Designer's Notes: GURPS Spaceships

GURPS Spaceships is the first book in a planned eight-book PDF series. It focuses on spacecraft design, space operations, and space travel, with the intent of producing a player-and-GM friendly set of game mechanics to support GURPS Space that provides an alternative to the more complex GURPS Vehicles Design System and modular design systems.

The first book on spaceships was actually twice as long as it was originally planned . . . but even so, a few sections were omitted for reasons of space, and some of these are presented below. (Others were moved to later books in the series, which will include both additional rules and plenty of sample spacecraft and discussion of different spacecraft classes.)

Reaction Drives: Behind the Numbers

The reaction drives in GURPS Spaceships are rated for their acceleration in gravities (G) per engine and their delta-V in miles per second (mps) per tank of reaction mass installed. Here's how these numbers were derived from real or speculative spacecraft engines.

Acceleration (G): This is equal to 5% of the thrust-to-weight ratio of the engine. For example, if an advanced nuclear rocket engine is able to produce a thrust of two pounds for every pound that it weighs (a thrust to weight ratio of 2:1), its acceleration per engine system in GURPS Spaceships would be 0.05 2 = 0.1G. Sticklers for realism can include the weight of heat radiators and other important components, not just the engine itself. Delta-V per tank of reaction of mass (mps): This is derived from the specific impulse (traditionally abbreviated Isp) of the engine, a measure of space drive efficiency. To get delta-V in mps per tank of reaction mass divide specific impulse by 3,000 (or alternatively, divide the exhaust velocity in meters per second by 29,400). For example, if a speculative nuclear rocket engine is known to have Isp 900, it would be rated for 900/3,000 = 0.3 mps of delta-V per tank. (Alternatively, use exhaust velocity/29,400).

Ares-Class Battle Cruiser (TL10^)

"Manufactured in the Deimos navy yards, these sleek, wedge-shaped vessels were the pride of the Free Martian Navy. In the dark days after the Terran star fleet was lost at Second Jupiter, they carried the fight to the enemy systems, harassing convoys and outpost star bases. Their design harkened back to old wet navy battleships, with a powerful all-beam armament of varying sizes, including a large tertiary battery for defense against small craft. However, they suffered at long ranges against missile firing ships."
-- Red Star Fleet: History of the Third Space War

This warship was originally supposed to appear in the sample spacecraft, but was cut to save space. It's an example of a limited superscience design: a very heavy cruiser-sized vessel is intended to kill destroyers and cruisers, and to raid commerce. Built with a SM+12 streamlined hull, it masses 100,000 tons and is about 1,000 feet long.

Front Hull

[1-3]

Nanocomposite Armor (total dDR 210).

[4-5!]

Major Batteries (each with a fixed mount 30GJ UV laser).*

[6]

Tactical Array (comm/sensor 13).*

[core]

Control Room (Complexity 10 computer network, basic array with comm/sensor 11, 20 control stations).*

Central Hull

[1-2]

Nanocomposite Armor (total dDR 140).

[3]

Defensive ECM.*

[4!]

Secondary Battery (10 turrets, with 300 MJ rapid fire particle beams).*

[5]

Habitat (310 cabins, two briefing rooms, three labs, eight offices, four minifac fabricators, a 30-bed sickbay, and 1,200 tons cargo).*

[6!!]

Super Stardrive Engine (FTL-2)*.

Rear Hull

[1]

Nanocomposite Armor (dDR 70).

[2-3]

Fusion Torch Engines (0.5G acceleration each).*

[4!]

Major Battery (turret with rapid fire 3GJ UV laser).*

[5-6]

Fuel Tanks (total of 10,000 tons hydrogen, total 30 mps delta-V reserve).

[core]

Antimatter Reactor (four Power Points).*

* 10 workspaces per system.
! high-energy system.
!! high-energy system requiring 1-2 Power Points.

Ares is designed with artificial gravity. Her basic complement are 190 crew: 20 control crew, 10 turret gunners, 10 clerks, six scientists, 120 technicians, and three medics.

TL

Spacecraft

dST/HP

Hnd/SR

HT

Move

LWt.

Load

SM

Occ

dDR

Range

Cost

Piloting/TL10 (High Performance Spacecraft)

10^

Ares-class Battle Cruiser

300

-2/5

13

1G/30 mps

100,000

1,262

+12

620ASV

210/140/70

2

$18.864B

Top air speed is 2,500 mph.

Drive Hazards

Antimatter plasma rockets and torches, fusion rockets and torches, super fusion torches, and all fusion pulse drives produce plenty of heat and hard radiation behind the spacecraft! Although the drives can be operated safely, environmentally conscious authorities are very likely restrict their operation near populated orbits or habitable worlds. This could either result in a total ban from use in inhabited areas, or it might just limit them to operations in out-of-the-way deserts, oceans, or high orbit. These drives may be LC2.

Antimatter pion and antimatter pion torch engines, and (probably) total conversion and super conversion torches will produce directional and highly lethal energy beams. Their danger zone may extend for hundreds or even thousands of miles behind the vessel! Such engines are likely to banned from operation anywhere near a settled planet's space. Space ports may be placed on distant asteroids, or ships may require the equivalent of tug boats to boost them out far enough that they can safely use drives. LC1.

Nuclear saltwater rockets and external pulsed plasma engines produce continuous or pulsed nuclear explosions outside the ship. If you take off from the ground with one of these drives, you'll be leaving behind a big radioactive crater . . . As above, but even more stringent restrictions: LC0!

Ramscoops generate magnetic fields at lethal intensities in front of the vessel, likely covering hundreds or thousands of miles. They're also LC0.

Space drives not listed above aren't free of hazard, but if properly operated aren't likely to raise the hackles of local spaceport authorities. However, there may be specific regulations ("make sure a mass driver uses very fine dust!") but these are likely to apply to any spaceship operations. Superscience reactionless drives and stardrive engines may be perfectly safe or have nasty side effects, at the GM's discretion.

The space combat maneuvers required to actually use high-energy reaction drives as weapon are covered in future volumes of the series.

Spacecraft Systems Optional Rules

A number of rules tweaks and options were developed late in the design process . . . some, like Cosmic Systems or water reaction mass, made it into the book; others came a bit too late to fit into the actual manuscript.

Ramscoops: Magsail Braking

If a ramscoop operating in interstellar space is activated at speeds above magsail velocitity (about 375 mps) but below minimum the ramscoop velocity (about 1,800 mps) it functions as a magsail (GURPS Spaceships, p. 25) to decelerate the spacecraft. Thus, a ramscoop-equipped ship can use its field to (slowly) decelerate from high sublight speeds without using reaction mass.

Handling of Multi-Stage Spacecraft

The rules allow a multi-stage spacecraft's lower stages may be controlled from a Control Room in a smaller upper stage. However, it should also suffer a Handling penalty since the attitude thrusters, etc., are also less massive. (This was ignored in the basic rules, since most multi-stage spacecraft don't maneuver much until they've ejected all their lower stages, instead usually just boosting in one particular direction!) To determine the penalty, find the SM of the stage containing the control room. Compare that to the SM of the lowest stage it's still attached to. This gives the Hnd penalty.

Example: A SM+8 spacecraft is a four-stage rocket. The last stage, with the Control Room, is a SM+5 spacecraft. While all four stages are attached, the spacecraft will suffer a -3 to Handling.

Phased Arrays

Advanced laser weapons may incorporate phased array optics, allowing a flat laser emitter composed of numerous cells that can project either a single powerful beam or multiple smaller beams, of variable intensity.

Any major battery equipped with a fixed mount laser or ultraviolet laser may be designated a phased array. Phased arrays batteries appear two TLs later than usual (TL11 for laser and TL12 for ultraviolet lasers) and cannot be combined with the improved, rapid fire, or very rapid fire options. A phased array laser or UV laser has the option of firing as a rapid fire weapon at one-tenth output or as a very rapid fire weapon at 1/100 output. In addition, a phased array can, if it does not fire, perform the active sensor (ladar) or laser communicator functions (only) of an equivalent SM tactical array.

Small Upper Stage [Front] (TL7)

This is similar to an Upper Stage, but it takes up two systems in the front hull, rather than an entire six-system front section. The upper stage spacecraft will be two SMs smaller; for example, a SM+10 spacecraft has a SM+8 spacecraft as its small upper stage.

(If a hit location roll indicates either of these two systems was struck, instead roll hit location and apply damage to the front hull of the upper stage spacecraft). Otherwise, use the normal rules for upper stages.

Notes for Deck Plans

GMs may wish to create deck plans for spaceships and space stations that follow the general design layout, with front, central, and rear sections divided into individual systems. Since spacecraft designs are based on their mass, the actual size of any system will vary somewhat due to differences in density. The table below shows the number of one-yard hexes per system:

Deck Plans Table

Hull

Armor

Other Systems

SM+5

neg.

2-3

SM+6

neg.

3-5

SM+7

0-2

6-15

SM+8

2-5

16-50

SM+9

6-15

51-150

SM+10

16-50

151-500

SM+11

51-50

501-1,500

SM+12

151-500

1,501-5,000

SM+13

501-1,500

5,001-15,000

SM+14

1,501-5,000

15,001-50,000

SM+15

5,001-15,000

50,001-150,000

Armor is "solid" spaceship hull. Cargo holds, fuel tanks, hangar bays, and open space systems will be 90% or more empty spaces, while habitats, and passenger seating will be 70-80% devoted to open space for the interiors of cabins, rooms, or corridors; the rest will be machinery. Factories will likely be about 50% machinery and 50% open space for assembly lines, etc. Most other systems will be 90% or more filled with machinery, with any remaining space devoted to rooms for workspaces, corridors, or ducts. One exception to the above are control room systems, in which (on larger vessels) most of the mass is distributed over the hull. A control room will generally take up at 3-5 hexes per control station; the rest of the mass is normally devoted to thruster and antenna systems outside the hull.

Drop Capsules

These tiny spacecraft are normally carried in hangar bays, though they may also replace 32cm or larger missiles and be carried in missile launchers. They are smaller than standard craft and have only attitude thrusters, but can reenter atmosphere as per Soft Landing System. See GURPS Ultra- Tech (p. 232) for detailed descriptions of their capabilities.

Life Pod (TL9): Four-person escape capsule with 90 man-days life support.

Drop Capsule (TL10): A basic landing capsule; not reusable; it breaks open a mile up to allow occupants (or packages) to descend via parachute, parawing, grav belt, etc.

Stealth Capsule (TL10): As above, but packed with countermeasures with a stealth hull. Either treat as if it had three Defensive ECM systems, or use the more detailed rules in Ultra-Tech. LC2.

Drop Capsules Table

TL

Vehicle

dST/HP

Hnd/SR

HT

Move

LWt

Load

SM

Occ.

dDR

Cost

9

Life Pod

5

-5/1

13

0.1G/0.3 mps

1

0.5

+2

4SV

2/10/2

$100K

10

Drop Capsule

5

--

13

--

1

0.5

+2

2SV

2/10/2

$10K

10

Stealth Capsule

5

--

13

--

1

0.3

+2

1SV

2/10/2

$50K




Article publication date: September 28, 2007


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