Advanced Propulsion Systems in Space
Exploration
Anand Natarajan
Alexander Dawson School
Undergraduate Space Research Symposium
April 3, 2004
Abstract ............................................................................................................................ 1
1
Introduction............................................................................................................... 2
2
Goals and Objectives ................................................................................................ 3
3
The Need for New Propulsion.................................................................................. 3
4
Related Research Projects ........................................................................................ 4
5
Advanced Concepts .................................................................................................. 5
5.1
Nuclear Fission ........................................................................................... 5
5.2
Nuclear Fusion ............................................................................................ 6
5.3
Antimatter ................................................................................................... 7
5.4
Beamed Energy Propulsion ........................................................................ 8
6
Conclusions............................................................................................................... 9
7
References............................................................................................................... 11
8
Appendix................................................................................................................... 1
1
Abstract
Using current chemical propulsion techniques, it is not feasible or useful to launch manned
interplanetary expeditions farther than Mars, due to strain on human health and on life
support systems. From analyzing data on conceptual designs of chemical propulsion
spacecraft it can be concluded that chemical propulsion is not the way to the future of space
exploration. The travel time is simply too long to facilitate a manned flight. Therefore, I
believe that some exotic new propulsion system is required for an interplanetary manned
spacecraft. A near term possibility for new a propulsion system is nuclear fission
propulsion. This would use a nuclear fission reactor to heat a working fluid and expel it
outwards, producing thrust. While useful in the near future, this method would use unsafe
radioactive fuels and would be very dangerous in the event of a failure. Another option is
nuclear fusion propulsion. This would use energy from a fusion reactor in various ways to
create thrust. This would be much more useful, as it would not be as radioactive and would
use cheap fuels. However, problems with current reactor designs would have to be
eliminated. A possible design would be Inertial-Electrostatic confinement, which uses
electric fields to confine fuel ions. A third example is antimatter propulsion, using the
annihilation of protons and antiprotons to produce energy. This is very far in the future but
is the perfect rocket engine, producing extremely high amounts of energy, with mass being
converted entirely to energy.
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1 Introduction
The current best propulsion systems used for launching space vehicles are chemical [1].
These use the combustion of fuels to provide thrust that propels the vehicle forward. While
these types of engines are useful for Earth orbit or Earth to Moon trips, any distance beyond
Mars is impractical for manned space exploration. Therefore, I believe that a new
propulsion system is necessary for manned exploration of the outer solar system. In this
paper I shall describe various methods that could be used to create an interplanetary
manned spacecraft.
These advanced propulsion systems will allow us not only to go to distant planets in our
solar system, but also make it much faster to travel to the lunar base when we build one.
With these advanced propulsion systems, it will be feasible to make regular human
missions to distant planets making space colony a reality. It is very important that we as a
nation develop a clear long term plan for our space exploration and start working on that
plan. It is very clear that any ambitious space plan would require a much more powerful
propulsion system than chemical systems. We are looking for an order of magnitude
improvement. We need to look at where we want to be in 20 and 30 years and work
towards that.
For the present, nuclear fission propulsion is a high performance available option (NASA Space Science - Project Prometheus). This is a very workable proposal but has problems
due to the radioactivity of the fuels and waste, causing radiation poisoning in the event of a
failure. I will also describe nuclear fusion propulsion, which combines two light nuclei into
2
a heavier one. This is a safe, clean, and efficient reaction that releases much energy but is
very hard to start and control. Many problems plague modern devices for fusion research
and these will have to be worked out before a workable fusion engine can be constructed.
The final possibility is an antimatter engine, which uses the complete annihilation of matter
with antimatter to generate enormous energy and thrust. An antimatter engine would truly
open up the galaxy to the human race.
2 Goals and Objectives
The objectives of this paper are to discuss major new propulsion concepts and evaluate
their merits and uses. I will discuss nuclear fission, fusion, and antimatter propulsion with a
note on beamed propulsion. I shall describe their key characteristics as well and will
describe their workings and when they will be technically feasible. I will also do a
comparison based on specific impulse, a measure of how long a rocket can sustain one
pound of thrust using one pound of fuel, and required technological breakthroughs if any.
Now we shall look at a description at the current state of the art.
3 The Need for New Propulsion
The current best in rocket technology is Liquid Hydrogen/LOX chemical rocket engines.
An example of one is the space shuttle main engine (SSME). It has a specific impulse of
about 450 seconds. This is about the best that a chemical rocket can get safely. Newer ideas
such as tripropellants are all either unstable, explosive, toxic, or a combination of these. At
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the best that we can do with chemical rockets, we will get to Mars in about 2 years, and a
trip to any farther destinations is impossible with human payload due to the simple fact of
the slowness of chemical rockets. New concepts, either with fuels with higher energy
densities, or even spacecraft with no fuel at all are required. These include nuclear fission,
fusion, antimatter, and beamed propulsion. Now we shall examine them in detail as well as
look at some related research projects.
4 Related Research Projects
Several current research projects are under way on nuclear and antimatter propulsion. One
of them is NASA’s Project Prometheus. Project Prometheus is a program to develop fission
reactors for power generation in space to run ion thrusters, as well as new RTG
(Radioisotope Thermoelectric Generators) technology for safer compact power sources for
space probes. Another research project is antimatter research at Penn State. This group has
developed a new trap that can store antiprotons for a longer time. The group has also
produced designs for antimatter spacecraft that are feasible with today’s production rates of
antimatter. These groups have all been receiving only a trickle of funding. I believe that
these nuclear research groups should receive more funding and NASA should go more in
depth with these projects. But now, let us look at the advanced propulsion ideas that these
groups are looking at.
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5 Advanced Concepts
5.1
Nuclear Fission
Nuclear fission has been a viable propulsion concept for decades. Project Orion (19581965) was a design to propel a spaceship by detonating small nuclear explosives behind the
vehicle [2]. The blast would impinge on a heavy metal pusher plate, providing both high
thrust and specific impulse. This would have given us the outer solar system by the 1970’s,
except for the fact that it was cancelled due to the ban of nuclear explosions in space [2].
Also, launching a spaceship using atom bombs made people worried, what if there is a
crash, and all the uranium and plutonium is released into the atmosphere? However, this
concept could be built easily with current technology. This concept is the only currently
existing viable interplanetary concept. Another concept worked on in the 1970’s was
Nuclear Thermal propulsion, basically running hydrogen gas through a reactor to heat it
and then expelling out the back for thrust. This concept is also feasible and can practically
be used. However the specific impulse is only about 1000 sec, twice the best chemical
rockets. The reactor was a standard design with graphite at the moderator running at very
high temperatures. The design, known as a solid core rocket, had the disadvantage that it
was limited by the melting point of the reactor materials. To improve this design, a core of
molten or even plasma uranium or uranium hexafluoride could be used. These designs
culminate in the Gas Core Nuclear Lightbulb [3]. A gas core reactor passes radiation
through a Silica Lightbulb casing into hydrogen propellant, and expels it out the back at
high speeds. The specific impulse would have been around 2000. Also, Nuclear Electric
Propulsion is a concept where a nuclear reactor generates power for an electric propulsion
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unit such as an ion thruster. This has an enormous efficiency but has a thrust equivalent to
one ounce [1]. Therefore it is not practical for human launch. Also, it requires enormous
amounts of electric power. Finally there is Fission Fragment Propulsion [3]. This is using
the fragments from fission reactions in Americium 242m flying off at high velocity to
generate thrust. This would be practical for interstellar precursor missions. However
Americium 242m is an artificial element and can only be made by reprocessing spent
nuclear fuel rods. I believe that efforts should be directed to Orion and nuclear thermal
systems as they are feasible using today’s technology and are of high utility in manned
missions to Mars and the Planets.
5.2
Nuclear Fusion
All of these concepts under nuclear fission share the problem of radioactivity. This is not
much of a problem in space but is within the biosphere or magnetosphere. A way to get
around this is Nuclear Fusion Propulsion. Fusion would basically be plasma in a magnetic
bottle that leaks at one end, which is somewhat easier than a power reactor due to the fact
that some leakage is necessary. The problem is that enormous amounts of energy are
required to get two nuclei to come close enough to fuse. Scientist have been pursuing the
tantalizing goal of controlled magnetic confinement for about 30 years now, and still are
working on it, saying that it will take another 30 years before a controlled reaction is
produced. There is another technique as well called inertial electrostatic confinement that
uses electric potential wells to accelerate ions to the center of a device of spherical
geometry where they have a high probability of colliding and fusing [4]. I believe that this
technique shows promise and should be given government funding and research. However,
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there are problems such as electrode bombardment reducing electrode life and causing
energy loss that needs to be solved. This means that fusion is still about 20 years in the
future. However, a simpler system such as VASIMR could help. VASIMR is a plasma
rocket being developed by NASA that can vary its specific impulse. VASIMR works by
heating hydrogen by microwaves in a magnetic bottle and then expelling it out the back.
This is somewhat like a fusion rocket but it does not quite go that hot. VASIMR based
engines could be used for interplanetary missions but still need some work in order to turn
them into flight hardware. However, VASIMR is low thrust and shares the problems of
NEP. Fusion could also be used in a pulsed manner like Orion, with H-Bombs being
exploded or pellets of deuterium and tritrum being bombarded by lasers to produce thrust
(inertial confinement) [3]. This is pretty much proven technology using hydrogen bomb
technology. This approach would not be very much better because of enormous lasers
needed to compress the pellet for fusion to occur. These fusion propulsion systems would
have much higher specific impulse, but have the technical issues that prevent them from
being currently realized, however they are a system that would find great use in the future.
For interplanetary manned missions these two technologies can do the job.
5.3
Antimatter
But when one starts talking about manned interstellar missions, fission and fusion won’t
work. They still don’t have enough energy to propel a space ship to another star within a
human lifetime. Therefore, something with much higher energy density is needed and one
of the only things that can do this is antimatter. Antimatter is equal to normal matter in
every way, except for the fact that it is opposite in electrical charge and whenever the two
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meet they annihilate. Electrons and positrons annihilate into pure gamma rays, which
cannot be harnessed for propulsion, but protons and antiprotons are messier, producing
pions and other charged particles [6]. These particles could be directed by a magnetic
nozzle to produce thrust. This particular type of engine is called a beam core engine, and it
has a specific impulse on the order of several million seconds [6]. This would be
enormously useful for exploring the galaxy except for one problem, the fuel. Antiprotons
are the equivalent of the universal solvent and containing them is a big problem.
Researchers at Penn State have built traps that can hold antimatter for 18 days [5].
However, this is not practical for a space mission. Also, a new way of producing antimatter
is needed since the current methods are extremely inefficient, with perhaps 1000 protons
for every antiproton made at best [3]. A way around these difficulties has been suggested
by researchers at Penn State. This is called Antiproton Catalyzed Micro Fission/Fusion.
The design uses electron beams and a puff of antiprotons to ignite a miniature H-bomb in a
pellet, powering an Orion type vehicle called ICAN-II. This design can be built using
current amounts of antimatter produced but requires breakthroughs in antimatter
confinement. But if a pure antimatter engine could be built, the stars would be ours to
explore.
5.4
Beamed Energy Propulsion
I would like to also give a brief description of beamed propulsion. Beamed propulsion
(using laser beamed energy or microwave beamed energy) is the concept of leaving the
engine at home and using it to propel the vehicle. Usually an enormous laser or maser is
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constructed that fires light at large sail which gets pushed forward by this light, particle
beams are also used in some cases. These designs are very elegant since the spacecraft itself
does not carry any propellant. However the supporting infrastructure required is
enormously complex and it would probably not be very practical for manned missions. For
example the lasers/masers required are simply too heavy to carry into orbit and the
acceleration is too small for a practical manned mission outside the solar system.
6 Conclusions
From these investigations of multiple propulsion systems it can be concluded that fission,
fusion, and antimatter are the only useful systems for manned exploration of the planets and
the stars. Fission is viable in the near term and should be more actively pursued by NASA.
Already Project Prometheus is investigating these technologies, but if a serious manned
exploration of the stars is to proceed, then more investigation needs to be done such as in
the area of flight testing. Fusion and antimatter are promising concepts but are not quite yet
practical for rockets. Fusion has problems with confining the plasma to heat it to fusion
temperatures. Antimatter propulsion doesn’t even have enough fuel, and the fuel,
antiprotons, is like the universal solvent, exploding with anything it touches. However, the
hybrid of all three, Antiproton Catalyzed Microfission/fusion, or ACMF, can be built as
seen in the ICAN design using today’s amounts of antimatter, making it possible to explore
the solar system. But if the pure antimatter system reached maturity, the stars would be
ours. This is why I believe that we should actively pursue these propulsion systems despite
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possible public disapproval, for the conquest of space. We need to educate the public about
these propulsion systems and the benefits of space exploration. If we seriously pursue these
technologies, I am sure that in our life time we will have manned missions to the far away
planets.
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7 References
1. James R. Wertz and Wiley J. Larson (editor), Space Mission Analysis and Design,
Space Technology Library.
2. George Dyson, Project Orion: The True Story of an Atomic Spaceship, Henry Holt
and Company.
3. http://www.islandone.org/APC
4. http://www.fusor.net
5. http://www.engr.psu.edu/antimatter
6. http://www.thespacesite.com/space/future/antimatter.php
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8 Appendix
Type
System
Advantages
Fission
Disadvantages
High Energy
Radioactivity
Solid Core
Easy to build
Limited in Temp
Gas Core
High Temps
Needs much work
Nuclear Pulse
Orion
High thrust and Isp
Fallout
Exotic
Fission Fragment
High Speed
Needs much work
Higher Energy
Difficulty
Nuclear Thermal
Fusion
MCF
Gasdynamic Mirror
Linear Geometry
Still not working
ICF
Fusion Orion
Proven Technology
Heavy lasers
IEC
Farnsworth Fusor
Relative Ease
Not enough research
Interstellar Travel
Getting Fuel
Antimatter
Pure Annihilation
Beam Core
Very High Isp
Amounts of fuel
Hybrid
ACMF
Low antimatter need
Traps and storage
1
2