Re: fusion powered interstellar rocket
- From: raphfrk@xxxxxxxxxxxx
- Date: 31 Oct 2005 11:54:41 -0800
pgarrone@xxxxxxxxxxx wrote:
> Thanks for the idea and the maths.
>
> The arks in my story are one-off journeys so unless I misunderstand
> this solution severely I cannot see the utility in laying out a railway
> line ahead.
>
> Suppose there was one reusable array in the solar system to accelerate
> the payload in any direction to 0.2C at 1 G taking 1200AU of distance.
> Would light photons accelerating millions of tonnes at something like
> 1G using a 10Kmx10Km array be cheaper than a similarly sized fusion
> rocket which would mostly be fuel tanks? I think the scale of the
> solutions is similar. Given the rocket technology has to be developed
> anyway for the deceleration phase, I dont see any overiding advantage
> for this application, unless the first stage starts being many
> gigatonnes, in which case I would reconsider.
Light pressure is the power divided by the speed of light.
F = P/c
To accelerate a 100000 tonnes ( 10E8 kg ) at 10m/s^2 requires
F = ma = 1E8 * 10 = 1E9 N
P = F*c = 1E9 * 3E8 = 3E17 = 300,000 TW of power
This is pretty big. To power it with solar at 1kw per m2 requires a
square collector of edge 17,000 km. On the other hand, the reactor on
your ship giving the same performance is (assuming I haven't messed up
the formula)
P = (ve/2)m(a) = 0.0677 * c / 2 * 1E8 * 10 = 10,000 TW or power
so you are already dealing with very large power generation components.
Also, depending on the level of tech of the Sol system, you might be
able to just buy that power from the "grid" or rent the generators for
the solar sail laser.
Also, the solar sail case can send lots of small payloads instead, so
you can reuse the laser. You could break your payload into say 100
smaller parts and then the power requirements drop accordingly. Each
payload would take around 70 days to get to full speed, so the entire
lauch would take years though. If you launched the later payloads
slightly faster than the first ones they could be made to catch up with
the faster ones after a few years and then dock. Aligning velocities
might be difficult if they are unpowered. However, since this is a one
shot launch, you still need rockets to break at the destination, so
they will likely be rocket powered anyway. You could also try magnetic
breaking, where you have a big super conducting ring that slows you
down against ions in interstellar space. However, I am not sure how
effective that is. Another option is to send the first piece at say
0.25c and the rest at 0.2c. The first probe would have a laser, solar
collectors and a breaking rocket. It will then fire the laser to bring
the rest to a stop. You could also have that probe be a von neuman
probe and build the braking array from raw materials in the target
system.
>
> Every action has an equal and opposite reaction. The transmitting array
> would be pushed in the opposite direction. The light source at the
> focus of the transmitting array would be intense. Assuming any sort of
> efficiency, one would imagine that a direct photonic drive would be
> possible. This solution is moving away from the big dumb low-tech
> cost-effective first-stage rocket technology I am trying to envisage.
The laser would move, but it is probably not a big issue. You could
just put it into an psuedo-orbit so that the Sun's gravity and the
force of the beam work together to keep the array in a stable orbit.
Worst case, you just fire two beams in opposite directions. The array
would be much more massive than the ship.
.
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