Sigtrygg said:
And how much of that ship is reaction mass...
You can see the fuel tanks in the picture, to accelerate for a month and to reach about 10% of the speed of light requires the full fuel load of a single stage, the bottom stage has to carry the top stage, and for the purposes of the ships hull, the top stage counts as part of the Bottom stage when it is launched. the top stage is accelerated fully fueled to 10% of the speed of light and then it seperated from the bottom stage and accelerates another 10% to 20% of the speed of light.
https://en.wikipedia.org/wiki/Project_Daedalus
What I am looking at is ballpark estimates of speed, probably you need closer to 60% of its mass as reaction mass/fuel to reach 10% of the speed of light.
1 week = 604,800 second, cruise velocity equals 3,024,000 m/sec = 1% of the speed of light, fuel requirement = 15% of hull volume
2 weeks = 1,209,600 seconds, cruise velocity equals 6,048,000 m/sec = 2% of the speed of light, fuel requirement = 30% of hull volume
3 weeks = 1,814,400 seconds, cruise velocity equals 9,072,000 m/sec = 3% of the speed of light, fuel requirement = 45% of hull volume
4 weeks = 2,419,200 seconds, cruise velocity equals 12,096,000 m/sec = 4% of the speed of light, fuel requirement = 60% of hull volume
This is roughly in the ballpark with fusion, but what if we used something slightly better than fusion, there is a concept called a "Nebula drive" it is used stricly for slowing down a starship using magnetic fields, an ionization laser and the interstellar medium, with that we could use 10% of hull volume in fuel to accelerate to 1% of the speed of light, the time spent accelerating would still be 1 week, about 3.5 days accelerating and 3.5 days slowing down, using magnetic breaking saves on the fuel requirements for slowing down, so we can use a greater fraction of it to speed up. By extending this relation ship, that means 6% of light speed can ultimately be achieved using 60% of hull volume for fuel/reaction mass.
A Nebula Drive is more commonly known as a magnetic sail.
https://en.wikipedia.org/wiki/Magnetic_sail