Two frame challenges.
1. Why are you opposed to reusing the same engine across classes? Engines are expensive to research; reusing components is usually a positive not a negative. If you are reusing engines, keeping round sizes and speeds is easy as you said.
2. Why should ships of different sizes have the same speed? Smaller ships are, all else equal, worse at any given task than a larger one (stealth aside). If you have multiple sizes of ships, they should probably be doing different things, and thus wanting different speeds.
Point 2 is a "perfect world" point, but still worth thinking about (perfect world, parallel building small ships lets you get tonnage faster, even if every ton is less efficient).
Gonna hit these real quick before main comment:
- If you can afford the research, a dedicated engine at each mass point is more fuel-efficient for heavier ships than using multiple lighter engines. For example, if you have two engines for a 10,000-ton ship, then for a 20,000 ton ship you could use four engines or design a new engine and use two of them to achieve the same speed. The latter case is more fuel-efficient by a factor of SQRT(2).
- Generally, while small or specialist ships should have different speeds, your main fleet ships should have similar speeds if they are intended to operate together, as in this case any ship with speed in excess of the fleet's maximum speed is wasting tonnage on engines or fuel that could be used on weapons or defenses. If I have 10,000-ton escorts and 20,000-ton cruisers, I don't need my escorts to move 1,500 km/s faster than the cruisers - I need them to stay with the cruisers and escort them!
Main comment:
It's not actually
hard to get a ship with a nice, round tonnage and speed. You just have to realize that you're probably sacrificing also having a nice round set of component sizes/parameter values. If you look at Steve's ships you'll see that some of them have rather odd-looking values for EP for example.
Generally speaking, pick a tonnage and divide it by 50 to get the size in HS. Pick a speed, Multiply HS and speed, divide by a thousand, and you know the total EP you'll need. Usually, in fact, you'll find that the EP you need is not difficult to come up with and the real decision is how to choose between different combinations of size, boost, etc. to get the best engines.
What Steve usually does, I think by his own admission in one of those AAR threads or another, is to keep the boost at 1.0x and pick an engine size at his current tech to get the needed speed with 2-3 engines, then throws a bunch of fuel on to get whatever range he needs, and I think this is what most people do albeit with some variations e.g. always using maximum boost and so on.
Alternatively, if you like optimizing your ships to get the most performance from your tonnage allocation, there is a fairly good approach for a "quick" calculation which exploits the fact that the optimal engine-to-fuel mass ratio is 3:1 in order to get the best combination of speed and range for a particular total engine and fuel mass:
1. The most common example, and the one likely relevant here, is where you have a specific ship size, speed, and fraction of mass you want to dedicate to either propulsion or engines (one implies the other - if you want 30% engine mass, that's the same as saying 40% propulsion mass once you add in the 10% for fuel). We have an expression for ship speed based on total engine power:
V = 1000 * EP / HS --> V * HS = 1000 * P0 * Me * Be (Eq. 1)
V = speed/velocity
EP = total engine power
HS = ship size in HS
P0 = base EP per HS from tech level
Me = total engine mass in HS
Be = engine power boost
If we selected a specific propulsion fraction, Fp (fuel + engines), the engine mass fraction is 75% of that (if you're designing for a specific engine mass fraction instead, then Fp is 4/3 of that value).
V * HS = 1000 * P0 * 0.75 * Fp * HS * Be --> V = 750 * P0 * Fp * Be (Eq. 2)
2. We can then select the necessary EP boost level:
Be = V / 750 / P0 / Fp (Eq. 3)
3. Given the boost level, the ship range can be calculated as:
R = V * (3600 * T) (Eq. 4a)
R = range
T = endurance (given in hours in-game)
3600 = seconds per hour to convert units consistently
T = Vf / EP / eff (Eq. 4b)
Vf = fuel volume = Mf * Cf
Mf = fuel mass in HS = 0.25 * Fp * HS
Cf = fuel capacity in liters per HS (usually 50,000 L/HS)
eff = fuel consumption efficiency, in liters per engine power hour (EPh)
The fuel efficiency is the product of three factors (four, if you're messing around with missiles, but that's another subject):
eff = e0 * eff_size * eff_boost (Eq. 5a)
e0 = fuel consumption efficiency tech
eff_size = SQRT(10 / mass of a single engine) = SQRT(10 * Ne / 0.75 / Fp / HS)
Ne = number of engines
eff_boost = Be^2.5
eff = e0* Be^2.5 * SQRT(7.5 * Ne / Fp / HS)
Back-substitute everything and you have an expression for the range which depends on the design parameters Be, Fp, and Ne (for Ne, I recommend always having at least 2-3 engines on any military ship larger than a FAC for redundancy in case of battle damage):
R = 9 * 10^4.5 * Fp^1.5 * Cf / e0 / Be^2.5 * SQRT(0.75 * HS / Ne)
4. Calculate the range you get for the value of Fp and Ne you have chosen as well as the value of Be you obtained from Eq. 3 to get your desired speed. If you're happy with the range you have, go ahead and build the ship/engines (remembering the 3:1 ratio of engine and fuel masses). If not, go back to Eq. 3 and play around with different values of the propulsion fraction to get a lower boost, or reduce the number of engines. If all else fails, you may have to settle for a lower speed until you tech up some more - 10,000 km/s might not be viable for a cruiser propelled by ion drives unless you're really willing to compromise on the range, for example.
Note that with this method, since it's purely analytical you will run into problems with the discrete nature of Aurora - for example, an engine boost of 1.23x is not possible, so you'll have to round to 1.2x or 1.25x or else change your input parameters a little bit. If you're not lucky enough to get round values some tweaking back and forth is probably going to be needed.
My suggestion is that if this all seems like a lot more than you bargained for, stick with 1.0x boost and use engine size as your design parameter until you get a feel for how the numbers work at different tech levels. This will give you good results and also let you get comfortable with building ships with 40% or 50% engine mass (and correspondingly less payload). Ultimately, you're going to need to try your ships in combat to get an intuitive sense of how much speed is enough versus how much engine mass is too much, which will vary depending on your tech, weapons, doctrine, and enemy fleet composition as well.
