SFRPG

Only three things have been constant about the FTL drive in my setting, Flat Black. It has always functioned on ships in space. It has always destroyed planets if you used it wrongly. And it has always been called "the Eichberger drive".

In the first version of the material it was just the IS drive from the starship rules in ForeSight. An instantaneous jump drive that worked best at a distance from all stars of 25AU times their mass in solar masses, which got more difficult and dangerous to use with increasing range, and which had its performance degrade after uses (especially long jumps) until it was overhauled using costly parts. Its characteristics implied the use of "outwell" ports, with non-interstellar ships shuttling passengers and cargo between up-port and out-ports as well as orbital lighters shuttling them between down-ports and up-ports. I didn't like the out-well facilities very much, it didn't lead naturally to the lost colonies that I wanted, and as I figured out how to optimise its use I found that most of the human sphere of settlement in "Flat Black" was within one Jump of the centre. The travel time to a colony depended on the mass of the original and destination stars, not on their relative location. That meant that no colony was as remote as I wanted some to be, and it led to government and manufacturing being too much dominated from the centre, with a hub-and-spoke arrangement.

The second version was another instantaneous jump drive, but it forced ships to go to a different place to get to each possible destination, and made the point of arrival uncertain, thus making out-well ports impractical. Ships had to put energy into their drives sufficient to overcome their gravitational binding energy to any nearby stars &c.; drives had limited storage capacity proportional to their mass, and were expensive. Thus a high-spec ship could jump from closer to a star than a cheap or lower-TL one could. On jumping, ships travelled instantaneously along lines of net gravitational force opposite to that they would have accelerated in, and re-appeared at the first point with equal gravitational potential to their starting point. Another reason for moving out into the Black before jumping was to reduce the significance of errors in position. That version did away with the out-well facilities, but when I crunched the numbers it turned out that navigational risk was nowhere near forcing prudent jumps to be short compared with the radius of settled space. Also, an astute reader pointed out that that made it much easier and safer to jump Coreward than Rimward, because of the Galaxy's gravitational field potential.

To make sure that I got the sense of astrography I want, to make the old worlds in the Core central and the new worlds on the Frontier remote, I decided to cut down drastically on the among of realspace travel at the beginning and end of each trip, and to make travel between the stars slow rather than instantaneous. I ran a planet generator over a star catalogue to figure out how big the sphere of settlement has to be to contain the number of worlds I want (one thousand worlds implies a radius of 180 light-years), then figured out how fast ships have to travel to make the frontier as remote as I want it to be. I reckoned that I want ships to travel at a speed in the ballpark of 1,000 times lightspeed to one parsec/day.

In the old days spaceships were designed using ForeSight rules, and optimising the design of second-version ones meant adjusting the ratio of IS drive mass to ship mass to alter the duration of trips by controlling the minimum distance from a star that the ship had to be at to jump. Unfortunately, OS and software upgrades have left me unable to open the old design spreadsheets. Meh! Comparisons are odious, I guess. I'm going to investigate the new version of Eichberger Drive performance using the design rules in GURPS Spaceships, and then see about adjusting things to suit my needs. The Eichberger Drive as currently imagined is a "hyperspace" drive in GURPS terms, and the desired speed of 1,000c is close to FTL-1 (i.e. one parsec per day, 1,191c), not that that matters.

This is where I'm starting out from.

• An Eichberger Drive requires FTL-1, i.e. one stardrive system per ship. I'll adjust drive sizes if I need to after doing some investigations.
• I'll start with the drive costs suggested by GURPS Spaceships and adjust if I need to to get the effects I want.
• This is like the Quantum I/Quantum II hyperdrive in Niven's Known Space: all ships travel at the same speed in hyperspace: bigger drives and higher tech ones don't help that. I'll eventually make the cost or mass of drives a diminishing function of tech level, but fiddle with that after preliminary investigations.
• To operate its drive, a ship needs to be gravitationally un-bound from the nearby planet and star, i.e. travelling at the joint escape speed, but the direction of travel doesn't matter.
• Momentum is restored on arrival, meaning that the ship needs the drive performance to deal with differences in the stars' space velocity as well as orbital speeds of the planets. Total delta-vee requirements are in the tens of kilometres per second. Trading off before-jump velocity and after-jump deceleration requirements is a problem for starship navigators, not PCs.

Any comments? Am I overlooking something that is going to come back to bite me?

One thing you could do to limit jump range is what I did in my setting (currently on extended hiatus):

Jump is only possible within a corridor that is directly between two sufficiently close star - so for example, there is a "jump corridor" between Sol and Barnard´s Star, extending theoretically directly from one star to the other, 1.4 million km wide on Sol´s end and 280,000 km on Barnard´s Star´s end. Jump only happens if the starting point, the destination point, and every point in between lie within that corridor. You *can* jump into deep space somewhere between the stars, but woe betide you if you stray outside the corridor while in deep space, because you´ll have a devil of a time finding it again.

The maximum distance between two stars at which there can be a jump corridor between them depends on the combined mass of the two stars. More massive stars would then be natural traffic hubs because they allow for longer jump corridors. These massive stars are fairly worthless otherwise from a colonization perspective, since they can´t have habitable world, so that would make them interesting in their own right, and give you more accessible and less accessible regions of space, depending on the presence and distribution of more massive stars.

Jump still isn´t possible too close to a major mass like a star or gas giant, so the practical end of a jump corridor could be a couple of AU away from the star - meaning that, if you´re just passing through, you´d have to do some in-system travel to get from one jump corridor terminus to the other, depending on where all those stars are located relative to each other.

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Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. Sir Frederick Hoyle
Earth is the cradle of humanity, but one cannot live in a cradle forever. Konstantin Tsiolkovsky
Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"

The Mote in God's Eye was one of my influences, it is something that I intended from the beginning to emulate. So the Alderson Drive with its jump lanes is certainly something that I considered. But I decided that I didn't want to have to do all the work of finding short paths through a network, especially one in which most of the nodes, representing uninhabited systems, were not themselves of interest.

I could do it. I used to work with computer models of the traffic flow in cities, specifically Adelaide, Brisbane, Canberra, Melbourne, Perth, and Sydney. But to get 1,000 inhabitated worlds I had to lash out on a sphere of settlement with about fifty thousand stars in it, 180 light-years in diameter. And out towards the edges the lists I have are significantly incomplete. They are mostly missing M and K stars that are unlikely to have inhabitaed planets. But those stars would still be significant to navigation in a jump network scheme. I'd have to expend a great deal of effort, and I'd get a poor-quality result because of data limitations.

Then there's the question of what it would get me. An interesting strategic situation for sure. But that's not what I particularly want this setting for. Probably travel that is even slower than I want, since a trip from the centre to the fringe will cost crossing 25 to thirty systems.

I built a handful of different liners using GURPS Spaceships and estimated the cost of providing transport services. The key decision turned out to be using cheap water propellant in fusion torch drives to get up to escape velocity cheaply. The giant automated 100,000-ton liners turned out to be more economical only because the mass of propellant ate proportionately less into their proportionately larger payload.

The results were as follows:

Freight rate, per ton:

• G$ 4.39 + G$ 3.64 per light-year in a large carrier
• G$ 5.14 + G$ 4.92 per light-year in a SM+9 feeder

Fare, per compartment, no service

• G$ 43.43 + G$ 48.53 per light-year in a large carrier
• G$ 45.00 + G$ 54.05 per light-year in a SM+9 feeder

Note that these figures do not include the ground-to-orbit and orbit-to-ground legs, port costs, docking charges, taxes, containerisation, loading, or insurance of the passengers and cargo. Listed fares are per-"compartment"—a bunk space four to each compartment, couchette or shared-stateroom is two to a compartment, stateroom is one to a compartment, and a luxury suite is two compartments. The listed fare covers air, water, sanitary and washing facilities and cooking and dining facilities, but not food, and no service. Also, these fares and freight rates assume full load at a 75% duty cycle. Actual freight rates and fares would have to be somewhat higher to provide food, pay the salaries of stewards, corporals, a doctor, a purser etc., and the fact that the ships cannot average 100% full.

From the owners' point of view the fares and freight rates calculated cover salaries, propellant, parts for maintenance, repair, and overhaul of the drives, straight-line depreciation over fifty years, insurance/losses at 1% per year, and a 5% per year real return on capital. A monopolist or oligopolist with significant market power would charge more.

I think I have probably pushed GURPS Spaceships at least to its limit. It's a rules system that rounds ship sizes off to the nearest half of an order of magnitude. Still, these are okay as ballpark figures. I feel confident in maintaining my position that travel and cargo transport using the Eichberger drive is kind of slow but not very expensive.