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Old 12-26-2020, 03:01 PM   #1
GURPS Fox
 
Join Date: Jun 2020
Default Cinematic Rocket Delta-V Rules

So, I've heard that there are rules for the various reaction rockets that make them more akin to the rockets used in cinema and/or games (like The Expanse or Battletech), and want to set things up for 'cinematic' rockets and a more realistic 'semi-cinematic' reaction drives.

I'm wondering because a setting that I'm working on would have present-day reaction drives to be considered 'semi-cinematic' reaction drives...
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Old 12-26-2020, 09:10 PM   #2
YankeeGamer
 
Join Date: Dec 2017
Default Re: Cinematic Rocket Delta-V Rules

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Originally Posted by GURPS Fox View Post
So, I've heard that there are rules for the various reaction rockets that make them more akin to the rockets used in cinema and/or games (like The Expanse or Battletech), and want to set things up for 'cinematic' rockets and a more realistic 'semi-cinematic' reaction drives.

I'm wondering because a setting that I'm working on would have present-day reaction drives to be considered 'semi-cinematic' reaction drives...
One way to make a drive more cinematic would be to give them more delta V. (With the ultra tech ones, the figures are speculative anyway. Perhaps your total conversion drive can eject its reaction mass at near lightspeed, though that has other complications...)
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Old 12-26-2020, 09:42 PM   #3
Fred Brackin
 
Join Date: Aug 2007
Default Re: Cinematic Rocket Delta-V Rules

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Originally Posted by GURPS Fox View Post
.

I'm wondering because a setting that I'm working on would have present-day reaction drives to be considered 'semi-cinematic' reaction drives...
It's easiest to do almost anything related to Delta-V in Gurps Spaceships so i recommend getting that.

The Real World/TL 8 rockets in that supplement give you 3 Gs of acceleration per 5% of ship's mass used for engines and 0.15 miles per second of Delta-V per 5% or ship's mass used to carry fuel.

There are dirves in Spaceships do will exceed one or the other or even both of those numbers and sometimes by a lot. Soem are even labeled Cinematic rather than Superscience (TL^) but we probably need more information about what you want. There just isn't a "standard" Gurps campaign or even a "Stadnard Gurps campign with Semi-Cinematic Rockets". Gurps is made to custom fit every game differently.
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Old 12-27-2020, 01:31 AM   #4
GURPS Fox
 
Join Date: Jun 2020
Default Re: Cinematic Rocket Delta-V Rules

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Originally Posted by Fred Brackin View Post
It's easiest to do almost anything related to Delta-V in Gurps Spaceships so i recommend getting that.

The Real World/TL 8 rockets in that supplement give you 3 Gs of acceleration per 5% of ship's mass used for engines and 0.15 miles per second of Delta-V per 5% or ship's mass used to carry fuel.

There are dirves in Spaceships do will exceed one or the other or even both of those numbers and sometimes by a lot. Soem are even labeled Cinematic rather than Superscience (TL^) but we probably need more information about what you want. There just isn't a "standard" Gurps campaign or even a "Stadnard Gurps campign with Semi-Cinematic Rockets". Gurps is made to custom fit every game differently.
Well, it's mostly due to the fact that my ships are double-digit kilotons at the minimum with accelerations going towards a dozen Gs at the most with propellant tanks that are around or less than half the ship's mass.

The setting is one of constant warfare across the solar system and one of the things that the various factions wanted is to have immense delta-v without compromising things like acceleration too much (most factions use water as the propellant, although some factions use methane/decane), for the ships have to have as long of a combat time as possible while being able to patrol between planets, planetoids, asteroids, and habitats. While this has lessened due to the invention of the Interplanetary Drive, most factions assume that it might be prudent to have the option to burn their way through Sol if needed.
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Old 12-27-2020, 05:50 AM   #5
Varyon
 
Join Date: Jun 2013
Default Re: Cinematic Rocket Delta-V Rules

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).
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Old 12-27-2020, 07:12 AM   #6
AlexanderHowl
 
Join Date: Feb 2016
Default Re: Cinematic Rocket Delta-V Rules

Cinematic rocket delta-v is either magic or superscience, just choose your poison and do not fret over the details, as they are no more or less realistic than reactionless rockets (and they generally have many of the same issues).
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Old 12-27-2020, 08:52 AM   #7
Fred Brackin
 
Join Date: Aug 2007
Default Re: Cinematic Rocket Delta-V Rules

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Originally Posted by GURPS Fox View Post
Well, it's mostly due to the fact that my ships are double-digit kilotons at the minimum with accelerations going towards a dozen Gs at the most with propellant tanks that are around or less than half the ship's mass.
Spaceships is serenely untroubled by raw size but your drive is probably going to have to be a Total Conversion drive. You use 21.6 miles per second of Delta-V to accelerate at 1 G for 1 hour.
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Old 12-27-2020, 01:08 PM   #8
Jinumon
 
Join Date: Mar 2013
Default Re: Cinematic Rocket Delta-V Rules

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?

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Old 12-27-2020, 01:30 PM   #9
Anthony
 
Join Date: Feb 2005
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Default Re: Cinematic Rocket Delta-V Rules

In general cinematic rockets behave as needed for the plot and do not have strange things like 'delta-V', but there's a wide range of cinematic rockets, so it really depends on what you're after.
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Old 12-27-2020, 02:25 PM   #10
Jinumon
 
Join Date: Mar 2013
Default Re: Cinematic Rocket Delta-V Rules

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Originally Posted by Anthony View Post
In general cinematic rockets behave as needed for the plot and do not have strange things like 'delta-V', but there's a wide range of cinematic rockets, so it really depends on what you're after.
IDK. I feel like that can be a lazy way of looking at it. I think the impetus of keeping a ship in repair, cargo in the bay, and fuel in the tank can be a great source of drama and story threads, even in a cinematic game.

I don't know about how others feel, but personally as a player I get sort of irked if my GM simply declares something like "you're out of fuel" or "a part on your ship breaks" solely for the purpose of creating tension. Like, "no it didn't, I've been succeeding my Mechanics rolls and paying for replacement parts, nothing should be wrong with my ship."

When real, "fair" disaster strikes because of legitimate misfortune ("Aww man, those pirates rolled a crit and knocked out our life support") or especially due to a lack of player forethought ("I thought we'd make it to port on the fuel we had but I forgot we were gonna make a detour to Hoban's Moon"), it feels more like a problem that deserves to be taken seriously.

And for those reasons, I feel like maintaining some of the minutia of operating a spaceship can be important, if that is the kind of game you want to play. Certain tweaks to realism may need to be made to make things like small, independent crews a viable part of the setting, but I don't think that necessitates a total hand-wavium of routine spaceship operations.

Jinumon

EDIT: Then again you did say there's a wide range of cinematic rockets, so maybe this point goes without saying.
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