Are you saying if you want a 1000 ton engine you want to put 400 tons of fuel on the ship (40% of 1000) in order to be efficient? Is this just to get the most range and speed per liter of fuel because more fuel slows the ship down?
I design engines for each set of parameters:
1. What size ship will this need to propel?
2. How fast will this ship need to go?
3. How long do I want that speed to be able to be maintained? i.e., how far should it be able to travel?
Then I fiddle. In the engine design window, leave it at your highest engine type (like nuclear pulse, ion, magnetoplasma, gas core antimatter) and best efficiency (50%, 30%, 90%). For now, leave the power/efficiency tradeoff bar at x1.00/x1.00. Now adjust the engine size until the EP is equal to:
EP = desiredSpeed*hullSpacesToCarry/1000 or desiredSpeed*tonsToCarry/50000
For instance, a 2000-ton ship going 5000 km/s would need an EP of 200.
Now take your desired range and speed, and determine the number of hours that you want the ship to be able to travel at maximum thrust.
Time = Distance/Speed, so
seconds = km/(km/s)
hours = km/(km/s * 3600)
Multiply the hours of travel by the fuel requirement per hour (
not the fuel requirement per EP-hour, that's different) to get the liters of fuel required. Now divide that by 1000 to get the tons of fuel storage needed to hold that fuel.
Note the engine size in tons (click the "show sizes in tons" checkbox at the bottom of the engine design window if it's something like 20 HS instead of 1000 tons) and multiply that size by 0.40 to get the tons of fuel storage that should be paired with the engine in the engine-fuel composite.
If your fuel storage weight is greater than this number, move the power-efficiency tradeoff bar to a
more efficient, less powerful setting. For instance, x0.80/x0.57. If the fuel storage is less than this number, move the power-efficiency bar to a
more powerful, less efficient setting. For instance, x1.30/x1.93. Once you've changed the power-efficiency bar, change the engine size until the EP is once again the required amount for your ship size and speed.
Repeat this, recording the power-efficiency setting and total ton size for each attempt. Then once you've found a setting that seems to closely match the 40% rule, test the two surrounding settings (for instance, if x0.35 fit well, test x0.30 and x0.40). Then look at every test's total engine-fuel size in tons, and pick the setting that gives you the smallest total engine-fuel composite size.
An explanation of why this works:
There are many ways to move a given object a given distance with a given speed. You could use a giant engine with almost no power, but, hey, it just needs one drop of fuel! Or you could use the most powerful engine in the world, taking up only the mass of my hand - but it requires a skyscraper full of sorium every hour. Obviously, these aren't optimal. This technique finds the smallest engine-fuel composite (that is, the mass taken up by both the engine and the fuel storage) for a given ship, speed, and distance.
Now, there are of course exceptions, as well as times you might want to ignore this rule. Shields take up fuel, so you might want to add a little extra fuel storage to your ship even if it's suboptimal. However, going for a wanted distance of a few million km beyond your actual wanted distance achieves the same thing, only optimized a little better.
If you are low on sorium or want to conserve it, using a less-powerful, more efficient, much larger engine may be strategically important. This technique assumes that sorium supplies are not a problem, though it does produce more efficient than other designs.
An explanation of why longer distances need more efficient engines:
Look at this technique as multiplying and dividing the sizes of the two parts of the engine-fuel composite. When you only travel a few million kilometers, the fuel will take up a very small portion of the ship size, so dividing the engine by 1/2 and multiplying the needed fuel by 2 will produce a better composite and a better ship.
But when you're traveling many billions of kilometers, the fuel starts to become significant, and decreasing the needed fuel for a small increase in the engine size becomes optimal.
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If 60% of your ship is just engines, and you add 40% of your engine tonnage as fuel, wouldn't that leave you with 84% of your ship made entirely out of fuel and engines, and only 16% of actual ship (before considering armor/crew)?
This is true. But when one remembers the current state of spaceships, this doesn't actually sound all that bad.
For spacecraft the payload fraction is often less than 1%, while the useful load fraction is perhaps 90%.
And perhaps the advice is bad. I don't design my ships around proportions, I design my ships around mission requirements. I need it to go this far, this fast. How much space do I need for engine-fuel composite? Let me calculate, etc.
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Or is all of this in relation to the actual power modifier versus engine size in some way?
I do not understand, but I hope that the above explanation clears up this question.
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Don't worry too much about answering, I'm probably just going to use the automatic engine designer thing anyways.
Too late.
Already wrote a thousand-word explanation. (That's a three-to-four pages double-spaced essay! Wish I showed this much effort for my actual assignments, heh.)
And what is this "automatic engine designer thing" you speak of?