Terrible writing advice discusses weapons in sci-fi

[pendell]

As seen here . It's a short ten minute youtube segment which covers many of the subjects we've talked about here, including lasers (and why they're a bad idea), the cube-square law, need for realistic tactics, and more.

Respectfully,

Brian P.

[wumpus]

In a thread discussing space battles, I computed that for any technology that used heat engines for power and radiators for heat. You need at least one square meter of radiator for each megawatt of power produced/consumed. And that is for "unobtanium tech" (Carnot engines running at 4000K, 100% efficient power conversion, 100% efficient thermal gradient for the heatsink...).

Spoiler: way too much detail cut and pasted

Show

The theoretical limits are pretty easy to calculate. The maximum possible efficiency of your power generator is limited by the Carnot cycle limit (and this actually limits real power plants, and even some research car engines are approaching it).

Efficiency<1-(Tc/Th) where Th is the heat of your steam (or whatever) going into a turbine (presumably a combined cycle). Assuming using Tungsten-Halfnium-unobtanium alloys throughout, this could be somewhere between 3000-4000K.

Tc= the temperature of your heatsink (in Kelvin). Note that for ISS, this is less than 200K (you could presumably generate power for the ISS at 93% efficiency).

Blackbody radation increases at the fourth power of temperature. So if you want to produce massive increases of power, you could presumably have the same size radiators and replace the ammonia with molten iron of something and have an efficiency of 10% and have the radiators at 3600K and the steam at 4000K (ok, this assumes your generators and lasers are 100% efficient, but still). This gives you a 324 fold increase in the amount of power you can produce with the same sized radiators. I optimized this a little more and came to the conclusion that Th/Tc should equal 3/2 and that for a Th of 4000 you would get 500 times the power over ISS using the same sized radiators (with an efficiency around 33%, but radiating a bit more than a third as much). The ISS radiators are about 1/10th the size of the solar panels, and not visible from any view directly facing the solar panels (what most pictures show), I'll use this for a "heat sink area unit" for theoretical spacecraft.

REALITY CHECK: This assumes theoretically ideal turbines crafted out of unobtanium (to withstand 4000C), impossibly perfect lasers, and equally impossibly perfect generators (it doesn't spare any heating back from the lasers, and I doubt that such an equation will fit on the envelope used for the above). And you still have to deal with radiators only a couple orders of magnitude smaller (per Watt) better than a space station lifted into orbit in the 20th century. So for an *absolutely perfect* system (optimzed to reduce the heatsink), you get ~60MW per ISS-sized-heatsink. These are fundamental physical limitations and the only ways around them are open cooling cycles, matter that remains solid at 4000K, or perpetual motion machine. No amount of other tech will change this. There's a reason I want to fire on the heat sinks.

Note this limitation is unlikely to be an issue outside of deliberate attack. It places no bounds on the radiators aside from shear size. You could presumably use gold leaf for the majority of you heatsink (not gold, you need something that won't melt at 2600K). It is only when you need something that can endure deliberate attack does the issue of raw surface area become an issue. Note that I suspect that with tech in barely doable (in parts, not all at once) 21st century tech we could bump this up to 30MW per ISS heatsink, but after that the asymptote gets *steep*. I'm guessing that any "missile based" laser will fire only briefly and use some sort of open cooling scheme. Open loop cooling works great if you need high-Isp "flanking speed', but also has the same tyranny as the rocket equation (bring *lots* of mass). Liquid hydrogen gives you great Isp, but little cooling. I suspect things like tungsten, halfnium, and depleted uranium (or just spent fuel rods) would be good for open loop cooling.

The point was that in a space battle, you can only angle your heat sinks orthogonal to two widely spaced attackers. A third will be able to bear on your heatsinks. Blasting the heatsink with your own (unattacked) laser would essentially shut down any attacker. And of course the heatsinks are obvious targets (1MW over 1m**2 should be a blinding light). Whether this would make lasers in space utterly impossible or rely on nearly all of your fleet hiding until it can bear on a heat sink and then give away its position.

Oddly enough, space armor is a real thing. Since micrometeroites hit spacecraft so fast they are a real danger and plenty of thought has gone into protecting against them. For those wondering if they discovered that the 2mm hole in the ISS wasn't caused by a micrometeor thanks to lack of an exit hole, there typically are no exit holes. During impact (with anything) a micrometero is vaporised (both the meteor and whatever it went through) and the rushing vapor quickly disperses. Expect "spacearmor" (for spaceships, if not Buck Rogers) to look more like a medieval gambeson, possibly with plenty of thin films arrayed in such a way that the resulting attacks could be deflected away from the hull of the ship (how this works with missiles that might detonate when encountering such is an exersize for the reader).

[keybounce]

I read a story, I think it was by Heinlein, where a space battle was described as letting one ship in front shoot a laser at the enemy while absorbing heat; when the ship got too hot, the second would move in front, absorb the laser, and let the first cool off. The winner was the one that did the dance better than the other.

Then, there was also the heat radiation system used in "A Mote in God's Eye" / "The Gripping Hand".

[pendell]

I read a story, I think it was by Heinlein, where a space battle was described as letting one ship in front shoot a laser at the enemy while absorbing heat; when the ship got too hot, the second would move in front, absorb the laser, and let the first cool off. The winner was the one that did the dance better than the other.

Then, there was also the heat radiation system used in "A Mote in God's Eye" / "The Gripping Hand".

I've read a lot of Heinlein but I don't recall that one. The space battles of his I read used nuclear missiles rather than lasers. Are you sure it wasn't Niven?

Respectfully,

Brian P.

[wumpus]

I've read a lot of Heinlein but I don't recall that one. The space battles of his I read used nuclear missiles rather than lasers. Are you sure it wasn't Niven?

Respectfully,

Brian P.

Considering the detail in "Have Spacesuit, Will Travel", I'd expect that Heinlein would know that doing such would only reduce your firepower in half when what you really needed to do is double the size of your heatsinks (see my analysis above before seriously worrying about spaceship laser battles).

I could see it being done in some sort of "line [now plane] of battle" large space war where each side would concentrate fire on a few ships to force them behind other ships and ceasing to fire, but on a whole you want your ships nearly always bearing on the enemy, and radiators are cheap (if easy targets to ships in at least 3 independent directions).

Taken to extremes, this really sounds like deliberately letting the enemy "cross your T", so I tend to doubt it came from Heinlein (who was after all a naval officer and only took up sci-fi after being invalided out of the Navy).

[poppy]

I watched the video, it's really amazing