VOLUME , NUMBER ei
PHYSI CAL REVIEW LETTERS
14 NOVEMBER 2003
Space propulsion: a look at ion drives and nuclear engines
Sean Kelly
Department of Physics, University of Ottawa, Ottawa, Canada
(Received 10 July 2003, Published 14 November 2003)
Interplanetary space travel has long been part of our imaginations. However, current chemical
rocketry provides only unsustained propulsion, relying on momentum for the majority of the voyage.
As a result, trips to even the nearest planets require several, if not tens of years. Presently, propulsion
technologies which could provide continued thrust throughout a journey are being actively investigated.
Two such technologies which are especially promising are ion drives and nuclear engines. A spacecraft
utilising an ion drive as primary propulsion, NASA’s “Deep Space 1”, has recently completed an
extremely successful extended mission, proving the worth of this type of engine. Nuclear fuels in space
can be categorized into either the already implemented non-propulsive type and the as-yet unflown
experimental propulsion systems. Though these technologies are still in their infancies, they are
promising, in that they will render solar system exploration more efficient.
DOI: 10.1103/PhysRevLett.42.256901
I.
CURRENT STANDARD: CHEMICAL
PROPULSION
At the present time, the overwhelming majority of
aerospace propulsion is provided by means of chemical
propulsion. There are a few exceptions to this rule once
in orbit, but lift-off is invariably carried out by chemical
rocket engines. Currently, these engines are the only ones
which provide the requisite amount of power and thrust
(force exerted by the engine) to propel a spacecraft
beyond the Earth’s gravity. However, there are limits to
the load they can lift. We are unable to carry sufficient
fuel to provide continuous thrust once in space. Thus, we
rely on a “initial push – momentum coast” technique of
travelling. Spacecraft do possess small “thruster” engines
to effect rotations and adjustments, but these are used
sparingly. Consequently, the velocity of travel is mostly
determined at the end of the thrust phase. Swing-by
manoeuvres, where the gravity of a planet is used to
further accelerate the vehicle, are common.
If there are humans on board, a large quantity of fuel
must be stored: for the return voyage and/or movement,
course changes, landings and (an important fuel
consumer) lift-offs.
Chemical propulsion at present cannot provide enough
energy per mass for such voyages over a much greater
distance that that between the Earth and the Moon. At the
limit, we might be able to attain Mars. In other words, we
can not transport such vast amounts of chemical fuel.
II. ION DRIVES
Ion drives, also known as Solar Electric drives, capture
solar energy to impart impulsion to ions. The ions are
created at one end of a cylinder. An electric field is
applied tin order to accelerate the ions towards the other
end of the cylinder, where they are ejected. Establishing a
flow of such ions would create a force on the source side
0031-9007/02/ (ei)/256901(4)$20.00
PACS number: 07.87.+v
of the cylinder.
For each singly charged ion of mass m, the kinetic
energy imparted by an assumed constant electric field of
potential difference V is:
  mv 2 2  eV
where v is the exit speed of the particle and e is the
elementary charge. Therefore
v  2eV m
(1)
Let μ be the mass being ejected per unit time. Then
from Newton’s 2nd law, we have F  v where F is the
force. With eq.(1) substituted for v, this becomes
F   2eV m .
(2)
To express this as a function of the measurable electrical
properties of the engine, we can use the fact that
ch arg e ch arg e particle mass .
Current I 



time
particle
mass
time
I  e m
Substituting this into eq.(2), we get
F  I 2m eV .
(3)
This equation (3) shows that for constant current and
voltage, the force is greater for a higher mass to charge
(m/e) ratio. This would suggest the usage of heavy ions.
There is however something else to consider. As the
concentration of the positive ions increases, the electric
field that they induce will begin to cancel the accelerating
electric field. This implies a maximum current density,
which is directly related to the ion density.
Using Poisson’s equation in the one dimension we are
exploring and substituting in the current density j by the
relationship j=ρu where ρ is the charge density and u is
the velocity of the ions, we have
d 2V
(4)
 j
dx 2
u 0
© 2003 The Americun Physical Seaniety
VOLUME , NUMBER ei
PHYSI CAL REVIEW LETTERS
where εo is the permittivity of the vacuum (remembering
that we are working in space). We also know that the
velocity of the ions u is given by the energy relationship
mu2/2 = e(Vo-V), where Vo is the source side potential
and V is the potential at a distance x from the source side.
The differential equation (4) becomes

d 2V
j 
m


2
 o  2e(Vo  V ) 
dx
1
2
(4’)
with the boundary conditions that dV/dx = 0 at Vo=V.
The final result after integration is that the maximum
current density is
3
j max
4 2
e V 2

o
9
m s2
(5)
where s is the distance between the electrodes.
What then is the maximum force density fmax we can get
within an ion thruster?
We can develop the force density as a function of know
quantities by
f = Force/Area = (mass*velocity)/(Area*time)
= (mass density)*(velocity)2
= (charge density)*(mass/charge)*(velocity)2
= (current density)*(mass/charge)*(velocity)
= j(m/e)v .
(6)
Carrying out two final substitutions of eqns. (1) and (5)
into (6), we obtain a simple expression:
8 V2 8
f max   o 2   o Eo2 .
9 s
9
(7)
The maximum thrust of an ion engine is therefore only a
function of the strength of the accelerating electric field.
High voltages and small electrode separation are therefore
advantageous. 1
To get an idea of the order of magnitude of the
thrust produced by an ion engine, let us take V o=10 kV,
s=0.1m, εo≈10-12C2N-1m-2 and an engine area of 1m2.
Therefore
Ftot  f max  1m 2
 10 12 C 2 N 1 m  2  10 8 V  10  2 m 2  1m 2
 10 N
-2
A higher voltage and surface area might then yield 1N
of total thrust. This is far from being capable of effecting
a lift-off. Again, we are led back to a dependence on
chemical rockets to put an ion engine into orbit. Once
there though, the ion engine could provide continuous
acceleration for a long time (several years).
In fact, a spacecraft using an ion drive as primary means
of propulsion has already flown in space. The National
1
This development has followed that of
Shepherd.
0031-9007/02/ (ei)/257901(4)$20.00
14 NOVEMBER 2003
Aeronautics and Space Administration (NASA) launched
“Deep Space 1” on October 24th, 1998. Its ion engine
worked on 2100W of electricity from the vehicle’s solar
panels, providing about 0.1N of thrust during some 677
days, consuming only 73.5 kg of Xenon propellant. It
was considered a very successful mission.
III. NUCLEAR POWERED SPACECRAFT
A. Non-propulsive Nuclear Energy Sources
For thirty years now, space vehicles have encorporated
radioisotope thermoelectric generators (RTGs). These
components transform the energy of radioactive decay
into electric power by solid-state converters, necessitating
no moving parts. The most commonly used radioisotope
is plutonium 238 (Pu-238). This isotope releases a
significant amount of heat during its disintegration,
making it unsuitable for use in weapons but ideal for the
thermoelectric generators. This nuclear fuel is stored as a
ceramic of plutonium dioxide. In this form, it is
insoluble, non-reactive and breaks into large pieces
instead of powdering in case of an impact. Furthermore,
the PuO2 is separated into compartments by iridium,
which has a melting point of 2410oC, providing additional
containment.
With this design, the RTGs are compact, safe and
reliable energy sources, capable of providing continuous
energy for many years. This allows long-distance space
probes to operate during their lengthy missions.
Presently one such spacecraft, the Cassini-Huygens
probe, is en-route to Saturn. Cassini uses the on-board
RTGs to power its instruments, as well as its antenna to
send back images and data which it collects.
Cassini has however relied on the initial thrust and
several swing-by manoeuvres to propel it towards its final
destination, Saturn. The total travel time will be about 6
years.
B. Nuclear Propulsion Systems
The main advantage of nuclear propulsion systems over
chemical ones is the vastly greater energy-to-mass ratio in
nuclear fuels. As well, nuclear engines could provide
more thrust. Therefore, the possibility of carrying more
fuel into space becomes more feasible.
There is a fundamental difference between the
mechanisms of the chemical and nuclear systems. In
nuclear engines, it is advantageous to have a gas
propellant in addition to the nuclear combustible to fully
utilise the thermal energy released by the nuclear
reactions. In chemical engines, generally the combustible
and propellant are one and the same, and hence
inseparable.
© 2003 The Americun Physical Seaniety
VOLUME , NUMBER ei
PHYSI CAL REVIEW LETTERS
One system which is being explored experimentally by
the Innovative Nuclear Space Power and Propulsion
Institute of the University of Florida is the nuclear
thermal rocket. In this engine, heat from the isolated
nuclear reactor is transferred to the propellant. This
causes the gas to expand and be ejected, much as it would
in a traditional chemical rocket. However, the specific
impulse (thrust per unit of weight of fuel) of nuclear fuels
is nearly twice that of the finest chemical fuels.
Another kind of nuclear propulsion system integrates
the nuclear combustible and gas propellant in the same
chamber. In their paper, Yigal Ronen, Menashe Aboudy
and Dror Reger (2000), show calculations which indicate
that thin films of 242m-Americium could sustain nuclear
fission reactions. This could be used in reactors into
which the propellant could be pumped directly, bypassing
the need for a heat exchange device. As well, this design
could take advantage of the products of nuclear
disintegration by releasing these high impulse particles as
propellant, instead of capturing them as do traditional
reactors. With solid films, the problem of not releasing
the nuclear fuel before it is entirely spent, while still
allowing the heated propellant to escape from the
integrated engine, might also be solved.
IV. CONCLUSIONS
Ion drives are already in use but are very slow to
accelerate and must be put into orbit by chemical
rocketry.
Nuclear thermal rockets could be in use in the next
thirty years but for the moment nuclear energy in space
remains only as an electrical power source in the form of
radioisotope thermoelectric generators.
0031-9007/02/ (ei)/257901(4)$20.00
14 NOVEMBER 2003
――—─────===─────―――
V. REFERENCES
DiChristina, Mariette, “Many From One”, Scientific
American Magazine (Scientific American,
Inc. ) Dec 31, 2001 [www.sciam.org].
Leutwyler, Kristin, “Mars in Two Weeks”, Scientific
American Magazine (Scientific American,
Inc.) Jan 3, 2001 [www.sciam.org].
Ronen, Y., M. Aboudy and D. Regev, Nuclear
Technology 129, 407 (2000).
Innovative Nuclear Space Power and Propulsion
Insitute, Why Nuclear Thermal
Propulsion?
[www.inspi.ufl.edu/research/ntp].
Spacecraft Power for Cassini, Office of Nuclear
Energy, Science and Technology (U. S.
Department of Energy, Feb 1996)
[www.ne.doe.gov/space/spacepwr.html].
Nonproliferation and Arms Control Assessment of
Weapons-Usable Fissile Material Storage
and Excess Plutonium Disposition
Alternatives, (U.S. Department of Energy,
Jan 1997) pp. 37-39.
“Iridium”, CRC Handbook of Chemistry and
Physics, 48th ed., (Chemical Rubber Co.,
Cleveland, 1967) p. B-116.
“Deep Space 1: Quick Facts”, Jet Propulsion
Laboratory website
[www.jpl.nasa.gov/ds1/quick_facts.html]
Shepherd, Dennis G., Aerospace Propulsion
(American Elsevier Publishing Company,
Inc., New York, 1972) Chap. 9.
Goodger, E. M., Principles of Spaceflight
Propulsion (Pergamon Press, Oxford,
1970) Chap. 4 and 5.
© 2003 The Americun Physical Seaniety