GURPS Spaceships and Traveller

[Pomphis]

David Pulver wrote an article about how to design Traveller ships with Spaceships in JTAS. That article was written earlier than Spaceships 7 and therefore could not use the Smaller Systems rule from Spaceships 7.

He states (IMO correctly) that Traveller ships are about 2 - 10 short tons per dt and that 5 st/dt may be a reasonable rough average approximation.

1 dt of hydrogen is 1 short ton. So if we use mass, not volume, each JNo should require about 2% of mass.

Jump drives were small in Traveller, the basic formula is 1%+(1% per JNo) plus an equal sized power plant.

With the Smaller Systems rule we can work with up to 60 systems per ship at about 1.6% of mass each.

So I am contemplating to build jump drives as stardrives of SM-1, with the requirement being 1 stardrive plus (1 stardrive per JNo). Jump Fuel requirement would be 1 Fuel Tank of SM-1 per JNo. The stardrives would need one "smaller" power point per JNo.

Any comments?

[DangerousThing]

I'm prepping a campaign inspired by Traveller.

Rather than use smaller jump drives, I would make define drives such that the base cost is for J1, double that for a J2, multiply by 5 for a J3.

[Rupert]

I don't know about GURPS Traveller, but in TNE and MegaTraveller Jump drives were also very dense. Thus while they use up little space in a volume based design system they would use up rather more in a mass-based system.

As for fuel, one thing to consider is that a high-jump ship will be considerably less dense than a low-jump ship.

If we assume that non-fuel systems average 5 tons/DTon, and of course fuel systems mass 1 ton/DTon the actual mass of 100 DTon ships of varying jump numbers looks like this:

Jump #   Mass   fuel mass %
0        500     0.0
1        460     2.2
2        420     4.8
3        380     7.9
4        340    12.8
5        300    16.7
6        260    23.1

If we assume the jump drives actually aren't especially massive, and add them to the fuel mass, the mass devoted to jumps is:

J1 4.2%
J2 7.8%
J3 11.9%
J4 17.8%
J5 22.7%
J6 30.1%

Depending on edition and book, a powerplant is also required, which adds to the minimum space/mass required for the jump drive, but in most cases normal space operations (manoeuvre drive, etc.) will require a power plant big enough to power the jump drive, so we can probably ignore it for this discussion.


However, I think a more relevant consideration is "How much of the useful design space does the jump drive plus fuel eat?"

In Classic Traveller that was 1 + (Jn x 11) percent of the total hull. However, some ship's systems were mandatory, such as the bridge (2%, minimum 20 DTons), computer, and accommodations. In a ship of a few thousand DTons and up these eat up 3-4% of the available volume. However, in Spaceships a Control Room requires 5% of the available mass (for the default case I think we can assume not using smaller control and their handling penalty), and a Habitat another 5% (using smaller systems a Habitat one SM smaller will do unless the ship has many guns, small craft, etc.). Thus even with smaller systems a SS design has a little less free space to work in than a CT ship. Also, many systems, such as turret weapons, are quite small in CT, while in SS weapons, armour, and electronics (not really covered at all in CT, aside from the computer(s), but small in MT and TNE) are very massy and take up a lot of the available slots.


So, another question is "How much design space should a jump drive and fuel use up?"

My thinking on this is that SS can't actually get this 'right' (as in, matching Traveller) for all types of ship. Civilian ships in Traveller decline in useful payload very quickly with increasing jump number, but large warships do so more slowly, because there are caps on their armour, screen, and weapon fit-outs that limit those to fairly small parts of their displacement.


My current rules are that a jump drive always takes up one system, and has a price as a Stardrive Engine, multiplied by (1 + Jn)/2. They require one power-point per jump number for twenty minutes, or double that for ten, to be able to initiate jump. Fuel requirements are one system per jump number.

I'm not convinced that this is the best, as it makes for quite capable J4 scouts. However, the five systems J4 eats make quite a difference to the capabilities of warships, and a greater cost would probably cripple them.

I have a feeling that your numbers would make starships very little less capable than non-starships, though YMMV on how much that matters.

[Fred Brackin]

The first time I looked at this question it seemed obvious to me that the number was 10% of the hull per Jump number.

The fact that you're talking hull volume in CT and Hull mass is Spaceships is a meta-game issue. CT ships are limited by their volume and mostly ignore mass. Spaceships vessels are limited by their mass and ignore volume.

So translating from one system to the other by taking the mass of a CT component and then using that mass for Spaceships looks like an attempt at slight of hand to me. Note that I had to run that last sentence through my internal politeness filter three times to get "sleight of hand" rathert than soem alternate term..

If you're trying to build a J-2 trader you ought to be committing 20% of your available ship to fuel. That's the way things have always worked in Traveller.

[Rupert]

Originally Posted by Fred Brackin

If you're trying to build a J-2 trader you ought to be committing 20% of your available ship to fuel. That's the way things have always worked in Traveller.

In MegaTraveller and TNE the fuel requirement was 5% x (Jn + 1)...

More to the point, as I said, many things take up a lot more space in Spaceships than they did in Traveller, and thus if you assume 10% of space goes to fuel per jump number, SS ships with high jump numbers end up very limited, even more so than high jump Traveller designs did.

[Fred Brackin]

Originally Posted by Rupert

In MegaTraveller and TNE the fuel requirement was 5% x (Jn + 1)...
d.

I knew there were reasons I never played (or even bought more than a sample for) those editions. The gross breaks in ship capability made possible by that rule would be one of thsoe reasons.

<shrug> I can build Spaceships designs by counting systems on my fingers til I get to 20 (twice around each hand). Then I look up exact numbers to get stats.

Somebody who wants some complex rule about how many things to have on each ship should be using the design system his complex rule orignated in.

[Pomphis]

Originally Posted by Rupert

My thinking on this is that SS can't actually get this 'right' (as in, matching Traveller) for all types of ship. Civilian ships in Traveller decline in useful payload very quickly with increasing jump number, but large warships do so more slowly, because there are caps on their armour, screen, and weapon fit-outs that limit those to fairly small parts of their displacement.


My current rules are that a jump drive always takes up one system, and has a price as a Stardrive Engine, multiplied by (1 + Jn)/2. They require one power-point per jump number for twenty minutes, or double that for ten, to be able to initiate jump. Fuel requirements are one system per jump number.

I'm not convinced that this is the best, as it makes for quite capable J4 scouts. However, the five systems J4 eats make quite a difference to the capabilities of warships, and a greater cost would probably cripple them.

I have a feeling that your numbers would make starships very little less capable than non-starships, though YMMV on how much that matters.

Thanks. My idea does reduce the size of the jump drive, yes. As you noted, some systems are "bigger" in Spaceships, especially armor. Even minimum armor takes 15% of the ship´s mass and it´s almost impossible to have more than 4 times minimum armor on even the heaviest warships. Compare that to the DR 100 minimum in GT and the DR 130,000 of a Tigress.I am still wondering whether I should permit force screens to help warships get decent dDR. Using one system as jump drive with only cost and power requirement changing by JNo is a good idea.

[Pomphis]

Originally Posted by Fred Brackin

The first time I looked at this question it seemed obvious to me that the number was 10% of the hull per Jump number.

The fact that you're talking hull volume in CT and Hull mass is Spaceships is a meta-game issue. CT ships are limited by their volume and mostly ignore mass. Spaceships vessels are limited by their mass and ignore volume.

So translating from one system to the other by taking the mass of a CT component and then using that mass for Spaceships looks like an attempt at slight of hand to me. Note that I had to run that last sentence through my internal politeness filter three times to get "sleight of hand" rathert than soem alternate term..

If you're trying to build a J-2 trader you ought to be committing 20% of your available ship to fuel. That's the way things have always worked in Traveller.

The J-2 Trader is not the problem. The problem is the Azhanti High Lightning with J-5 (11+ systems), 2 G (1 system), armor (3 systems), a spinal mount (3 systems), other weapons (1 system), hangar (1 system), control room (1 system), habitat (1 system), nuc damper (1 system), meson screen (1 system), and 24 systems even without a power plant.

[AlexanderHowl]

Strangely enough, I usually design spacecraft on the assumption that each metric ton of mass provides sufficient space for approximately one metric ton of liquid hydrogen (~14 cubic meters per metric ton) in order to make them compatible with some of the existing real spacecraft designs. It actually fits fairly well with the dimensions given in Spaceships. Since armor takes up very little space (0.125 cubic meters per metric ton for steel to 1.1 cubic meters per metric ton for ice), any spacecraft with armor will likely have more internal space that their mass indicates.

A streamlined SM+10 spacecraft is probably going to be a cylinder 200 meters long and 20 meters in radius, giving an internal volume of ~168,000 cubic meters (assuming the top 50% is conical). An unstreamlined SM+10 spacecraft is probably going to be a cylinder 100 meters long and 20 meters in radius, given an internal volume of ~126,000 cubic meters. If they both have three components of steel armor, they have 1500 metric tons of steel, which only takes up <200 cubic meters (sloping allows the greater volume of the streamlined vehicle to receive equivalent DR to the unstreamlined vehicle). Of course, this means that the steel armor is only 13 mm thick, which means that it is a very strong steel (something like 2800 Maraging Steel).

[Pomphis]

Looking at armor again, a TL 10 SM+5 USL Spaceship needs about 4.5 Systems to get dDR 10. That´s 22.5% of mass. It costs 675 K$. Using VE2 and the 5:1 ratio, it has 6dt = 3,000 cf = 1,250 sf. At TL 10 expensive metal for DR 100 has a mass of 3.75 tons = 12.5% of mass and Costs 45 K$. Advanced Laminate has a mass of 2.5 tons = 8.33% and costs 500 K$.