Now we’re moving to the interesting topic of space propulsion, and which one to use. Since GURPS Spaceships offers a large number of them, I’m only going to look at the best ones - some, like ion drives or chemical rockets, are simply going to lack in either thrust or efficiency to be useful.

The Drives

ISP Optimization There are three drives which form a distinct group of best ISP. Those are the Antimatter Plasma Rocket (350,000s), the Advanced Fusion Pulse (290,000s) and the Fusion Rocket (175,000s). However, the APR and the AFP both use expensive fuels ($12M and $50k per ton respectively), while the Fusion Rocket uses hydrogen at $2k per ton. Therefore, it will be superior for almost anybody not looking to optimize delta v at all costs.

High-Thrust There are several drives offering high thrust at the expense of nearly everything else: Orion Drives combine thrust and ISP, but are Orion Drives. Almost as good are the limited-superscience Fusion Torches and Nuclear Salt-Water Rockets. Lastly, there’s HEDM and chemical rockets - high thrust but definitely lacking in ISP.

Cheapest This one probably goes out to the Fusion Torch (depending on whether you’re optimized for mass or cost, choose hydrogen or water). If we’re lacking in limited superscience, the choice should probably be the fusion rocket - at an ISP of 175,000 (that is 90km/s per tank) and using hydrogen, you’re going to have quite some range.

Combat Those should ideally combine acceleration and ISP, with cost as only a secondary concern. It appears clear that the Fusion Torch, filled with water, represents one of the best choices - it’s cheap, it gives 1.5G acceleration per system, and 10km/s per fuel tank should be sufficient for most combat situations. The hydrogen-powered Fusion Torch has the disadvantage of bulky propellant, though that isn’t modelled in Spaceships. Should no superscience be available, the choice is more difficult, and is then between the Orion Drive, HEDM propulsion, and the Fusion Pulse Drive. The Orion seems really bad for flying in formation, HEDM provides almost no delta v, and the Fusion Pulse Drive has a very anaemic acceleration. Still, it looks as if the Fusion Pulse would win in the end, augmented by HEDM as boosters or in auxiliary spacecraft.

Torch or no Torch?

For this, let’s look at the necessary delta v in this universe: First of all, we require ca. 5km/s per jump, with roughly five jumps needed to go between interesting systems. That’s 25km/s dV. A water fusion torch would need 7 fuel systems to make it there without stops, a hydrogen-fuelled torch slightly over two. In comparison, the Fusion Rocket will be able to execute 9 jumps per tank of fuel, and the Advanced Fusion Pulse Drive 15 - although that fuel is far more expensive.

However, the main advantage is the acceleration it gives you, while retaining competitive delta v. Where the Fusion Rocket grants only 0.005g acceleration, the water-propelled Fusion Torch grants 1.5g, and the hydrogen-propelled still 0.5g. That’s a huge advantage in combat, and Fusion-Torch-equipped space forces will probably wipe the floor with those that aren’t.

And even outside of space forces, there is another issue: A fusion rocket cannot achieve orbit after a jump with our FTL numbers; it’s simply lacking in acceleration. This just decided it - fusion torches are available, and are used in almost every craft.

Military craft are definitely going to use water torches - the bulk penalty for hydrogen is far too big for anything with armour. For civilian craft, it’s murkier. If there are no available stops between interesting systems, a water torch will have to spend seven systems of fuel, while the hydrogen torch only has to spend two systems. On the other hand, the hydrogen torch will cost almost thirty times for that trip - pretty sure that it’s not going to be worth it. Water torches still profit from having stopovers at less important systems, which also explains why there’s infrastructure there.

A perfect example for a transport craft would probably be the Zhongguan HSTV (Spaceships 8, p. 20) - replacing one cargo hold with an FTL drive, it would have sufficient delta v for one interesting-system-travel, transporting 11,000 tons of cargo for $210,000 of fuel between systems. On safer routes, four of the seven fuel tanks could be replaced by more cargo holds, increasing transport capacity to 23,000 tons - but that requires waystations selling fuel.

Space Travel

Now that it is decided that there are torch drives, how does the space travel in this setting look like?

First of all, interfaces. To get off planet, the usual way is to use a water-torch powered shuttle (usually winged, but that is optional). Tickets are fairly cheap - $500 for a trip to LEO. On some major worlds, there are space elevators. Those aren’t being built much, since shuttles are much more flexible, but can be used for freight, at a competitive $20 per ton. Once in orbit, you either dock with a station, a High Port in Traveller parlance, or a spacecraft parked in orbit. That’s what you’d do to refuel when there’s no fuel service in orbit - use your own shuttle to transport water from the ground, possibly from an automated extraction-and-purification-installation.

Now that you’re in orbit, you want to get somewhere. To do so, you get onto a passenger transport like the Meizi PSV from Spaceships 8, p. 25. If you’re in luck, you’re sufficiently wealthy to travel in one of the 35 luxury cabins, otherwise you’ll get one of the 154 standard cabins assigned. Those are located in spin capsules, and for most of the time those will spin. However, they will be three times per day, before the acceleration phases that precede and follow each jump (burning into escape orbit, transfer orbit, and parking orbit). The Meizi would be used for less-used routes, and the only planets you’d get to see are the major stops once per week.

Military spacecraft, on the other hand, do not use spin capsules. They are also significantly more crowded and utilitarian than the PSVs. Most of the military spacecraft patrol during peace time, since the FTL setup actually allows piracy.

Should there be a need for operating on the surface, military lift shuttles can easily transfer personnel and equipment, operating LEO-surface-LEO. They will probably be more heavily armoured than the commercial ones, and might have some short-range weaponry to suppress landing zones - the light ones, at least. The heavy shuttles probably aren’t built for deployment into hostile fire, though. Tech also allows for more direct air support, using heavily-armoured gunships. Oh, each of them will probably be VTOL-capable, using their torches to stand in the air. That consumes one fuel tank every 12.5 minutes, though - gunships and shuttles operating in atmosphere should probably mount ram-rockets, which will allow greater endurance.