TL9 Antimatter pion rockets

[Anthony]

Originally Posted by Sunrunners_Fire

Portability/energy-density.

Doesn't apply to the case of 'escaping the system' (you only need it in one place), and in general interstellar transport of antimatter isn't likely to be efficient, and there are few uses for antimatter pion drives other than interstellar travel. It has some theoretical application if you want to send a probe to an uninhabited star system and then have it make its own return trip, but for launch purposes lightsails are pretty consistently superior.

[lexington]

Originally Posted by Sunrunners_Fire

Also, people get really itchy when you start waving around (fast-than-light!) lasers powerful enough to push around lightsails in other star systems ...

But a nice safe material like antimatter doesn't bother anyone.

[Jovus]

Because it's easier and less supersciency to build an antimatter pion drive for interstellar travel than a lightsail.

[Anthony]

Originally Posted by Jovus

Because it's easier and less supersciency to build an antimatter pion drive for interstellar travel than a lightsail.

Urr...what? Lightsails are probably the only non-superscience option for interstellar travel on less than geological timescales.

[Jovus]

Originally Posted by Anthony

Urr...what? Lightsails are probably the only non-superscience option for interstellar travel on less than geological timescales.

I'll just point you here: http://physics.ucsd.edu/do-the-math/...ded-resources/

[Sunrunners_Fire]

Originally Posted by lexington

But a nice safe material like antimatter doesn't bother anyone.

Heh. I expect that it would bother people; anti-matter makes a wonderfully destructive weapon. So does nuclear material. Shipping such materials is quite regulated in the real world but isn't completely banned; I don't imagine anti-matter being any less regulated. There is also probably an expectation that you can defend yourself against an anti-matter bomb by blowing up whatever is transporting it to your location before it gets to your location; or maybe there isn't. It gets rather campaign-specific, rather quickly. :grins:

[lexington]

Originally Posted by Sunrunners_Fire

Heh. I expect that it would bother people; anti-matter makes a wonderfully destructive weapon. So does nuclear material. Shipping such materials is quite regulated in the real world but isn't completely banned; I don't imagine anti-matter being any less regulated.

Nuclear material doesn't cause enormous explosions if handled improperly. Uranium is fairly inert until you deliberately make it into a weapon, just don't eat it or dump it into the water supply. If you lose containment on a few kilograms of antimatter the explosions kills everyone on the ship and everyone on anything its docked with. Regulation would be vastly tighter outside of libertarian fantasy settings.

[Anthony]

Originally Posted by Jovus

I'll just point you here: http://physics.ucsd.edu/do-the-math/...ded-resources/

That's merely pointing out that space travel in general is awkward. Antimatter-pion is vastly more difficult than lightsails. To get a lightsail up to 0.1c requires 4.5e+15 J/kg (of spaceship mass), divided by the efficiency of your photon source; for an antimatter-pion drive, at the quoted Isp of 0.69c, requires 7e+15 J/kg, divided by the efficiency of your antimatter production. The efficiency of light sources can plausibly exceed 10% (90% for a klystron, though that forces use of long wavelengths and extremely large focusing elements), even extremely optimistic assumptions on antimatter production are on the order of 0.01%, so we're dealing with 3-4 higher orders of magnitude. In addition, the above AM-pion rocket requires storing 0.078 kg of antimatter per kg of ship mass, which is a storage efficiency we have no reason to think is possible. By comparison, the lightsail requires an ultralight sail (limit of suggested designs is about 0.1 g/m^2, or 100 kg/km^2; with 50% sail mass, a 10 km x 10 km sail is 20 tons total) and a drive laser capable of 35 kW/m^2 over the sail area -- that requires a power of 3.5e12W and, for a 1 micron laser, a diffraction-limited focal array somewhat over 500 meters in diameter. These are some pretty challenging numbers, but compared to the AM-pion they're trivial.

[Johnny1A.2]

Originally Posted by Anthony

That's merely pointing out that space travel in general is awkward. Antimatter-pion is vastly more difficult than lightsails. To get a lightsail up to 0.1c requires 4.5e+15 J/kg (of spaceship mass), divided by the efficiency of your photon source; for an antimatter-pion drive, at the quoted Isp of 0.69c, requires 7e+15 J/kg, divided by the efficiency of your antimatter production. The efficiency of light sources can plausibly exceed 10% (90% for a klystron, though that forces use of long wavelengths and extremely large focusing elements), even extremely optimistic assumptions on antimatter production are on the order of 0.01%, so we're dealing with 3-4 higher orders of magnitude. In addition, the above AM-pion rocket requires storing 0.078 kg of antimatter per kg of ship mass, which is a storage efficiency we have no reason to think is possible. By comparison, the lightsail requires an ultralight sail (limit of suggested designs is about 0.1 g/m^2, or 100 kg/km^2; with 50% sail mass, a 10 km x 10 km sail is 20 tons total) and a drive laser capable of 35 kW/m^2 over the sail area -- that requires a power of 3.5e12W and, for a 1 micron laser, a diffraction-limited focal array somewhat over 500 meters in diameter. These are some pretty challenging numbers, but compared to the AM-pion they're trivial.

Note that the energy necessary to do either has implications in a hard SFnal setting, a society able to muster such energy levels will have weapons, industry, etc, to scale.

[sir_pudding]

Originally Posted by Johnny1A.2

Note that the energy necessary to do either has implications in a hard SFnal setting, a society able to muster such energy levels will have weapons, industry, etc, to scale.

The really big laser is a weapon, as are those storage densities of antimatter.