James and Gregory Benford’s Starship Century

by

Manjula Menon

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James and Gregory Benford need no introduction to followers of space technology. From major contributions to SETI, to founding the Mars Society, to scholarly academic work, to writing hard science-fiction, their work is widely admired.ⁱ I will not attempt to summarize their substantial oeuvre, but will instead focus on their 2013 anthology, Starship Century, that offers essays, short fiction and even poetry, all around the theme that humans have a destiny in the stars.ⁱⁱ

There is no talking about space travel without talking rocketry, and there is no talking rocketry without talking about the Tsiolkovsky rocket equation. The equation is named after the Russian scientist Konstantin Tsiolkovsky who derived it in 1903. It is widely seen as the most important equation in rocketry, maybe even one of the most important in all of engineering. All rocket propulsion is governed by this elegant equation, where “rocket” refers to launched vehicles that expel reaction mass for propulsion. The rocket propulsion process begins with a chemical or nuclear reaction in the rocket’s engine that releases energy in the form of heat. A portion of this heat energy is converted into the kinetic energy of the gas particles exiting the rocket’s nozzle. The ejected gas particles exert an opposing force onto the rocket, and propels the rocket in the other direction, as per Newton’s third law of motion.

The Tsiolkovsky rocket equation, also called the ideal rocket equation, connects the movement of a rocket ΔV, with its exhaust velocity, V_e:

ΔV=V_elnM₀/M_f 

Although the velocity change ΔV is described in units of speed, km/s for example, it can be thought of as a measure of the energy required to maneuver the rocket. V_e, the effective exhaust velocity, is also described in units of speed, but can likewise be thought of as describing the kinetic energy of the exiting hot gas, ΔV²m/2, which imparts thrust onto the rocket.

The term lnM₀/M_f is the crushing constraint for rocketry, as it describes an exponential relationship between ΔV and the “mass ratio” or the ratio of initial to final mass. A very rough way to think about it is that if you want to double the speed of the rocket, you need at least four times the propellant, and if you want to triple the speed of the rocket, you need at least nine times the propellant. Additional propellant is required for additional speed, and that propellant is now part of the initial mass, which in turn requires more propellant to lift and so on.

The term “specific impulse” or I_sp, is often used as a stand-in for V_e. Specific impulse, measured in units of seconds, describes how much time a rocket will take to burn through one pound of propellant while delivering one pound of thrust as described by Geoffrey Landis in his essay for Starship Century, “The Nuclear Rocket: Workhouse of the Solar System”. Specific impulse is a useful yardstick by which to measure relative propellant efficiencies for ascent from sea level, given Earth’s gravity is a constant 9.8m/s². V_e = I_sg₀: increasing specific impulse means more V_e for the same amount of propellant expended. An example of a rocket that uses a chemical reaction is SpaceX’s raptor engine, that combusts liquid methane with oxygen to produce carbon dioxide, water vapor and heat. Starship’s published thrust is 1500tf (the weight in tons that the rocket can lift from sea-level). Although the specific impulse of the raptor engine is proprietary information, original specs show sea-level ascent capability of an impressive 330 seconds.ⁱⁱⁱ

Nuclear rockets are even more powerful, not only because the energy stored in nuclear bonds is far greater that that stored in chemical bonds, but also because unlike chemical rockets, the ejected reaction mass is not the same as the source of energy. For example, a nuclear thermal rocket splits apart fissionable material, usually uranium, inside its nuclear reactor core, releasing heat. This heat is transferred onto the “working fluid”, usually liquid hydrogen, which expands and accelerates out of the rocket nozzle, generating thrust. A fusion rocket even dispenses with the working fluid. Inside the core of the nuclear rocket atoms of perhaps helium-3 are being fused with perhaps deuterium to produce very hot ionized gas, which is then ejected to generate thrust.

None of this is new technology. As the Benfords note: “In 1978 the British Interplanetary Society produced the most detailed starship concept called Daedalus, aircraft carrier big, using inertial fusion, still undemonstrated today … Lasers crush a pellet of hydrogen isotope so pressure increases so much that the nuclei to fuse which releases energy that is expelled as exhaust that creates thrust. Lasers fire rapidly, giving pulsed thrust.” Project Icarus, son of Project Daedalus, produced interesting variants.^iv Many who worked on these projects moved on to other things, including working on different propulsion methods.^v No nuclear thermal or fusion rocket has ever flown (note that nuclear rockets would only be used in space anyway, given the vast amounts of energy involved) but the point is that these are still rockets, and still governed by the Tsiolkovsky rocket equation. Geoffrey A. Landis’s essay in Starship Century, “The Nuclear Rocket: Workhorse of the Solar System” offers an accessible and thorough analysis of the technology.

There are means other than rockets to get ourselves to the stars. As James Benford writes in his essay “Sailships” in Starship Century: “I begin with a prediction: The first starship will be a sail—a sailship, driven by a beam of photons, attaining very high velocities and flying through the Kuiper belt, the Oort Cloud and into Deep Space.” He explains why he’s so bullish: “… sailships have a singular advantage: they leave the engine behind. So we can build a spacecraft that consists of only payload and structure—no fuel at all. The propellant is light itself, so sails reflect light waves, whether visible or microwave or laser produced, from a beam generated elsewhere.” ^vi

Imagine in the far future, a sailship whizzing by you in the darkness of space, its kilometers-long, gossamer-thin, reflective sails unfurled towards the sun. You note that there is no reaction mass to provide propulsion. Instead, trillions of massless photons are being reflected by the ship’s mirrored sails, thus imparting forward momentum to the ship as per the Law of Conservation of Momentum. As the sailship disappears into the dark, another whizzes by, and then another, till an entire flotilla has raced silently onward, towards the stars.

Three years before the publication of Starship Century, the Japanese sailship Ikaros had successfully sailed to Venus. Ikaros had first hitched a ride to space on an H-11A Mitsubishi rocket, before spinning open its micrometers-thick, 20-meters on the diagonal, high-performance-plastic, kite-shaped sails. Attitude control was provided by light-reflecting liquid crystal display panels, while thin-film solar panels had been embedded into the sails to power the dust-counters that were part of the scientific payload. Using nothing but sunlight, Ikaros sailed all the way to Venus.

Current commercial entities like the Paris-based Gama Sails aim to be part of the support infrastructure that others rely on for deep space missions. For its maiden test, a Gama Sails satellite rode a SpaceX Falcon-9 rocket to LEO, where it successfully demonstrated unfolding its solar sail. Other tests are planned: “… Beta will be launched at twice the altitude and focus on “navigation”, going from A to B using only photonic propulsion and proving all key elements of the technology.” ^vii

Far more ambitious projects, involving not just solar sails, but also beam-powered-propulsion for powerful, sustained acceleration are in the works. Breakthrough Starshot, for example, is a project that James Benford is involved with (although it is currently on pause). Starshot’s destination goal is the Alpha Centauri star system, for a contingent of tiny “spacecraft on wafer” ships or “starchips”, each propelled by a sail of over 4X4 meters. The starchips would be accelerated to relativistic speeds by focused beams of electromagnetic waves from what James Benford (along with others) refers to as “beamers”. As he writes in “Sailships”, “Beamers would look like the microwave dishes we currently see listening to satellites, only very much larger …”

Benford, who has published extensively in the field of high-powered-microwave, or HPM, has already demonstrated feasibility of beam-powered-propulsion for sailships under laboratory conditions, writing in Starship Century, “I led the team that in 2000 demonstrated first flight of microwave-driven carbon sails using microwave beams to produce several-G accelerations …” As for the not insubstantial budget estimated for Starshot, he writes, “Like the nineteenth-century railroads, once the track is laid, the train itself is a much smaller added expense.” He envisions the beamers would initially be Earth-based but could later be built in space, “…from materials mined from the moon or asteroids. Beamers might be positioned close to the sun, where they could run on strong solar power.”

Other fascinating and exotic propulsion methods are discussed in Starship Century, but in addition to the question of how to get there, we must also figure out how to keep ourselves alive through the journey. As James and Greg Benford write in Starship Century, “For crewed missions, protection from the hazards of space, or even to live well, is challenging.” James Benford offers a design for a laser-driven colony sailship in his “Sailships” essay that features a rotating (for gravity), torus-shaped habitat with segmented sails.

Thinking about how to get frail humans with their blink-of-the-eye lifespans across the dark recesses of space also occupied the mind of the legendary pioneer of space travel, Robert A. Goddard. ^viiiAs Adam Crowl writes in his Starship Century essay “Starship Pioneers”: “Goddard kept his starship discussion private, putting them in a 1918 sealed letter titled The Ultimate Migration, recently republished by the British Interplanetary Society.” If voyages could not be completed within human timescales, “Goddard imagined multimillennial journeys carried out by crews bred to be able to undergo reversible mummification, revived for course-corrections. Eventually all the ship’s passengers would be revived after perhaps a million years of slowly crossing the galaxy.” And if fragile human bodies could not be transported across the deep recesses of space by any means, then Goddard thought that “… humanity might send forth cells able to re-evolve humanity after flying through the void for billennia to suitable planets.”^ix

In their essay introducing Starship Century, “Starships: Reaching for the Highest Bar” James and Gregory Benford write with excitement about a newly discovered exoplanet Alpha Centauri Bb: “This rocky planet around our nearest neighbor has ignited interest in sending probes there, as a first venture into interstellar space.” Alpha Centauri B is part of the Alpha Centauri three-star-system that also includes Alpha Centauri A and Proxima Centauri. It is the closest star system to us and therefore of particular interest. While Alpha Centauri Bb was announced to be a false positive in 2015, other exoplanets were discovered orbiting Proxima Centauri. With massive advances in imaging and statistical methods, and the advent of space telescopes like James Webb, as of 2024, over 5,600 exoplanets have been confirmed, a number that will keep rising. It was once thought that the octet in orbit around our sun was a rare kind of thing, but now we know that exoplanets are common and have been found orbiting everything from neutron stars to brown dwarfs.

The question thus becomes not whether there are planets outside our solar system, but whether we can build a starship that can get us there. There remain difficult physics and engineering problems to be solved, but we will make it by, as Isaac Newton put it, “… standing on the shoulders of giants”, James and Gregory Benford among them.

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