Junghalli wrote:This is only the thrusts normal to the surface if you are pointing your solar sail head on to the star. However, you cannot shed speed like this.
Why not? If you're headed straight for the star its light pressure is applying force directly opposite your direction of motion.
Because you're traveling on a hyperbolic trajectory about the star, not crashing into the star. When such a trajectory comes close to the star, the star is off to the side of your trajectory. The other component to the thrust is working to push you out; that is, helping pay off your potential energy. That's not what you want — you want the star to capture you gravitationally.
Junghalli wrote:Is this the velocity gained or lost during each phase of acceleration/decelleration?
Unaccelerated. I used them to find how long the probe would spend in each phase of the thrust, which I figured would be correct to first order.
Junghalli wrote:If it is I get a delta V of 1427.3 km/s if you start decelleration beyond 2 AU and 1338 km/s if you start decelleration at 1 AU.
The pick-up in speed is due directly to the fact that you're deeper in the gravity well.
Junghalli wrote:As you can see, the change in impulse is almost linear with each step, although each step is half the distance of the last. The combined impulse is 3.0231284225098e8 kg m/s per km² area, and imparted on 1000 tonnes (1e9 kg) this translates into .302 m/s·km². Solar sails are not really effective on the outbound transit, so we may safely ignore it. To be captured by the star, we need to shed 14 km/s over the course of the thrusting, you need a sail of about 47,000 km² in area.
Is 1000 tons neglecting the mass of the sail itself?
It's supposed to include the mass of the sail.
Junghalli wrote:According to this an advanced carbon nanotube solar sail might be made as light as 30 kg/km^2. Assuming that weight-bearing structure doubles this a 47,000 km^2 sail might have a mass of 2820 tons, without this assumption we get 1410 tons. Obviously with a solar sail it would be good to keep the payload mass as small as possible.
Then we have a problem. The trajectory we want requires a sail of greater mass than the total mass of the probe. We have to choose another trajectory, most likely one where the hyperbolic excess is less than 30 km/s. It must be noted that once you are captured by the star, assuming you can survive being put to sleep for long periods again, you can afford to wait while another chance to shed speed comes around.
Junghalli wrote:The human brain manages to run a human-level intelligence on around 20 watts and 1.4 kg of mass, and it's probably not the most efficient computing system possible by a long shot...
It's also not a very good intelligence, period. When was the last time one guy managed to plan out a mission for creating a
complete manufacturing infrastructure in the smallest detail? We're certainly not doing it, yet this is exactly the kind of careful preplanning that such a mission requires. We can get away with it because we have the help of our fellow humans and are just being carried along the wave of advancement this creates with only the
vaguest idea where we'll end up. The probe has quite a different problem.
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