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Issue 20 June 2020

TENNESSEE VALLEY INTERSTELLAR WORKSHOP . WEBSITE: www.TVIW.us , EM AIL: [email protected] ,

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THE INTERSTELLAR RAMJET AT 60 BY A. A. JACKSON

The interstellar ramjet conceived by Robert Bussard may have launched more physics careers than any other propulsion concept. Numerous scientists were captivated by Poul Anderson’s treatment of the idea in his novel Tau Zero. Al Jackson takes a look at Bussard’s concept in today’s essay, referencing its subsequent treatment in the literature and adding a few anecdotes about Bussard himself. The original paper was submitted on February 1, 1960 to Astronautica Acta, then edited by Theodore von Kármán (a ‘tough judge,’ Al notes) and published later that spring. Although the ramjet faces numerous engineering issues, its ability to resolve the mass-ratio problem in interstellar flight makes it certain to receive continued scrutiny. – Paul Gilster

Writers of science fiction prose noticed the difference between interplanetary flight and interstellar flight earlier than anyone. Various fictional methods of faster-than-light (FTL) were invented in the 1930s, John Campbell even inventing the term ‘warp drive’. Asimov’s Galactic Empire is only facilitated by FTL ‘jump-drives’. Slower than light interstellar travel made an appearance in Goddard and Tsiolkovsky’s writings in the form of ‘generation ships’, usually called ‘worldships’ now.

As far as I know, the first engineer to look at the very basic physics — quantitative calculations — of relativistic interstellar flight was Robert Esnault-Pelterie; he made relativistic calculations before 1920 that were published in his book L’Astronautique (1930). The first derivation of the relativistic

rocket equation occurs in Esnault-Pelterie’s writings. This was long before Ackeret (J. Ackeret, “Zur Theorie der Raketen,” Helvetica Physica Acta 19, p.103, 1946). The classical mass ratio rocket equation of Tsiolkovsky showed the difficulty of space travel. The relativistic rocket equation showed that interstellar flight was even more difficult.

Eugen Sänger, who had been interested in interstellar flight in the 1930s, addressed the interstellar mass ratio problem in 1953 with a paper on photon rockets, “Zur Theorie der Photonenraketen” (Vortrag auf dem 4. Internationalen Astronautischen Kongreß in Zürich 1953). Sänger, more than almost anyone before him, studied the hard physics of antimatter rockets and relativistic rocket mechanics. Using the most energetic energy source, antimatter, would require tons of it in a conventional rocket. There was sore need of a better method.

Bussard

Robert W Bussard was a rangy man who looked like he walked the halls of power. I had dinner with him at a San Francisco section of the American Institute of Aeronautics and Astronautics meeting in 1997. We had invited Poul Anderson, author of Tau Zero ; Anderson and Bussard had never met. Over dinner Bussard told me he started working on nuclear propulsion at Los Alamos in 1955, and that he and R. DeLauer wrote the first monograph on atomic powered rockets in 1959 [1]. He also said he had been looking at work at Lawrence Radiation Laboratory in 1959.

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NEWSLETTER CONTENTS

The Interstellar Ramjet at 60 by A. A. Jackson .......................... 1 An Overview of the Presentation Content from TVIW’s 6th

Interstellar Symposium by Paul Gilster ..................................... 1 The Interstellar Ramjet at 60 (cont’d) by A. A. Jackson ............. 6 Limitless Space Institue Student Paper Contest in Partnership

with TVIW .................................................................................. 9 Use AMAZONSMILE to Benefit TVIW ....................................... 9 Upcoming Interstellar and Space Events ................................... 9

AN OVERVIEW OF THE PRESENTATION CONTENT

FROM TVIW’S 6TH INTERSTELLAR SYMPOSIUM

BY PAUL GILSTER

TVIW’s 2019 symposium in Wichita, KS took as its theme “The Next Giant Leap -- Ad Astra,” a nod to spaceflight past and starflight to come. If Neil Armstrong’s ‘small step’ at the Sea of Tranquility dazzled us with how far we had progressed in just 66 years of powered flight, a journey to a star surely trumps anything conceived by our restless species. From a nearby Moon to even the nearest star moves us roughly 260,000 astronomical units at a time when traveling a single AU, the distance between Earth and Sun, pushes our technologies hard. Let’s not forget that it has taken Voyager 1 fully 43 years to attain its current position at 118 AU.

Small steps move eventually to giant leaps, but we are, as Greg Matloff reminded us, creatures in which the sizzle of adventure mingles with the demand for species survival, both imperatives that have taken humans to new terrain despite distance, politics and even ice ages. The author of The Starflight Handbook has countless

papers to his credit over the decades, illuminating the

interstellar options, modes that include everything from thermonuclear pulses (Orion) to nuclear fusion, solar- and microwave- or laser-beamed sails and exotic fuels like antimatter.

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Greg Matloff

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We must guard against what Matloff called ‘magical’ thinking in his keynote, while remaining optimistic as we look into propulsion modes even more at the edge of physics. But work pushes ahead with what we know now, as witness Breakthrough Starshot, the ongoing examination of a small beamed sail driven by an enormous laser array on Earth, one that would sustain brief acceleration in the tens of thousands of g’s to reach 20 percent of lightspeed, facing formidable issues of miniaturization and communication with a payload on the order of a single gram.

The emergence of Starshot, whose work engaged a number of the Wichita speakers, initially galvanized the interstellar community, but as a project designed for a minimum of 30 years (and much longer if a successful mission is achieved and data return taken into account), Starshot’s real significance may be the introduction of serious funding into a field that has for most of its life operated on a shoestring. Whether a mission flies from this work or not, the initial five-year study to assess the possibilities is itself justification for the effort, as it will produce papers and make discoveries that will drive future investigations.

Breakthrough Initiatives’ Pete Klupar explained Starshot’s genesis and the calculations that reduced the current options for reaching the nearest stars to a beamed lightsail. Sails we already know how to do, but fitting them for the stresses of laser bombardment, as

needed to reach 20 percent of lightspeed, is another matter. Challenges abound: How to combine huge numbers of lasers to produce a 100 GW total output for acceleration coherently? How to choose sail materials, sail shape, the profile of the beam to drive it? How to ‘rid e’ the beam?

The issues are complex, perhaps intractable. Some of them may be investigated through near-term demonstration missions. But there are enough challenges here to drive research for decades. Geoffrey Landis addressed an obvious one: Where does the power come from to operate a probe the size of a computer chip? Landis has analyzed the possibilities. Radioisotope thermal power systems fail to scale to such small sizes, while other radioactive decay devices may not support the lifetime required. Kinetic energy from the craft’s motion through the destination star system using its plasma and magnetic environment is a difficult solution in need of further analysis. Perhaps a bigger star is a better target.

And when our spacecraft has made its flyby of, say, Proxima Centauri? Now we have to get data back to Earth, a communications challenge further exacerbated by that power question. David Messerschmitt reported on preliminary studies of a downlink from a ‘swarm’ of small probes, finding no physical constraints but noting the problems of

multiplexing communications from numerous probes, maintaining attitude for pointing accuracy back to Earth, and dealing with the kind of optical filters and detectors needed. Doppler shifts owing to uncertainty in the velocity of the probes adds to the difficulty, outlining the challenge for designers.

But this is exactly how interstellar studies must proceed, identifying the major challenges, in this case of constructing a downlink system using available technology. Such analysis points the direction of future research. Construct a mission profile, in other words, then figure out what is needed, from which you learn where the deficiencies in current knowledge exist. A research program to address them moves forward from this, one that must operate, in the Breakthrough Starshot case, on numerous levels from beamer engineering to sail and probe design.

A craft, even one as small as 1 to 3 grams, moving at 20 percent of lightspeed invariably faces issues of dust and gas impact on its multi-decade journey. Alexander Cohen looked at ‘sputtering,’ which results from a material being bombarded by energetic plasma or gas. Damage to the spacecraft’s leading edge gives way to compromised internal components, and secondary particles can be produced that cause even more damage than the impacts themselves. Cohen calls for shielding strategies to protect the probe as it moves through the interstellar medium.

But not all impacts occur between the stars, as Andrew Higgins pointed out in referencing impacts on the sail in near-Earth space, degrading its performance when its flight under the burst of laser energy from Earth has just begun. The acceleration phase is measured in minutes, meaning the sail will traverse a region

dangerously high in dust as it comes up to speed. Higgins noted that displacing dust in the flight path via laser light transmitted through the sail may be possible, while a ‘fault-tolerant’ sail that would block effects from spreading would demand gaps in the sail, wasting much of the laser’s power. This preliminary study points to needed laboratory work in re-directing charged particles, something that has been demonstrated in the realm of particle accelerator physics.

The geometry of the sail itself will, of course, be crucial, for it must remain under the laser beam for several minutes, facing huge loads and rapid acceleration. Jacob Erlikhman told the Wichita audience about his analysis of mass distributions and beam configurations that result in parabolic and conic sail shapes that can provide stability in the acceleration phase. He noted that both the sail geometry and spacecraft center of gravity become the primary parameters for stability. Ahead for Starshot’s ongoing analysis will be simulations of sail tearing, deformations, vaporization due to absorption and impacts of the sort Higgins examined in the early phases of flight.

The good news here is that stability seems to be feasible with proper design in both sail and laser beam. As with so many of the concepts presented within Starshot, the stability work is at Technical Readiness Level 1, meaning it is only now progressing into more serious analysis. But the topic of lightsails itself continues to draw the attention of the interstellar

Pete Klupar

Andrew Higgins

David Messerschmitt

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community, as it has since the days of Robert Forward, an early advocate of beamed energy concepts whose work on interstellar missions using them remains a touchstone for all future work. It’s likewise worth noting the highly influential role that Forward friend and keynote speaker Greg Matloff has played in the development of interstellar sailcraft concepts ever since the 1980s.

Artur Davoyan acknowledged the Starshot approach in a talk on sail missions, looking at materials properties in light of the need for thermal stability and the sail’s response to the beam. A subset of ceramic and high-index superconductor materials suggests itself for this kind of beaming, producing ultrathin sails applicable not only to laser propulsion but missions approaching the Sun closely in an ‘Oberth maneuver,’ where the craft makes a close pass and deploys the sail just as gravitational forces are flinging it out into the Solar System. Machine learning, optimization and experimental testing point the way to future flight demonstration.

On the matter of sail materials, Joseph Meany pointed out the need to reduce sail mass, which in the best modern reflectors consist of thin film substrates that are too thick for the kind of performance an interstellar mission will demand. If metal films on a polymer substrate can be considered the current state of the art, Meany argues for graphene as a replacement substrate. Advantages like durability and low areal density can be coupled with advances in optical metamaterials, the latter necessary because graphene by itself is all but transparent. The combination offers extremely thin, low mass sails that can maximize thrust from a beam.

And where do we stand today with regard to sail technologies? Les Johnson brought us up to speed with a report on the kind of sails that have flown to date, thin films that take advantage of photon momentum. The soon-to-launch Near Asteroid Scout is NASA’s mission to reach an asteroid with an 86-square meter sail, to be launched as a cubesat mission in 2020. Solar Cruiser ups the size considerably, reaching 1200 square meters in a sail braced by lightweight, composite booms and including electro-

optical thin films to vary the reflectivity of the sail.

This last is an interesting point. The Japanese IKAROS sail varied its reflectivity to put torque on the spacecraft, allowing for maneuverability that will be useful in accomplishing mission objectives. Solar Cruiser will provide early warning techniques for solar storms as well as non-Keplerian orbits, which would allow ‘pole sitter’ positions from which to do solar science. As Johnson pointed out, we can see the beginnings of an interstellar roadmap here as we build toward the huge sails demanded to reach another star. Building progressively larger sails opens the path toward order of magnitude changes in short periods of time.

From where we are today to where we want to go takes us into a realm of considerable uncertainty. It would be useful to have a sense for which options are the most likely to bear fruit, which is what Marc Millis has been working on in a study of breakthrough propulsion funded by a NASA grant. The initial step in the study

is an assessment of existing interstellar propulsion concepts, moving toward a database allowing valid comparisons between these approaches to be made. Rather than selecting a mission and its technology, the study will highlight ways to choose research directions in light of the relative merit of these concepts.

This is the dilemma of researching a field as nascent as interstellar propulsion, where energy demands dwarf all that has preceded them and concepts vie for credibility only in the realm of theoretical papers. Divergent methods are hard to compare -- Millis noted as an example the metric of specific impulse vs. beam divergence -- while readiness levels vary widely and development times are comparable to historic technological revolutions. The study will identify which research paths are the most likely to produce results, using common mission scenarios and calculating performance with defensible figures of merit and consistent analysis.

Such a methodology may help us evaluate new concepts as they continue to emerge in the literature and in conferences like this one. Jeffrey Greason brought one such forward, what he describes as a ‘reaction drive’ used as a second stage for interstellar flight. Here, building on plasma magnet work sponsored by NASA in the 2004-2005 frame, Greason proposes a drive in which reaction mass is expelled from the vehicle using power drawn from its motion through the surrounding medium, whether that medium be the solar wind or interstellar plasma.

Usefully, such a reaction drive allows for braking after fast transit to destination. One year to Neptune with braking at the target is one possibility, but there is a clear interstellar application. Greason envisions interplanetary uses coupling a plasma-magnet device with modern superconductors, while as a second stage on an interstellar mission initially propelled by a fusion engine, the technology would offer velocities in the range of 10 to 20 percent of lightspeed at modest mass ratios. The path forward is to explore fast flight to Mars by extending the

Les Johnson

Mark Millis

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equations and taking the work into laboratory testing of scaled-down versions of the drive.

Although interstellar flight is a common goal for TVIW symposium attendees, the need to build up a Solar System-wide infrastructure is apparent, allowing the development of technologies like these as we look toward scaling up their principles for much longer journeys. Brian McConnell presented a platform for long-duration crewed flights within the Solar System called the Spacecoach, using water and waste gases as propellant in a solar-electric propulsion system.

The Spacecoach transforms consumables traditionally considered as dead weight into propellant mass, with the additional advantage that when used with a future fusion powerplant, this kind of electric propulsion system can reach 10 percent of lightspeed, while retaining a high mass budget for crew consumables. The evolutionary path goes through existing electric propulsion methods to eventual interstellar capabilities in a system where over 90 percent of the propellant mass is water. The analogy baked into the name is clear. The Spacecoach is conceived as having the same relation to space as the covered wagon did in opening up the west, a readily usable, crew-sustaining and affordable means to explore a frontier.

Along the way, we’ll consider options like the inflatable designs Jamey Jacob presented in Wichita, saving weight in habitat modules while optimizing weight and volume. Such inflatables have already been tested in space and offer potential in everything from deep space habitats, airlocks, boom and truss structural structures to components for long-duration, even generational missions, including gardens and reservoirs aboard the craft. Structural elements even include cellular structures or connected rings forming cylinders and other arrangements, meaning inflatable technologies can be exploited in a wide range of component designs.

Thinking beyond even nuclear fusion, Gerald Jackson alerted the audience to advances in antimatter research. Antimatter has long been of interest given the potential for releasing energies far beyond fusion, but limited production and problems in storage have slowed development. Even so, small probes powered by antimatter used to initiate fission reactions, a so-called ‘antimatter sail,’ have been well studied, with Jackson discussing how the concept could be altered to focus fission products into a coherent exhaust stream. The amount of antimatter needed drops sharply while velocities in the range of 10 percent of c emerge.

Jackson thinks in terms of an unmanned Proxima Centauri probe with deceleration at target, one capable of returning data for decades. More significantly still, he believes antimatter can be produced at levels far higher than today, with dedicated facilities generating 10 grams per year (33 kg for every kilogram of spacecraft dry mass would be needed to reach 10 percent of lightspeed in his envisioned Proxima Centauri probe). Enhanced antimatter production is the key to exploring these concepts, but one already at TRL 4 in terms of readiness.

Can we move beyond even antimatter as we look toward spreading beyond the Centauri stars and into the galaxy? Geoffrey Landis returned with a consideration of matter with negative mass, or ‘exotic matter,’ a concept that is consistent with Einstein’s General Relativity. We have no idea whether it could ever be manufactured, but its propulsion implications would include its use in creating and sustaining a wormhole through spacetime, the kind of shortcut depicted in science fiction that allows spacecraft to move between distant points as if they were directly connected. Landis noted that some properties of negative mass are being accepted in mainstream physics, making it a continuing unknown but one that is no longer off the charts.

Nor are traversable wormholes themselves. Gerald Cleaver presented his calculations and analysis of wormholes that could theoretically be traversable by a human, a possibility that goes back to work in classic papers on wormhole characteristics but is also a not infrequent trope in science fiction. A wormhole side-steps the faster-than-light ‘barrier’ by connecting two otherwise distant points in spacetime, making for all but instantaneous transit. Cleaver used methods developed for the study of black holes to study a variety of warped spacetime surfaces. He investigated four example traversable wormholes along with possibilities for ‘warp’ drives.

Wormholes turn out to be, at least theoretically, traversable, assuming they exist at all, opening an option perhaps available to species advanced enough to exploit them. That’s good news for those thinking of reaching beyond the nearest stars. But maybe human travel to the stars is possible in ways we haven’t considered yet. That was the gist of Alex Ellery’s look at self-replication: A small number of interstellar probes with self-replicating technologies could colonise the entire galaxy within 23 generations, for self-replication (using methods evolving today as 3D printing) acts as what Ellery calls an ‘exponential amplifier’ of both value and cost amortization. Experiments in 3D-printed cubesats are an early way to explore the concept.

Thus the possibility that our first starships will be self-replicating probes, with a yet more remarkable speculation that advances in 3D printing of biological organs could lead to replication of entire humans at destination, making the idea of a ‘worldship’ obsolete. Ellery went on to note that probes like these could make the transition from a Kardashev Type 1 civilization into Types II and eventually III relatively rapid, suggesting the possibility of searching for physical evidence of prior visitation of such intelligence in our own Solar System.

There is, in fact, plenty to be examined within our system, and a major theme of the Wichita sessions involved what we can do near to the Earth right now as well as in the more distant future. Timothy Swindle reviewed the literature on interstellar matter that is near at hand, ranging from interstellar dust grains up to larger objects like ‘Oumuamua, the first interstellar object discovered entering our Solar System. Because we are likely to discover many more such objects as new surveys are enabled, they become promising areas for study, though Swindle pointed to the difficulty in launching even flyby missions due to the high

Gerald Cleaver

Geoffrey Landis

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encounter velocities. Nonetheless, this category of object offers the chance to study interstellar materials without waiting for the propulsion breakthroughs that will power flight between the stars.

Bringing us back to today’s needs, Katelyn Greene discussed human health in space, looking in particular at musculoskeletal issues as found in astronauts on long-duration missions. Problems with vertebral bone deterioration and muscle atrophy make it clear that we have much to learn about alleviating the effects of microgravity on human health. Greene’s ongoing study will eventually include biomedical imaging on crewmembers from NASA, using custom algorithms to analyze musculoskeletal health and develop relevant metrics. Even early in the study, the clear decreases in muscle volume make the need for in-flight countermeasures apparent.

We’ll need to resolve these matters if we’re to build the in-system infrastructure that will underlay any attempt to reach nearby stars. Catherine Smith brought molecular biology into the near-term mix, a subject spotlighted by microbial experiments (and unexpected hitchhikers) on the International Space Station. Smith advocated a management strategy developed for agricultural systems as a way to deal with hitchhiking organisms, but noted that to achieve and maintain healthy microgravity environments, we will need to plan for the unexpected, such as the extremophiles that have been shown to persist in experiments aboard the ISS in what we once thought were unsurvivable conditions.

As technologies advance here in the Solar System, we may want to consider maximizing available resources through terraforming, which was the subject of a working group as well as a talk by Ken Roy. The latter outlined the possibility of terraforming Venus by constructing a material shell in the Venusian atmosphere about 40 kilometers above the surface, with the building blocks brought to altitude and deployed as floating cities until they eventually can be positioned to form a solid geodesic spherical shell. Over a millennium, atmosphere

above the shell is adjusted to Earth-normal, with the shell entirely supported by the Venusian atmosphere below and not connected to the surface. Roy’s calculations show that such herculean efforts could become a model for terraforming rocky planets like Venus in other stellar systems.

Robert Hampson’s report on the terraforming working track showed how many questions the issue raises while pointing to profound

benefits from a successful transformation of Mars. Discussion ranged from the need for planetary protection through a magnetosphere to the need for an atmosphere sufficient to perform necessary radiation protection for the surface. How existing atmospheres would be transformed to accommodate human populations over millennia is a major issue, but the group discussed cloud cities as a way to begin and explored ways to secure sources of water through comets. The issues are perhaps best discussed at this point through science fiction as the whole idea of terraforming becomes known to the public.

The matter of resources occupied James Schwartz, who focused on space mining and raised questions about the exploitation of near-Earth asteroids. Schwartz noted that when practical limitations such as delta-V requirements are put into play, the resource pool is smaller than it might appear, and the resources are non-renewable. Thus how we use them in the short term affects our long-term goals. The amount of water available in asteroids and on the lunar surface turns out to be relatively small. Early exploitation of these resources must, then, reserve some of them for expanded spaceflight operations or more energetically distant resource pools will remain out of reach. A carefully regulated period of space mining is the way to proceed.

How best to proceed with the expansion of the interstellar idea? Reaching the public through symposia and websites is part of the process, but Ad Astra Kansas founder Steve Durst advocated institutional growth. Specifically, an ‘interstellar university’ in Kansas with the motto ‘Ad Astra per Aspera’ could support, inspire and direct future efforts with a curriculum tuned toward astronautics and specifically the challenges of deep space. Durst sees this as building a long-term home for interstellar research and development along themes of observation, communication and transportation, thus encompassing everything from exoplanet research to SETI and work on advanced propulsion, topics covered by Ad Astra Kansas since its inception.

Philosophy and moral responsibility surface at interstellar symposia because people living aboard long-duration spacecraft must find ways to co-exist. David Burke brought ethics into play in a talk exploring the factors that affect human/machine relationships in mission-critical scenarios. Machines, he argued, must understand moral responsibility and make decisions based on this framework, creating what he called an ‘ethical congruence’ with humans. Burke used experiments involving Amazon’s Mechanical Turk program to explore trust granting in humans and scenarios demanding ethical guidance from humans and machines. Ethical congruence is not demonstrated by intelligence alone, and machines must be able to demonstrate that they understand the human stakes involved in complex ethical dilemmas.

Catherine Smith

Steve Durst

David Burke

Ken Roy

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And what of the role of religion aboard a starship? Kelly Smith asked whether humans could preserve the beneficial aspects of religion while removing the conservatism of supernatural metaphysics. He advocated a transcendental method drawn from Immanuel Kant which could construct a balance between rationalism and faith, preserving the positive aspects of each. A universe that creates complexity may provide a sense of purpose analogous to religious faith without supernaturalism, enabling a long-term human future in space driven by a sense of purpose and meaning of the kind that seems demanded by humans throughout history.

Deana Weibel likewise examined the implications of faith in her talk on the religious elements underlying space exploration, drawing on the anthropology of pilgrimage. The ubiquity of religion on Earth means that we’re almost certain to take it with us as we venture into the galaxy. Weibel described her work as qualitative and ethnographic, involving interviews with astronomers, engineers, astronauts and others involved in space research at workshops, space centers, laboratories and universities. Many of her subjects express concepts drawn from religion, but even in the non-religious, a sense of a destiny in space as inevitable is common. Religious or not, confidence in this destiny encourages problem-solving in many fields.

The security question inevitably follows space development as we place it in the context of superpowers wrestling for dominance in the high frontier. Michael Massa analyzed the risk factors involved in sending human crews on long-duration missions to the stars, drawing on extensive experience with Navy SEALS and a background in risk management. Analogs like extended submarine patrols and isolated research stations like those in Antarctica suggest that starship crews will need to accept considerable constraints on cultural matters like religion, sex, privacy and hygiene. Modifications to existing cultural norms in these areas will be necessary to ensure the survival of the mission.

Near term, deep space technologies involve a different kind of risk. C. Joel Mozer looked at national security issues in a keynote informed by his experience as chief scientist for Air Force Space Command. Mozer said that the pillars of US policy have remained the same for decades: Maintain military superiority, work closely with allies, develop international commerce and inhibit the spread of nuclear weapons. All these extend into space, which will become a place of competition among the major powers. Looking ahead to 2060, a recent workshop at the US Air Force Academy saw the human presence in space ranging from limited exploration and a small amount of tourism to an upper bound of thousands of people actively working in space, including habitats on the Moon and Mars.

The global value of the space economy is expected to reach $3 trillion in the next 10 years, according to NATO studies. Mozer walked through scenarios that maintained US leadership in space, ceded power to other nations like China, or kept space managed by international consortia. Eight scenarios were explored from science fiction and history. Working these and other alternative futures allows Space Command to develop a strategy toward the future we seek and build long-term

roadmaps to attain it. The workshop report recognizes that space will be a major engine of national, political and economic power for whatever nations organize to exploit its potential. Failure to remain a leading space power will put US national power at risk.

Thus the need for a comprehensive long-term space strategy, as outlined in the Future of Space 2060 report now available at www.afspc.af.mil. Economically as well as philosophically, the decisions we make today impact the human role in deep space, and must be considered as foundations for what lies ahead. The value of interstellar conferences like TVIW 6, it seems clear, is to weigh possible outcomes and the kind of technologies that can make them happen, while presenting the cultural and philosophical framework within which these decisions must be made. There is no easy jump to a human presence among the stars. Rather, persistent effort and long-term planning produce the conditions needed for interstellar dreams to be realized.

See the full presentations at:

https://tviw.us/2019-presentation-video-archive/

THE INTERSTELLAR RAMJET AT 60 (CONT’D) BY A. A. JACKSON

(…continued from page 1)

Bussard told me he had always been interested in interstellar flight. One day at breakfast at Los Alamos he got a tortilla rolled up with scrambled egg in it. That cylinder made him think of a fusion ram starship! I have to wonder if that story is true, for had he been looking at Livermore’s lab papers he probably saw Project Pluto, the nuclear powered atmospheric ramjet.

Bussard sat down in 1959 and wrote the paper “Galactic matter and interstellar flight,” published in Astronautica Acta in 1960. This paper is thoroughly technical; Bussard summarizes Ackeret, Sänger and Les Shepherd’s studies of interstellar flight [2]. Sänger had shown that even using antimatter one still had a mass ratio problem with a conventional rocket. Bussard then presents an amazing new concept that solved the mass ratio problem [3]. He notes that one can scoop interstellar hydrogen and fuse it to produce a propulsion system.

The treatment is rigorously special relativistic; using conservation of energy and momentum he derives the equations of motion of an interstellar ramjet. He accounts for the energy production and propulsion efficiency of the vehicle in general terms. He uses the most energetic fusion mechanism, the proton-proton fusion reaction which converts .0071 of the rest mass of collected protons to energy. Bussard derives the property that the ramjet will need to be boosted to an initial speed.

Bussard discusses the engineering physics problems; the difficulty of using the p-p chain is enormous. He notes that interstellar hydrogen can be unevenly distributed, there being rich and rarefied regions. He gives a simplified model for scooping and sometimes it is missed that he mentions magnetic fields as a ‘collector’. Bussard also notes both radiation losses and radiation hazards during the operation of the ramjet.

Sagan

The Bussard Ramjet got a boost in 1963 when Carl Sagan noted that there was a solution to the mass ratio problem for interstellar flight [4]. Sagan summarized this paper in Intelligent Life in the Universe in 1966 [5], probably the best popularization

Deana Weibel

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of the Ramjet. Sagan also noted that ships accelerating at one gravity could circumnavigate the universe, ship proper time, in about 50 years. He references Sänger in the paper version [4] and the calculation of the mechanics of a 1g starship. As far as I know, the 1957 paper of Sänger [6] is the first exposition of a constant acceleration starship and the consequences of time dilation when extreme interstellar distances are traveled. Bussard mentioned, very briefly, a magnetic field as a scoop, but Sagan describes such a collector in a more elaborated though qualitative way.

Fishback

John Ford Fishback published his MIT bachelor’s thesis in Astronautica Acta in 1969 [7]; this was supervised by Philip Morrison. Morrison and Cocconi were the fathers of radio SETI. Morrison seems to have taken an interest in Sagan’s mention of Bussard’s ramjet — I’m not sure if it was Morrison or Fishback who suggested the study. The paper is a remarkable marshalling of electrodynamics, charged particle motion, plasma physics, the physics of materials and special relativity.

Fishback constructs a model for the magnetic scoop field taking into account the fraction of hydrogen ingested and reflected. Using conservation laws, he derives the most detailed equations of motion accounting for mass and radiation losses that had been published anywhere. In the scooping process, Fishback examines the statistical distribution of gas in the galaxy and derives a relativistic expression for ship proper acceleration with ‘drag’. An important consequence, expressed for the first time, is the mechanical stress on the scoop field magnets. He derived an upper limit on the maximum Lorentz factor that can be obtained as a ramjet accelerates at 1 g for a long time due to stress on the source of the scoop field.

Martin

In 1971 [8] and 1973 [9] Tony Martin reviewed Fishback’s paper, making useful clarifying observations. Martin provides details of calculation that Fishback leaves to the reader on the relation of the fraction of particles that are magnetically confined to the reactor intake as a function of the confining field and the starship’s speed. In his second paper, Martin corrects a numerical error by Fishback showing that the cutoff speed due to the stress properties of the magnetic source is 10 times larger than was calculated. Martin also gives a nice calculation of the size of the magnetic scoop field. Fishback and Martin’s papers account for the ‘drag’ due to reflected particles; this result seems unknown to later critics of the ramjet.

Whitmire

I met Dan Whitmire in 1973, when we were both working on doctorates in physics at the University of Texas at Austin. Dan and I were talking about interstellar flight one day and I showed him Bussard’s paper. Dan was in the nuclear physics group at Texas and took an immediate interest in the problem with proton-proton fusion as had been pointed out by Bussard and Martin. Then he came up with an ingenious solution: Carry carbon on board the starship and use it as a catalyst to implement the CNO fusion cycle [10]. The CNO process is 1018 times faster than the PP chain at the fusion reactor temperatures under consideration. This reduces the fusion reactor size to 10s (and more) of meters in dimension. Since carbon cycles in the process, in theory one would only need to carry a small amount; however it is not clear how under dynamic conditions one would recover all the catalysis needed.

Later Developments

The above are the core studies of the interstellar ramjet. Hybrid methods occurred to several researchers. Alan Bond [11] proposed a vehicle that carried a separate energy source yet scooped-up interstellar hydrogen not as fuel but simply as reaction mass, this is known as the augmented interstellar ramjet. Conley Powell [12] presented a refined analysis of this system. The author [13] presented a study using antimatter added to the scooped reaction mass for propulsion as an augmented method. Relevant to the augmented ramjet is antimatter combined with matter for propulsion as studied by Forward and Kammash [14, 15].

T. A. Heppenheimer published a paper in the Journal of the British Interplanetary Society [16] noting the problems with the p-p chain for fusion without citing Dan Whitmire’s solution. Heppenheimer notes radiation losses but does not cite Whitmire and Fishback, who addressed the problems of bremsstrahlung and synchrotron radiation in the reactor and the scoop field.

Matloff and Fennelly [17] have interesting papers on charged particle scooping with superconducting coils. Cassenti looked at several modifications and aspects of the ramjet [18].

Recently Semay and Silvestre-Brac [21, 22] re-derived the equations of motion of the interstellar ramjet, first done by Bussard and Fishback. They find some new extensions with solutions of the relativistic equations for distance and time.

Dan Whitmire and the author [23] removed the fusion reactor by taking the energy source out of the ship and placing it in the Solar System. If one scoops hydrogen but energizes it with a laser system it is possible to make a ramjet that is smaller and less massive. Such a system probably has a limited range similar to laser pushed sails.

An excellent survey of interstellar ramjets and hybrid ram systems can be found in the books by Mallove and Matloff [24] and a recent monograph by Matloff [25], see these books and the references listed in them. See also Ian Crawford’s paper [26].

The Interstellar Ramjet in Science Fiction

It seems the Bussard Ramjet first appeared in a Larry Niven short story called “The Warriors” (1966). Later Niven used the Ramjet in his other fiction, inventing, I think, the term Ram Scoop. However I think the best known use of the Ramjet is Poul Anderson’s Tau Zero [26]. The core story in Tau Zero is not the Interstellar Ramjet but the constant acceleration circumnavigate-the-universe calculation first done by Eugen Sänger.

My guess is that Anderson only saw Carl Sagan’s exposition on this in Intelligent Life in the Universe. The Greek letter ‘Tau’ was introduced by Hermann Minkowski in 1908; it is the time measured by the travelers in the starship Leonora Christine, while the time measured by people back on earth is t. Special relativistic time dilation leads to (ship time)/(Earth Time) going to almost zero. Accelerate at one g for 50 years and one covers a distance of about 93 billion light years that is roughly the size of the universe.

The Bussard Ramjet Leonora Christine sets out for Beta Virginis, approximately 36 light years away. A mid-trip mishap robs the ship of its ability to slow down. Repairs are impossible unless they shut down the ramjet, but if the crew did that, they would instantly be exposed to lethal radiation. There’s no choice but to keep accelerating and hope that the ship will eventually encounter a region in the intergalactic depths with a sufficiently

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hard vacuum so that the ramjet could be safely shut down. They do find such a region and repair the ship.

Anderson then introduces the mother of all twists. The Leonora Christine has accelerated for so long that the crew discover relative to the universe a cosmological amount of time has elapsed. The universe is not ‘open’ but fits the re-collapse model, it is zoing for the big crunch. I know of no other science fiction novel with more extreme problem solving that this hard SF story.

Anderson’s cosmology for Tau Zero seems to come totally from George Gamow [28]. Gamow and his students did pioneering work on early time cosmology, an elaboration of earlier work done by Georges Lemaître. When Poul Anderson wrote the novel, he may have been aware that Big Bang cosmology had evolved beyond Gamow’s models …. However, having his starship eventually orbit the ‘Cosmic Egg’ or Ylem was a solution to the crew’s problem. Alas, even in Gamow’s cosmology the ‘Ylem’ is the universe, so no way to ‘orbit’ it. Poetic license for the sake of a Ripping Yarn! (An intersecting exercise is to see what the trajectory of the Leonora Christine’s plot problem is in current accelerating universe cosmology.)

After Niven and Anderson, the Bussard Ramjet became common currency in science fiction, although it has faded somewhat in recent times. Recently a fusion ramjet, SunSeeker, appears as an integral part of the Bowl of Heaven series by Greg Benford and Larry Niven [29].

Final Thoughts

There seems to be a thread of pessimism about the Bussard Ramjet centered around drag on the ramjet due to interaction with the scoop field. This is an issue that Fishback deals with in his analysis; he shows one cannot just use a dipole magnetic field. A more complex collector field is needed. Fishback and Martin do show there is a fundamental physics limitation. Even using the strongest material theoretically possible, there is an upper limit to a mission Lorentz factor, probably equal to 10,000. Above this one will bust the scoop coil due to magnetic stress. The cosmological peril of the Leonora Christine depicted in Tau Zero is not physically possible.

The main show stopper for the ramjet is the engineering. There is no way with foreseeable technology to build all the components of an interstellar ram scoop starship. Several aspects should be revisited. (1) The source of the magnetic scoop field, Fishback [7] derived one, Cassenti elaborated another [20]; (2) the fusion reactor — the aneutronic fusion concept is direct conversion of fusion to energy [30]; (3) hybrid systems, especially laser-boosted ramjets.

Since basic physics does not rule a ramjet out, it is possible that an advanced civilization might build one. Freeman Dyson [31] pointed out many times that what we could not do might be done by some advanced civilization as long as the fundamental physics allows it. An interesting consequence of this is that interstellar ramjets may have been built and might have observable properties. Doppler-boosted waste heat from such ships might be observable. Plowing into HII regions in the galaxy, a starship’s magnetic scoop field might produce a bow-shock which could be observable. Isolated objects in this galaxy with Lorentz factors in the thousands would be unusual and if they are accelerating even more unusual.

The idea of picking up your fuel along the way in your journey across interstellar space may be the optimal solution to the mass ratio problem in interstellar flight. The interstellar ramjet warrants more technical study.

Appendix

Because Robert Bussard sketched a ramjet with a physical ‘funnel’ …all the many illustrations I have seen since seem to have some kind of ‘cow catcher’ on the front. Though it is reasonable that such a structure is the source of an electromagnetic device, I think it more likely that the ‘scoop’ field will be produced by a magnetic configuration that directs the incoming stream into the mouth of the reactor without any extra funnel-like forward structure. Here is a rough schematic done for me by artist Doug Potter. There is a ‘bulb’ representing the magnetic source field (maybe the parabolic magnetic field calculated by Fishback), a reactor section and an exhaust. Not a very elegant representation of the ramjet but a suggested configuration.

References

1. Bussard, R. W., and R. D. DeLauer. Nuclear Rocket Propulsion , McGraw-Hill, New York, 1958

2. L. R. Shepherd, “Interstellar Flight,” Journal of the British Interplanetary Society , 11, 4, July 1952

3. R.W. Bussard, “Galactic matter and interstellar flight,” Astronautica Acta 6 (1960) 179–195

4. C. Sagan, “Direct contact among galactic civilizations by relativistic interstellar spaceflight,” Planet. Space Sci. 11 (1963) 485–498

5. Sagan, Carl; Shklovskii, I. S. (1966). Intelligent Life in the Universe . Random House

6. Sänger, E., “Zur Flugmechanik der Photonenraketen.” Astronautica Acta 3 (1957), S. 89-99

7. Fishback J F, “Relativistic interstellar spaceflight,” Astronautica Acta 15 25–35, 1969

8. Anthony R. Martin; “Structural limitations on interstellar spaceflight,” Astronautica Acta , 16, 353-357 , 1971

9. Anthony R. Martin; “Magnetic intake limitations on interstellar ramjets,” Astronautica Acta 18, 1-10 , 1973

10. Whitmire, Daniel P., “Relativistic Spaceflight and the Catalytic Nuclear Ramjet” Acta Astronautica 2 (5-6): 497–509, 1975

11. Bond, Alan, “An Analysis of the Potential Performance of the Ram Augmented Interstellar Rocket,” Journal of the British Interplanetary Society , Vol. 27, p.674,1974

12. Powell, Conley, “Flight Dynamics of the Ram-Augmented Interstellar Rocket,” Journal of the British Interplanetary Society , Vol. 28, p.553, 1975

13. Jackson, A. A., “Some Considerations on the Antimatter and Fusion Ram Augmented Interstellar Rocket,” Journal of the British Interplanetary Society , v33, 117, 1980.

14. R.L. Forward, “Antimatter Propulsion”, Journal of the British Interplanetary Society , 35, pp. 391–395, 1982

15. Kammash, T., and Galbraith, D. L., “Antimatter-Driven-Fusion Propulsion for Solar System Exploration,” Journal of Propulsion and Power , Vol. 8, No. 3, 1992, pp. 644 – 649

16. Heppenheimer, T.A. (1978). “On the Infeasibility of Interstellar Ramjets”. Journal of the British Interplanetary Society 31: 222

17. Matloff, G.L., and A.J. Fennelly, “A Superconducting Ion Scoop and Its Application to Interstellar Flight”, Journal of the British Interplanetary Society , Vol. 27, pp. 663-673, 1974

18. Matloff, G.L., and A.J. Fennelly, “Interstellar Applications and Limitations of Several Electrostatic/Electromagnetic Ion Collection Techniques”, Journal of the British Interplanetary Society , Vol. 30, pp. 213-222, 1980

19. Matloff, G.L., and A.J. Fennelly , B. N , “Design Considerations for the Interstellar Ramjet,” 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 2008

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20. Cassenti, B. N , “The Interstellar Ramjet,” 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 2004

21. Claude Semay and Bernard Silvestre-Brac, “The equation of motion of an interstellar Bussard ramjet,” European Journal of Physics 26(1):75, 2004

22. Claude Semay and Bernard Silvestre-Brac, “Equation of motion of an interstellar Bussard ramjet with radiation loss,” Acta Astronautica 61(10):817-822, 2007

23. Whitmire, D. and Jackson, A, “Laser Powered Interstellar Ramjet,” Journal of the British Interplanetary Society Vol. 30pp. 223-226, 1977

24. Mallove, E. F., and G.L. Matloff, The Starflight Handbook , Wiley, New York, 1989

25. Matloff, G., Deep-Space Probes , Praxis Publishing, Chichester, UK, 2000

26. Ian A Crawford, “Direct Exoplanet Investigation Using Interstellar Space Probes.” In Handbook of Exoplanets Springer 2017

27. Anderson, Poul. Tau Zero . New York: Lancer Books (1970) 28. George Gamow, The Creation of the Universe (1952) 29. Benford, G. and Niven, L., Bowl of Heaven series, Macmillan. 30. Benford, G., Private communication. 31. Dyson, F. J., “The search for extraterrestrial technology,” in

Marshak, R.E. (ed), Perspectives in Modern Physics , Interscience Publishers, New York, pp. 641–655

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UPCOMING INTERSTELLAR AND SPACE EVENTS

May 29, 2020. SpaceX’s Crew Dragon Demo 2 launch to the

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July 14, 2020. Mohammed Bin Rashid Space Centre’s Hope

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