Interstellar craft ideas?

OK so I'm writing a story (or I will be after I finish exams November 5) and it will involve the first interstellar craft. But I want it to be practical, not something of PURE science fiction with an explanation no better than saying it was magical. How do you think early interstellar craft will work? I'm trying to get it so that it can travel with an average speed of at least 0.5C, though if it's a constant acceleration craft then that means it reaches the speed of light. Perhaps it won't be a constant acceleration craft then. How about it gets arbitrarily close to the speed of light and then stays there for a while before slowing itself down again?
The plan is that I want it get to Epsilon Eridani (about 10 light years away) in 20 years or under.
So far my plan is that it will be a two-stage-non-rocket, using a nuclear blast in an Orion type fashion: http://en.wikipedia.org/wiki/Project_Or ... propulsion)
This will start it off, but the Orion concept's maximum velocity is in the single digit, or less, percentages of c, so that will take too long for the story. So what I imagine is that it will shed off a lot of it's mass once the ship reaches the top speed the Orion propulsion can get it, as an Orion craft needs a lot of mass to avoid lethal accelerations (or you could just reduce the force of the explosion I suppose) and because the next form of propulsion requires minimal mass. A Bussard ramjet:
http://en.wikipedia.org/wiki/Bussard_ramjet
So now you'll have the magnetic sail using the solar wind to push it along (and as long as they're in the heliosphere they might use solar sails as well, minimal mass involved there) before leaving and switching the sails forward to pull interstellar medium into a fusion generator.
So that's the plan so far, but what do you guys think? Have you got some more original ideas for getting a craft across the interstellar 'void'?

You would never reach the speed of light with conventional Newtonian propulsion, no matter how long or fast you accelerate. Mass increases with velocity, and with relativistic effects (time slows down) the amount of energy required to accelerate grows exponentially the faster you go. .5 C would be really good performance out of a drive system over 10 ly, probably unrealistically so (don't feel like doing the math). What you really have to worry about besides shoving things along (which is the easy part believe it or not), is radiation and running into things.

At high velocities, normally benign wave lengths of energy such as the visible light blue shifts towards more energetic and dangerous types. And the high end of natural radiation (Alpha and Gamma) shifts upward into the ?, bizarre energies.

Likewise running into matter as small as individual atoms of interstellar gas at relativistic velocities can be dangerous because these collisions would pack a tremendous amount of differential kinetic energy. Interstellar space is not empty, in fact it may be quite densely populated by fine dust and comet like objects. At high velocities, detection and maneuver become difficult and energy intensive (relativistic effects again).

To solve these known problems, you need either handwavium technology (shields, deflector fields, etc) or extremely thick, or tough "armor", and extremely fast and good sensors and maneuver systems.

We aren't anywhere near ready to venture from the sheltered lagoon of our solar system into the open ocean of interstellar space.

Hey thanks, I admit I didn't think too much about running into things! Shielding of course will have to be taken care of.
And you're right, I should have clarified, I'm aware of mass dilation but what I meant when I said 'reaching the speed of light' I meant reaching velocities arbitrarily close to the speed of light.
So do you have any suggestions as to how radiation and collision issues could be overcome?

For radiation shielding, probably a thick leading layer of very dense material. Someday it may be possible to gravitationally or magnetically bend EM energy away from a ship, this would protect it from radiation.

For collisions, a strong magnetic field to divert particles and stray gases. For bigger stuff, you'll need a "scout" leading the ship by a few light minutes equipped with radar to detect and warn with enough time and distance for the main ship to be able to move laterally to avoid the object.

Just ideas already used quite often in sci-fi...

Yeah I'm thinking magnetics for radiation shielding, it's lighter (even if it would use a lot of energy) and it's not quite as fictional as gravitational bending, perhaps that technology could be developed later in the story...

Sounds good though, thanks!

before we even leave the system we migt want to find a way to travel between the plantets faster. would multiple injens work like the starwars blocade runner or would you just need to build a more powerful injen?

Well, that's if your talking about the real world, but luckily I'm writing science fiction!
So that problem has been overcome to a degree in the time the story is set in. But even in the real world we are developing faster methods of flight through the solar system.
The issue is often not how big the engine is or how many you have but how efficient it is. It doesn't matter if you strap 10 Saturn-V rockets together, the chemical rockets will still be highly inefficient over a long voyage. Solar sails could prove valuable for interplanetary flight, though I'm not sure how well they'd work for a manned mission with a ship big enough to keep a crew alive for the voyage.
But at present there are a number of options for thrust in space which are being researched. You just have to be aware that a 'fast' trip to Jupiter may well never be something in the order of hours like in the movies (not to say it isn't impossible). It's going to become a lot more like the original explorer's faced when traversing the globe with trips taking months or even years one way. We'll just have to see how we go!

I notice that you limit yourself to current physics and don't include any spacecraft engines which rely on bending spacetime into interesting shapes or anything like that. Is that a point of principle for you, or just the way you've decided to go for this particular story?

Aahh well you've predicted my story too many times Rick! I'll have to be more creative...
But yes, I was planning on introducing such ships later. This interstellar craft I'm thinking of now would be the very first to carry humans. Originally I planned for them to be heading to Epsilon Eridani, but in light of recent discoveries, Gliese 581 might be a more interesting option! Or perhaps to spite people, g will be a barren world.
But yes, I'm going to bring in 'warp ships' very soon after the Bussard style. Maybe even beating them to their destination! A main issue with the warp engines is the huge amount of energy they would hypothetically need. I'm wondering if perhaps they'll discover an efficient way to produce antimatter for energy?
Thoughts on an energy source?

That would make for an interesting conflict point to hang the rest of the story. Although (I hate to say it), "its been done" already.

Maybe... Our group of stalwart first mover colonists in their slow boat wake up in the Gliese 581 (don't you think as significant a system as its turning out to be deserves its own name, not a catalog #?), to find that its already inhabited by a humanoid intelligent life. Then you go thru the whole story only to twist it at the end to reveal that the "natives" in fact are human decendants who mastered FTL (which might have had a time side-effect) to beat them to the promised land and biological engineering to adapt themselves to the local environment. Its "been done" as well, but not as much, and gives you lots of room to play with the story and lots of social commentary to make.