No use drawing spacecraft designs until we know how to produce gravity. Humans need constant g to survive any reasonable length of time. Take Startrek, they must have some sort of g generator under each floor. Has it ever been mentioned?
We know how to produce simulated gravity. Think 2001 A Space Odyssey (1968). Aside from that film there are quite a few films that use spin for simulated gravity. The problem with humans in space is not gravity, its radiation exposure over duration. The ISS is very close to Earth. Close enough to still be protected a bit by Earth's magnetosphere.
In Star Trek ~
Artificial gravity
Artificial gravity was created using
generators embedded in
gravity plating on the floor. Gravity levels in corridors and crew quarters could be adjusted.
In real world our current technology will use spin to produce artificial gravity.
In the far future, if we advance to high Type 1 or Type 2 civilization we might harvest star material to produce gravity in star ships.
A
teaspoon of neutron star material would
weigh about 10 million tons.
NEUTRON STAR
One could use neutron star dust or just use chunks of less dense dead stars like white dwarf material.
The difficulty working with such materials would be reduced by our civilization technology level.
A
Type I designation is a given to species who have been able to harness all the energy that is available from a neighboring star, gathering and storing it to meet the energy demands of a growing population.
a
Type II civilization – can harness the power of their entire star (not merely transforming starlight into energy, but controlling the star).
Then, there are some other technologies that could lead to gravity control. High Energy Physics or even Quantum Gravity control. If we are able to traverse those great distances even at .9c and have the technology to build large ships in space chances are we will be much higher advanced in the sciences than we are today.
Who knows what discoveries might be made by that time?