The starship with black sails

Avast ye hearties; Paul Gilster is talking aerographite, for solar sails. I'd thought lightsails were supposed to be reflective. Aerographite is absorptive - it is black. But it can also be stretched thinner than known canvas for mirror-sails. What the black sail loses by inelastic momentum, it gains by... not existing (as much).

Gilster cares because Gregory Matloff and Joseph Meany are talking interstellar. Gilster has also cared about the solar-lensing region, or maybe out to wherever Planet Nine is out to these days. For the Baghestan, aerographite sailships can be used for non-Tsiolkovsky small package sailshipping, closer to home.

[SCALE 10/5: one gram for 32 hours to Mars. The sail itself is likely gram-scale; for reference, a penny is 2.5g. Suppose we are sending a dozen sail-loads at once. We might get some appreciable cargo to my NEO-station in about a week: flashdrives and seeds/pollen, I think. That timespan further depends upon synodic-period, like Hohmann and cyclers. But those are major cargo; we are talking quick postage here.]

Gram-level sailshipping demands lasers - or at least an initial push sunward. The authors suggested 0.1 AU peri' (21.5 solar radii); but now going down to 0.04 AU (8.6 Rsol). I worried a bit about aerographite... melting. To that end, Gilster relates Matloff and Meany: maximum operational temperature of 3,500 K and a payload mass that is one-tenth of the sail’s. The authors expect absorptivity of 90%; albedo of 0.1. 8.6 Rsol is "only" 1655 K; there's a chart which points to 319 g (3130 m/s²) for a speed of 0.02 C, skating Einstein. Down there, I'd worry more about the cargo melting. Or the sheer disc of the visible Sun, firing photons from almost half the sky. Or coronal eruptions.

A laser hitting 1655 K for 3130 m/s² might miss the cargo, just hitting the sail. Still: 3130 m/s² would seem to rule out fragile cargo, like Laika the Russian goodgirl... and like the sail. Matloff and Meany therefore prefer a laser at 1 AU (Lagrange? Luna?); a gentler push can get the sail up to 0.033 C. The laser would be "1.8 km" (long?).

If there's a destination; this would, its own self, also want a light-source - to decelerate the sail. Maybe the Q-drive, where ions; or aerobraking. Rubblepiles might offer a cushion - lithobraking. Alpha Centauri of course has three lightsources. For local destinations, they should be supplied with their own honkin' lasers. Anyone up for pursuit-curve maths?