Power Points, Megawatts and Spaceships

[Trixbat]

I was looking at some forum estimates for "value of a power point" in GURPS Spaceships but most seem to have some dubious math (like forgetting that a system is 1/20 of the total mass of a ship, rather than equal to mass...)

Now, taking it as a given that the rules are intended to be abstract and some elements (like the lower-TL electric drives or fuel cells) are said to be fudged for playability, we can avoid back-calculating from the dries and are left with the hard numbers of the beam weapon outputs. )

Working from these values here is my estimate:

1 Power Point = about 50 kW per ton of total ship mass.

Here's how I got this value working from beam weapon numbers and one guess on weapon efficiency.

p. 26, Major Battery, says a +8 (1000 ton) ship beam weapon is 300 MJ and costs 1 Power Point. This ratio is common for all sizes of ship.

A ship can have 20 systems; each is 1/20 of the total mass. 1/20 of 1,000 tons is 50 tons. So the beam output is 300 MJ / 50 = 6 MJ per ton.

A beam weapon normally fires 1 shot every 20 seconds, but there is an option for +1 TL higher "improved" beams to double RoF without reducing output, so that is 1 shot per 10 seconds. We thus divide 6 MJ per ton by 10 to get 0.6 MW of power per ton: a power requirement of 600 kW per ton.

But even the best beam weapon is unlikely to be 100% efficient!

We know that the +1 TL improved weapon is twice the efficiency of the earlier models. We can pick any efficiency 100% down, but a look at descriptions of beam weapons suggests somewhere between 15% and 75% is likely for advanced, highly evolved spaceship weapons.

Let us assume a 60% or 0.6 efficiency at the improved beam weapon (which means the earlier beam weapon with half the rate of fire must be 0.3 efficiency). Since lasers are TL9, that means a TL10+ laser is 60% efficient, which seems reasonable for what is likely an advanced solid state or free electron design hundreds of years in the future.

If so, 600 kW / 0.6 = 1,000 kW power per ton of system mass = 1 power point of power required.

Since this requires 1 power point, it also means that a 1 power point power plant would generate 1,000 kW per ton of power plant system mass.

Thus, a TL9 fusion reactor (2 power points) produces 2,000 kW per ton of system mass, or 1 kW per pound of weight.

Since such a system is 1/20 of the total tonnage, to determine the "value' of a Power Point "per ton of ship" we divide by 20 systems. 1,000/20 = 50 kW.

1 Power Point = 50 kW per TON OF SHIP
1 Power Point = 1,000 kW PER TON OF SYSTEM (1/20 ship mass).
kW per lb. of power plant mass = power points /2.

Thus, if a 1,000 ton ship has a fusion reactor with 2 power points, the reactor is producing 1,000 * 50 kW * 2 = 100,000 kW, or 100 MW.

This also generates the following values:

TL8+ Fission: 2 lbs. per kW. (probably assumes a particle bed reactor)
TL10+ Fusion: 1 lb. per kW.
TL11+ Super Fusion: 0.5 lbs. per kW.
TL10+ Antimatter: 0.5 lb. per kW.
TL12 Total Conversion: 0.4 lbs. per kW*
TL7 Fuel Cell: 2 lbs. per kW. (Fuel is included, however, so this value must be higher, but
like the ion drives etc. this is probably fudged to give low-TL values for refineries and such rather than beam weapon powering.)
TL9 MHD: 1 lb. per kW.

Cosmic systems are said to be 1,000 * as powerful so a "cosmic power point" is 1,000x more. Total conversion is quite likely cosmic, so that would be 0.004 lbs. per kW.

I think it would be reasonable to assume a counterpart of Cosmic - the "mundane power point" - that is used with the TL7-8 power and drive systems: solar cells generate them, as do fuel cells and TL7-8 reactors (but not TL9+ ones). Mundane power points power things like realistic-speed refineries and mining, ion drives and mass drivers, etc. but you need, say, 10 of them to power a TL9+ system like a beam weapon, or a high-speed refinery/mine. Perhaps habitats and sensor arrays have a power requirement that can be measured in mundane power points and so you can explain why a TL7-8 space station has big solar arrays.

(That's with 0.6 eff assumption. Given other beam weapons in the setting, I think anything from 0.5 to 0.9 is valid, which will change the numbers slightly)