Good point on phase 6. For phase 3, smaller changes in velocity further out are fine, but I still think that even with less velocity changes, you’ll still have difficulty finding an engine that gets sufficient delta-V that isn’t fission/fusion/antimatter based. (also in the meantime I realized that neutron damage over those sorts of timescales are going to be *really* bad.) For phase 5, I don’t think a lightsail would provide enough deceleration, because you’ve got inverse-square losses. Maybe you could decelerate with a lightsail in the inner stellar system, but I think you’d just breeze right through since the radius of the “efficiently slow down” sphere is too small relative to how much you slow down, and in the outer stellar system, light pressure is too low to slow you down meaningfully.
Assuming acceleration occurs over a 40 light year distance and uniform acceleration (because why not; we have a variable power source), the ship would experience a constant acceleration of ~0.3m/s^2 (convertalot.com/relativistic_star_ship_calculator.html ).
If we wanted the same peak deceleration using only lightsail and a sun-like star, we’d get a deceleration of 83km/s (back of envelope calculation analogizing photon pressure as a reversed gravitational well), so we’ll need 72 stars in total.
That is quite reasonable considering the star density in the galactic core. The only problem here of course is that your lightsail might be so small that gravitation dominates, in which case you have to look for stars with higher photon-pressure-to-mass ratio, which are less densely populated. It’s a trade-off between peak acceleration, destination constraint and sail size. Our sun for example would be among the worst targets for decelerating an incoming intergalactic spaceship.
also in the meantime I realized that neutron damage over those sorts of timescales are going to be *really* bad
Is it though? Radiation in general tends to attenuate exponentially in matter, so a merely linear increase in shielding should solve the problem completely.
Btw this sequence has been a very enjoyable read; I’m glad I’m not the only speculating about Clarketech-level space travel in free time.
Good point on phase 6. For phase 3, smaller changes in velocity further out are fine, but I still think that even with less velocity changes, you’ll still have difficulty finding an engine that gets sufficient delta-V that isn’t fission/fusion/antimatter based. (also in the meantime I realized that neutron damage over those sorts of timescales are going to be *really* bad.) For phase 5, I don’t think a lightsail would provide enough deceleration, because you’ve got inverse-square losses. Maybe you could decelerate with a lightsail in the inner stellar system, but I think you’d just breeze right through since the radius of the “efficiently slow down” sphere is too small relative to how much you slow down, and in the outer stellar system, light pressure is too low to slow you down meaningfully.
Assuming acceleration occurs over a 40 light year distance and uniform acceleration (because why not; we have a variable power source), the ship would experience a constant acceleration of ~0.3m/s^2 (convertalot.com/relativistic_star_ship_calculator.html ).
If we wanted the same peak deceleration using only lightsail and a sun-like star, we’d get a deceleration of 83km/s (back of envelope calculation analogizing photon pressure as a reversed gravitational well), so we’ll need 72 stars in total.
That is quite reasonable considering the star density in the galactic core.
The only problem here of course is that your lightsail might be so small that gravitation dominates, in which case you have to look for stars with higher photon-pressure-to-mass ratio, which are less densely populated. It’s a trade-off between peak acceleration, destination constraint and sail size. Our sun for example would be among the worst targets for decelerating an incoming intergalactic spaceship.Is it though? Radiation in general tends to attenuate exponentially in matter, so a merely linear increase in shielding should solve the problem completely.
Btw this sequence has been a very enjoyable read; I’m glad I’m not the only speculating about Clarketech-level space travel in free time.