Bussard Ramjet: Efficacy as an Interstellar Space Transport

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.