LJK said:

>My only question is what kind of engineers do you think an ETI
>would use? I am wondering if they would need deceleration
>engines if, say, they used laser sails or some other method of
>interstellar travel.
>
>And are you assuming an actual, biological crew? I am becoming
>more and more swayed to the idea that any starships roaming
>the galaxy will only have AI for a "crew".

Laser sails don't work for interstellar travel. Robert Forward's laser sail designs are essentially rubbish. Key concepts: Light sails are based upon photon momentum transfer. Photon momentum scales with photon wavelength. As the vehicle approachs the speed of light, the laser light from the projecting laser red shifts and the photon momentum goes to zero.

The best starship design was from the Daedalus Project by the British Interplanetary Society. Unfortunately the Daedalus design is flawed. It is based upon inertial fusion using a magnetic mirror for the nozzle. This is a very clever idea. However the design also assumed use of a laser for pellet compression and a pure He3 combustion cycle. Both concepts are no-starters. He3 is too hard to ignite (a deuterium combustion cycle is better despite the neutron radiation penalty) and neutral ion beams are a better way to compress the fuel pellets.

Classical biological interstellar travel is impossible. The cosmic radiaition dose and multiple century travel times prohibits it. However the transfer of biological (genetic) information is possible. A starship would use an artificial intelligence with biological information stored in its memory.

To get as close to the speed-of-light as possible requires that the starship be almost all deuterium fuel. The vehicle would be multistaged with at least three stages. The first two stages would accellerate the vehicle to its operating point. The last stage would decellerate the vehicle from near light speed to where it could orbit a ringed gas giant in the target star system. Upon arrival, the vehicle would be extremely battered by century long collisions with interstellar grains and be extremely radioactive due to cosmic ray and neutron activation. Dumping the deceleration stage would be the first objective of the autopilot (a machine intelligence) in order to reduce its radiation dose. After dumping the deceleration stage, the vehicle would limp around the star system using electric propulsion, e.g. plasma thrusters powered by nuclear energy. The starship would then construct a self replicating von Neumann machine to build new parts for the starship and exploration hardware. The starship would then carry out its mission in the star system, refit itself and then leave to explore a new star system. The battered deceleration stage and maybe some working von Neumann machines would be left behind. Whether ET would tidy up after itself is an impossible call to make. My guess is it wouldn't bother assuming that Mother Nature would tidy up for it (we did not tidy up after Apollo). Also there would be the possibilty of the starship malfunctioning thus requiring return to the star system for repair.

Gary