i find so many people thumping science fiction for a means of interstellar travel, but till you throw actual engineers at the problem you will never know. it aint gonna be no small construction thats for sure. my idea is something i like to call the iceball of doom:
pluck an ice dwarf out of the kuiper belt, fit it with very large nuclear water rocket engines (or nswr, or pretty much any nuclear-electric engine technology that can use that will accept the melted ice, and not necessarily water-ice, as propellant), and using the ice dwarf itself as propellant, and radiation shielding for internal habitation. this of course requires surface mining facilities (using a melt, filter, feed process). surface ice is fed into a melting furnace, filtered for harmful impurities and pumped directly to the engines. surface mining facilities and engines would be mobile so that you can move them as you clear areas of ice or need thrust in different directions.
habitation would exist in centrifugal modules to provide earthlike gravity (in addition to natural low gravity) and buried underground to shield them from space radiation. they would include hydroponics facilities, livestock farms, factories, etc. there would be many of these habitats, rotating in different directions to maintain torque neutrality, taking one off-line for maintenance requires taking a complementary habitat off line or changing the speeds of other habitat's rotation temporarily. they could also be built with integrated anti-torque systems such as high angular velocity flywheels.
given the mass of the object any undesired angular momentum should be avoided. since it will be more efficient to relocate the engines and mining facilities than it would be to spin up/down the entire ice dwarf, angular momentum will be kept at a set value and engines will be gimballed slightly to help maintain it. however rotation is not necessarily a bad thing. as you want to keep surface mining even to avoid creating hazards to future engine moving operations. rotating the ice dwarf allows you to move rotation lagging engines forward to lead the rotation to ensure even surface consumption.
power will be all nuclear as the engines and mining facilities would use the lions share of the power and will require their own reactors. habitation modules would require much smaller reactors. it might be more efficient to run a power grid through a network of sub surface habitation access tunnels which would provide transportation access between habitats and allow miners and other surface workers as close to the burn site or other surface facilities (science or communications for example) as possible before emerging to the hostile surface environment. the power gird would be fed by centralized high output nuclear or (technology allowing) fusion reactors. nuclear reactors should be complementary so that byproducts of reactor 1 could be fed into reactor 2 and so on to maximize use of fissile material. if feasabe the grid could also power mining systems and engines depending on their technologies.
ice dwarfs may also contain other materials useful for agricultural and industrial applications, and these could be extracted (filtered from ice or mined from deposits within the ice or extracted from the core) and utilized. given the nature of the vehicle and the need to keep it frozen, it does not allow direct access to the habitable zones of the target solar system. so smaller spacecraft would need to be brought along for the ride (or built on-board) to explore habitable zones of target solar systems, while the mothership itself maintains a high solar orbit. during exploration and science missions, other inhabitants will dispatch themselves on resource gathering missions. parking the "ship" near asteroid belts or in rings of gas giants would aid these operations. you might also transplant the entire colony into a new ice dwarf it propellant supply or resource availability is low. this would allow for exploration of multiple solar systems on a multi-century survey of multiple star systems, you could even leave behind seed colonies on potentially habitable or terraformable world.
this is a concept of a sustainable generation ship and is capable of seeding colonies, gathering information, and is also a viable long-term space colony, because it can move about and exploit new resources. it would require some degree of social engineering to keep continuous generations on the original mission and to avoid war-scale mutinies from wasting essential resources and potentially destroying the viability of the colony. population control will also be necessary to control the expenditure of available resources, and given the limited size of the gene pool would need to prevent or control inbreeding or potently use selective breeding and genetic engineering to engineer a more space-worthy human (or a human suitable for a particular environment for a seed colony).
i have not actually attempted to determine what kind of delta-v such an iceball of doom colony would have and more data on candidate ice dwarfs is required to determine if this idea is viable.
tldr: use ice dwarfs as space ships
