The engineering and science we have now, and assumed we will have in 2050, will change. Fusion, fission, relativity, quantum mechanics, and a host of other fundamentals of current physics; were all discovered within the last hundred years. We can conservatively expect physics to change far more in the next hundred years, then it did in the last hundred. What technologies we will have on hand in a century or two are impossible to guess. We could have matter conversion, hyperlight drives, new understandings of inertia and kinetic energy, nanotech, hyper intelligent A.I.s, or all those and far more. Any of these would dramatically effect our ability to travel between the stars. So even though we can't come up with any practical ideas for exploring the stars now, we can be sure our descendants will find it far easier than we imagine.
The reason we in the L.I.T. group assumed few new technologies, is that we could quickly wind up in a science fiction argument as apposed to a starship design project. Not only don't we know which revolutionary technologies will developed by 2050 (50 years ago, would you have believed the incredible stuff we have now?), but assuming any major advance changes everything else in the project.
For example: a couple of technologies frequently talked about are: Nanotech, self replicating machines, and Artificial intelligences with human or greater levels of intelligence. These obviously are related fields, but the effects they'd have on the rest of the mission are dramatic.
Nanotech is a set of technologies currently under research that would build machines the size of complex integrated molecules. In theory; such machines working together, could tear a mountain full of ore down, atom by atom, and reassemble it into manufactured products. They could do this so quickly the mountain would flow into its new forms like it melted. Virus sized robots could cruise through our cells and repair anything from radiation induced mutations to any form of disease or injury. They may allow virtual immortality and eternal youth. And their promoters expect them to be commonly available by 2050.
This could allow the ship to become fully self repairing down to the molecular level. A semi living machine that could continue indefinitely without any concern for wear and tear. Populated by near immortal, superhuman, crews. Able to manufacture almost anything, to any scale, with little if any human assistance. Need a massive infrastructure in the target star system? Drop a set of these and they will transmute a continent to build it for you.
Obviously this starts to eliminate almost any normal physical limits of the ship and crew. The side effects on human society are incalculable. Would such super humans: be to preoccupied to explore the stars? To powerful and impatient to take a long slow flight, or so long lived that adding a few decades to the trip would mean nothing to them.
Self replicating machines
Say you can't build molecular sized machines, but you can build small adaptable robots that can make copies of themselves. They can still revolutionize automation. Can still be dropped on a world and told to restructure a continent to serve our purposes, or mine anything we need in whatever amount. That would be very valuable if you need to large scale mining or infrastructure construction to get home, or do extensive exploration of all parts of all the worlds.
Note that this doesn't assume the machines are intelligent or completely autonomous. After all Bees can build pretty complex structures by following a few rules. Perhaps self replicating computers can do as well. On the other hand we've never come close to making a factory that could run unattended for very long, much less fix or rebuild itself. Some studies by NASA show that such things might not be possible without humans to keep everything running. But things might be a lot different in half a century.
Artificial intelligences with human or greater levels intelligence.
We already build and sell super computers with more power and data capacity then a human brain. In 20 years that should be the capacity of a good home computer. What we can't do is figure out how to teach these brains to think. Will we be able do that by 2050? Will our ship be totally automated? Assembled by robots, and capable of doing all the exploration itself? Will it be filled with a crew of robots, or a mixture of robots and humans?
Unless we said otherwise we assumed none of these technologies is in use on this project. The implications just get to bizarre. Any of the technologies listed above would turn all of Earth culture inside out. Or any of the dozens of other similar technologies We didn't discus, or think of. After a lot of arguing we decided to assume that no fantastic discoveries are made in science and technology in the next 50 years. Which we all agree, is the least likely assumption we could possibly make.