Now we have somewhere to go…
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!