by Paul Gilster | Dec 14, 2011 | Culture and Society |
I normally scan through various news items for the Notes & Queries posts, but in this case I’ve been trying to catch up on my reading. In particular, I’ve been looking at books that could be useful in inspiring young people to get interested in astronomy and engineering. Here’s a look at three titles that more or less fit that bill. The budding rocket scientist will love the revised Zubrin, particularly with its infectious and expansive sense of what’s possible, but younger students may find their minds tweaked by the two other selections, both of which I’d recommend for a high-school audience. We never know what can launch a career, but scientists are always reminding me of particular books they read when they were kids that made all the difference.
Back to the Red Planet
Toward the end of Robert Zubrin’s The Case for Mars (revised edition, 2011), the author looks at three possible models for getting humans to the Red Planet. It’s a significant section because Zubrin recognizes the political and economic realities of our time, and knows that to make something audacious happen in space will require either a commitment from government of the kind we haven’t seen in decades or a substantial infusion of interest from the commercial sector.
Zubrin’s ‘JFK Model’ is what you would expect, a call upon America to rise to the challenge of interplanetary flight. It would involve the nation embarking on what he thinks would be a $30 billion program that would create jobs and spin off technologies while reinvigorating the exploratory spirit. The rival ‘Sagan Model’ would be an explicitly international effort drawing upon collaboration from all space-faring nations, with the potential benefit of easing global tensions in pursuit of a common goal. Finally, the ‘Gingrich Model’ draws on Zubrin’s conversations with the former Speaker of the House, which resulted in Zubrin’s working out a Mars prize that would be funded by government but would result in intense commercial competition for a privately-built mission. All three are out of vogue given the economic realities, but the case for each is interesting, and the right approach could wind up saving quite a bit of money.
For Zubrin is able to get the numbers down even lower than the surprising $30 billion he discusses for the JFK model by using the private sector’s various economies. He’s a persistent critic of government waste with a career in aerospace that has taught him how it happens:
…the major aerospace companies contract with the government to do a job on a ‘cost plus’ basis, which means that whatever it costs them to do the job, they charge the government a certain percentage more, usually 8 to 12 percent. Therefore, the more it costs the major aerospace companies to do a job for the government, the more money they make. For this reason their staffs are top heavy with layer after layer of useless, high-priced ‘matrix managers’ (who manage nothing), ‘marketeers’ (who do no marketing), and ‘planners’ (whose plans are never used) and whose sole apparent function is to add to company overhead. Of course, since the government needs proof that the expenses claimed by the aerospace companies are actually being incurred, vast numbers of accounting personnel are also employed, to keep track of how many labor hours are spent on each and every separate contract. To give you an idea of how bad it is, at Lockheed Martin’s main plant in Denver, where I used to work, and where the Titan and Atlas launch vehicles are produced, only a small minority of all personnel actually work in the factory. The fact that Lockheed Martin is cost competitive with the other large aerospace companies indicates that the rest of them are operating with similar overhead burdens as well.
A new kind of aerospace industry based on minimum-cost production methods — this is what you get by posting multi-billion dollar prizes of fixed amount for breakthrough space accomplishments — is one way to make a Mars mission more tenable than the $450 billion colossus once estimated to pay for President Bush’s 1989 Mars initiative. The logistical nightmare created by multiple launches, on-orbit assembly and propellant-hauling to the Red Planet seemed to define our prospects until Zubrin started attacking those numbers with refreshing new concepts. He also supplies a philosophical case for making them happen.
The Case for Mars was a bracing read when it appeared and its revised version has the same combination of exuberance and inventive zeal that resulted in Mars Direct, a plan that uses technology we have today to land fuel factories on Mars that would supply later manned missions with what they need to get back to Earth. Those of us with an interstellar bent will appreciate Zubrin’s tireless work in favor of a human future off-planet. Whether you’re coming to The Case for Mars for the first time or read it when it first appeared, the new version is worth your time. Travel light, live off the land, make it happen now — that’s the spirit invoked here, which is why Sagan once wrote “Bob Zubrin really, nearly alone, changed our thinking on this issue.”
Reaching the Budding Scientist
The Japanese form of comic art known as manga seems a surprising way to teach tough concepts, and I was more than skeptical when I first took a look at The Manga Guide to Relativity and The Manga Guide to the Universe (No Starch Press, 2011). But these are cleverly designed books, co-published with the technical publishing house Ohmsha in Tokyo and part of a Manga Guide series devoted to technical and scientific subjects. Each guide is written by a specialist in its field and illustrated by a professional manga artist. I started thumbing through the Manga Guide to Relativity and found myself drawn into a storyline that fed into the deeper content.
I’m thinking these guides would be a good way to reach young people with an approachable story that leads to some meaty physics. In the relativity book, a student is challenged to pick up the basics of special and general relativity over the course of a summer and works his way through these with an attractive young teacher who teases and prods him into understanding. The book alternates between manga and written text and manages to work through everything from time dilation to the twin paradox, examining why length contracts and mass increases at relativistic speeds and how mass affects spacetime, with results we can measure. I won’t say that the concepts here are taught painlessly — Einstein puts plenty of demands upon anyone’s imagination — but they’re creatively presented and made exciting.
The Manga Guide to the Universe is a thicker volume that takes three high school girls through a course in basic astronomy and astrophysics, with the aid of a kindly professor and the context of a play they are trying to present. The alternating manga and text cover theories of the universe’s origin and the discoveries that helped us understand its scale, working up to the cosmological microwave background and the mysteries of dark matter and dark energy. There’s enough meaty concepts here to challenge a young student and perhaps fill his or her head with ideas that might lead to a career. The Manga Guides aren’t textbooks in the classic sense, but you could consider them enticements, keys that might unlock much tougher texts down the road.
A TV Universe Now in Print
If I were trying to inspire young people with the beauties of astrophysics, I’d also make sure to show them Brian Cox’s Wonders of the Universe (HarperDesign, 2011). This is a superb coffee-table book with the kind of gorgeous photography I associate with large, glossy science books, but it’s also a well-written and thoughtful presentation of current thinking on the cosmos. Cox (University of Manchester) is a particle physicist who has brought his teaching skills to the fore in a television show of the same name. A keyboardist in the UK pop band D:Ream in the 1990s, he brings a certain brio to the task that never undercuts the clarity of his exposition.
I also see him compared to Carl Sagan, which seems relatively apt given that both were gifted popularizers who could make difficult concepts accessible. For the book and TV series, Cox was able to travel to exotic locations around the globe to make his points, going to the Valley of the Kings, for example, to talk about celestial alignments and ancient concepts of the heavens, or to the Bagmati River in Nepal, where he sees the funeral pyres that line its banks and discusses the Hindu beliefs about the recycling of the elements, a line of thought that will lead him on to examine the death of stars and the explosive birth of new elements.
The photographs and diagrams truly carry this book, but Cox gets across the wonder of what he does for a living, as he does here in a bit once again reminiscent of Sagan (in Pale Blue Dot):
Our story is the story of the Universe. Every piece of every one and every thing you love, of every thing you hate, of every thing you hold precious, was assembled in the first few minutes of the life of the Universe, and transformed in the hearts of stars or created in their fiery deaths. When you die those pieces will be returned to the Universe in the endless cycle of death and rebirth. What a wonderful thing to be a part of that universe – and what a story. What a majestic story!
All of this in large print on the side of a double-page spread showing an artist’s conception of a supernova that blazes out in fantastic wraiths of color. This kind of thing makes for good TV and it’s solidly expressed in the text, which offers a crash course on our latest discoveries in the cosmos even as it sketches in the historical contributions that have gotten us to our current understanding. I would aim this book at the student who has already decided he or she is interested in the heavens and wants to learn more. Reading through Cox’s description of entropy — as he walks the sands of the Namibian desert and uses the ruined structures of old settlements to explain it — connects theory with the palpable world in ways that can reach young minds.
by Paul Gilster | Dec 13, 2011 | Sail Concepts |
Minimizing the cost of a project is no small matter because, as Jim Benford points out in the paper we’ve been examining over the past several days, cost determines how we decide on competing claims for resources. In the case of a beamed sail mission and its infrastructure, the cost is largely the reusable launcher or ‘beamer,’ which is the beam source and the antennae needed to radiate the beam. Benford is able to derive general relations for cost-optimal transmitter aperture and beam power, from which he can estimate capital cost and operating cost using today’s parameters. He can then study the economics of high-volume manufacturing.
How to get from today’s economics to tomorrow’s? This is where the concept of the ‘learning curve’ comes into play — it is the decrease in unit cost of hardware with increased production. A 90 percent learning curve means that the cost of a second item is 90 percent the cost of the first, while the fourth is 90 percent the cost of the second, and as Benford explains, the 2Nth item is 90 percent of the cost of the Nth item. Usefully, the author notes that the learning curve in power-beaming technologies is well documented. Antennae and microwave sources have an 85 percent learning curve based on large-scale production of antennae, magnetrons, klystrons, etc.
Image: Jim Benford, whose focus on beam-powered propulsion led to the first laboratory demonstrations of microwave-driven carbon sails and showed their ‘beam-riding’ stability under power.
Yesterday we saw these principles applied to an interstellar precursor mission, a 1-kilometer sail moving at 63 kilometers per second, and found that a capital cost of $144 billion could be reduced to $21.6 billion as we factor in economies of scale. But consider how intractable a true starship moving at 10 percent of the speed of light is even when all factors are cost-optimized. Here’s we’re pushing up against serious problems, not the least of which is the high acceleration demanded, about 50 g’s at an acceleration distance of roughly 1 AU. As we’ve seen with beamed sail concepts, acceleration is strongly temperature-limited. As Benford writes:
Not even carbon can survive the heating due to absorption at this acceleration. Sail reflectivity would have to be close to perfect to allow such acceleration. How to change this concept, via the assumed parameters, to bring down the acceleration is left as an exercise for the reader. Cost is in the T$ range, even with economies of scale, and is larger than any past human project.
Some (but not all) of Benford’s observations based on applying cost-optimized scaling to beamed sails:
- Given that the highest costs are found in the antenna and power source elements, it’s important to note that both are proportional to velocity, transmitter diameter and frequency. Costs can therefore be reduced through larger sails, lower mass sails, and higher frequencies.
- Reducing the cost of power will be more important than reducing the cost of antennae.
- Halving the transit time by doubling the speed will cost 2.5 times as much.
Charting a way forward for directed energy propulsion involves building on what we already have by working through a sequence of applications. To make power beaming economic, we need to move power from places where it is cheap to places where it is scarce:
Previous work has shown that it is often more economical to transmit power than to move the equipment to produce power locally. Modern power systems are complex, but if power for space can be located where it is easily accessible and adjacent to where the required skilled people are located, i.e., on Earth, then it becomes more practical.
We use microwave and millimeter-wave antennae in astronomy already, and gyrotron sources at high frequencies are being developed for fusion. Benford thinks that orbital debris mapping could be an early objective, as could recharging of satellite batteries in LEO and, eventually, GEO. He also notes microwave thermal thruster concepts that could provide economical launches of supply modules to low-Earth orbit. Given the economic realities, it is clear that technology development must demonstrate commercial applicability, incremental commercial growth in these areas leading to a space-based infrastructure.
Beamed propulsion offers one path to precursor and eventual interstellar missions. What Benford has been tackling in this paper are the questions of engineering and cost that take economies of scale into consideration. But he also notes that we need to apply this kind of cost analysis not only to past interstellar mission designs, many of which could presumably be improved by working out cost issues, but also to that other possible application of sails — as a method for decelerating a probe when it approaches a destination stellar system. In the broadest sense, determining what is feasible not only in physics but also in economics makes our interstellar thinking more focused and provides needed tools for assessing new concepts.
The paper is J. Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy.” The paper is now available on the arXiv site.
by Paul Gilster | Dec 13, 2011 | Culture and Society |
Be aware that the History Channel show The Universe will air an episode at 2200 EST tonight (December 13) in which Richard Obousy will discuss interstellar propulsion concepts. The air time will probably vary depending on your cable provider, so be sure to check. Richard, once project leader of the Icarus effort and still actively involved at every stage of the Icarus design, is a Texas-based physicist whose work we have often discussed in these pages. I was there for his presentation on Icarus at the recent Oak Ridge interstellar workshop and look forward to seeing him on the tube.
Addendum: This episode is now available online.
by Paul Gilster | Dec 12, 2011 | Missions |
Although we frequently talk about beamed sails for interstellar missions, the fact is that spacecraft on the scale Robert Forward used to talk about that could take us to Alpha Centauri in 40 years won’t come out of nowhere. The evolution of the solar sail into the beamed sail will involve all kinds of experimentation and a variety of mission concepts developed for use right here in the Solar System. Consider just one, a microwave-driven sail that could reach Mars in one month, and Pluto in five years. I wrote about this one in A Microwave-Beamed Sail for Deep Space.
The idea comes from Jim and Greg Benford, who discussed it in a 2006 issue of the Journal of the British Interplanetary Society. The scenario involved a phenomenon the duo had discovered in their laboratory work on microwave beaming. Experimenting with a 7.5 g/m2 carbon sail, they had uncovered the fact that molecules evaporating from the sail created accelerations beyond what would have been expected from photons alone. Painting various compounds on the sail would allow it to take advantage of this ‘desorption’ to produce an extra kick.
A mission like this would be a mixture of the familiar and the unknown. The sail would be deployed by conventional rocket into low Earth orbit, then pushed by a microwave beam from Earth to cancel its solar orbital velocity and create a close flyby of the Sun for a gravitational slingshot boost augmented by deploying the sail at perihelion. The Benfords work has shown that no sail damage results from desorption of the materials painted onto the sail. Having expended its desorbed materials, the sail would then operate as a conventional solar sail for the rest of its interplanetary journey, still using a solar push as far out as the orbit of Jupiter.
Near-Term Uses of Beamed Sails
Fast missions to Mars are desirable, but let’s consider may be the first practical assignment for microwave beaming: Shortening the amount of time a solar sail needs to escape Earth orbit. Let me quote from Jim Benford’s new paper on microwave beaming:
Computations show that a ground-based or orbiting transmitter can impart energy to a sail if they have resonant paths — that is, the beamer and sail come near each other (either with the sail overhead an Earth-based transmitter or the sail nearby orbits in space) after a certain number of orbital periods. For resonance to occur relatively quickly, specific energies must be given to the sail at each boost. If the sail is coated with a substance that sublimes under irradiation, much higher momentum transfers are possible, leading to further reductions in sail escape time.
Benford believes that resonance methods can reduce escape times from Earth orbit by over two order of magnitude compared to a solar sail using only the pressure of solar photons. And while microwave transmitters require much larger apertures for the same focusing distance than lasers, they make up for this by producing higher acceleration. This is why the carbon microtruss material that the Benfords used in their laboratory work is so significant. It can be heated to high temperatures without damage, allowing a stronger beam and higher acceleration. That means more velocity in less distance, which in turn allows the aperture size to be reduced.
A number of missions for microwave beaming are in the literature that develop the idea for use here in the Solar System, with 175 kilometer per second speeds produced during a relatively short period of acceleration. Missions like these could be critical supply channels for getting small payloads to human crews on Mars or the asteroid belt, with deceleration by aerocapture in the case of Mars, or perhaps through a decelerating microwave beam. Here we’re talking about potential travel time to Mars as low as 10 days, though not for human crews. Scientists have explored missions to the outer planets as well, including Jordin Kare’s work on a beamed energy mission to Jupiter, and Benford envisions 250 km/sec for interstellar precursor work.
Assessing an Interstellar Precursor
But Jim Benford’s studies of cost-optimization offer a number of examples, beginning with a slower precursor mission moving at 63 km/sec. The 1-kilometer sail is driven through a ground-based array supplied by power from the Earth grid from a site chosen to optimize the effect of the 100 GHz waves — this implies a high altitude location with low humidity. A capital cost of $144 billion can be reduced by high-volume manufacturing and economies of scale to $21.6 billion, in a range comparable to Flagship missions like Gaileo and Cassini. Benford works through the hardware and plots the ‘learning curve’ — the decrease in unit cost of hardware with increasing production — that accounts for increasing economies in the various technologies the mission will require. The cost savings ratio (CSR) is what brings the mission within reach.
Moreover, this kind of precursor mission creates a lasting infrastructure:
The operating cost, i.e., the electrical cost to launch out of the Solar System is 17 M$. This is surely far less than the capital cost of building the sail itself, so once built, the beaming facility can send many probes into the interstellar medium. With a launch cost less than the cost of the 1 km sail, the strategy will be to use the system to launch sequences of sails in many directions to sample the Interstellar Medium and flyby Kuiper Belt and Oort Cloud objects, such as Sedna. As the facility grows, the sails will be driven faster and can carry larger payloads.
Several other mission concepts are examined in this paper, and I’ll deal with them briefly tomorrow, along with some of Jim Benford’s observations on cost-optimized scaling. Ultimately, we’re interested in a development path that will support what he calls ‘directed energy propulsion,’ and his paper is a clarification of what a roadmap for sail technologies leading to beamed energy sail missions must include. Further work on cost-optimization should help us examine other interstellar concepts from Robert Forward, Greg Matloff and others to find the optimal development path.
The paper is Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy,” soon to be made available on arXiv. Also relevant is G. Benford and P. Nissenson, “Reducing solar sail escape times from Earth orbit using beamed energy,” Acta Astronautica Vol. 58, Issue 4 (February 2006), pp. 175-184. The Benford brothers’ paper cited at the top of the article above is “Power-Beaming Concepts for Future Deep Space Exploration,” in the Journal of the British Interplanetary Society Vol. 59 No. 3/4 (March/April 2006), pp. 104-107.
by Paul Gilster | Dec 9, 2011 | Sail Concepts |
There is a natural path through solar sails, which are now flying, toward beam-driven propulsion, and it’s a path Jim Benford has been exploring for the last eighteen years. In my Centauri Dreams book I described how Jim and brother Gregory ran experiments demonstrating that carbon sails could be driven by microwave beams back in the year 2000. We learned that the theory worked — a sail could indeed be propelled by a beam of photons — and moreover, we learned that the configuration of the craft and propulsion system allowed it to be stable.
Now we’re talking about beam-riding, which the Benfords were able to demonstrate in later experiments. For it turns out that the pressure of the beam will keep a concave-shaped sail in tension, and as Jim pointed out in a recent email, the beam also produces a sideways restoring force. His work showed that a beam can also carry angular momentum and communicate it to the sail, allowing controllers to stabilize the structure against yaw and drift. This is as far as our microwave-beaming experiments have taken us so far, but as solar sails become less an experimental than an operational technology, we can move to space-based experimentation.
Image: A near-term sail experiment under microwave beam. Courtesy of James Benford, Microwave Sciences.
Robert Forward’s name always comes up in such discussions. An old friend of Benford’s, Forward developed enormous interstellar mission concepts using beamed propulsion, ideas that physicists like Geoffrey Landis and Robert Frisbee were able to tweak, just as Jim did, to produce smaller systems. Jim went on to take a cost-optimized approach to the issue, understanding that even the most ingenious of starship designs will be driven by economics. His new paper discusses the matter and notes that a design project using his methods called Project Forward will be undertaken by Icarus Interstellar, the group that manages Project Icarus.
Benford’s notions are solid and based on long experience. As he wrote recently:
I feel beam-driven propulsion is more firmly grounded, more thought through and quantified than nuclear propulsion methods at present. We should put more of our effort into beam-driven sails in this era of little funding. The on-going development of solar sails will tell us how to deploy and control sails, so we will keep close links with that community. This will lead to beam-driven experiments and simulations. Let’s get on with it!
Let’s talk for a moment about the experimental work on beam-driven sails, which was enabled by the invention of carbon microtruss material that is both strong and absurdly light. The material from which a sail is made is critical given that a certain fraction of the power the beam provides the sail will be absorbed and must be radiated away. Given that acceleration is strongly temperature-limited, materials with low melt temperatures like aluminum, beryllilum and niobium are ruled out for beam-driven missions, no matter how useful they may be for standard solar sailing, which uses solar photons rather than concentrated beaming to drive the spacecraft.
Carbon mesh materials work admirably for beamed-sail experiments because carbon has no liquid phase and sublimes instead of melting, as Benford explains in his new paper. These materials allow a sail to operate at temperatures up to 3000 C, allowing them to be ‘launched’ in a vacuum chamber here on Earth without burning. The Benfords were able to push ultralight sail materials at several g’s of acceleration, with the sails reaching temperatures in the range of 1725 C from microwave absorption while remaining intact. Bear in mind that various mission concepts call for lower power densities than the scientists used here. Operating on Earth, they needed a powerful push to get the forces needed for liftoff within a gravity well.
Robert Forward’s interstellar concepts were awesome in their scale, but Benford points out that there is a path to be followed before getting to the interstellar stage. From the paper:
It’s important to realize that for large-scale space power beaming to become a reality it must be broadly attractive. This means that it must provide for a real need, make business sense, attract investment, be environmentally benign, be economically attractive and have major energy or aerospace firms support and lobby for it. Therefore, we include missions that could lead to Starwisp missions, from an infrastructure base developed for smaller-scale missions.
Starwisp was another Robert Forward concept that came out of a time when the scientist moved from laser propulsion ideas to microwaves, whose longer wavelength allowed the sail to be little more than a grid — the wavelengths involved are comparable to the human hand, as Benford told me in an interview some years back, whereas lasers operate at minute wavelengths. A microwave sail, in other words, could be far lighter than the sail required for a laser push because the microwaves are stopped by a conducting surface with gaps smaller than a wavelength. From this, Forward came up with the ultralight ‘starwisp’ design.
Imagine a wire mesh about a kilometer in diameter that weighs no more than sixteen grams. You’ll want data return from the spacecraft so Forward included microchips at each mesh intersection. The craft would be so light and insubstantial that it would be invisible to the eye, but it could be accelerated at 115 g’s using a 10 billion watt microwave beam, taking it to a cruising speed of 20 percent of the speed of light within a few days. Forward’s Starwisp paper included his usual love of gigantic objects, including a beaming lens 50,000 kilometers in diameter.
Geoffrey Landis has shown that the wrong materials would cause a Starwisp to be fried by the powerful microwave beam thus generated, which is why people like Benford are looking at entirely new sail materials as they explore closer and more practicable missions. And practicality — a realistic path forward through solar sails to beamed propulsion — is what I want to discuss on Monday, when I’ll run through the mission concepts Jim Benford has looked at from the standpoint of cost-optimization. Because if we’re going to move beamed sailing out of the realm of science fiction, we’ll need missions that are near-term and offer a clear and economical way to deep space.
The paper is Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy.” I’ll post the link when this paper becomes available online.
