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/m^{3}, 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 10^{11} A/m^{2}, 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!