GURPS Spaceships Power Plants

[Diomedes]

Was playing around with GURPS Spaceships, and decided to work out the power levels of various ship sizes, based on weapon use.

a SM+5 Major Weapons Battery uses one power point to power one 10MJ beam weapon. It can fire this weapon once per 20 seconds, and I'm assuming that the designers are using capacitors to store energy from the power plant so they can use the entire amount of the power point over that period.

10 MJ/20sec gives you 500kW – but only if the weapon is (somehow) 100% efficient. In reality, there's going to be heat loss, so the power required is that 500kW divided by the efficiency.

Scaling to the various size classes gives:

Size Class     40% efficiency    33% efficiency    20% Efficiency 10% efficiency
SM+5              1.25MW             1.5MW             2.5MW            5MW
SM+6              3.75MW               5MW             7.5MW           15MW
SM+7              12.5MW              15MW              25MW           50MW
SM+8              37.5MW              50MW              75MW          150MW
SM+9              125MW              150MW             250MW          500MW
SM+10             375MW              500MW             750MW          1.5GW
SM+11             1.25GW             1.5GW             2.5GW            5GW
SM+12             3.75GW               5GW             7.5GW           15GW
SM+13             12.5GW              15GW              25GW           50GW
SM+14             37.5GW              50GW              75GW          150GW
SM+15             125GW              150GW             125GW          500GW

I ran the numbers for several efficiencies to find one that "worked" best, but using more than one column in a single game would result in mismatches.
An Improved beam weapon is simply one that is twice as energy-efficient, allowing two shots of a given power for each 20 second increment.

Under the 33% rules, a SM+7 Fusion Reactor would be rated for 100MW, a massive SM+14 Antimatter Reactor would be 600GW, and (extrapolating to the low end) a 10-pound Fuel Cell for a SM0 sized would give 5kW.

[Crakkerjakk]

Nice. I seem to recall reading somewhere that a beam would produce twice as much waste heat as it fired out the barrel. I have no idea if thats accurate or not, but if it is, the 33% efficiency would be the one to go with.

[jacobmuller]

They don't fire once per 20 seconds, you just assess their hit/s once per game turn which happens to be 20 seconds. Actual ROF is something else.
So, if ROF is really 1/s...
+5 = 30mw
+6 = 100mw
etc

[Crakkerjakk]

I don't think thats right. Given that you get a bonus based on RoF, and there are set values for that RoF(pg 58) for different time scales, I believe that firing once per 20 seconds for a regular beam is correct.

[Kale]

Yeah, wasn't there a '20 minute' rule from Traveller or something? The RoF in Spaceships (sort of) depends on the turn scale you choose for your specific space combat.

[Crakkerjakk]

Yes, the table on pg 58 covers what the RoF of different weapons are at different time scales. It says 1 for a 20 second time scale, and 3 for a the minute time scale for ordinary beams. Ergo, a beam fires about once every 20 seconds.

[Diomedes]

Originally Posted by Crakkerjakk

Nice. I seem to recall reading somewhere that a beam would produce twice as much waste heat as it fired out the barrel. I have no idea if thats accurate or not, but if it is, the 33% efficiency would be the one to go with.

It seems to depend what kind of laser you use. From Atomic Rocket:

Note that laser cannon are notoriously inefficient. Free-electron lasers have a theoretical maximum efficiency of 65%, while others are lucky to get a third of that. This means if your beam power is 5,000 megawatts (five gigawatts), and your cannon has an efficiency of 20%, the cannon is producing 25,000 megawatts, of which 5,000 is laser beam and 20,000 is waste heat! Ken Burnside describes weapon lasers as blast furnaces that produce coherent light as a byproduct. Rick Robinson describes them as an observatory telescope with a jet engine at the eyepiece. Laser cannons are going to need seriously huge heat radiators. And don't forget that heat radiators really cannot be armored.

The messy alternative is to use open-cycle cooling, where the lasing gas is vented to dispose of the waste heat. Not only does this endanger anything in the path of the exhaust, it limits the number of laser shots to the amount of gas carried.

But Troy Winchester Campbell brings to my attention a recent news item. In 2004, a company named Alfalight, Inc. demonstrated a 970 nm diode laser with a total power conversion efficiency of 65%. They are working in the DARPA Super High Efficiency Diode Sources program. The goal is 80% electrical-to-optical efficiency in the generation of light from stacks of semiconductor diode laser bars, and a power level of 500W/cm2 per diode bar operating continuously

So yeah, 33% is good. It also gives me an excuse to use its rounder numbers, unlike the 40% solution.

[vicky_molokh]

Does all this assume beam efficiency stays the same across TLs?

[Anthony]

Originally Posted by Molokh

Does all this assume beam efficiency stays the same across TLs?

Other than Improved beams, yes. However, that seems to be what Spaceships is assuming.

[Diomedes]

Originally Posted by Molokh

Does all this assume beam efficiency stays the same across TLs?

It pretty much has to; otherwise the PP to fire a laser wouldn't fuel the same RoF for a particle beam, for instance, which would be a major accounting headache if your ship has both.

I suppose you could say that a ship with more-efficient, higher TL equipment could be granted an extra power point every X rounds or so, but the Spaceships system is meant to be simple.