Well... 1/3 engine is not always optimal... it depends on the engine technology or rather the engine cost. If you look at a standard transport for example...

Nuclear thermal engine transports. The below ships are 100kt cargo, 50 billion range with different total engine (50% power) size.

NT 20% engine size, speed 486km/s cost 896BP (312 Gallicite), 1.84 BP per speed (0.64 Gallicite per speed)
NT 30% engine size, speed 753km/s cost 1241BP (562 Gallicite), 1.65 BP per speed (0.74 Gallicite per speed)
NT 40% engine size, speed 999km/s cost 1659BP (875 Gallicite), 1.66 BP per speed (0.87 Gallicite per speed)
NT 50% engine size, speed 1271km/s cost 2325BP (1375 Gallicite), 1.83 BP per speed (1.08 Gallicite per speed)

The NT ships above should be more expensive as the armour technology was too high so the engine portion cost is too high in general, but still it is a relatively good comparison. So the TN ship above should in general favour slightly bigger engine portion for efficiency.


 Below are the same ship just swapped out the NT engine with an ION version.

ION 20% engine size, speed 1217km/s cost 1369BP (781 Gallicite), 1.12 BP per speed (0.64 Gallicite per speed)
ION 30% engine size, speed 1888km/s cost 2079BP (1406 Gallicite), 1.10 BP per speed (0.74 Gallicite per speed)
ION 40% engine size, speed 2498km/s cost 2977BP (2187 Gallicite), 1.19 BP per speed (0.87 Gallicite per speed)
ION 50% engine size, speed 3177km/s cost 4387BP (3437 Gallicite), 1.38 BP per speed (1.08 Gallicite per speed)

The trend is... the more expensive the engine the less efficient it is to have a larger engine in the Transport ship.. obviously there is still the benefit of getting things faster to the point of interest so speed still have some advantage, especially in very long travels. The longer the trip the more benefit fast ships will become. I also included the Gallicite cost per speed as well as that can be quite important at times as well, at least as much as the total build cost.

If you then swap out the 50% powered ION engine for a 30% power ION engine you can have a slightly higher engine portion for the more efficient ship and better fuel economy and much cheaper ships so way more efficient in BP per speed, thus more transport ship. The 30% ION engine actually is just a tad bit cheaper than the 50% NT engine but 50% more powerful (56 BP versus 62 BP). The best BP per speed for the 30% engine gets down to about 0.95 BP per speed at around 35% engine and speed around 1500km/s. More importantly the Gallicite cost for the 30% powered engine is only about 0.48 Gallicite per speed, so substantially cheaper than the more powerful engine.

 This is obviously not the same as a TUG as that have different priorities.

Quote from: Jorgen_CAB on July 13, 2022, 07:10:02 AM

The tug probably should be around 25-35% the size of the thing it drags if at 50% efficiency, larger if less efficient engines.

Did you come to this conclusion by analysis, or by intuition and experience?

Also, you are saying "efficiency" but I think you are meaning "power." This makes it hard to follow your reasoning at times.

I probably was a bit inconsistent...

I obviously meant engine power such as 30% being of the more optimal design choice with the best speed to fuel efficiency trade-off, in my opinion any way. I have not done the math as that depends highly in the tech levels you currently have and I will not sit down and do it that methodically to be honest. The speed you want needs to be a trade-off between how important it is to get the thing to the place you need. Speed is also a strategic considerations as well as consideration of fuel efficiency.

I also meant that the Tug should be around 1/3 of the "total" size of the thing it drags (at 50% power), that seems a good trade-off between cost and efficiency. At 30% power you would want it to be almost at 50% of the total size so the tug would then be as big as the station it drags. The bigger the tug it also will burn more fuel just moving around itself.

In general I would not want to reduce speed by adding a less powerful engine, I would just make the tug larger in that case. In general a 30% power engine tug will be cheaper than a 50% power tug even if it is larger, both to build and move around at the same speeds... unless you want insane speeds that is for some reasons.

Did you come to this conclusion by analysis, or by intuition and experience?

Intuition and experience count for quite a lot in Aurora.

In this case, having a tug class ~1/3 the size of the stations being tugged is a balance between speed and fuel conservation. A larger tug will move stations more quickly, but you have to pay with fuel to move the extra tug displacement. A smaller tug will conserve fuel to an extent but takes longer in transit, which means your station is unproductive for longer. It is a balance of different factors which defies general analysis and often defies specific analysis as well, at least if you want to spend more time playing the game than doing math.

Generally, it is the case that optimizing a single metric in Aurora is not really the best approach. In this case we can analyze fuel consumption to death, but without any other considerations it is meaningless. How quickly can we move things from point A to point B? Is the value of lost production worth using a slower or smaller tug class? Tugs are also used for other purposes very often, so optimizing for a single use case may not represent the best overall design - in some cases speed is more important, in others fuel conservation becomes a more prominent concern.

We also are not considering the bottom line of economics here; that is, is the fuel usage from a single tug to get a FHS into position significant compared to the production from that FHS once it is deployed? It can easily be the case that fuel production easily exceeds demand, and fuel transport logistics are the dominant consideration. If a tanker or tug transporting fuel uses 2x as much fuel to go faster, but the consumption is only 2% of fuel carried compared to 1% with the more efficient design... well, would you rather have 99% of your fuel transported in 3 months, or 98% transported in two months? These are important considerations which are highly situational and usually defy straightforward analysis unless you are a trained logistics engineer.

But you also need to look at overall total consumption of fuel as well as fuel is not actually infinite either, so both consumption and production efficiency is important overall. It is OK to loose some production now in order to get more out of fuel harvesters over time. So we can't completely disregard the importance of fuel economy in terms of absolute numbers completely. What I mean is we can't just stare our self blind on one type of efficiency.

A 10+ million ton gas giant is functionally infinite - I've never had one of those run out, and I get all my fuel from orbital refineries as soon as possible.

I bet a 50% fuel engine will almost always win this as the production capacity of the harvesting station will vastly outperform the fuel cost of delivering the station to it's destination.

I guess it depends on the relative size of the tugs and the station.

A 50% engine uses about 75% more fuel per EPH than a 40% engine.
A tug using 50s instead of 40s will reach the destination in 20% less time.
In that amount of saved time, will a station produce that much extra fuel?

My slow tug uses ~557kl/yr.
My fast tug uses ~979kl/yr.
Both are roughly the same weight as my harvester station, which has ten modules and can produce about 600kl/yr (with my naval admin mining bonuses accounted for).

Suppose it takes the fast tug a year to deliver a station, and six months for the return trip.
Then the slow tug would need 15 months to deliver and 7.5 months to return.
That's a 1.5-year round trip for the fast tug, consuming ~1.47ML of fuel.
The slow tug takes 1.875 years, consuming ~1.04ML of fuel.
So the fast tug needs 0.43ML more fuel. What does the faster trip gain me?
Just 3 months of fuel production from the station.
In that time, the station produces ~0.15ML of fuel.

The slow tug is better.
After all is said and done, delivering the station with the slow tug leaves me with an extra ~0.28ML of fuel.

But if my tugs were much smaller, the faster tug might be better, because the 20% time savings equals much more time, while the 75% extra fuel usage equals much less fuel.

If you cut the tug sizes in half, then the trip takes 50% longer and uses 2/3 as much fuel.
The station would then be harvesting ~0.225ML more fuel when delivered by the fast tug, and the fast tug would be using ~0.29ML more fuel per trip than the slow tug.

Cut the tug sizes in half again, and the faster tug will be the better option.

Of course, all this does not factor in the value of finishing the return trip sooner.
If you are going to build stations as fast as you can tug them all away, then using faster tugs means you get more stations built and delivered sooner.
But it costs you more fuel to do it that way.

If you are going to build stations at some given rate, then you will want to build enough tugs to handle that build rate, so that you always have a tug ready when a station is completed.
This will require 25% more slow tugs than fast tugs.
But the fast tugs are about 30% more expensive, so this really only saves ~2.5% of the cost of tugs for this job.

...As long as you can ship as many mines to the destination per year as you are building, you should be golden.

If cargo in transit were as useful as cargo at the destination, then speed would not matter per se. It would only matter as a component of throughput (which is speed times capacity over distance).

But installations in transit aren't providing value for us.
Installations that have reached their destinations are providing value for us.
So it stands to reason that, all else being equal, we should prefer that our installations reach their destinations sooner rather than later.

Example:
You are deciding between two options for transporting automines from a production planet to a mining planet.
The production planet will make 100 automines per year.
The automines produce nothing while at the production planet.
Option A will use 100 ships, each making 1 round trip per year carrying 1 automine.
Option B will use 1 ship, making 100 round trips per year carrying 1 automine per trip.

Both options have the same throughput: 100 mines per year.
But if the total cost is the same for both options (accounting for ship cost, fuel cost, fuel transport cost, amortized shipyard cost, and whatever else applies), then option B is far superior.

With Option A, for each year that passes you will have 100 mines in transit for half a year each--a total loss of 50 mine-years of production.
With Option B, for each year that passes you will have 100 mines in transit for about 1.8 days each--a total loss of about 0.5 mine-years of production.

49.5 mine-years of production per year is worth a lot more than zero, for sure.
How much it is worth, exactly?
I'm pretty sure it's worth exactly as much as it costs to make 49.5 mines.
Because that's what you have to pay to get 49.5 mine-years of production per year.

So, in this contrived example, I'm willing to pay up to the cost of 49.5 mines more to implement option B.

I'm not sure what the conclusion is meant to be here.

You sound like me at the end of nearly every student paper I ever marked. 

Think of this piece as more of a case study than a persuasive essay.
The intent was not to discern certainty, but to present thoughts to a candid world, and hope for thoughts in return.

A major consideration, which I do not see here, is that not all BPs are created equal, as the same BP cost may reflect very different mineral requirements and not all TNEs are rated equally. For example a tug with 40x engines at 50% efficiency may generate the same engine power as a tug with 50x engines at 40% efficiency, but the latter will actually cost only 80% as much gallicite as the former. Even though the bigger tug will add additional build costs in extra armor, etc. requirements based on size, saving 20% of your gallicite can be instrumental in keeping the economy afloat while continuing to expand your navy full of gallicite-hungry fast warships.

This is a fair point.
I tend to simplify the "cost" side of things by just considering the BP required.
That simplification seems to work fine for my empire; I keep a close eye on stocks, income and outlay of all minerals, and I expand my mining operations with a particular attention to circumventing any shortfalls.
A more granular analysis of cost would certainly benefit empires feeling a crunch in a particular mineral.

Minor point about relative costs: a comparison of tug/freighter options shouldn't be concerned with the relative cost per ship, but about the relative cost per throughput.
The tug with more, less powerful engines costs 20% less gallicite than the other one and has the same total engine power, but it outweighs the other one by more than 20%, meaning you have to build more of them (and more barges) to move the same amount of cargo per year.
On a throughput basis, the savings for the big slow option is closer to 10% for my scenario with the ~128kt barge.

I think you get a closer comparison if you let design B have 50 engines instead of 40. For a tug this makes sense, as you want to have the same engine power available.

The effect directions are similar, but the magnitudes get smaller.

It turns out to be slightly worse that way.
I've added TugC to the post.