The Orion drive will provide a (very expensive) platform for moving large amounts of men and materials quickly across interplanetary distances.  Ships built around less effective drives will be cheaper but much less capable than the Orions.  It seems unlikely that any private concern would, in the near future, have the resources or need to build Orion drive commercial ships.  Most private, and non-military government transport will use rockets, ion drives or solar sails.  Sails will be especially favored by private concerns because of the cheapness of operation � absolutely no fuel costs.  Faster transportation for VIPs or urgent cargos will be provided by souped up, stripped down nuclear thermal rocket powered craft.

If a power wishes to impede the shipping of a rival, non-Orion warships will be the most cost-effective commerce raiders.  These ships would operate like earthly submarines, and it would be well within their power to effectively attack enemy shipping, or engage in anti-�submarine� warfare.  Reconnaissance, intelligence gathering, lurking, stealthily inserting commandos � these are other missions that they might conduct.  They could even serve as a sort of destroyer screen for a force of more capable ships.  As escorts for friendly shipping, they would be useful in warding off the predations of enemy commerce raiders.  But these light warships would be less well suited to the other missions that a space navy would be called upon to fight. 

[wik] Side note: in talking about the relative usefulness of Orions and other warships, I am imagining a time when the solar system is somewhat well settled, and rival powers have emerged, and space warfare has had time to evolve.  Initially, combat between the smaller classes of warships would be the leading edge � until the first Orion warship is built.  I think that the first Orion would be like the British Dreadnought, taking naval warfare to an entirely different level, and possible igniting an arms race.  The first interplanetary warships will be commercial or government ships originally designed for other purposes and retrofitted with weaponry.  Indeed, ships like that will still be part of navies for a long time after the first purpose-built warships are laid down.  But eventually, someone will become sufficiently frustrated with the limitations of conventional ships, and build that first Orion.

Battleship or Carrier?

Since we�ve been so free with analogies to naval warfare, let�s throw out a few more.  If the smaller class of warships, using conventional drives, are to be likened to submarines, what is the proper analogy for the Orion drive warships?  The obvious choices are Aircraft Carriers and Battleships.  Which one it ends up being depends a lot on weapons technology.

On earth, the battleship was surpassed by the carrier because of the advantages of aircraft.  The best carrier without its dive-bombers, fighters, and torpedo planes would be a sitting duck for even an awkward, adolescent battleship.  Why did aircraft have such advantages?  Speed and range.  Battleships were not only the largest of warships, they were the fastest and longest ranging.  Aircraft trumped that by being able to fly above the water at speeds ten times or more faster than the fastest ship, and then drop bombs on the battleship with impunity from thousands of feet up.

Can we imagine an analogous vehicle in space?  We have already seen that an Orion powered ship will be faster and have longer range than any smaller ship.  While an Orion-powered ship could indeed carry fighter-equivalent spacecraft, dispersing your firepower into a bevy of smaller and slower ships does not seem to be as great an advantage as it was for wet navies.  The same logic that drove the development of ever larger, ever more heavily armed battleships seems to apply to spaceships as well.

However, another consideration might yet result in Orion carriers rather than Orion battleships.  The development of autonomous reconnaissance and (very soon) combat drones is well under way.  There is no reason to believe that these developments will not be carried into space � in fact, all of our robotic space probes could be considered non-combat autonomous drones.  The advantages of a non-crewed warship would be many � greater tolerance for acceleration, no need to waste mass on life support and a vulnerable but clever meatsack, and less concern if the drone is lost as opposed to a piloted warship.  I don�t think that the big warships will ever be unmanned, as the limitations placed on communications by the speed of light will require that humans be present at the battlefield.  But that does not mean that drones will not be present on the battlefield.  As I mentioned earlier, the line between weapon, sensor, and drone will grow vague.  Each ship will be attended by a network of drones, feeding sensor data back to the mother ship; and if opportunity presents - deploying itself as a weapon.  A big part of battle management will be the handling of these networks of drones.  (I think that will be true here on earth in a very short time as well.) But these drones � be they weapons platforms akin to fighters, sensor drones, or x-ray lasers, will not make the Orion warship into a carrier.  The primary focus will I think remain on the primary weaponry of the warship; if only because the autonomous drones of various types could never keep up with the mother ship.  It does not pay to deploy millions of dollars of equipment that could be rapidly left behind by a fast moving battle, and play absolutely no part in the battle itself.

So the Orions will be battleships, queens of space.  The generous payloads of Orions will likely see them armed with powerful generators, lasers and masers, particle beam weapons, railguns and metalstorm cannon.  Bundles of lasing rods like those used in the standoff X-ray lasers could be dropped overboard with propulsion nukes, literally gaining more bang for the buck.  The powerful weaponry of an Orion battleship, powered by an onboard fission reactor, would likely outrange as well as outpower any smaller ship.  (Just like with traditional battelships, which could shoot farther than any other.) Armor will be possible, making the battleship resistant to many of the weapons capable of being carried by smaller warships, and even to those mounted on orbital bases.  (An Orion battleship is in effect a mobile base � considering its size.) Crew complement for an Orion Battleship might number in the hundreds � mostly for damage control, but also to manage all the weapons, sensors, drones and communications that would be required by such a vessel.

Next bit will cover what might happen in an actual space battle.

[also wik] Side note: The only reasonable variant on the basic battleship that seems likely is an assault version.  It would perform the traditional naval missions of projection of force and covering assaults.  This vessel would be used to rapidly transport space marines and the means to get them into whatever they�re attacking � winged landing craft, zero-gravity assault boats, or whatever is required.  This type of ship would also favor the types of weapons that could be used to bombard planetary surfaces.  In time, as space navies build more Orions, variations in size and relative power might eventually be grouped into traditional categories such as frigates, cruisers and battleships.  Or we might come up with altogether new names.

[also also wik] I think that in the long run, the traditions of the Navy will be more suited to space warfare than those of the Air Force.  But since the Air Force is closer to space - they will likely get there first.  And we’ll have generals in command of our space fleets.  And that would suck.