[GURPS Fox]

Originally Posted by Varyon

Looking at Spaceships, if you want around 12 G's with water as the reaction mass, you're looking at superscience. TL9+ Nuclear Thermal Rockets do come close, however - using water as the reaction mass, they've got 1.5 G each, for a maximum of 9 G with 6 systems (total 3/10ths of the vessel's mass). With 10 fuel tanks of water (half the vessel's mass), you'll get 2.1 mps delta-V which isn't going to get you much of anywhere fast. You can arbitrarily multiply the delta-V by whatever factor you'd like for your cinematic rockets, of course. Another option might be to allow for high-thrust, like the various fusion reaction engines have access to - this is double acceleration, half delta-V, or 3 G per system (18 G max) and a total of 1.05 mps delta-V when using water; again, you'll need to markedly increase delta-V for this to work out.

If you're willing to go to some limited superscience, the TL 10^ Fusion Torch is an option. Using water, and again with half the vessel's mass in fuel, you're looking at 1.5 G per system and 70 mps delta-V at TL 10^, 210 mps delta-V at TL 11^, and 700 mps delta-V at TL 12^. High-thrust is canonically an option here, for double acceleration (3 G per system) and half delta-V (35, 105, 350 mps, respectively). As an example of what you can manage, let's go with the worst-case - high-thrust water at TL 9^. To go from Earth orbit to Mars orbit, first you've got to burn 2.1 mps to break orbit. You've got 33.9 mps left to play with, but you need to account for both acceleration and deceleration. Deceleration is 0.93 mps less than acceleration (you just need to slow down to orbital velocity, not to a dead stop). If we use all but 0.93 mps delta-V (leaving a little room for error, and making the math easier), that means we accelerate to 16.95 mps, cruise for a while, then burn another 16.02 mps to slow down to orbital velocity. If we accelerate at a comfortable 1 G (we only need the high acceleration for combat), this takes around 45 minutes (12 G would take around 4 minutes), during which time we travel 0.00025 AU, which is pretty much a rounding error compared to the 1.5 AU between Earth and Mars, so we'll ignore it for the next calculation. Traveling 1.5 AU at 16.95 mps takes a bit over 95 days (95 days, 5 hours, 16 minutes, and 48 seconds), then another ~45 minutes to slow down to 0.93 mps. You're probably going to want higher delta-V. One option might be to have an adjustable drive - it only suffers the halved delta-V when used in high-thrust mode (such as in combat). I'll leave it to you to work out the math on that and the other delta-V's (although IIRC, Pyramid #3/79 comes with a spreadsheet that can do most of the work for you).

Thank you for this information.

Originally Posted by Jinumon

This may be a somewhat unpopular opinion, but when dealing with cinematic or superscience propulsion systems, I actually find it's easier and more efficient to work backwards.

I took a crack at this a while back for my own space opera setting (which never got off the ground but that's neither here nor there). Instead of looking at the available options in the various Spaceships books and building a setting around it, I decided on certain standard metrics and then made my own custom propulsion drives based on those metrics. Things like:

• How long do I want it to take a ship to travel across a star system?
• How long should a ship be able to burn for on one fuel tank?
• How many Reaction Engines/Fuel Tanks should be the standard on typical freighters, q-ships, etc?
• How expensive should it be to refill a fuel tank?

Once you've got a decent amount of answers for these questions, not based on existing drives but based on how easy, fast, and economical you want space travel to be, then you can start assigning numbers to things like:

• How much delta-V does a single fuel tank provide?
• How many G of acceleration does a single reaction engine provide?
• How expensive is a ton of fuel?

I find that when it comes to mechanics, it's more important for them to fit the style of game you want to play rather than adhere strictly to realism. So long as it remains internally consistent and gets you the results you want, who cares, right?

Jinumon

Well, at the core, is a war setting, where there is no peace, only a cease-fire until the next war. Anywhere within the solar system, there are at least two factions fighting it out.

One of the core things of the setting that it is a 'future-history' setting, where things progress, however slowly. Originally, the various factions of humanity used NTRs and nuclear lightbulbs to travel between planets, but due to the fact that peace is no longer a thing in the setting, it was found that the rockets aren't good enough in the D-V department to head to somewhere, fight, and then head out to somewhere else. So the various factions invested in researching ways to improve the delta-v to maximize effectiveness.

This led to a high-efficiency fusion drive similar to Transhuman Space's High-Efficiency Plasma Recombustion Fusion Rocket (HePlaR). They're not Battletech efficient (~40 days at 1G for only 1.6kTons of hydrogen!?!) but they're enough to turn a trip that would take years (Earth to Jupiter) and reduce that to a handful of months... and that is before the invention of the 'Interplanetary Drive' (making a trip that would have taken years to a literal handful of months).

Originally Posted by Anthony

It's worth asking yourself what cinematic means to you, and whether it's what you actually want out of a game. It sounds like you want unrealistic but not cinematic.

'Unrealistic' is usually termed 'cinematic' from what I've heard...