Turning a source of heat – such as concentrated sunlight – into useful power (say, electrical power) is not an easy proposition. There’s a dizzying array of options – thermal engines using different thermodynamic cycles, photovoltaic arrays, thermoelectrics and thermionic conversion. The last was used extensively in early space power generators using small reactors or radioisotope heat sources, but left behind by thermoelectrics and Stirling cycle free-piston systems in more recent work. Now a new approach to “thermionic” conversion, focussing on electrons (thus thermoelectronic), has shown promising behaviour in experiments and out-standing performance in theory.

Highly-Efficient Thermoelectronic Conversion of Solar Energy and Heat into Electric Power

Thermionics previously had efficiency limitations due to “space current” – build-ups of electrons mutually repelling each other and choking the flow of current – so the new system uses external electric or magnetic fields to get the electrons going in the right direction. The system promises a high fraction of the Carnot Limit can be converted directly into electrical power. The Carnot Limit is a measure of how much useful work can be extracted from a thermal cycle – if the heat source temperature is T_in and the heat-sink temperature is T_out, then the Carnot Limit is:

CL = (T_in-T_out)/T_in

…say the source is 2000 K and the sink is 500 K, then the Carnot Limit is (1500/2000) = 0.75. In practice realistic thermal engines achieve a fraction of the Limit and thermionics & thermoelectrics achieve a low fraction. Efficiencies of 5-10% are typical. The new thermoelectronic approach promises efficiencies in the high 40-50% range, achieving the latter by acting as a “topping cycle” to a lower temperature steam system. For example a coal furnace burns at ~1500 C (1773 K), but a steam turbine runs at 700 C (973 K) and outputs at 200 C (473 K). Thus there’s significant loss due to the mismatch between furnace and steam power-cycle. A thermoelectronic converter covering the 1773-973 K range will add significantly to the overall power extracted by the power-plant pushing its efficiency above 50%. In this case a 45% efficient coal plant can be pushed to 54%, thus increasing the power output for no additional fuel costs and NO MOVING PARTS.

Switching to solar-power applications, imagine a thermoelectronic converter at the centre of a concentrator system which focuses sunlight to 500 times its normal intensity (temp ~1900 K.) By using a Photon Enhanced Thermionic Emission (a cousin of the Photoelectric effect) the system can convert raw sunlight to electrical power at over 40% efficiency. While maintaining a hard vacuum around the emitter-collector system is difficult here on Earth (but easy enough given the right engineering) imagine such a system in space! Hard vacuum everywhere! Even the densest squall of the Solar-Wind is a harder vacuum than a Thermoelectronic system needs here on Earth. Concentrators have to remain pointed at the Sun, but this isn’t excessively onerous engineering either.

One problem is the trick of efficiently losing excess heat to maintain the temperature differential that drives the system, but even this isn’t intractable engineering in space. Given the right “heat-pipes” the whole system can be built without moving parts, eliminating the main failure-point for mechanical thermal-cycle converter systems that have been proposed in the past.

Father & Son team, William and Arthur Edelstein discuss one of the dangers of near lightspeed travel in their paper published just last month: Speed kills: Highly relativistic spaceflight would be fatal for passengers and instruments [citation: Edelstein, W. and Edelstein, A. (2012) Speed kills: Highly relativistic spaceflight would be fatal for passengers and instruments. Natural Science, 4, 749-754.doi: 10.4236/ns.2012.410099.] They highlight the lethality of the high-energy proton head-wind that the Interstellar Medium (ISM) becomes when moving at near light-speed, which they define as above about ~0.9c.

I hadn’t realised the Edelsteins finally published their work until a Facebook friend, Jay Real, sent me a link. Of course these issues have been discussed in the literature for years so their discussion is nothing new – but welcome nonetheless as an explicit statement of the problem. High relativistic speeds are difficult to achieve, so most vehicles would probably stay below ~0.9c unless something exotic appeared, like an easy way of making one of Sonny’s warp-drive fields for rapid sub-light travel. In our part of the Galaxy the proton flux is much lower than the 1.8 protons/cc assumed by the Edelsteins. Some hot bubbles in the Local ISM go down to ~0.01-0.05 protons/cc and the local clouds are ~0.1-0.2/cc. This doesn’t change the results very much, but does lessen the local applicability.

Their analysis focuses chiefly on mass-shielding – big enough chunks of material to absorb the incoming flux. Magnetic shielding is mentioned dismissively, but I think that’s premature. Workable designs using known materials exist which can deflect 10 GeV cosmic rays, the equivalent of flying at 0.995c. Advanced superconductors, which will be needed for antimatter containment, plasma nozzles, magnetic-sails, will allow even higher protection levels. Thus I submit the Edelsteins’ negativity is premature.

The energy flux of interstellar matter hitting the ship can cause a lot of heating. If the ISM is just 100,000 atoms per cubic meter the flux is equivalent to 536 K temperature at 0.866 c. Peak temperature during re-entry is 2700 K for a moonflight – that level is reached at about 0.997c. Of course a starship wouldn’t just absorb that heat on its forward surfaces. A magnetic deflector would channel most of it away- but deflecting particles makes them lose momentum as high energy photons (x-rays) which would need to be shielded against. And the shield would get HOT! Fast starships would need to be long and narrow to minimise the energy absorbed. An x-ray reflective diamond coating could be used, but will need to be keep highly reflective while operating. Maintenance will be tricky!

As an example of the kinds of particle energies we can handle the Large Hadron Collider regularly bends a high energy stream of particles into a circle – the protons in the beam have a speed of 0.99999999c when it’s at full power. Cosmic-rays can reach much higher energies and need protection against. However the very highest energy cosmic rays are very rare, so only lower energy particles need deflecting in a crew habitat. The ones of biological concern, due to their numbers, are in the 1-10 GeV range. If we can deflect 10 GeV protons coming at us from our motion through space, then cosmic rays aren’t an issue.

Aberration comes into play at such high-speeds – the direction of origin of incoming particles and photons starts piling up directly in front of the starship. I would suggest the best protection at very high speed might be a “diffuser” – a high intensity magnet held far forward of the starship’s main hull which deflects the charged particles and creates a “shadow cone” behind it. The faster we go, for the same magnetic intensity, the further forward we put the diffuser. We fly, in safety, in its shadow thanks to aberration concentrating all the radiation to directly in front of us.

If we can deflect particles up to LHC energies, then how far can we accelerate at 1 gee? The acceleration distance required to increment the time-distortion/gamma factor (call it the TDF) by 1 is about 1 light year at 1 gee. At 0.99 c the TDF is about 7. So it takes about 6 light-years (because we start with TDF = 1) to get to 0.99c. To reach 0.9999c (TDF = 70) takes about 69 light years. Thanks to the time distortion, on ship the trip-time is much less. Remember a light-year is a distance, but as we’re flying so close to light-speed the ship is seen to take about 70 years to travel 69 light-years. A speed of 0.999999c (TDF = 700) takes 700 years Earth-time and 699 light-years of distance, but on the ship only just over 7 years have passed. If we decide to stop, then another 7 years ship time, 700 Earth-time, and 699 light years is needed – meaning we’ve flown 1398 light years in 14 years ship-time. But let’s push on. We’re pushing to TDF = 7,000 (0.99999999c) so the distance is 6,999 light-years, 7,000 years Earth-time, about 9.5 years onboard ship. Thus we could travel 13,998 light years and stop, in 19 years of our time, if can protect against proton energies equal to the LHC.

In a 2005 paper Craig Williams and crew, from the NASA Glenn Research Center in Cleveland, Ohio, improved on their 1998 fusion propelled Outer Planets vehicle – and dubbed it the “Discovery II”, inspired by the fictional “Discovery” from “2001: A Space Odyssey”. The improved version massed 1,690 tonnes fully loaded with propellant, some 861 tonnes of slush hydrogen propelled to several hundred kilometres per second by fusing 11 tonnes of D-He3. Full throttle and the “Discovery II” promised a trip-time of 118 days to Jupiter and 212 days to Saturn, which is faster than the fictional version.

Source SPACE.com: All about our solar system, outer space and exploration

SpaceX is proving that spaceflight can be economical, by eschewing corporate bloat and reducing cost with in-house component manufacture. Plus the company has a mission – affordable access to Mars!

If we’re sufficiently patient, M31 is coming towards the Milky Way and should arrive in about 3 billion years or so. Intergalactic Travel is easy, given aeons.

However, if we’re talking mere megayears, then the trip to M31 and beyond requires boosting the transit speed. If we can accelerate at a continuous acceleration – undergoing so-called “hyperbolic motion” – then the ship-board time can be reduced to arbitrarily low values. With the proviso we can supply sufficient energy and protect ourselves from the high-energy photon/particle bath that cosmic-rays and the Cosmic Microwave Background both become. Aberration – the distortion apparent direction of objects moving towards the observer – means the incoming radiation becomes ever more restriction to dead-ahead, making mitigation somewhat easier.

Slower trips, at constant fractions of the speed of light, require the passengers/payload to remain in some kind of stasis, else the billennia will inexorably erode their viability. Alternatively a World-Ship is sent, sufficiently well provisioned to last several million years. Back in 1987 Burruss & Colwell proposed such a concept, with a vast 1,000 km wide World-Ship, 50 billion passengers, and a cruise speed of 0.4c. The antimatter fuel required would be the equivalent of several days worth of the Sun’s total luminosity, so it would require at least a Kardashev Type II Civilization dedicated to the task to achieve it.

A World-Ship or a whole World? What if we sent an Earth-mass planet, using tricky orbital maneuvering around the 4.2 million solar-mass black-hole in the Milky Way’s Core as our accelerator? A Type III Civilization, with control over the Galaxy’s resources, would surely be able to arrange such a minor rearrangement of masses in the Core, flinging the Intergalactic Planet-Ship outwards at 0.5c. But what would it require to stop in the target Galaxy?

Given the right materials a magnetic-sail might do the job. We can slow an Earth-Ship from 0.5c to 0.005c in about 550,000 years (11% of the trip-time) over a braking distance of about 36,000 light-years. The sail would be 13.4 AU in radius with a super-current of 68 giga-amps and a mass of about 15.4 quadrillion tonnes (if its density is about that of carbon nanotubes.) Thus immensely BIG and probably immensely strong. At the “wire” (1.5 metres in radius) the field strength is 9,240 tesla, which is about 100 times higher than the highest critical magnetic field strength of known super-conductors. Thus not material we presently possess.

Now we have somewhere to go…

Image courtesy of Steve Bowers, for the Orion’s Arm shared Universe.

Now that we have somewhere to go around Alpha Centauri, with good odds of more clement planets too, then the question of getting there faster becomes more pertinent. In Part 1 I discussed the Mag-Sail equipped Laser-Sail, based on the advanced mission parameters discussed in this paper by Zubrin & Andrews: Use of magnetic sails for advanced exploration missions, from NASA, Lewis Research Center, Vision-21: Space Travel for the Next Millennium; p 202-210.

A limitation not covered by Zubrin & Andrews directly is the Critical Magnetic-Field strength of the superconductor used – using their specific characteristics (density 5,000 kg/m³, current 1.36 MA, mass 950 tonnes, 3,100 km diameter) the magnetic field at the wire is over 100 tesla. Modern High-Temperature Superconductor (HTS) wires struggle to reach 20 T critical field strength. However they did specify a very high critical current of 10¹¹ A/m², which suggests a high critical field strength.

Zubrin & Andrews discussed two options – deceleration via mag-sail to 0.01c (3,000 km/s) and terminal braking via a fusion rocket, or pure mag-sail braking to 0.00167c (500 km/s) which is sufficiently low to allow pure mag-sail braking in the destination star’s stellar-wind and thus orbital capture. The fusion-rocket option is significantly heavier by 438 tonnes, so let’s look at the pure mag-sail case first. So how well does the pure mag-sail braking do? With a 0.5c cruise speed the trip to Alpha Centauri takes 25.9 years. However the magnetic-braking takes 79% of the total trip-time! Dropping to just 0.25c increases the trip-time to 33.2 years, but reduces the total energy expenditure to just 25% of the 0.5c cruise speed.

With the additional fusion rocket, mag-braking to 0.01c and 0.5c cruise speed, the trip-time drops to about 20 years. This might make the fusion rocket worth-while, assuming we can build a fusion rocket light enough that is!