This is a paper that I wrote involving the notion of the Bussard ramjet and its theoretical use in science and in literature. I think that it's quite interesting. Enjoy!
Bussard
Ramjet: Efficacy as an Interstellar
Space Transport
By Matthew
Michel
Abstract
The
portrayal of the Bussard ramjet in popular fiction is generally positive; it
allows humanity to travel to other stars under a velocity nearing the speed of light. The concept uses interstellar gases to
generate nuclear energy, which in turn propels a starship. Several popular works of fiction will be
examined and then juxtaposed against scientific papers proposing the
effectiveness of the Bussard ramjet. Based
on both scientific probability and proposed use in fiction, a verdict will then
be formed regarding the efficacy of the Bussard ramjet as an interstellar space
transport.
Introduction
The Bussard ramjet is an
interstellar vehicle first proposed to the scientific community by Robert W.
Bussard in 1960 in his work entitled Galactic
Matter and Interstellar Flight. In
its simplest form, a ramjet is a theoretical fusion rocket that uses
electromagnetic fields to scoop up hydrogen from the interstellar medium. The
hydrogen is then used to propel the vessel through thermonuclear fusion
(Bussard 4). The ramjet seeks to provide
an alternative method to rocketry for interstellar travel, which require a
source of fuel and therefore limits missions by both distance and time. Since hydrogen is readily available in the
interstellar medium, the ramjet proposes a kind of reactor that would be able
to utilize the H-H reaction and the proton-proton chain. Up until this point the vehicle has remained
entirely theoretical due to several technology limitations, which will be
discussed later in this paper.
The Bussard ramjet has remained
steadily popular in the public imagination, starting with its conception in the
early 1960s and leading up to the present day.
In his television series and book Cosmos,
which was debuted in 1980, Carl Sagan proposed the Bussard ramjet as the
vehicle for future interstellar travel.
In fiction, a number of authors have utilized the idea, including Larry
Niven, Isaac Asimov, Vernor Vinge, and Poul Anderson. Scientists have also been drawn to Bussard’s
idea, and many have written their own works on the ramjet, exploring modifications
and limitations on the proposed use of the ramjet. In an effort to reconcile science and
literature, this paper seeks to compare and contrast views presented in both
the scientific and literary communities and thereby ascertain a complete understanding
of the efficacy of the Bussard ramjet as a device for interstellar travel.
The Ramjet in
Science
In Galactic Matter and Interstellar Flight, Bussard defines the initial starting conditions of the
ramjet. In order for the electromagnetic
ramscoop to start operating, the ramjet must reach an initial starting
velocity. Bussard proposes an equation
and states the following about the initial starting velocity: “For any flight we must accelerate our ramjet
vehicle by rocket boosting to some finite initial velocity, however there
appears little incentive to strive for starting velocities as high as those
which might be attained by relativistic rockets” (Bussard 8). By this statement he means the initial starting
velocity that must be achieved by the vehicle is well within the purview of
modern science. Therefore, today’s
current technology is capable of producing rockets that will accelerate the
ramjet to a velocity in which it can begin to collect hydrogen form the
interstellar medium.
Bussard also puts forth one of the
first problems with the ramjet design.
He calculates that in order for the fusion reactor to function
efficiently at normal Earth gravity it must reach ratios of
[(gm/
)/(nucleon/
)]. He goes on to say that ramjets must be
extremely large in size and possibly unwieldy to construct unless areas of the
interstellar medium can be found with high concentrations of hydrogen (Bussard
8). This is one of the difficulties that
anyone determined to construct a working ramjet would face. Undoubtedly, such a large vehicle would not
be able to be constructed on the Earth, but would have to be made in space. This further complicates the matter, because
then it is plausible that an extensive space station or some other construct
would be needed to facilitate the manufacture of the ramjet.
In his paper exploring the effect
that acceleration and velocity have on the time that it takes for space
missions to succeed, Frisbee provides a figure for the magnitude of the size of
the electromagnetic field that a ramjet would need to generate in order to
function properly. Assuming an average
sized ramjet, and based on the scant distribution of hydrogen (the area of
space is quite large, and atoms are quite small), he proposes the dimensions of
the ramscoop as 30,000 km long and 6,000 km in diameter (Frisbee 9). This is an extremely large number. In order to properly visualize this, let us
compare the dimensions of the ramscoop to the moon. The moon’s diameter is approximately 3,475
km. This means that the diameter of the electromagnetic
field that would need to be generated by the ramjet would be 1.73
moon-diameters. Needless to say, current
science does not have the technology to generate an electromagnetic field of
these proportions.
Bussard also acknowledges that
hydrogen has a low reaction cross-section, but that one alternative fuel source
is deuterium. The process of energizing
deuterium is not nearly as slow as that of hydrogen. The following diagram shows the cross-section
of both hydrogen and deuterium (Bussard 10).
As
is seen in this diagram, deuterium is likely more efficient than hydrogen as a
source of fuel. However, Bussard points
out a few problems with using deuterium.
It is not nearly as pervasive as hydrogen in the interstellar
medium. In addition, because deuterium
is denser than hydrogen, the construction of the ramjet itself would be more
difficult. Bussard concludes that the
creation of a reactor for the fusion of hydrogen might be more complicated than
one for deuterium, but that the construction of the ship itself would be more
difficult if the fuel source was deuterium (Bussard 10). At the time of Bussard’s work on the ramjet, much
of the knowledge about deuterium was entirely theoretical. However, it is important to construct a
framework upon which future scientists can implement their own ideas. Now that more is known about deuterium, and
its percolation throughout space, in 2009 Whitmire refuted the idea of using
deuterium as a possible alternative fuel source. He points out that the magnetic scoop on a
deuterium-collecting ramjet would have to be at least
times larger than its hydrogen-gathering
counterpart, which
is simply not feasible (Whitmire 2, 3). If
we once again use the numbers proposed by Frisbee, this would mean that the
deuterium-gathering ramjet would have a diameter of
,
or 6,000,000,000 kilometers. This would be an equivalent of 1,726,619 moon
diameters. While
scientifically possible, from a human standpoint the construction of a
deuterium-utilizing ramjet is outside of the reach of any foreseeable future
technologies.
Whitmire goes on to propose a method
of increasing the fusion of hydrogen in a Bussard ramjet, as that is one of the
largest issues from a scientific view.
He describes a catalytic nuclear ramjet in which the CNO Bi-Cycle is
used as a catalyst to increase the efficiency of the ramjet (Whitmire 4). In the end, Whitmire’s work is proved to be
entirely theoretical. As he so
eloquently states, “There are no practical fusion reactors of any kind at
present and the prospects for a workable heavy ion reactor with our assumed
parameters are more remote” (Whitmire 6).
However, he points out that this is merely a technological problem and
that his ideas are sound scientifically and merit attention in the future.
This is by no means an exhaustive
representation of the science behind the Bussard ramjet, but the author has
attempted to bring together different scientific accounts and demonstrate a few
of the challenges that the ramjet faces as its theoretical conception moves
forward.
The Ramjet in
Fiction
The Bussard ramjet has been partly
popularized in the romantic imagination through the use of fiction. These fiction pieces are known as “hard”
science fiction novels, because they use plausible scientific facts and
elements. One of the reasons that
science fiction is important is because it allows us to use the imagination to
portray futuristic inventions and societies.
The Bussard ramjet, which falls into the category of a theoretical
invention, can be explored through fiction.
In many of Larry Niven’s Known Space novels, the Bussard ramjet
is one of the principal methods of travel.
In particular, in the novel Protector
a Pak (alien) uses a ramjet to travel thirty-one thousand light years to
Earth. The novel brings up a number of
interesting points about the ramjet.
First, a ramjet could potentially outrun any type of vessel that uses
traditional rockets as propulsion (Niven 39).
This is interesting because it suggests that ramjets could be used for
warfare over long distances, although Niven does not take time dilation into
account here. Another interesting point
that Niven brings up is that a crew on a ramjet would have to be able to
recycle air, liquid, and waste products on any long-term voyage (65). This is a problem that must be solved for any
interstellar flight that proposes a lengthy journey. Towards the end of the novel, multiple
ramjets engage in a type of interstellar warfare. Niven describes how this might be possible:
A directed
magnetic field would churn the interstellar plasma as it was guided into a
Bussard ramjet. As a weapon it might be
made to guide the plasma flow across the ship itself. The gunner would have to vary his shots, or
an enemy pilot could compensate for the weapon’s effect. If the local hydrogen density were uneven,
that would hurt him. If the plasma were
dense enough locally, the enemy could not even turn off his drive without being
cremated. Part of the purpose of the ram
fields was to shield the ship from the gamma ray particles it was burning for
fuel (Niven 173).
This
passage suggests that an additional magnetic field could be used to disrupt an
enemy’s ramjet, and thereby produce some sort of feedback in the ship’s
drive. If the field of a ramjet were to
malfunction, then any protective measures that had been put into place to
protect the passengers would fail. This
is interesting because human explorers could use this system as a potential
defense against hostile aggressors in the interstellar medium.
In Vernor Vinge’s novels A Fire Upon the Deep and A Deepness in the Sky, the Bussard
ramjet is used as a method of vehicle propulsion. It is particularly interesting to examine its
use in A Deepness in the Sky, because
the novel suggests use of the ramjet not only for exploration, but also as a
method of establishing a trading empire.
The Qeng Ho are a group of traders who move across the stars with
ramjets in order to find new economic opportunities. This suggests that ramjets could be used to
link a colonized planet with mother Earth, and perhaps even ferry resources
between two worlds.
The most extensive exploration of
the Bussard ramjet in fiction takes place in the novel Tau Zero by Poul Anderson.
The story follows a shipload of colonists who are travelling in a ramjet
through the interstellar medium in order to reach a new world to colonize. One interesting fact that the novel brings up
is that extensive automation would be necessary in order to properly control
the ramjet fields:
Unpredictable
variations occurred in the matter content of space. The extent, intensity, and configuration of
the force fields must be adjusted accordingly—a problem in ? million factors
which only a computer could solve fast enough (Anderson 43).
This
suggests that intricate computer systems would be needed in order for the ramjet
function at top efficiency. This brings
up two other problems—the need for a technician on the voyage to service these
computers, and the possibility of the computers malfunctioning.
As the novel progresses, it
becomes apparent that there is a certain point at which the Bussard ramjet must
start deceleration, decreasing its speed further and further from the speed of
light. In addition, Anderson describes
the Tau factor, or that which governs the velocity of both the spaceship and
the speed of light. Anderson concludes
that, “the faster she [the ship] travels the more massive she is, as regards
the universe at large” (59). This comes
into play later in the novel when the ship hits a small, extremely dense nebula
in space and so loses the ability to decelerate (72). Without the ability to decelerate, the ship’s
crew cannot turn off the electromagnetic field or they will instantly be
annihilated by harmful radiation (88). As
the ship continues to travel, she becomes more and more massive. Essentially, the men and women aboard the
ship must find a way to reach an area of the universe where gases are extremely
rare. One of the characters comes up
with this solution: “If we can find a
region where gas is particularly nonexistent, we can safely shut down the
fields, and our engineers can go outside and repair the decelerator system”
(93).
However, the crew comes across
another problem when they reach this area of low-density matter. They find themselves in a part of the
universe without habitable planets of any sort.
In a winner-take-all kind of gamble, the crew decides to continue
accelerating in order to witness the birth of a new universe. This is possible because as their velocity
increases, and their Tau continues to approach zero, time dilation increases
and the universe around them experiences time at an incredibly fast rate. In the end, the crew finds a new planet in a
new universe.
A number of interesting
ramifications appear from analyzing this novel.
One is the psychological effect that the journey has on the crew. The crew knows that they must be the last
surviving members of the human race, as the outside universe speeds by over the
millennia. As a result, many of the crew
become depressed and even suicidal at points.
Without specific goals to live for, many others suffer from extreme
boredom.
The second, and perhaps more
general, point is that no one can predict what might happen in space. Space is extremely vast and we still know
comparatively little about it; therefore, crews in a ramjet would face unknown
dangers while exploring space. Fiction
has allowed us to explore some of these potential dangers, and has given us a
more encompassing view of the Bussard ramjet and its possible uses.
Conclusions
The Bussard ramjet is currently
outside of the purview of modern science and therefore remains entirely
theoretical. It is not possible to build
a reactor or generate the powerful electromagnetic fields that such a vessel
would require. However, the scientific
principles behind the ramjet are sound. With
current technology, a hydrogen-collecting ramjet is more feasible to build than
a deuterium-collecting one. In order to
circumvent some of the problems that would appear once a human crew is added to
the vessel, such as time dilation and psychological wellness, it is recommended
that space exploration using ramjets should first be undertaken by robotic
vessels at fairly close distances to Earth.
Works Cited
Anderson,
Poul. Tau Zero. New York: Lancer Books, 1970. Print.
Bussard,
Robert W. “Galactic Matter and
Interstellar Flight.” Astronautica Acta 6.4 (1960):
1-14. Web. 15 April 2013.
Fribsee,
Robert H. and Stephanie D. Leifer. United States. American Institute of Aeronautics
and Astronautics, Inc. Evaluation of Propulsion Options for
Interstellar Missions.
Pasadena: California Institute of Technology, 1998.
Web. 15 April 2013.
Frisbee,
Robert H. United States. American Institute of Aeronautics and Astronautics,
Inc.
Impact
of Interstellar Vehicle Acceleration.
Sacramento: Joint Propulsion
Conference
and Exhibit, 2006. Web. 15 April
2013.
Niven,
Larry. Protector. London: Galaxy Publishing Corporation, 1967. Print.
Niven,
Larry. Ringworld. USA: Holt, Rinehart, and Winston, 1977. Print.
Vinge,
Vernor. A Deepness in the Sky. New
York: Tor Books, 1999. Print.
Vinge,
Vernor. A Fire Upon the Deep. New
York: Tor Books, 1992. Print.
Whitmire,
Daniel P. “Relativistic Spaceflight and
the Catalytic Nuclear Ramjet.”
Acta
Astronautica
2 (2009): 407-509. Web. 15 April 2013.
.jpg/200px-Ringworld(1stEd).jpg)
