CT Only: Book 2, power plant fuel requirements

[pesco]

Hi all,

As I mentioned in another thread, I only recently came across Traveller and have been making my way through my "Little White Books" (Facsimile Edition of 1981 CT).

Regarding the ship construction rules of Book 2, specifically the formula for fuel tankage (p. 15),

0.1 M Jn + 10 Pn​

where M is the mass of the ship, Jn is the "jump number" and Pn is the "power number". I am looking at this in conjunction with the Drive Potential Table (p. 22), which yields the power number for a given hull tonnage and power plant type (letter). The same table is also used for jump number and maneuver drive acceleration.



Cutting to the chase, it seems strange to me that power number, and thus fuel requirements, decrease as hull tonnage increases. All other things being equal, a bigger ship requires less fuel to maneuever!? I am looking at the right-hand side of the equation (10 Pn); the left is specifically stated to be (only) for jumping. For example, a 400 ton ship with a type M power plant requires 60 tons of fuel tankage for regular operation. Increasing the ship's tonnage to 1000 tons (padding it with bulkheads and filler bunnies, presumably) drops the maneuvering fuel requirement to 20 tons, the same as a 100 ton type S Scout/Courier. Yay filler bunnies!

I can only speculate that this may simply be an artefact of not wanting to introduce yet another table, but it irked me enough that I spent some time thinking about it and I came up with the idea to use something like

0.1 M Jn + 4 P​

or

0.1 M Jn + 5 D + 3 P​

for the fuel formula, where P is just the power plant's letter rating (A - Z sans I and O) converted to a number (1 - 24), same with D for the (maneuver) drive. Small ships would benefit and bigger ones would take a hit. But in any case, the problem with these formulas is probably their linearity which tends to feel unnatural. So maybe something like this instead:

     A  B  C  D  E  F  G  H  J  K  L  M  N  P  Q  R  S  T  U  V  W  X  Y  Z
Pn   1  2  2  3  3  3  4  4  4  4  5  5  5  5  5  6  6  6  6  6  6  7  7  7

with the original

0.1 M Jn + 10 Pn .​

Anyway, is this a stupid idea? Has it been rehashed to death? Am I completely missing something (aside from revised rules in other books)? Or is it something that could reasonably be made to work?

 

[McPerth]

Well, that's the way MgT1E CB went, making fuel dependant on PP letter instead of number...

Once it was decided that the MD was reactionless, though, the MD does not consume fuel, only power-

 

[Spinward Flow]

Regarding the ship construction rules of Book 2, specifically the formula for fuel tankage (p. 15),
0.1 M Jn + 10 Pn

It's an artifact of the design rules from an earlier era (late 1970s to be clear) where the assumption was that power plants would be "fuel" maneuver drives (rather than power them). Maneuver drives were assumed to be basically reaction mass rockets, rather than reactionless contra-gravity thrusters. Rockets consume fuel, reactionless drives merely consume electrical power.

Combine that with a premature optimization of the design rules and you wind up with the 10Pn fuel formula that makes little to no sense when you think about it for more than 5 minutes.

There's a reason why LBB5.80 moved to a much more formulaic fuel requirement of 10MJn+1MPn (where M=Hull Tonnage) instead in order to automatically scale the fuel requirements with hull tonnage. It just made so much more sense and was a much "cleaner" method of handling the problem.

 

[Grav_Moped]

The 10Td per Pn rule was the way all (non-jump) drives worked in the first edition. Fuel burn was in kg per G acceleration per turn (ships from the drive table had a higher kg/G-turn cost -- if memory serves it was something like small craft were 10kg per G-turn, ships were 33kg per G-turn).

Critically, this was not proportional to ship size! A 10Td fighter doing a 1 G-turn burn used the same amount of fuel as a 95Td shuttle doing a 1 G-turn burn. (And a 100Td Type S Scout doing a 2 G-turn burn used the same amount of fuel as an 800Td Type C Mercenary Cruiser doing a 2 G-turn burn.)

Obviously, this makes little sense, but that what they'd written.

The starship power plant allocation was supposed to provide something like 2 days continuous maximum acceleration; that is, more than you'd ever need, so in practice you just counted it all as burned every single trip to save on bookkeeping.

Second Edition LBB2 changed small craft fuel burn rates and requirements (mostly by using High Guard rules to build the small craft). For ships using the Drive Tables, the purpose of the 10Pn fuel allocation changed from "2 days full thrust" to "4 weeks of normal operations", but the formula didn't change -- I'd guess that was mostly to keep from "breaking" all previous LBB2 ship designs, at the cost of retaining seriously skewed power plant fuel requirements across the board.

 

[infojunky]

Hi all,

As I mentioned in another thread, I only recently came across Traveller and have been making my way through my "Little White Books" (Facsimile Edition of 1981 CT).

Regarding the ship construction rules of Book 2, specifically the formula for fuel tankage (p. 15),

0.1 M Jn + 10 Pn​

where M is the mass of the ship, Jn is the "jump number" and Pn is the "power number". I am looking at this in conjunction with the Drive Potential Table (p. 22), which yields the power number for a given hull tonnage and power plant type (letter). The same table is also used for jump number and maneuver drive acceleration.

View attachment 2396

Cutting to the chase, it seems strange to me that power number, and thus fuel requirements, decrease as hull tonnage increases. All other things being equal, a bigger ship requires less fuel to maneuever!? I am looking at the right-hand side of the equation (10 Pn); the left is specifically stated to be (only) for jumping. For example, a 400 ton ship with a type M power plant requires 60 tons of fuel tankage for regular operation. Increasing the ship's tonnage to 1000 tons (padding it with bulkheads and filler bunnies, presumably) drops the maneuvering fuel requirement to 20 tons, the same as a 100 ton type S Scout/Courier. Yay filler bunnies!

I can only speculate that this may simply be an artefact of not wanting to introduce yet another table, but it irked me enough that I spent some time thinking about it and I came up with the idea to use something like

0.1 M Jn + 4 P​

or

0.1 M Jn + 5 D + 3 P​

for the fuel formula, where P is just the power plant's letter rating (A - Z sans I and O) converted to a number (1 - 24), same with D for the (maneuver) drive. Small ships would benefit and bigger ones would take a hit. But in any case, the problem with these formulas is probably their linearity which tends to feel unnatural. So maybe something like this instead:

     A  B  C  D  E  F  G  H  J  K  L  M  N  P  Q  R  S  T  U  V  W  X  Y  Z
Pn   1  2  2  3  3  3  4  4  4  4  5  5  5  5  5  6  6  6  6  6  6  7  7  7

with the original

0.1 M Jn + 10 Pn .​

Anyway, is this a stupid idea? Has it been rehashed to death? Am I completely missing something (aside from revised rules in other books)? Or is it something that could reasonably be made to work?

I am note quit sure what you are asking.

Yes, what you are describing is in the post 81 rules.

Bear in mind there has been a bunch of discussion about variant rules. Mostly about bringing Book5 items into Book2.

 

[mike wightman]

It's an artifact of the design rules from an earlier era (late 1970s to be clear) where the assumption was that power plants would be "fuel" maneuver drives (rather than power them). Maneuver drives were assumed to be basically reaction mass rockets, rather than reactionless contra-gravity thrusters. Rockets consume fuel, reactionless drives merely consume electrical power.

Combine that with a premature optimization of the design rules and you wind up with the 10Pn fuel formula that makes little to no sense when you think about it for more than 5 minutes.

There's a reason why LBB5.80 moved to a much more formulaic fuel requirement of 10MJn+1MPn (where M=Hull Tonnage) instead in order to automatically scale the fuel requirements with hull tonnage. It just made so much more sense and was a much "cleaner" method of handling the problem.

Your formula is wrong...
M is actually (hull displacement x 0.01}

 

[AnotherDilbert]

Anyway, is this a stupid idea?

No, of course not.

I might call it overly complicated, though.

Has it been rehashed to death?

Yes...

Am I completely missing something (aside from revised rules in other books)? Or is it something that could reasonably be made to work?

Presumably it is designed to punish small ships. LBB2 is quite clear that larger ships are more efficient and small ships are marginal. Compare the payload of the Scout, Free Trader, and Subsidised Merchant as a percentage of total hull size to see what I mean.

 

[pesco]

Hi again, thanks for the enlightening comments! McPerth mentioned MgT1E ("Mongoose Traveller, 1st edition", I assume), and I've heard High Guard, which is CT Book 5, right?

So in High Guard, I take it from the comments by Spinward Flow and mike whiteman, we have the formula

.1 M Jn + .01 M Pn​

where M is hull tonnage. So the 100 ton ships gain a lot, and others' requirements scale up. Thanks for quoting that!

MgT1E is the one under OGL and it should equal Cepheus Engine exactly, right? I have the latter here and find a Power Plant Fuel Requirements Table on p. 124 of the Cepheus Engine SRD. Converting it to a formula for four weeks of operation, to make it comparable to '81 Book 2:

.1 M Jn + 4 P​

where, again P is power plant letter converted to a number (1-24). Interesting! That is exactly the first formula I suggested in the first post, so I guess it's not stupid. But, as mentioned above, I wonder if the linear nature makes it undesirable. Maybe somebody can say something about that from their experience with MgT1 / CE! Note that the High Guard formula pulls in nonlinearity with the table lookup of Pn.

 

[Dragoner]

For me, I have known it since I have had LBB 2 & 5; though I have not ever changed much, so as to not have to re-configure ships needlessly. Though I have flipped fuel requirements before, and made that the nav skill is a little more relevant it that they roll (1d6+1)-nav for objective time in jump. The fuel consumed is basically for the m-drive being a fusion rocket engine. That works ok, though I also don't have grav plates, so no inertial compensation in that way. I did have a Clarke-tech inertial suppressor, and maybe some sort of orthogonal force generator, just to rationalize the horizontal decks.

 

[McPerth]

MgT1E ("Mongoose Traveller, 1st edition", I assume

You assume right. See that what I told was for CB (Core Book), as in High Guard it worked differently...

MgT1E is the one under OGL and it should equal Cepheus Engine exactly, right?

As I don't know CE, I cannot be sure, but I guess that, while they took MgT1E as a basis, there are changes...

where, again P is power plant letter converted to a number (1-24).

Well, in different words but yes. In MgT it's 2 x PP letter (expressed as a number) for two weeks (So i guess we can say PP letter )expressed as a number) tons of fuel per week (so allowing you to choose how much endurance you want for your ship)...

 

[Spinward Flow]

Perhaps the simplest reconciliation between LBB2 and LBB5 for fuel formulas would be this:

1. Within 10 diameters to the nearby gravity wells, reactionless TL=8+ G-Drive can used for propulsive thrust in normal space. G-Drive requires only electrical power (EP) from the fusion power plant to operate and is therefore extremely fuel efficient.
2. Within 1000 diameters to the nearby gravity wells, reactionless TL=9+ M-Drive is used for propulsive thrust in normal space. M-Drive requires only electrical power (or energy points if you prefer) from the fusion power plant to operate and is therefore extremely fuel efficient.
3. Beyond 1000 diameters to the nearby gravity wells, TL=10+ HEPlaR is typically used for propulsive thrust in normal space. HEPlaR requires expenditure of fuel in addition to electrical power from the fusion power plant to operate and is therefore less fuel efficient when using fuel for (literal) rocket thrust.

So the LBB5.80 fuel formula covers the M-Drive case while not being all that interested in detailing the HEPlaR use cases for starships.
The LBB2.77 fuel formula started with the assumption of detailing the HEPlaR use cases but bungled the formula and explanation.

The best solution then would be to use LBB5.80 for power plants as the "default assumption" for all drives (standard and custom) and then also come up with a "HEPlaR fuel consumption formula" that scales with craft tonnage and acceleration Gs per unit of time. The basic idea being that "most" interplanetary maneuvering will not require consumption of extra fuel to provide HEPlaR thrust (1000 diameters of Sol reaches past the orbit of Saturn, for example, but not all the way to Uranus) ... but there would be "edge cases" where having an extra fuel supply for sustained HEPlaR thrust maneuvering beyond 1000 diameters to the nearby gravity wells in the outer reaches of solar systems becomes necessary/advisable.

Such an approach would create a "two tiered interplanetary maneuvering environment" for craft, with 1000 diameters being the dividing line (which is also the Hop Drive Shadow dividing line, for those who are reading ahead).

The key point would be that both standard LBB2 and custom LBB5 maneuver drives would naturally incorporate BOTH technologies (M-Drive and HEPlaR) into their maneuver drive requirements once sufficient tech level thresholds have been reached (as noted above). The only distinction that would need to be made from a gaming rules perspective would be that the "fuel formula" for maneuvering CHANGES from inside the 1000 diameter limit (reactionless, no extra fuel needed) to outside the 1000 diameter limit (reaction thrust, rocket fuel consumption required).



Accomplish that relatively minor tweak to the rules and everything starts working together relatively seamlessly.

Type-S Scout/Courier have "excessive power plant fuel" (20 tons instead of only 2 tons) because their mission tasking may force them to rely on HEPlaR thrust beyond 1000 diameters more often than other types of craft. The 10x normal fuel allocation also allows the 3 ton cargo bay to be stocked with additional life support reserves (150 person/weeks per ton) for extended exploration and/or survey missions that require most of a year between refueling opportunities so as to have extremely lengthy loiter times on station.

Basically, you change the explanation for the "excessive" fuel requirement from being one of "the power plant is stupidly inefficient and wasteful" to being one of needing fuel reserves "far in excess of normal" in order to support operations outside the norm for other mission types required by other starships. That large fuel fraction reserve then becomes an important factor in the Type-J Seeker modification for both HEPlaR thrust support and extended loiter time for prospecting missions.

Turn what would be assumed to be a negative in the previous context into a positive in a new design paradigm context.

Once maneuver drives are assumed to have two "modes" of operations (reactionless/reaction) that use different fuel formulas, everything starts falling into place relatively neatly.

 

[daryen]

So in High Guard, I take it from the comments by Spinward Flow and mike whiteman, we have the formula

.1 M Jn + .01 M Pn​

where M is hull tonnage. So the 100 ton ships gain a lot, and others' requirements scale up. Thanks for quoting that!

If you are trying to stay purely in Classic Traveller, then using the above formula for fuel requirements fixes the excesses of both the tiny ships (e.g. Scout) and the ships larger than 1000 tons. It's purely based on CT and keeps things super, super simple. (You're already using percentages for jump fuel. It can't be that hard to do the same with power plant fuel.)

However, if you don't like that, then, yes, you have lots of options you can take from any edition of Traveller out there. Knock yourself out and use whatever you want and makes the most sense to you.

Personally, I use two solutions in Classic Traveller*:
- If I want to only use Books 1-3/TTB as the foundation, I use the quoted fuel formula, but keep everything else pure Book 2. Any ships that can't be made by Book 2 (like actual warships) exist, but don't have any statistics. Their combat factors are "run now, surrender, or die; choose one".
- If I want more detail, use Book 5 for starship construction and replace all Book 2 designs with equivalents remade using Book 5.
But this is just me. Use what makes most sense to you.

[*] Again, this is when I use CT. If I am using a different system, I use that system's construction rules.

 

[aramis]

Hi again, thanks for the enlightening comments! McPerth mentioned MgT1E ("Mongoose Traveller, 1st edition", I assume), and I've heard High Guard, which is CT Book 5, right?

Yes, MGT or MgT refers to Mongoose Traveller. (Certain disparagers still use RTT , just in case you go looking at the posts from 2008-2012 or so).
High Guard can mean any of the following:

• CT-Book 5, 1979 edition
• CT-Book 5, 1980 edition
• A rules section in the MegaTraveller Referee's Manual.
• MGT1 Book 2: High Guard.
• MGT2 High Guard.

 

[pesco]

Hah, OK, thanks. No such thing as a simple answer, right. ^^

 

[Spinward Flow]

The best solution then would be to use LBB5.80 for power plants as the "default assumption" for all drives (standard and custom) and then also come up with a "HEPlaR fuel consumption formula" that scales with craft tonnage and acceleration Gs per unit of time.

And ... I finally found what I needed in Beltstrike.

Copy/cross-posting from The Xboat is a HG2 design thread for consolidation of topic and clarity of information relevant to the conversation.

---

CT Beltstrike, p5:

Fuel Is the other factor. Fuel use during a prospecting and mining expedition is significantly lower than in normal operations, since constant acceleration is rarely undertaken.
The fuel consumption table on page 11 shows the requirements of various types of maneuvering in terms of fuel use per hundred tons of ship. Basic power is used at at times, including when maneuvering. Every maneuver (matching course with an asteroid, for instance) uses at least one hour's fuel at the 1G rate. The referee and/or players should keep track of a ship's fuel supplies: the ship should not be permitted to run out of fuel. It is possible to refuel by locating ice chunks, skimming gas giants, etc.

CT Beltstrike, p5:

Excess Fuel Use: fuel use has been higher than necessary, due to wasteful maneuvering. Subtract 1D x .001 tons from the ship's remaining fuel.

CT Beltstrike, p11:

Added for reference.

Note that the table is in fuel consumption per 100 tons of craft.
At 0.002 tons of fuel per 100 tons of craft per G of acceleration per hour ... you have an equivalency of 50,000 G-ton-hours per 1 ton of fuel.

50,000 G-ton-hours per 1 ton of fuel means:

• 100 ton ship = 500 G-hours per ton of fuel spent maneuvering
  • 500 hours @ 1G
  • 250 hours @ 2G
  • 125 hours @ 4G
  • 100 hours @ 5G
• 200 ton ship = 250 G-hours per ton of fuel spent maneuvering
• ... etc.

If we assume that 4 weeks or 28 days=672 hours (28*24), then that means that a 100 ton ship can "usefully consume" no more than 1.4 tons of maneuvering fuel (700 hours, so a +4% safety margin on 100% usage) per 1G of acceleration during those 4 weeks. If the safety margin were increased to 750 hours (a +11.6% margin) it would make the basic computations simpler at 1.5 tons of maneuvering fuel.

Therefore, a 100 ton Type-S Scout/Courier performing a continuous 2G acceleration burn for 4 weeks would consume 2 tons of fuel for power plant fusion (LBB5.80 formula) and 2.688 tons of fuel for the HEPlaR reaction thrust (Beltstrike formula) with a recommended 3 ton fuel allocation for adequate reserve (my quick houserule eyeballing the situation) over and above the basic power plant fusion support requirement (LBB5.80 formula).

In other words, a 100 ton Type-S Scout/Courier can consume up to 5 tons of continuous 2G maneuver+power plant fuel per 4 weeks ... or up to 3.5 tons of continuous 1G maneuver+power plant fuel per 4 weeks.

• (Tons/100) * (Pn + G*1.5) = continuous acceleration for 4 weeks using HEPlaR reaction thrust fuel consumption maximum

This would then necessarily mean that a Type-S Scout/Courier with 20 tons of fuel reserve not needed for 1J2 could have about 4 months of continuous 2G endurance or over 5.5 months of continuous 1G endurance before exhausting their fuel supply ... a very "healthy" fuel reserve for long duration surveys and exploration missions.

 

[Spinward Flow]

I would also note that in practical terms, starships ought to be capable of microjumps with their jump drives (basically within the same map hex) which in turn means that for all intents and purposes a starship should not need to to HEPlaR reaction thrust burn for more than 1 week (2 at the most) in order to reach their destination under nominal conditions, rather than needing a full "one month subscription" for HEPlaR fuel consumption ... a single week ought to suffice for most starships (Seekers and Scout/Couriers being a rather obvious endurance exception).

Non-starships ans small craft will still need to have sufficient HEPlaR reaction fuel capacity to reach their destination (with continuous acceleration being optional, since they could coast on momentum/inertia part of the way).

---

So if you want to modify the above fuel formula for a variable number of weeks of HEPlaR endurance allocation instead of a default 4 weeks, you wind up with this fuel formula instead:

• (Tons/100) * (Parsecs*10 + Pn + G*0.35*weeks)

Tons/100 is usually going to be the hull tonnage, but with external loads this could potentially be circumstantially increased and thus is not necessarily a constant.

Parsecs*10 is the jump fuel needed to transit the specified number of parsecs before needing to refuel.

Pn yields 4 weeks worth of power plant operations at full power and includes reactionless maneuver thrust within 1000 diameters of a gravity well.

G*0.35*weeks determines the HEPlaR endurance allocation dependent upon the number of weeks at continuous thrust multiplied by the acceleration Gs of the maneuver drive which need to be used beyond 1000 diameters of a gravity well.

 

[Spinward Flow]

So if you want to modify the above fuel formula for a variable number of weeks of HEPlaR endurance allocation instead of a default 4 weeks, you wind up with this fuel formula instead:

• (Tons/100) * (Parsecs*10 + Pn + G*0.35*weeks)

For an even more thorough flexibility for the fuel formula for broader use than just 4 weeks of power plant endurance, I suppose it ought to be this instead:

• (Tons/100) * (Parsecs*10 + Pn*0.25*weeks + G*0.35*weeks)

1. Tons/100 is usually going to be the hull tonnage, but with external loads this could potentially be circumstantially increased and thus is not necessarily a constant.
2. Parsecs*10 is the jump fuel needed to transit the specified number of parsecs before needing to refuel. 0 parsecs is an option for craft 100+ or mandatory for craft below 100 tons. It is possible to have more jump fuel allocated to a starship than can be used in a single jump, allowing multiple jumps before needing to refuel.
3. Pn*0.25*weeks worth of power plant operations at full power and includes reactionless maneuver thrust within 1000 diameters of a gravity well. 4 weeks is the standard minimum requirement, although long duration prospectors.exploration, survey and system defense craft however may have a need for more than the minimum 4 weeks of endurance.
4. G*0.35*weeks determines the HEPlaR endurance allocation dependent upon the number of weeks at continuous thrust multiplied by the acceleration Gs of the maneuver drive which need to be used beyond 1000 diameters of a gravity well. 0 weeks is an option for craft that have no need for maneuvering capacity beyond 1000 diameters. Starships will rarely require more than 1 or 2 weeks of HEPlaR endurance allocation before refueling due to the ability to microjump within a star system. Long duration prospectors, exploration, survey and system defense craft however may have a need for a more than a 1-2 week reserve of HEPlaR maneuvering capacity before needing to refuel.

 

[Spinward Flow]

And finally, perhaps the last bit of refinement I can think of for the CT fuel formula to make it as flexible as possible.

• (Tons/100) * (Parsecs*10 + Pn*0.25*weeks + G*0.05*days)

 

[mike wightman]

And Beltstrike postdates HG80 - from which we can vbe assured that the GDW crowd still considered the m-drive a reaction drive, despite the wording, and it was a DGP error/retcon that gave us the reactionless drive.

 

[AnotherDilbert]

Nothing in Beltstrike even hints at reaction drives or reaction mass.

You need fuel to run the power plant to produce power for the M-drive, as usual.


But, yes, Beltstrike fuel consumption is obviously based on HG, not LBB2, giving small LBB2 ships extreme mission duration, but large LBB2 ships extremely short mission duration.

E.g.: A 5000 Dt PP-1 LBB2 ship requires a 10 Dt fuel tank.
It would consume 5000 / 100 × 0.05 = 2.5 Dt fuel per week with no manoeuvre and 2.5 Dt fuel for each hour of acceleration.
So, a 2 week trip would take 5 Dt fuel for basic power, leaving 5 Dt or 2 h worth of acceleration and deceleration.
A 4 week trip would take 10 Dt fuel, leaving 0 h worth of acceleration. Basically unusable...


IMTU I would not touch this rule with a 10' pole. It directly contradicts the powering down rules, gives ridiculous results, and requires silly levels of book-keeping; but that is just my house rule, of course.