400 tons of mini carbon nanotube solar sails can be used to accelerate a 200 ton starship to 4.5% of lightspeed in 156 days

Adam Crowl and Crowlspace details “Sail-Beam” or “Macron Beam” propulsion of humans in spaceships to about 4.5% of lightspeed.

Quarter-wave sails made of Carbon Nano-Tubes (CNTs) can achieve high speeds by slingshotting near the sun and then pushed by the solar energy of the Sun. Dropping to 0.019 AU, the final velocity is 5.6% of light – dropping to 0.00465 AU (skimming the photosphere) would allow a speed of over 0.11c (11% of lightspeed), but the material might not be up to the beating. Crewed vehicles would not endure the extreme acceleration – 84,000 gee at peak – so the speeds that might be achieved by solar-sailing star-travelers would be limited to 1,000 year flights to Alpha Centauri, with just 17 gee peak acceleration.

Higher accelerations could be endured if the travelers were suspended in a liquid solution.

If CNT quarter-wave sails prove as agile as Christensen, Zubrin & Spieth have described, able to accelerate at 18 m/s2 at Earth’s orbit, thus having a thrust/mass ratio of ~3,000, then they could form the basis of a naturally energised “Sail-Beam” or “Macron Beam”. The most energy efficient ratio of macron-particle to space-vehicle velocity is 2:1, which allows a macron beam total mass of 1/2 the space-vehicle to be used. If the peak speed is limited to 0.056 c, then the most efficient starship speed is 0.028 c (2.8% of lightspeed). But we can go faster if we have plenty of sails, approaching the macron beam speed asymptotically. In theory a Macron Beam of mini-sails could push an Icarus Probe, with a payload of 150 tonnes, to the preferred mission speed of ~0.045 c (4.5 % of lightspeed).

If we assume a perfect reflection (e = 1), then double the mass of the vehicle in Macron Beam mini-sails is needed to get to 0.045 c, with a Macron Beam speed of 0.056 c. As the mini-sail approaches the ship it’s zapped by a laser tuned to a frequency at which it absorbs strongly, quickly blasting it into plasma. Alternatively it is heated by smashing into the vehicle’s magnetic field at a high relative speed. Then the plasma is reflected from a magnetic mirror arrangement on the starship. Some will stream forward at the centre of the magnetic mirror, reducing the reflection efficiency slightly.

If the starship accelerates at 1 m/s2 then it’ll need a final mass-flow of ~0.03 kg/s to push a 200 tonne starship. This doesn’t seem onerous. To reach 0.045 c (4.5% of lightspeed) will need 13,500,000 seconds – just over 156 days. The real trick is keeping the sails on course over ~600 AU when the acceleration finishes.

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