Primary Drive System:
multi-cycle Ram Augmented Interstellar Ramjet (RAIR)
The first drive idea I came up with, and used as the assumed system for the Explorer Class of ship, is A multi-cycle Ram Augmented Interstellar Ramjet (RAIR). It would scoop up reaction mass from interstellar space like a pure ram scoop, but it would only use it as reaction mass, not fuel. But the scoop system could simultaneously scoop up fuel thrown ahead of the ship by a fixed launcher back in our solar system.
The system is basically an electromagnetic accelerator running through the core of the ship, powered by onboard fusion reactors. The accelerator could accelerate scooped-up interstellar matter (or ram flow), or reaction mass carried in the ship. Again, this kind of system is different from a true Bussard ramscoop drive in that the ram flow is not fused for power. Indeed, it normally isn't even slowed down. The engine functions in various ways at various speeds.
At low speed interplanetary runs, the drive works like a conventional ion rocket or mass driver. Stored reaction mass is feed into the electromagnetic or electro static accelerator core. Unlike normal thermal rockets, an ion thruster works more efficiently with heavy atom ions. So I'll assume we are storing iron for reaction mass. Specific impulse varies depending on the exhaust velocity of the expended mass.
For acceleration out of the Sol system the ship will deploy its ramscoop and switch to the externally fueled RAIR configuration I came up with. As mentioned above, a normal RAIR doesn't try to fuse anything it scoops up. It just ionizes it (with a laser), scoops it (interstellar gas and dust) up, and accelerates it as reaction mass in a linear accelerator. This gets around the Bussard Ramscoop problem of getting more drag trying to fuse the interstellar matter than you get back as thrust (and the fact we can't find enough in interstellar space to run it), and could put any interstellar matter to best use. But you still need a lot of fuel to power the accelerator.
The idea I came up with to get around this fuel problem is to launch the fuel ahead of the ship with orbital accelerators (thus the externally fueled part of the name). The accelerators back home throw out HUGE quantities of frozen pellets of whatever fusion fuel is selected, in the path of the ship (called the acceleration track). The fuel is launched at measured and timed speeds, so that the ship will encounter a fairly steady stream of fuel during its run down the fuel loaded acceleration track. As the ship accelerates, it catches up with fuel going at increasing speeds, but always at speeds slightly slower than the ship itself. This way the ship doesn't have the drag a true ramscoop would have accelerating interstellar fuel that is going much slower than the ship is. All incoming mass is hit with a short length of electric or magnetic forward thrust once it enters the ship. The fuel, being close to the speed of the ship, will be accelerated to the ship's speed and can be transferred to temporary fuel tanks. The interstellar mass, being at a far different speed and direction than the ship, will blow past the magnetic catcher into the main thrust accelerator (powered by the soon to be burned fuel).
If you can load a 1/4th light year track with enough fuel to keep the ship accelerating at 1g, the ship will (after 6 months) exit the track at half of light speed (0.5c). If a pure ramscoop can work at any speed, it should work at this speed. But assuming we can't make a pure ramscoop work (as we are) the ship can accelerate the external mass using power supplied by fuel stored on-board, or the ship can coast to the target star. Assuming Alpha Centauri, in the later case it will coast for about eight years.
Now for the bad news - you have to slow down. We can't pre-load the deceleration course track with fuel at the target star because it would be virtually impossible across interstellar distances. Carrying enough fuel / reaction-mass to decelerate at the target star would be prohibitive at any great speed. Fortunately, there may be a solution to this dilemma, from a rather unlikely source.
G. E. Seger found a 1978 paper by Heppenheimer on ramjets. Heppenheimer applies radiative gas dynamics to ramjet design and eventually proves that radiative losses (via bremsstrahlung and other similar synchrotron radiation-type mechanisms) from attempting to compress the ram flow for a fusion burn would exceed the fusion energy generated by nine orders of magnitude (IE. about a billion times!). By conservation of energy laws, those energy losses have to be made up from somewhere. The only suitable source around is the starships kinetic energy. So a bad Ram Scoop drive makes a great starship brake.
By reconfiguring the RAIR fields to attempt to compress and initiate a fusion burn in the ram flow, the starship instead will be generating a tremendous braking force. The resulting plasma (fused or unfused) can then be run through the main accelerator (rerigged as a generator) and used to generate electricity. Which in itself will cause further drag on the ship. Or, if we power the accelerator in reverse and force the pre-fusion plasma back down the throat of the oncoming ram flow, it will serve as a reversed plasma drive.
Once in the target star system the ship once again becomes a fast Plasma or Ion drive ship. Its first target will no doubt be the outer star system and a asteroid, moon, or comet rich in fuel and reaction mass. After that it can shuttle back and forth in the star system. Though it will be crawling compared to its high speed interstellar run. Its 1g thrust capacity will allow it to rapidly reach any part of the star system.
If we drop most of the exploration equipment and eat the consumables, the ship should be a lot lighter on the return leg. This should make it easier for it to accelerate without an externally loaded acceleration fuel track. The ship may be able to load itself with massive amounts of extra fuel, burning all the fuel, even most of the fuel that would have gone into decelerating the ship, into accelerating it. But this must be worked out in more detail. (See Internally fueled fusion rockets.)
Otherwise, the crew could be forced to construct an automated fuel launcher system at the target star. This seems a clumsy and unreliable method, but I haven't thought of another.
The ship can not only use its ramscoop brake to slow down, it can use the Externally fueled RAIR scooping up fuel from a deceleration track at the edge of Sol, preloaded with fuel. This works like when we first left the Sol system, except that the fuel is blasting right down the throat of the ship at high speed. The fuel will be the mass that's the hardest to accelerate to the ships speed, causing very high drag, further slowing down the ship.
Hopefully between the high fuel drag, ram drag, and the engines in full reverse, the Explorer class ship will reenter our home star system Sol. Decades older, probably worn out, and definitely antiquated, but loaded with data banks full of information.
The problem with using an interstellar scoop, is their isn't much in interstellar space to scoop up. We found papers that proposed1000 km diameter scoops that only weighed 200 tons. Assuming your moving at 1/3rd the speed of light (100,000 kilometers per second) with a scoop area of 1000 km across (pi*R^2=pi(50,000,000cm)^2 = 7.854E15 cm^2). You'd be scooping up the mass in 7.854 E25 cubic centimeters of space.
A big question is the composition of interstellar space. A classic assumption is that there is nothing out there but about 1 atom of hydrogen per each cubic centimeter of space. More recently, people guess it might as little as .054 atoms per cubic centimeter, or as much as 10. Even more recently than that (say the last few months) it has been proposed that there may be a lot of long-chain carbon molecules in space. Perhaps 60-200 atoms / molecules. These small, dark, heavy molecules might be the missing 90-99% of the mass of the galaxy (euphemistically called "dark matter").
So far, no one really knows. This is unfortunate, because the composition of the interstellar medium makes a hell of a difference in the design and feasibility of a Ramscoop or RAIR-based starship. If we assume one hydrogen atom per cubic centimeter (at a proton mass of 1.673 E-27 Kg), and assume we're running at 1/3rd light speed (0.333c). Our 1000 km in diameter scoop, would scoop up a ram flow of 131.4 grams per second. That's about 345 tons a month. That doesn't sound to bad unless you realize how much fuel our ship would need, and remember that the scoop weighs 200 tons (and you really want the mass at slower speeds). Given the amount of time the scoop could be used it couldn't scoop up enough to be very helpful. So this stardrive would go into the impractical bin.
If on the other hand, you assume that each cubic centimeter had a hundred atom carbon molecule in it, the same scoop would give us over a thousand times as much mass to work with. Making a RAIR drive powered from stored fuel far more attractive. But since we don't know. Its hard to recommend this drive.