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)

[Diomedes]

Back in my very first post, I ran a similar calculation, and ultimately got the same 1MW/ton figure you did.

[Ulzgoroth]

I think we've been told that the fuel cell/MHD values are fudged for game purposes? (It came up in relation to the perpetual motion machine of high tech fuel cell+refinery.)

[Crakkerjakk]

Mr. Pulver has said that the beam weapon names for energy strengths aren't actually Joules, but just descriptive names to mark varying weapon strengths. So the math doesn't work unfortunately.

[Anthony]

Another way of calculating this is to work out reaction drives. The power requirement of a perfectly efficient electric thruster is 0.5 * exhaust velocity * thrust. For 1 G and 1 mps/tank, that works out to 8890N/ton * 32,180m/s * 0.5 = 143MW/ton. Ion drives give 0.0005G and 3 mps/tank, so that's 215 kW/ton. Mass driver drives give 0.01G and 0.3 mps/tank, so that's 429 kW/ton.

[David L Pulver]

Originally Posted by Crakkerjakk

Mr. Pulver has said that the beam weapon names for energy strengths aren't actually Joules, but just descriptive names to mark varying weapon strengths. So the math doesn't work unfortunately.

What I meant was that I needed a descriptive name for the weapon levels of power, and joules was as good as any. However, within that constraint they are indeed intended to represent actual beam power outputs and to be reasonably consistent with GURPS weapon technology estimates.

Ultra-Tech, in contrast, is less rigorous. By editorial mandate the book was required not to include specific power outputs, etc., though I tried to follow a broad scaling sequence when calculating weapon and power effects.

[David L Pulver]

Originally Posted by Anthony

Another way of calculating this is to work out reaction drives. The power requirement of a perfectly efficient electric thruster is 0.5 * exhaust velocity * thrust. For 1 G and 1 mps/tank, that works out to 8890N/ton * 32,180m/s * 0.5 = 143MW/ton. Ion drives give 0.0005G and 3 mps/tank, so that's 215 kW/ton. Mass driver drives give 0.01G and 0.3 mps/tank, so that's 429 kW/ton.

That seems reasonable. I do remember the electric drives were fudged to make them about 5-10x as efficient so as to have mass-driver powered manned ships that were actually worth taking. For realism reduce G thrust by about 1/5 (allowing some efficiency losses) for ion and by about 1/10 for mass driver.

In short, the intent is that the beam weapons are broadly correct, but the electric drives are unrealistic.