Engine Power/Efficiency

[JRHaggs (OP)]

I'm just getting around to designing a naval fleet. 

A survey ship was blown to bits in a neighboring system, prompting the militarization.  Shortly thereafter, about a dozen wrecked vessels of unknown origin were detected in a second neighboring system.  No sign of whatever did the wrecking of my survey vessel or the unidentified alien ships.  At this point they are assumed to be different entities.

Sorry, got a little carried away.

Anyway, I've been playing with engine power and efficiency.  The wiki's example fleets are all from earlier versions of the game.  Has fuel consumption been dramatically increased since the 5. xx versions? The example ships all seem to use something like an order of magnitude less fuel than I can manage to engineer. 

Recent posts of ships in the forums suggest specs more in line with what I'm able to produce, but I guess there are some basic things I still don't get about engine design.

What power multiplier and engine size do you folks generally go with on naval vessels? Or do you shoot for a specific fuel consumption rate?

I've tried defaults of both.  I've tried 50% power modifier, size * 2.  I've tried 2. 0x power modifier, size/2.  What am I missing? I'm confused.

[TheDeadlyShoe]

Yes, fuel consumption rates have increased dramatically since earlier versions of Aurora. Fuel production is now a significant industrial strain, at least until massive numbers of Sorium Harvesters are deployed.

Multipliers are a complicated question.  Freighters, Colony ships and very large utility vessels (like gate ships) generally want the biggest, lowest multiplier engines available.  But for military ships there's just too many factors to give a right answer.

A good starting rule of thumb for is to pick a battle-line speed you are happy with and try to make sure fleet combat vessels adhere to that standard.  You have as much engine power on your design as it takes to reach that speed.  Therefore, the real questions are how much space you are willing to tolerate for engines, and how much range you want on your ships. A higher multiplier gets you more space, but reduces your range.  But further complicating this is varying sizes of engines.  If you are experimenting, I would advise largely ignoring engine size considerations by using size 1, 5, or 10 engines.  These engines are simple to research and you can add or remove them as desired to adjust your designs.  Larger engines impose significantly greater research burdens (especially with good multipliers) and drastically reduce design flexibility, even if they are noticably more fuel efficient.

Here's an example of that in action.

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Skagit class Missile Cruiser    9 000 tons     223 Crew     1473.68 BP      TCS 180  TH 825  EM 0
4583 km/s     Armour 5-38     Shields 0-0     Sensors 12/16/0/0     Damage Control Rating 5     PPV 39.5
Maint Life 4.43 Years     MSP 512    AFR 129%    IFR 1.8%    1YR 42    5YR 632    Max Repair 144 MSP
Intended Deployment Time: 12 months    Spare Berths 1   
Magazine 189    Cryogenic Berths 200   

Sorium-Argon Ion Torch (11)    Power 75    Fuel Use 132.76%    Signature 75    Exp 12%
Fuel Capacity 500 000 Litres    Range 7.5 billion km   (19 days at full power)
This defensive ship uses 11 size 5 ion engines with a 1.25x multiplier (55 HS total) to hit its design speed.  It has short legs though, and burns through fuel rapidly.

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Brasilia class Missile Frigate    9 000 tons     234 Crew     1542.18 BP      TCS 180  TH 810  EM 0
4500 km/s     Armour 5-38     Shields 0-0     Sensors 1/16/0/0     Damage Control Rating 6     PPV 45
Maint Life 2.39 Years     MSP 643    AFR 108%    IFR 1.5%    1YR 154    5YR 2312    Max Repair 405 MSP
Intended Deployment Time: 12 months    Spare Berths 1   
Magazine 189   

Xenon-Ion Sorium Torch (1)    Power 810    Fuel Use 84.7%    Signature 810    Exp 13%
Fuel Capacity 400 000 Litres    Range 9.4 billion km   (24 days at full powe)
This slightly more advanced ship uses a single 50 HS engine with a 1.35x multiplier. It still hits its design speed of 4500 km/s, with noticable size savings and substantial fuel savings. There's an important caveat though: even with more space dedicated to engineering spaces, it has a far shorter estimated maintenance lifetime owing to huge cost of its single engine.  (Owing to the vagaries of dice rolling, single hugely expensive components make a ship way more vulnerable to maintenance failures.)

In terms of real practical advice, I design military engines to range from 1.0x to 1.5x  or so. Multipliers higher than that are usually used only on parasite warships or fighter craft owing to the extreme fuel usage.   I do sometimes use commercial or quasi-commercial engines on some designs - specialists like long range carriers, orbital defenses, patrol cruisers, or jump point monitors.

[Iranon]

Default is pretty much the one thing I *don't* use.

Cost scales quadratically with power multiplier up to 1, linearly afterwards.
Since I value both speed and fuel efficiency highly, tonnage efficiency has to go. I'm usually happy with my ships allocating 50-60% to size 50 engines, with fairly low power multipliers (e.g. 0.3 commercial, 0.9 for my beam fleet). For some roles, speed is critical and efficiency can slide.

I don't generally go much higher than 60% because you approach a soft cap, there's so much tonnage taken up by engine that the fuel cost per km and useful payload can't be improved on very much and the increased build costs aren't worth it.

[Prince of Space]

In my opinion, if you are designing warships to respond to hostile aliens, then your design parameters should be based on their known specs. Ideally you would be faster than them, so you could chase them down if they flee, or flee yourself if things go badly. You will also need enough range to get to where the fight will take place, do some maneuvering, possibly fire up shields if you have them, and then get back to a tanker to refuel.

Once you have the speed and range worked out, follow Shoe's advice and derive engine size and power mod from there.

If I was designing a warship with complete ignorance of the enemy, here is what I would do. Take  your available engine tech's power per HS value and multiply it by 375. That is the target speed for the design. Take the distance from the jump point into the alien system to the wreckage of your surveyor. Multiply that by 2.5. That is the target range for the design. If my rule of thumb puts your range below 15 billion km, consider going with 15 billion instead. That will get you from Sol to Pluto and back again with a little room to spare, which is a fine range for a warship, to my mind.

It would probably be wise to use multiple smaller engines, rather than one large engine. You'll take a hit on fuel efficiency, but if you lose an engine to a lucky hit you will still be able to limp around.

[JRHaggs (OP)]

OK.   I think I have to dramatically rein in my range expectations.   I was hoping to get 10/20k ton vessels going 4000km/s with a 100b km range.   

I'll halve the range and see what I come up with. 

Thanks, folks.   Experimentation resumes this evening. 

Edit: I would totally spec my designs so that they exploit known enemy tech, but I have exactly no information about what I'm up against.  In Luhman 6, my survey vessel reported a bunch of hits before exploding, but it was only equipped with geo- and grav-survey sensors.  Whatever took it down remains unknown to me.  Then I jumped into the Barnard system, immediately saw the wrecked ships, and got the hell out.

So all I know is that there is(was?) a civilization out there that used vessels weighing between 7 and 23 tons, but I don't know anything about the civilization that did the wrecking.

[Steve Walmsley]

Don't forget you can have tankers supporting your warships. At the moment I seem to be designing warships with ranges of around 15-25 billion kilometres.

[sneer]

my usual power multplier for military engines used in capital ships varies from 1.35 to 1.65 with 1.5 most often hit - depends on role , scouted enemies around and other factors
range is 20+ bn km for early ( till magneto or 1st fusion technology ) and longer for later engine types . Also i try to use 50hs engines for additional efficiency

[Prince of Space]

You don't need information on enemy tech (though it's nice to have). If you had picked up an enemy ship on active or passive sensors before your surveyor got atomized, your civilization's intelligence service would estimate the vessel's speed for you. That was what I would have based the target speed on. But it's no big deal that you didn't pick that up. The mystery adds to the narrative tension once you go back.

My range prescription was based on the presumption that you would have a tanker available to support your warships: warships and tanker(s) get to the jump point leading to Luhman 6; warships refuel from tankers and go into Luhman alone; warships fight; warships go home the way they came in, topping off their fuel once they rendezvous with the tankers. If you need to carry your fuel with you for the whole trip to and from Luhman 6, that's tonnage that can't be used for weapons or armor.

[AL]

I find myself favouring max-size, fuel efficient (low multiplier) engines lately - with this setup it is possible to get the 100bkm ranges that you mentioned. While space efficiency and battle-line speed does take a hit (alongside maintenance, research and whatnot), I prefer this due to the much lower reliance on tankers your ships will have.

[Jorgen_CAB]

In the way that fuel efficiency increase with size linear and research cost increase even more with it as well as less design flexibility and more maintenance required than big engines are generally too expensive.

I see no reason to design a size 5 engine over a size 1 engine outside of role-playing, there is only a 5% gain on fuel efficiency... it gain even worse as size get bigger since the same engine becomes less flexible in different sizes of ships. I rather spend the saved research into overall better fuel efficiency.

For civilian engines where fuel actually matters more I simply design the biggest and most fuel efficient engine I can get my hands on.

When it comes to a military ships engine power efficiency it's more about the ships role and my overall fuel situation.

[Iranon]

Regarding engine size: I tend to go for size 1 or 50, because of the way efficiency scales. We don't get much for smallish-but-not-tiny engines, the difference between big-ish and 50HS is considerable. Most of my real ships get size 50 engines. While research cost is an issue,  I feel the penalty to maintenance life is overstated here:

1) For most military ships, the additional engineering bays to reach desired maintenance life take up less space than the fuel savings while still reaching desired range. Engineering bays are a one-off expense, higher fuel use continues to be a pain.
2) Maintenance lives aren't equal, those additional engineering bays aren't wasted. A single ship with a single huge engine may have a much shorter expected maintenance life than one with many small engines... but that's due to variance. Much less pronounced in a fleet-to-fleet comparison where ships can exchange supplies. If you compensate with additional engineering bays, your fleet will last longer even though this isn't apparent from the design screen.

*

If you design your ships to fixed requirements like speed, range and mission tonnage, engine power multiplier and corresponding engine size to achieve the desired speed give us a trade-off between fuel efficiency (better with low multipliers) and cost/size efficiency (better with high multipliers).
Surprisingly, the default 1.0 multiplier is awkward here, because of how costs scale.

0.9 power engines cost 0.81 as much as 1.0 engines (quadratic below 1.0) , 1.1 power engines cost 1.1 as much (linear above 1.0).
Dipping slightly below 1.0 power multiplier will save fuel without a noticable increase in build cost.

Go too low on power multipliers (with correspondingly massive engines) and your fuel efficiency will eventually drop because you spend most of your fuel hauling around oversized engines and have very little mission tonnage compared to total size. Even before that, additional fuel efficiency is expensive in terms of size and build cost. I'm often fine with 60% engines, but rarely with 70%. On the other hand, most designs I see on these boards are too stressed for my tastes - I only go for less than 1/3 engine weight when compactness is very important (e.g. missile fighters which are expected to launch without being detected themselves) or I'm limited by technology (already on my lowest engine power multiplier, ship doesn't gain much from speed).

Sometimes, it depends on your overarching plan. Assume you want some properly fast warships.
If they're intended to be a fleet staple, you need to watch fuel consumption... having them consist mostly of medium-power engines makes them big and expensive for their capability, but you can actually use them without running dry.
If you intend to shove them into a hangar until their perfect quarry shows up (like a certain spoiler, which the right design will humiliate utterly), you care more about build cost and required hangar space than fuel use: less/smaller but more stressed engines.

[TheDeadlyShoe]

I get a little tired of size 50 everything.  Honestly the engine size range is probably too small; if i could wave a magic wand i'd put it size 1<-> size 1000 (just like jump engines).

That way to get maximum fuel efficiency you're looking at 100,000 tons (size 2000) minimum ship size, or thereabouts.

Though I kinda feel like over the long arc of Aurora history there's been an inflation of ship sizes.  It might just be me?   But i think a lot of things factor into it, like the addition of intended deployment times, the reduction in fuel efficiency, and the engine design rework... All of them have conspired to push up cost-effective ship sizes.  So 50HS just isn't very impressive anymore.

I'm not sure how one would fix the lack of meaningful difference between size 1 and size 10 engines though, unless its going to be on a hella step curve like missile engines.

[alex_brunius]

I get a little tired of size 50 everything.  Honestly the engine size range is probably too small; if i could wave a magic wand i'd put it size 1<-> size 1000 (just like jump engines).

That way to get maximum fuel efficiency you're looking at 100,000 tons (size 2000) minimum ship size, or thereabouts.

Though I kinda feel like over the long arc of Aurora history there's been an inflation of ship sizes.  It might just be me?   But i think a lot of things factor into it, like the addition of intended deployment times, the reduction in fuel efficiency, and the engine design rework... All of them have conspired to push up cost-effective ship sizes.  So 50HS just isn't very impressive anymore.

I'm not sure how one would fix the lack of meaningful difference between size 1 and size 10 engines though, unless its going to be on a hella step curve like missile engines.

I made a suggestion for a crude new engine size vs efficiency formula here almost a year ago based along similar lines that the missile engine size impact efficiency:

http://aurora2.pentarch.org/index.php?topic=7448.msg75697#msg75697

Formula: sqrt(10/HS)

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HS Consumption
1,0 316%
1,2 289%
1,4 267%
1,6 250%
1,8 236%
2,0 224%
2,2 213%
2,4 204%
2,6 196%
2,8 189%
3,0 183%
3,5 169%
4,0 158%
4,5 149%
5,0 141%
5,5 135%
6,0 129%
7,0 120%
8,0 112%
9,0 105%
10,0 100%
11,0 95%
12,0 91%
13,0 88%
14,0 85%
15,0 82%
16,0 79%
17,0 77%
18,0 75%
19,0 73%
20,0 71%
22,0 67%
24,0 65%
26,0 62%
28,0 60%
30,0 58%
32,0 56%
34,0 54%
36,0 53%
38,0 51%
40,0 50%
45,0 47%
50,0 45%
55,0 43%
60,0 41%
65,0 39%
70,0 38%
75,0 37%
80,0 35%
85,0 34%
90,0 33%
95,0 32%
100,0 32%
As a reference today the consumption is 99% for size 1, and 50% for size 50.

[joeclark77]

Lately I have tended to favor missile ships with good sensors and long range, and in those cases I don't need high speed.  Rather, I prefer to slowly but surely approach the enemy and force him to either flee or find himself in my circle of death.  I tend to use 0.8 or 0.9 power engines, 5HS per 1000T (i.e. 25% of my tonnage).  On the other hand, if I'm at a tech level where I can come close to matching my enemy, I may go with overpowered engines in order to exceed his speed.

For civilian designs, I usually use a 40 or 50 HS engine at around 0.3 power.  I create fast tankers with 5,000,000 gallon fuel tanks and slap on a bunch of engines.  They come in at less than 20000T and can generally go almost as fast as my combat ships.  Therefore, combat ships need only have a short range, and I can just add tankers up to the range the fleet needs.

Lately I find that I want to try beam ships, and those need speed.  I'm leaning toward a model where large, efficient carriers haul short-range fighters and FACs into enemy territory, so I can use gas-guzzling engines on the beam combatants.

Now, if you want to talk about *really* long range warships.... (go here)