Monthly Archives: August 2012

Fastest Time to Alpha Centauri

Currently I am working on a paper & presentation for the 100 YSS Symposium in Houston, to be presented by an Icarus colleague. I am examining the effectiveness of using a magnetic-sail to brake to low-speeds in the target system, but part of that is a comparison with a pure fusion rocket. As it is still the most detailed design for an interstellar fusion rocket I am using the performance characteristics of the “Project Daedalus” star-probe. The most economical use of propellant for pure-fusion is to boost up to cruise speed using the 1st Stage, drop the spent stage, then brake using the 2nd Stage after a period of cruising. “Daedalus”, due to its ignition system and the tricky physics of implosion ignited fusion, had two different exhaust velocities for the stages – 1st Stage was 10,600 km/s and 2nd Stage was 9,210 km/s.

A limiting variable on the possible mass-ratio was the mass of the cryogenic tankage required to keep helium-3/deuterium fuel at a chilly 3 K storage temperature. For the 1st Stage the tankage was 2.85% of the fuel mass stored and 4% for the 2nd Stage. As a critical mass-ratio is approached the required mass of propellant goes asymptotic – runs off to infinity. Thus there’s a maximum cruise speed for a single stage using “Daedalus” style storage systems. It works out as 0.1c for the 2nd Stage engine. To achieve that speed requires infinite propellant mass, so it’s not really practical.

A more practical question is the fastest trip to a given destination. Rockets are limited in how quickly they can burn their fuel – Stage 1 burns it at 0.72 kg/s and Stage 2 burns it at 0.0711 kg/s. To achieve higher speeds requires burn-times that are asymptotically rising, when the critical mass-ratio is factored in.

Alpha Centauri is 4.36 light-years away. A two-stage “Daedalus” vehicle can travel there in 68 years at a maximum speed of 0.075c and then brake to a halt at the destination. However the amount of fuel required is about 300,000 tonnes. Going a bit slower – arriving in 71 years – can reduce the fuel required to just 140,000 tonnes. “Daedalus” carried an immense payload by modern standards – 450 tonnes, the equivalent of the International Space Station. The recent paper on boot-strapping a robotic economy on the Moon only required delivery of 41 tonnes to kick-start things. A large exo-solar industrial base could be sent to other star systems in a decent time frame to build, in advance of human arrival, large laser or mass-beam facilities to decelerate a human-carrying star-ship. Such would allow much faster trip-times.

The Way to K-II …Update

The preprint I mentioned is available online (thanks to Brian Wang’s “Next Big Future” post) and it’s here:
Affordable, rapid bootstrapping of space industry and solar system civilization

More discussion is coming. One preliminary idea is that 100,000 teleoperated robots on the Moon could have a volunteer Army controlling them to help build the Solar System economy. The pay-off for all involved would be long-term energy/resource security – and potential wealth.

ISV Venturestar

The only realistic Interstellar Starship from Hollywood so far

The ISV “Venturestar” is an example of “poly-propulsion”, using a Forward laser-sail to boost to 0.7c, and brake to a halt, in Sol-space, then using matter-antimatter, Powell/Pellegrino “Valkyrie” style, to brake at Alpha Centauri, then boost for the trip home.

The Way to K-II

No. Not the mountain in the Himalayas. Kardashev II Civilization status – a civilization using the energy output of its star. Earth intercepts just 2.2 billionths of the Sun’s energy and presently we use ~1/10,000th of what Earth receives. Thus the plateau of K-II seems a long way off. However we could boot-strap our way there by developing an automated space economy. And the first step isn’t huge.

Philip Metzger and Robert Mueller have both been busy developing a Map of the way to K-II via quasi-self-replicating robotics on the Moon.

Here’s Phil’s 2011 100 YSS Presentation: Nature’s Way of Making Audacious Space Projects Viable
Abstract

Building a starship within the next 100 years is an audacious goal. To be successful, we need sustained funding that may be difficult to maintain in the face of economic challenges that are poised to arise during these next 100 years. Our species’ civilization has only recently reached the classification as (approximately) Type-I on the Kardashev scale; that is, we have spread out from one small locality to become a global species mastering the energy and resources of an entire planet. In the process we discovered the profound truth that the two-dimensional surface of our world is not flat, but has positive curvature and is closed so that its area and resources are finite. It should come as no surprise to a Type I civilization when its planet’s resources dwindle; how could they not? Yet we have gone year by year, government by government, making little investment for the time when civilization becomes violent in the unwelcome contractions that must follow, when we are forced too late into the inevitable choice: to remain and diminish on an unhappy world; or to expand into the only dimension remaining perpendicularly outward from the surface into space. Then some day we may become a Type-II civilization, mastering the resources of an entire solar system. Our species cannot continue as we have on this planet for another 100 years. Doubtless it falls on us today, the very time we intended to start building a starship, to make the late choice. We wished this century to be filled with enlightenment and adventure; it could be an age of desperation and war. What a time to begin an audacious project in space! How will we maintain consistent funding for the next 100 years? Fortunately, saving a civilization, mastering a solar system, and doing other great things like building starships amount to mostly the same set of tasks. Recognizing what we must be about during the next 100 years will make it possible to do them all.

He presents a stark choice and though it’s based an arguably finite resource base, the road to freedom surely lies with not being restricted to one planet.

Metzger, Mueller and their NASA colleagues have submitted a technical paper to the “Journal of Aerospace Engineering”:

Affordable, Rapid Bootstrapping of Space Industry and Solar System Civilization

Abstract:

Advances in robotics and additive manufacturing have become game?changing for the prospects of space industry. It has become feasible to bootstrap a self-sustaining, self-expanding industry at reasonably low cost. Simple modeling was developed to identify the main parameters of successful bootstrapping. This indicates that bootstrapping can be achieved with as little as 12 metric tons (MT) landed on the Moon during a period of about 20 years. The equipment will be teleoperated and then transitioned to full autonomy so the industry can spread to the asteroid belt and beyond. The strategy begins with a sub-replicating system and evolves it toward full self-sustainability (full closure) via an in situ technology spiral. The industry grows exponentially due to the free real estate, energy, and material resources of space. The mass of industrial assets at the end of bootstrapping will be 156 MT with 60 humanoid robots, or as high as 40,000 MT with as many as 100,000 humanoid robots if faster manufacturing is supported by launching a total of 41 MT to the Moon. Within another few decades with no further investment, it can have millions of times the industrial capacity of the United States. Modeling over wide parameter ranges indicates this is reasonable, but further analysis is needed. This industry promises to revolutionize the human condition.

Robert Mueller presented on the Plan at several different meetings, his presentation slides being available here:

Robotic, Self-Sustaining Architecture to Utilize Resources and Enable Human Expansion Throughout the Solar System

I got in touch with Phil and will hopefully have more to discuss in Part II of this blog post.