Gunboats operating in orbital space around, say, Earth will have powerful, high thrust engines and limited facilities for life support.  They will be based in orbital forts, or perhaps launched atop disposable launch vehicles like the Gemini or Apollo rockets of the sixties.  The life of the crews of these warships will be more like that of an Air Force fighter pilot than that of a submariner � which I think will be the closest analog for long duration deep space warships.

Gunboats, operating in the constrained space around a planet, will engage at shorter distances than their deep space cousins.  In most respects, their armament and sensors will be very like that of a modern jet fighter.  In fact, they will probably look something like a modern fighter � as being able to enter the atmosphere (at least the upper reaches of it) will be a very useful thing.  Aero-braking, skip-jumping along the top of the atmosphere, and similar tactics will all save fuel while increasing the range and maneuverability of the ship.  And being able to land on Earth will be a happy alternative to dying in space in the event of damage to the ship.

Looking beyond the descendents of a marriage between the space shuttle and an F-15, other types of orbital gunboat can be imagined.  Light sail ships, boosted by ground or space based lasers might also be developed.  Heavier warships, analogous to coast guard cutters might linger in orbit for weeks at a time, before returning to base.  If scramjets are ever perfected, then warships operating at the interface between space and the atmosphere might become common.  All of these types would have some capacity to attack targets on the ground, and in fact some might be designed around that mission.  Erwin Sanger, an Austrian designer in the forties, imagined a rocket-powered bomber that would skip along the top of the atmosphere.

In combat within the gravity well of a large planet, altitude will be the most important tactical consideration.  Like the wind gauge for sail-powered warships, gravity gauge will be the dominant factor.  Having the advantage of position will be crucial, in that a position higher up the gravity well translates to more options for maneuver.  Also, shooting up the gravity well is inherently harder than shooting down.  The first pilots of these warships will have to learn the somewhat paradoxical logic of orbital mechanics � slowing down speeds you up, and vice versa.  For pilots used to the straightforward maneuvers within an atmosphere will have to adapt quickly. 

Deep Space Design Tradeoffs

Deep space will offer vastly different challenges to warship designers.  All of the propulsion systems that might be available in the near future have serious limitations.  Two tradeoffs will determine the design of all warships.  The first is mass/acceleration; the second is power/stealth.  I noted in the first part the tradeoffs required by stealth.  Most of the tradeoffs for mass and acceleration will push ship design in the same direction.

The major propulsion systems that could be constructed with current or very near future technology are chemical rockets, nuclear fission rockets, nuclear pulse drives, ion drives and solar sails.  The first three are high thrust, short duration drives; while the last two are low thrust, long duration.  With the exception of nuclear pulse, which I will discuss separately, all of these systems impose the same limitation on warship design: every ounce of mass will reduce the total acceleration the warship is capable of.  Space types refer to this as delta-v, or change in velocity.  It is a measure of the total change in velocity (speed plus direction) that the ship is capable of with a given drive and fuel supply.  It doesn�t matter whether your ship accelerates really fast and then coasts, or if it makes a long slow burn, since delta-v measures the total change.  This makes it a useful comparison between ships even of vastly different design. 

(While solar sails will have effectively infinite delta-v, because they use the solar wind for propulsion, solar sails will not be well suited for combat since the sails are so visible and so fragile.  Warships will be confined to the other drives.)

Ship designers will always be striving to make the ship lighter.  This will allow engines of a given capacity to achieve a higher delta-v.  However, there are things that a warship must have in order to be effective.  Weapons, armor, sensors and stealthing; crew, and food, water and life support for voyages lasting months or more; a storm cellar to protect the crew from solar flares; fuel or reaction mass; these are all things you will need to bring along.  Rockets and ion drives are low energy, and this balance will place a premium on low mass weapons, small crews (and thus lessened life support requirements) and little or no armor.

Weapons that require vast power plants will be right out.  (Both for mass and heat/stealth loss reasons.) Weapons that are themselves heavy will be right out.  Missiles will not be very useful in long-range engagements, due to the fact that a rocket capable of propelling a warhead to a target tens of thousands of miles away in time to affect a battle will be almost as large as a small space ship.  This would seem to put a premium on beam weapons.  However, as we discussed in the previous part, and as Clueless mentioned, power plants capable of powering lasers, masers, and particle beam weapons will be heavy and produce lots of heat.

So, it may very well be that early spaceships will be armed with rapid-fire cannon and machineguns.  With some effort, a high velocity, rapid-fire cannon could be developed for use in spaceships.  Rate of fire would be important, as I discussed in the first part.  The more rounds put in the general vicinity of the target will increase the chance of a hit.  One of the most promising technologies is the Metalstorm system invented by the Australian O�Dwyer.  This system stacks bullets in the barrel, and fires them electronically.  By bundling several barrels together, it can achieve rates of fire approaching millions of rounds per minute.  Gunners on warships would fire hundreds of rounds at a time, laying patterns that would (hopefully) intersect the course of the target.  Variations might include sub-munitions, target seeking or sensor rounds, and explosive rounds.  After firing all its rounds, individual Metalstorm units could be discarded, increasing available delta-v.  Rapid-fire, self contained, requiring effectively no external power, and disposable after use � Metalstorm cannon seem an ideal fit for spaceships.

As technology advances, smaller and more efficient power plants will allow warships to move toward beam weapons that will be more accurate than the cannon described above.  Unless radically better drives are developed, missiles will remain the weapons of orbital gunboats, and not deep space navies.  The mass penalty for missiles with adequate range will simply be too great.  Warships of these types will be armed with cannon; and, if they can be developed, standoff x-ray lasers. 

Deep space warships built around rockets or ion drives will tend toward small.  Small is better for mass and stealth both.  In all likelihood, they will be narrow, to provide a smaller radar and IR signature for enemies to detect.  (That is, as long as the ship is pointing in the right direction.) They will be covered with stealth materials, and the rear of the ship will have complicated and fragile fractal heat radiators as well as the drive exhaust.  Weapons will be concealed beneath the stealth covering.  Life for the crew will be hard, living in cramped spaces for months at a time.  I imagine it will be rather like a submarine.

Orion Drive

The exception to much of the mass considerations discussed above is the nuclear pulse, or Orion drive.  This concept involves building a very large ship with a heavy base plate attached to the back of the ship by some very serious shock absorbers.  Then, you light off a small nuke behind the ship.  Repeat as necessary.  This is an over-the-top propulsion scheme.  With this, you could accelerate very large masses very quickly.  Ships using an Orion drive would simply have to be big just to make the acceleration survivable.  Since you need a big ship; adding armor, huge power plants, or anything else you want is not such a big deal.  An Orion powered warship would be a huge hulking brute.  It would not be subtle, and stealth would be a lost cause. 

No other type of spaceship (based on current technology) could match the Orion for speed and payload.  It will be in a class by itself until and unless someone invents fusion or antimatter drives.  Meanwhile, the inherent limitations of the other propulsion types will limit the kinds of warships that can be built around them.  (As will the existence of Orion powered warships.) And given the requirement for (large numbers of) nuclear devices for propulsion in an Orion, and the stupendous expense of putting that much mass in orbit will probably mean that only governments will ever have them. 

Life for a crewman on an Orion warship will be easy, by comparison.  The generous payloads of an Orion will make for more comfortable quarters, and better life support.  Large amounts of armor will likely contribute to the peace of mind of the crew as well.  Rotating crew quarters providing artificial gravity might even be possible.  The speed of Orion will also mean shorter journeys � weeks instead of months between planets.

In the next part, we�ll look at strategic considerations, and how these ships might be employed.