The ship design process is really an iterative one: design a ship, find flaws, adjust the design, find more flaws...
I like to design a mission package first, since it doesn't matter if I get the engine to fuel ratio right if i can't get enough missile launchers and fire controls to the battle. Then, I've found that the mission package is generally about 40% of the total tonnage, so I estimate the total tonnage and allocate engines and fuel using a spreadsheet like the one posted here:
http://aurora2.pentarch.org/index.php/topic,5482.msg56842.html#msg56842
That gets me most of the way, but it still takes some tweaking. It doesn't work as well for ships with very long or very short deployment times, or that need to fit within a given tonnage restriction, such as fighters or any other ship that fits within a hangar.
Not sure if I am doing something wrong or if there is an error in this spreadsheet but the results are not coming out correctly when put into practice, here is an example.
I tell the sheet I want a 50k ship to do 8k speed with the following specs. (this is for multiple engines)
Engine tech - Internal confinement 32EP/HS
Fuel consumption 0.25 per hour
Total ship size - 50,000 tons
Desired speed - 8000km/s
Desired range - 150b/km
This then tells me I need the following.
2x 50-HS engines with Power Modifier x0.8, plus 38.18 HS of fuel, totaling 6909 tons of engine and fuel space.
However when I put this in a 50k ship the results are vastly inferior to the desired ones, here is a quick copy of a design I had with some extra rubbish thrown in to get to the exact 50k mark.
Typhoon - Copy class Command Carrier 50,000 tons 1049 Crew 9200.4001 BP TCS 1000 TH 2560 EM 0
2560 km/s Armour 6-120 Shields 0-0 Sensors 288/288/0/0 Damage Control Rating 107 PPV 199.4
Maint Life 1.95 Years MSP 5405 AFR 425% IFR 5.9% 1YR 1868 5YR 28025 Max Repair 576 MSP
Intended Deployment Time: 12 months Flight Crew Berths 1
Flag Bridge Hangar Deck Capacity 10000 tons Magazine 3699
1280 EP Inertial Fusion Drive (2) Power 1280 Fuel Use 7.16% Signature 1280 Exp 8%
Fuel Capacity 2,000,000 Litres Range 100.6 billion km (454 days at full power)
Donlan Heavy Industries 25cm Far Ultraviolet Laser (1) Range 144,000km TS: 8000 km/s Power 16-8 RM 5 ROF 10 16 16 16 16 16 13 11 10 8 8
Quad Mcguyer & Briano Gauss Cannon R40k - F4/5 Turret (4x16) Range 40,000km TS: 20000 km/s Power 0-0 RM 4 ROF 5 1 1 1 1 0 0 0 0 0 0
Mcguyer & Briano Phalanx CIWS (4x8) Range 1000 km TS: 32000 km/s ROF 5 Base 50% To Hit
Sadberry Foundation Fire Control R72k-T40k (FTR) (2) Max Range: 144,000 km TS: 40000 km/s 93 86 79 72 65 58 51 44 37 31
Mcguyer & Briano Size 12 Missile Launcher (3) Missile Size 12 Rate of Fire 75
Size 5 Anti-ship Missile S30 - W16 - R14 (164) Speed: 30,700 km/s End: 7.9m Range: 14.5m km WH: 16 Size: 5 TH: 225/135/67
Sadberry Foundation Active Search Sensor MR5000 - 259m (1) GPS 14400 Range 259.2m km Resolution 100
Sadberry Foundation Thermal Sensor TH288 (1) Sensitivity 288 Detect Sig Strength 1000: 288m km
Sadberry Foundation EM Detection Sensor EM288 (1) Sensitivity 288 Detect Sig Strength 1000: 288m km
ECM 30
Missile to hit chances are vs targets moving at 3000 km/s, 5000 km/s and 10,000 km/s
This design is classed as a Military Vessel for maintenance purposes
As you can see neither the speed nor range is matching up to what the sheet is saying using the engine design suggested and slightly more fuel than was suggested. Have I made an error somewhere in understanding how this sheet works?