Hello OP, I for one commend you for your work -- I've always felt that the figures don't quite add up, but I had no idea it's
that bad. I.e. that even with the tightest, heaviest / most massive module packing possible, the big ships come out at 1/6 the density of styrofoam.
(Note that it's quite different for small ships; Eagle and Viper for instance are about the size of a Space Shuttle (orbiter), and actually about twice as massive)
A possible meta-explanation might be that the medium and large ships were originally designed somewhat smaller, then upscaled to make them look bigger, without changing the stats.
For added lulz, calculate the density of a Jumpaconda. =D
the exit velocity of the helium atoms is similarly high resulting in an insanely high specific impulse while maintaining a much higher fuel efficiency as well. Aka woowoo space magic" that is actually solvable by advanced enough technology.
Nah it would still remain magic. For that tiny, tiny amount of fusion ash created by those power plants (something like 1-3t/h) to create that kind of thrust that can accelerate (in realspace) a ~1000t-ship at the rates we observe, the drives would have to accelerate these particles to extreme velocities. This opens not one but several cans of worms:
1. the kinetic energy of the exhaust can't be greater than the energy put into them, or we'd be looking at a perpetuum mobile machine of the first kind. Disclaimer: I'm a bit rusty with this kind of calculations, but I'll do my best to get the units right and not mix in too many tenfold errors:
Example: ship mass 750 tons
Acceleration - for simplicity's sake - 1G = 10m/s^2
-> required thrust = 750t = 750.000kg = 7.500.000N
Let's say the ship uses 3.6t of fuel per hour (again, KISS), or 1kg/s straight.
In order to get 7.5MN of thrust out of 1kg of propellant, we'd have to speed the propellant up to 7.500km/s (2.5%c). Now this isn't too grotesque for a far-future SF I guess; well within the theoretical capabilities of fusion drives. But!
In order to squeeze out the propellant at this velocity, we need to charge it up with the appropriate amount of energy.
E_kin = 1/2 m * v_e^2;
plugging in, we get 1/2kg * 7.500.000m^2/s^2 = 28 Terajoules per second, or in other words, 28 TERAwatt.
Compare that to the figures given for the power plants and engines in ED... 28MW here, 8MW there... it doesn't just not add up, it doesn't add up by
six orders of magnitude! It's like filling a gallon of petrol into your car and expecting to drive 1000 times around the world with that.
2. Now the apologists might say "So maybe the units are borked, so what", but again, it isn't that easy. ED actually does a good job not entirely ignoring the problem of waste heat. Typical power plants emit something on the order of 1MW of heat for 2MW of electricity (in in-game notation this would be called "0.50 efficiency". If only the "units were borked", the PP and thrusters would have to be a million times more powerful and, you get it, generate a million times as much heat.
Now this would be a huge problem for the ship itself. In the above example, those 28TW of drive power would produce something like 14TJ of heat per second, every second. For comparison, that's one Hiroshima bomb every four seconds. Yes you heard that right, imagine that every four seconds there'd be an atomic bomb exploding
inside your ship. You see the problem.
3. And even ignoring that, the figures simply don't match up with the rest of the ship's systems anymore. Everything consumes power (and creates heat) on the order of megawatts, and these figures are probably already a bit on the high end comparing them to real life applications, but close enough. (Let's not think too much about the "docking computer" that draws
750.000 Watts, or probably about 1500x as much as your home computer system's peak draw)
It's just not plausible that we can feed a 28TW thruster no problem, but have to squeeze fractions of megawatts out of our other systems to keep everything running.
And worst of all,
Jon's Law rears its ugly head: with that kind of thrust power at our disposal, we're being stupid for even carrying weapons, especially when those weapons have only a millionth the power output of our engine. It's like a Challenger 2 main battle tank whose main gun has been replaced with an air rifle. Yes you can plink away at things with the air rifle, but it'll be a lot more efficient if you just roll over the target with your 65 tons of heavy metal.
Or in the case of a spaceship with a multi-terawatt engine, just point the drive plume at your enemy and watch it evaporate in a puff of plasma.
(IIRC that Expanse show demonstrates a few examples of this)
So, long story short: it IS space magic. Somewhere along the way there's a perpetual motion machine that creates 1 MW worth of thrust power for every 1 W of electrical power we put it, but without any of the side effects such a thrust power would bring with it.
The only "solution"
I see is that the realspace / sublight engines work by an entirely different (and in itself magical) principle, maybe similar to Star Trek's "Warp" bubbles that reduce the mass of a body in realspace and thereby make it easier to accelerate. This approach would probably also be easiest to bring inline with the ridiculous speed limits imposed on the ships in realspace, or the fact that they bleed velocity even when all thrusters are off.
--
As insult comes to injury, these maths operate in exactly the opposite direction as the OP's complaints about mass, density and carrying capacity. If we/FD tried to make the mass figures for the ships shown more believable by increasing them, that would mean we'd have to increase the thrust values even more. If we increased the carrying capacity to more believable values, that would move the gameplay effects of owning a Sidewinder vs a Cutter way, way further apart than they already are, and that is most likely not desired by the devs.
The most reasonable approach to bring the numbers to more plausible levels would be to ... make the ships smaller. As they were probably designed at some point in the past (before release). However, that again would make space combat much harder, at least if we were still expected to aim fixed weapons manually at suddenly much smaller targets zipping by with what would be +/- Mach 2 here on Earth.
