Day: March 12, 2012

The Centauri project: Manned interstellar travel

A plan from 1990 for an antimatter powered starship of gargantuan size. The starship is a “terraformed” asteroid, wrapped in a multi-layer artificial “sky”/Whipple Shield. With just ~100 people to start with, the thing must mass multi-million tonnes, all just to be nice and homey for the crew. Are such colossii really needed to carry humans to the stars? Other designs have appeared from time to time and appeal more to the claustrophile than the claustrophobe. Dana Andrews [1] estimates a toroidal starship 200 metres round, but only a few metres wide in its rim, might house ~20 people for a mass of ~200 tonnes. His vehicle uses a magnetic field and an electron stripper – a thin sheet of durable material perpendicular to the direction of travel – to protect againt the ISM and cosmic-rays. Space dust and neutral particles pass through the electron stripper, which ionizes them and thus allows them to be deflected by the magnetic field. The toroidal habitat is large enough for a mini-menagerie and dwarf trees, and provides a nice running track through the middle, though I do wonder how 20 people would manage in such a confined space for ~20 years of starflight. Yet if the first star-farers aren’t “Flatlanders” (to borrow Larry Niven’s phrase for Earthlings) but “Belters” who are used to space’s yawning void, then such a space might seem very homey indeed.

Ref:
[1] Andrews, Dana G., “Things to do While Coasting Through Interstellar Space”, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida. AIAA 2004-3706.

Feasibility of interstellar travel

Spencer & Jaffe, in 1962/3, produced this JPL study which examined the feasibility of interstellar probes, finding there was no physical reason why fusion or fission propelled multi-stage vehicles could not approach the speed of light. With five stages and a mass-ratio of ~240,000, a D-He3 fusion propelled vehicle could reach ~0.8 c. However the initial mass would of necessity be very large and the engine power levels required would be unlike anything in previous experience. However the physics was clear, interstellar travel was feasible. But how fast would a probe accelerate to burn-out speed? Heating limitations meant the acceleration that could be achieved would be low – a nominal 5 light-year mission would require five stages and 55 years. Fusion plasma containment vessels and high-temperature radiators could only handle so much waste heat from the reactions – about 20% would be lost as chiefly x-rays via bremmstrahlung.

Dwain Spencer explored the characteristics of fusion-propelled probes in a more focussed piece:
Fusion Propulsion System Requirements for An Interstellar Probe
His work reinforced the original finding that fusion propelled probes would be hampered by heat-rejection issues. In one sense nothing has changed since, as magnetic confinement fusion reactors inherently absorb a large fraction of waste heat from the reactions they contain. Newer materials might reduce the intercepted x-rays, instead returning them to the fusion plasma, but waste heat needs to be efficiently handled for fusion rockets to achieve reasonable voyage times.

Freeman Dyson, based on his work on “Project Orion”, led him to sketch a high performance interstellar version of “Orion” with a wholly different relationship to reaction heat. Instead of absorbing and re-radiating the heat, blow it away with the propellant in a massive thrust, with a very short heating period. Interstellar “Orion” used pure deuterium fusion devices with very high burn-up fractions (“burn-up” is the fraction of fusion fuel that actually fuses) which allowed high-performance. Dyson argued, for reasons of energy efficiency, that the mission velocity be kept to a low multiple of the effective exhaust velocity – his fusion devices had an exhaust velocity of 15,000 km/s (0.05c) and his “Orion” had a mass-ratio of just 4, meaning a total delta-vee of ~20,000 km/s. This meant trips to Alpha Centauri lasting ~130 years. A slightly higher mass-ratio would drop that to just 100 years – “Project Icarus” time-frames.

Helium-3 Mining Aerostats in the Atmospheres of the Outer Planets

Daedalus assumed He3 sourced from Jupiter, but the other three gas giants would be better due to Jupiter’s massive gravity-well. Uranus seems the best choice – as I jokingly put it, He3 from the Gas Mines of Uranus – for accessing this very attractive fusion fuel. However the energy requirements reported in this NASA paper are quite at odds with the figures derived by Bob Parkinson for Daedalus, a discrepancy I can’t yet explain. There’s more helium and helium-3 than what Parkinson assumed, so the task is easier, but the energy levels are an order of magnitude higher. This means extracting sufficient He3 is a mammoth task and a significant fraction of any interstellar effort.

For that reason I advocate developing deuterium propulsion. Pure deuterium fusion puts out about ~1/3 its energy as neutrons (versus 4/5 for D-T fusion) making it challenging to operate at high power-levels. However the prospect of ultra-dense deuterium, 1 million times denser than liquid deuterium, makes this potentially a very good choice indeed. Quite possibly ultra-dense deuterium forms naturally inside super-Jovian planets, which makes for the intriguing possibility of planetary collisions causing deuterium fusion explosions, with all sorts of exotic isotopic anomalies arising… much like what’s seen in our Solar system.

Fusion energy for space missions in the 21st Century

Norman Schulze’s massive study covers the major options and their state of development as of 1991. 20 years on and his good advice has been largely ignored, not for technical reasons, but because the vision of a long-term interplanetary future seems unpalatable for the powers-that-be. Why?

Massive amount of information on all the fusion options, as of 1991, including the Field Reversed Configuration, which is a favourite approach of Rostoker and co. Schulze computes the performance of different fusion rocket concepts, though the performance figures are surprisingly low. An Isp of 270,000 seconds for a probe to Alpha Centauri, which means multi-century mission times. Dyson’s “Orion” and “Daedalus” got 1,000,000-1,5000,000 seconds, though the designs aren’t as elaborated as Schulze’s.