a repost from Facebook.
?”Daedalus” had a top speed of ~0.122c, though some variants could hit 0.138c for an extra 10,000 tonnes of fuel or so. This makes for a 36 year trip to Alpha Centauri – but no way of stopping. Equipping “Daedalus” with a magnetic sail and enough propellant to brake downwards from 1500 km/s, when the mag-sail performance drops significantly, lets us contemplate braking to a halt. But, as always for realistic rockets, there’s a trade off between how fast the fuel can be expelled – the mass-flow rate – and the cruise speed. Too high a cruise speed means the time spent accelerating drags out and actually reduces the average speed.
Throwing in the relevant characteristics and model parameters means that I can compute the total flight time for a range of speeds, and then search for the minimum time. I’ve assumed a 1,000 tonne mag-sail which is about equal in mass to the “Daedalus” 2nd Stage with enough propellant for the final brake phase, 1100 tonnes. The mag-sail is 800 km in radius and carries a super-current of several hundred kiloamps. The maximum magnetic field in the wire is about 16 tesla, which is high, but not as high as the critical field of some present day SCs.
What results is a minimum flight time of 45 years – not much more than the bare minimum. The cruise speed is a higher 0.1388c, while the initial mass is 181,480 tonnes. In the original “Daedalus” plan mining 50,000 tonnes of propellant from Jupiter would take 20 years. To mine the extra 130,000 tonnes needed for a faster probe could require ~60 years. However going a bit slower means a 50 year flight needing only 66,040 tonnes initial mass.