%.
Ion thrusters are modular. If you have more cargo or want to
speed up your mission or slow it down to conserve fuel, then you can
add or subtract thrusters and solar cell array units.
Ion drive engines have long lives, being subject to a much less
stressful environment than chemical rocketry. Ion drive engines are
also easier to work on, consisting of simple electrical components,
in contrast to the high performance mechanical pumps bolted into
chemical rocketry.
Ion drive was developed in laboratories in the 1960s, and there
were the SERT1 and SERT2 experiments in space which proved that the
drive would work in space for long periods of time and deliver
significant propulsion to a spacecraft. When the space program
shrunk due to poor political leadership after the Kennedy-Johnson
era, ion drive was one area that saw research and development wane.
However, some private communications satellites in geosynchronous
orbit incorporated ion drive into their stationkeeping system once
the satellite was delivered there by a chemical rocket and its solar
cell array deployed to power the ion drive engine.
The NASA Lewis Research Center is developing a lower power
version (about 700 Watts) of the DS1 NSTAR ion drive engine system.
However, large scale space industrialization will use larger ion
drive engines, or else many low power units together in a modular
craft, the latter offering spacecraft security in case an engine or
two fail.
In any case, ion drive is looking as if it will become a routine
means of interorbital propulsion within the next 5 years for low
cost scientific missions as well as some kinds of industrial
missions.
Hall-Effect Stationary Plasma
thrusters (Russian-French)
In the 1990s, an electrically powered propulsion technology used
by the Russians was marketed outside of Russia, particularly in
France. It was kept fairly confidential by those interests, but was
referred to by various names, e.g., the Russian Stationary Plasma
thrusters, the Hall-Effect thrusters, and the Russian Anode Layer
thrusters.
Then came the announcement of the European SMART-1 probe to
launch in October, 2002, to demonstrate this technology in a lunar
orbiter probe. The home page for SMART-1 is sci.esa.int/smart-1/ but skip over to
the subpage sci.esa.int/content/doc/10/2320_.htm for
a good description.
The following explanation is a 1990s quote thanks to John
Schilling (schillin@spock.usc.edu) of the University of Southern
California's Aerospace Engineering:
"The thruster consists of a cylindrical chamber ~10cm in
diameter with a central spike. Both spike and cylinder are made of
ferrous material, and incorporate magnetic windings so as to
create a transverse (radial) magnetic field across the exit. The
walls are insulated, the base serves as an anode.
"External to the thruster is a separate, hollow, cathode. About
95% of the propellant (currently Xenon or Krypton) is metered into
the thruster, and 5% to the cathode. In operation, a discharge is
struck between cathode and anode. As electrons are much lighter
than ions, one would expect most of the current to be carried by
the electrons, but that same lightness makes it hard for them to
cross the transverse field across the thruster exit. Some manage
to trickle through anyway and keep the discharge running, but
there is always a surplus of electrons hanging around outside the
thrust chamber.
"This creates a strong electric field (~100 V/cm) which sucks
out ions from the anode discharge region. Being fairly heavy, they
don't so much notice the magnetic field, and with the field
strength involved they end up moving at 15-25 kilometers per
second. They pick up electrons from the accumulated surplus as
they pass, not slowing for that any more than they did the
magnetic field, and the resulting bulk-neutral plasma proceeds
ballistically towards infinity.
"Oxygen would be a less effective propellant than Xenon, but it
would probably work. You'd still need 5% of something
non-oxidizing to run the cathode discharge on, but using 95%
oxygen is not bad.
"The latest edition of Sutton's Rocket Propulsion
Elements has a section on the Stationary Plasma thrusters,
which is reasonably good."
MPD and other electric
thrusters
Magnetoplasmadynamic (MPD) thrusters (aka magnetohydrodynamic
(MHD) thrusters without the hydrogen association) and other kinds of
electric propulsion techniques have been developed but not
incorporated on spacecraft yet.
The page on the Southhampton University MPD research and
development project describes the basics of the MPD thruster
concept, gives a history of the Southhampton Univ. Dept. of
Aeronautics and Astronautics MPD project, references, and lots of
pictures of their MPD thruster research and development, thanks to
kind web work by Alexander Fitzhugh.
Mass driver not considered
seriously
PERMANENT gets a lot of e-mail suggesting we cover using a mass
driver for interorbital transport. Basically, the mass driver has
merit for lunar launch but not interorbital propulsion, compared to
alternatives, in a near-term scenario. The moving parts entail
considerable risk. Overall, it's too complicated. For more
information on a mass driver for lunar launch, however, see the
section on long-term propulsion where we have a section on the mass driver.
Passing through medium Earth
orbit radiation belts
A significant problem with interorbital transfer between low
Earth orbit and high Earth orbit is the Van Allen Belt of trapped
high-energy particles (i.e., radiation) due to the Earth's magnetic
field. The Van Allen Belt exists in a middle-level Earth orbit
located below geosynchronous Earth orbit and above low Earth orbit.
This radiation degrades the solar cells a little bit each time they
pass through. This requires repair of the solar cells by an
annealing process (heating and recrystallization) after a number of
trips. Alternatively, interorbital vehicles could receive beamed
power from a satellite stationed in high orbit, as discussed in the
section on "Solar Power Satellites". A third option is to use
nuclear electric vehicles.
The Van Allen Belt radiation affects only electric vehicles with
solar cell arrays which are cycling between low Earth orbit and high
Earth orbit. It will not affect cargos being hauled between two
different high Earth orbits, or between the asteroids or lunar orbit
and high Earth orbit.
Final words on electric
propulsion
To date, the main drawback to electric vehicles has been the need
to have an electric power plant. It is most attractive for
fuel-efficient stationkeeping of satellites with sizeable electric
power capacity, for deep space missions on a low budget which must
get the most mileage out of their fuel, for satellites which already
have sizeable power plant needs for other purposes (e.g., Russian
military space based radar), and in the future as a reusable
interorbital vehicle. Notably, DS1 is demonstrating low mass/high
efficiency solar panels that will make a 3 kW power plant within
reach of most spacecraft. DS1 is not a big spacecraft, and the whole
scientific mission which includes studying an asteroid is being
accomplished on a nearly record low budget. However, it is debatable
if electric vehicles are the most economical means of propulsion for
purely scientific missions which don't otherwise need large
quantities of electric power. Nonetheless, electric vehicles are
attractive for interorbital haulers in a large scale space
industrialization scenario.
Links to other websites on
electric propulsion
Hughes Aerospace Company produces the Xenon Ion Propulsion System (XIPS)
which was first used on their previous generation 601 satellites
with great success, and is heavily marketed in their current
generation 702 satellites. You can also see a
diagram of the XIPS. Hughes claims that their latest model
of the XIPS is 13 times more efficient than conventional fuel
propellant. The following is paraphrased from their web
advertisement: XIPS needs only 5 kg of fuel per year for
stationkeeping, a fraction of what bipropellant or arcjet systems
consume. As a customer option, using XIPS as an upper stage "to help
raise the spacecraft into final orbit" can save even more launch
mass. Customers can apply the savings in launch mass to launch
additional propellant to prolong satellite service life, or to
increase the satellite mass to enhance its revenue-generating
potential. Or, the savings in mass can be used to shift to a less
expensive launch vehicle. They also suggest that using XIPS could
allow some customers to add another payload or two to a given launch
vehicle.
It's worth mentioning that Hughes' leadership in space
communications launched an entire industry communications
satellites. Hughes launched the Early Bird satellite more than 30
years ago, the world's first commercial communications satellite.
Since 1965, HSC has launched more than 100 satellites, more than 40%
of the commercial communications satellites currently in orbit --
and achieved a 99% on-orbit reliability record. Hughes has boldly
led the way without sacrificing reliability, and this is the case
with ion drive as well.
The Italian CENTROSPAZIO is conducting research in
a variety of electric propulsion systems, including
magnetoplasmadynamic (MPD) propulsion, arcjet propulsion, field
emission electric propulsion (FEEP), and free electron drift
propulsion (Hall thrusters). (Their vacuum facilities are also
utilized for other research.)
If you know of any other sites on electric propulsion which are
not listed here, please send a message to
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