Spaceships: Ultralite Fighter
Since we want it manned, we pick out the smallest possible bridge, or in this case, cockpit; admittingly, there's only one one and a half tonne option. Could it be more cramped for that minus one modifier? Maybe at one tonne, which is sufficient for a turret, but let's stay canonical.
The computer takes up no volume; basic sensors take up no space, the fighter would be relying on more dedicated sensor cee three eye platforms, and in any case, has to close in order to engage any target.
Rules clearly say that you need minimum one tonne manoeuvre drive, and a one tonne power plant, which brings you to three and a half tonnes. Integral life support is twenty four hours, plus vacc flight suit, so in theory, you only need sufficient fuel for forty eight hours. For a one tonne power plant, you only need a hundred kilogrammes per four weeks, though the rules say minimum one tonne; fusion plus is like watering your plants, so I'd say this is a gray area.
A firmpoint is by default a fixed mount, which takes no volume; missiles and sandcaster canisters do, but use no energy, so it's balancing off the power plant against the fighter's magazine.
For the ground attack variant, I'm not sure if it's a hard rule that a minimum of one tonne has to be dedicated to infantry and vehicle weapons, though logic seems to indicate anything upto two hundred and fifty kilogrammes can be used to fill one slot, four slots per tonne. Ironic, if the missile rack or laser take up no volume.
At this point, your basic variant ultralite fighter weigh in at three point six tonnes, though with forty eight hour endurance, closer to three point five zero seven. This would also be the size of a civilian speedster, minus the weapons.
I think even after four millenia, no one voluntarily sits for any period just in front or over a fission reactor, though presumably a drone can raise no objections, and endurance would be what, five years Traveller or twenty/thirty years for super carriers and boomers?
Performance for a three point five zero seven tonne fighter is, using a tech level eight fusion plant is ten scotts, or nine point two nine eight six scotts available after deducting basic energy requirements. one tonne manoeuvre drive generates a hundred tonnes of thrust, which translates to twenty eight point five ratio. Or twenty eight gees.
You can't use reaction drives, the fuel just takes up too much space, unless we're going for point defence interceptor designed for a one or two hour mission.
I haven't figured out how high burn thrusters work in this edition.
For nine gee thrust, you need three point one five six three scotts, which leaves us with six point one four two three scotts, more than enough to power up that energy weapon. Maybe overclock it.