Volume-Based GURPS Spaceships: The Rules
The following is a rough rules document for a modification of GURPS Spacship’s design system that makes it volume-based. It is not complete—you’ll need the original book to construct your own spacecraft.
The system follows a small number of steps, as with the original:
- You choose the size of your spacecraft, which informs volume, SM, and measurements.
- (NEW) You choose the structural strength of your spacecraft, which informs its hit points.
- (NEW) You choose the armour strength. Armour isn’t modeled as components but added to the three sections separately.
- You add components.
- You compute statistics.
1. Choose SM
As in the original system, choose your spacecraft’s SM on the following table. This table is an extension to the original table in Spaceships:9.
💡 Designer Notes: How did we get the table?
Originally, I wanted to make the table simply based on the volume: An SM+5 spacecraft hat a length of 15m (because that’s SM+5 on the Size and Speed/Range Table). But that means what’s previously been SM+10 in Spaceships is now SM+8, meaning I’d have to change all the numbers for all of the systems.
Instead, I assume that we don’t have a perfect cylinder as a shape but rather an imperfect and non-smooth hull shape. That hull shape makes it easier to target. So a 15m x 5m cylinder would be SM+5 going simply by its longest dimension. But, as Basic:550 says, “Box-, sphere-, or blob-shaped objects or characters add +2 to SM”. Both of these make a good argument for the 15m x 5m cylinder SM+7, which gets us back to the Spaceships numbers.
Width is assumed to be a third of that. Estimated mass is based on an unarmoured spacecraft.
Remember that Robustness is just SM + 7. An SM+5 spacecraft has a Robustness of 12. An SM+10 spacecraft one of 17.
| SM | Length (m) | Width (m) | Estimated Mass (t) | Base Mass per Component (t) | Base Cost per Component |
|---|---|---|---|---|---|
| +4 | 5 | 1.5 | 10 | 0.5 | $10K |
| +5 | 7 | 2 | 30 | 1.5 | $30K |
| +6 | 10 | 3 | 100 | 5 | $100K |
| +7 | 15 | 5 | 300 | 15 | $300K |
| +8 | 20 | 7 | 1,000 | 50 | $1M |
| +9 | 30 | 10 | 3,000 | 150 | $3M |
| +10 | 50 | 15 | 10,000 | 500 | $10M |
| +11 | 70 | 20 | 30,000 | 1,500 | $30M |
| +12 | 100 | 30 | 100,000 | 5,000 | $100M |
| +13 | 150 | 50 | 300,000 | 15,000 | $300M |
| +14 | 200 | 70 | 1,000,000 | 50,000 | $1B |
| +15 | 300 | 100 | 3,000,000 | 150,000 | $3B |
| +16 | 500 | 150 | 10,000,000 | 500,000 | $10B |
| +17 | 700 | 200 | 30,000,000 | 1,500,000 | $30B |
💡 Designer Notes: What’s an MP and CP?
We need to decide what the “default density” is for components. Vehicles to the rescue! Almost all components in that book are “weight/50 cf”, i.e. 50lbs/cubic foot. That’s almost exactly 800kg/m³ (it’s 800.9, which is so close I’m astonished).
Therefore, 1 MP (i.e. the Base Mass per Component) is 800kg/m³.
Do note that you’ll also often see 40kg/m³—that’s because this is how much a component contributes to the total spacecraft density (i.e. it allows us to use addition instead of the mean to find the total density).
How much does everything cost? The important thing here is to find a baseline. The Spaceships book gives us something between $2000/ton (Hangar Bays, for example) and $2M/ton (Ramscoops). Weapons are $120K/ton (so $96K/m³).
Ultimately, there’s only one real choice for me: We’ll want a base price that alternates with the same 1/3/10 progression mass does. I’ll set the base price at $20k/ton, then. That’s $16k/m³, more specifically. At SM+10, base price for a component is $100M. That means 1 CP is $20,000/ton.
💡 If you’re interested in the concrete volume numbers, they start at 12.5m³ for SM+4 and 37.5m³ for SM+5 and increase by a factor of ten every two SMs. But the actual numbers don’t matter because they’re never used but abstracted away.
💡 In computing the estimated mass in the table, I assume one habitat component and normal structural mass; those two roughly cancel out. That’s hugely inaccurate, though—a passenger spacecraft will have much less mass than that; a heavily-armoured military spacecraft much more.
2. Structural Strength
Contrary to the Spaceships system, we also introduce structural mass. That’s the backbone and frame of a spacecraft, strengthening it. We’re using this to easily allow cargo spaces or open spaces to have no mass at all—that’s because they profit from structural mass.
By default, a spacecraft has a structural mass of 1 MP, which costs 1 CP. This gets modified by structural strength as follows:
| Strength | Mass | Cost | Robustness |
|---|---|---|---|
| Light | 0.25 | 0.25 | SM + 3 |
| Medium | 1 | 1.0 | SM + 7 |
| Heavy | 2 | 5 | SM + 11 |
💡 Designer Notes: Where did we get these numbers? The original references for structural mass are from Vehicles:18f. These are dependent on the surface area, i.e. scale with mass^(2/3). THS:175 instead scales it by volume. That’s repeated in Vehicle Expansion:5. THS numbers give M x Hull Spaces / F; using a medium frame of Foamed Alloy, that’s 5/8 x Hull Spaces tons. With Hull Spaces being 500 cubic feet (about 14m³), that’s 45kg/m³, fairly close to our 40kg/m³ definition of a CP, and so I define Medium Strength as simply 1 CP. Mass multipliers result directly from THS’ numbers. Note, though, that I don’t allow modifying the structural material because it introduces significantly more complexity.
💡 Optional: More Structural Strength Variety: I’ve based the options on the frame strengths from Vehicles/THS, but I’ve eliminated Super- and Light, and Heavy Strengths (and renamed Extra-Light to Light and Extra-Heavy to Heavy). That’s because I assume you’re operating in one of three regimes:
- You have a normal spacecraft. Just use Medium.
- Every gram counts. You’ll most often use extra-light, because Light isn’t much worth it. And you’re not a balloon-tank Atlas rocket, so Super-Light is out.
- You’re a military spacecraft. You take all the HP you can get, and therefore Extra-Heavy.
3. Armour
Next up, you will need to choose armour, and choose so separately for the Front, Sides, and Back.
Decide on the mass you want to invest into armour: That defines how thick your Armour is, relative to your SM. This differs significantly between front/back and side armour: The latter is far harder to armour because its area is so much bigger. The table also shows you the Armour relative to the spacecraft’s Robustness assuming Heavy Structural Mass (the assumption is that all armoured craft have Heavy Structure). Remember from my Conditional Everything post that armour doesn’t matter if it’s Robustness - 7 or less (because you don’t take any damage from these attacks anyway).
If you streamline your spacecraft, you get one less armour potential for the same mass.
💡 Example: An SM+10 spacecraft has a Robustness of 17. Heavily armouring its front with 5 MP of Carbon Composite gives it Armour 22. Attacks of up to Wound Potential 22 are simply ignored. At WP 23, we reduce the wounding by 3, still receiving Severity+2 (Mortal) wounds. If you power through front armour without an armour divisor, the craft stands no chance. If you use the same amount of armour on the side, it will only give you Armour 19, which reduces WP 20 from Severity+3 to Severity+0, saving you from the first death check.
Of course, if you armour your spacecraft like this, it takes 15 MP (front, back, sides). That’s almost as much mass as the rest of it will probably be—combined. It will also set you back 15 CP.
Carbon Composite Armour Table
| MP | Front/Back Armour (Relative to Robustness) | Side Armour |
|---|---|---|
| 0.5 | SM + 6 (R-5) | |
| 0.7 | SM + 7 (R-4) | |
| 1 | SM + 8 (R-3) | |
| 1.5 | SM + 9 (R-2) | SM + 6 (R-5) |
| 2 | SM + 10 (R-1) | SM + 7 (R-4) |
| 3 | SM + 11 (R+0) | SM + 8 (R-3) |
| 5 | SM + 12 (R+1) | SM + 9 (R-2) |
| 7 | SM + 13 (R+2) | SM + 10 (R-1) |
| 10 | SM + 14 (R+3) | SM + 11 (R+0) |
💡 Designer Notes: How did we get these numbers?
I first took a specific size, SM+10, and looked at the dimensions. Length is 100 metres, width is 30. That makes front and back area (assuming a cylinder) about 700m² each and side area about 9500m², i.e. the side area is about 14x as large as the side areas.
However, armouring just the front still makes is possible to shoot around that armour. If you’re off-axis by 10° or so, you can aim for the side. You’ll pass through more of the side armour because it’s at an angle, but the sides are usually much lighter armoured. With some assumptions (you’ll want to be armoured such that a 20° off-axis attack still hits your front armour), you end up with a virtual front armour diameter of 64 meters, which is about 3200m² of front area, ⅓rd of the side area. Therefore, the same mass in side armour gets you 3 Armour Levels less than for front armour.
At SM+10, base mass per component is 5,000t. 5,000t of steel (at about 8t/m³) is 625m³. If you apply this equally to our front area, you get about 20cm of armour which, according to GURPS’s default assumption of 70DR/inch for RHA (I’ll call it 30DR/cm, though 27.5 would be more accurate), is DR 600. That’s exactly on the threshold between Armour 14 or 15, which is SM + 4 or +5. We’ll make it SM + 5; that means Steel armour of 1 MP is SM + 8 Armour.
For costs, I looked at THS again. Carbon Composite has a C of 0.02, meaning $20,000/t. That’s exactly 1 CP/MP. Therefore, Carbon Composite is our default armour, with 1 CP/MP. That gives another +3 to Armour (see below for other armour materials).
Applying the same armour to the sides, we get significantly less armour potential: Dilution by a factor of 3x is equivalent to 3 steps on the SSR table, so that’s 3 Armour less for the same mass.
To get some more variety, choose the armour material on the table below. The Base Material is Carbon Composite. Use the table below to account for material.
| Material | Armour | CP/MP |
|---|---|---|
| Slag | -7 | 0.003 |
| Steel Alloy | -3 | 0.03 |
| Titanium Alloy | -2 | 0.1 |
| Foamed Alloy | -1 | 0.3 |
| Carbon Composite | 0 | 1 |
| Metal-Matrix Composite | +1 | 3 |
| Nanocomposite | +2 | 10 |
| Diamondoid | +3 | 30 |
💡 Designer’s Notes: How thick is your armour? In many cases, it’s interesting to see how thick your armour is. The best way of determining it is looking at the Armour Potential you’d have if your armour were Carbon Composite. That means if you’re mounting Diamondoid, subtract 3 from your actual armour potential; if you’re mounting Steel, add 3.
Armed with this, we can first build this for steel: At roughly 30DR/cm, 1cm of steel is Armour Potential 7. Accordingly, Armour Potential 7 is 1/3rd of a cm of Carbon Composite. Armed with this knowledge, we build the following table, in which you can find your armour thickness. To get RHA-equivalent, take the thickness of your actual Armour Potential + 3.
Example: The thickly-armoured example above has AP 22/19/22 of carbon-composite. That has 100cm/30cm/100cm of thickness, with RHA-equivalents of 300cm/100cm/300cm.
| Carbon-Composite Armour Potential | Armour Thickness |
|---|---|
| 10 | 1 cm |
| 11 | 1.5 cm |
| 12 | 2 cm |
| 13 | 3 cm |
| 14 | 5 cm |
| 15 | 7 cm |
| 16 | 10 cm |
| 17 | 15 cm |
| 18 | 20 cm |
| 19 | 30 cm |
| 20 | 50 cm |
| 21 | 70 cm |
| 22 | 100 cm |
| 23 | 150 cm |
| 24 | 2 m |
| 25 | 3 m |
| 26 | 5 m |
| 27 | 7 m |
| 28 | 10 m |
| 29 | 15 m |
| 30 | 20 m |
4. Components
As in the base system, all components have a cost, and might have a power or workspace requirement. Because my volume-based and the Spaceships mass-based SM are the same, we can just reuse the statistics from the book—with a few exceptions.
You also need to track Mass points now. Most components have 1 MP of empty and 1 MP of loaded mass (i.e. their mass does not change). Exceptions are noted in the descriptions.
Cargo Hold
A cargo hold has no empty mass; that is included in the structural mass above. Loaded mass greatly depends on the goods being carried. For “generic” freight, assume 0.5 MP. Maximum is 2 MP regardless of actual density: You can’t just load the cargo space full of iron.
Below, TEU is the number of Twenty-Foot Equivalent Units (shipping containers) you can fit.
| SM | +4 | +5 | +6 | +7 | +8 | +9 | +10 | +11 | +12 | +13 | +14 | +15 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Cargo (m³) | 0.6 | 2 | 6 | 20 | 60 | 200 | 600 | 2K | 6K | 20K | 60K | 200K |
| TEU | - | - | 1/5 | 2/3 | 2 | 6 | 20 | 60 | 200 | 600 | 2K | 6K |
| Approx. Loaded Mass (t) | 0.25 | 0.75 | 2.5 | 7.5 | 25 | 75 | 250 | 750 | 2,500 | 7,500 | 25K | 75K |
💡 Designer Notes: For loaded mass, we use the assumption from 3e’s Traveller of 5t per cargo module (500cf), which is 0.5 MP. (Actually, it’s 0.4, but 0.5 is much easier to work with.)
A single TEU is about 33m³. I assume 30m³, such that we can fit two into one SM+8 component. I assume a TEU carries 12.5t.
Control Room
Use the same numbers as with the Control Room system. Density is 1 MP. However, this does not include control stations—use Passenger Seats (see Habitat) for that.
💡 Designer Notes: Why not include control stations? That’s because it doesn’t mesh well with the other numbers. At SM+5 in the original system, it has 1 control station, while a passenger seat has 2, meaning the control station takes up half of the system. At SM+ 10 (15 stations, 600 seats), control stations only make up 2.5% of the system. That’s why I eliminated them.
However, it will not break too much if you keep including them for simplification.
Engine Room
Treat this as 1/2 MP. It’s half access space, half tools.
External Clamp
External clamps no longer exist as a separate system because they don’t have a volume. Rather, their empty mass is 1/10th of the equivalent MP of the docking spacecraft. Loaded mass is then equivalent 1.1MP. Ignore empty mass if it’s less than 0.1 MP.
Example: An SM+10 spacecraft wants to have a cradle for a 100t AKV. At SM+10, Base Mass Per Component is 500t, so 10t (the cradle mass) is 0.02 MP, which we ignore. Loaded mass is 0.2MP. Loading five AKVs would get us an empty mass of 0.1 MP, and a loaded mass of 1.1MP.
💡 Designer Notes: This is based on THS’s external cradle. These are 1/10th of the mass of the carried spacecraft; obviously, that’ll need you to calculate MP yourself.
New Option: External Container Frame
This is a new option: It’s essentially a girder onto which containers can be docked/hung. In contrast to a cargo hold, this only works for vacuum-sealed containers (Type 05/8 rather than Type 00/8, in Traveller: Far Trader’s parlance, which cost triple), you can’t access it in flight, and it’s not covered by your armour or aerodynamical hull. In exchange, you get much more cargo capacity for a smaller footprint.
| SM | +5 | +6 | +7 | +8 | +9 | +10 | +11 | +12 | +13 | +14 | +15 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Cargo (m³) | 40 | 120 | 400 | 1,200 | 4K | 12K | 40K | 120K | 400K | 1.2M | 4M |
| TEU | 1 | 4 | 12 | 40 | 120 | 400 | 1200 | 4K | 12K | 40K | 120K |
| Approx. Loaded Mass (t) | 15 | 50 | 150 | 500 | 1500 | 5K | 15K | 50K | 150K | 500K | 1.5M |
| Cost | 30K | 100K | 300K | 1M | 3M | 10M | 30M | 100M | 300M | 1B | 3B |
💡 Designer Notes: We base this on an external cradle from THS: It’s about 15m³ of structural support per 125t carried, and masses 12.5t empty. That’s a density of 800kg/m³ (so 1MP) empty, and a density of 10 MP loaded. It costs $20K/ton, so exactly 1 CP.
Fuel Tank
In some ways, fuel tanks stay the same: Their cost certainly does. However, they are now (surprise!) volume-based. They mass 0 MP empty; their loaded MP depends on the fuel.
💡 Designer Notes: We start with the fuel tank from Vehicles 2e: A TL8+ tank weights 0.5 lbs per gallon, takes up 0.15cf per gallon, and costs $5 per gallon. Volume assumes 90% is usable for fuel; I’ll treat it as 100% for easier math (the structural support supplies the actual mass). Mass is 65kg per m³, for about 0.075 MP. Light or ultra-light tanks (as used in THS) are half or 1/10th that mass. Indeed, if you look at the cost of a Fuel Tank in Spaceships, a 5t tank costs $30K; that’s 1100 gallons of water, so $5500—the fuel tank in Spaceships might already be ultra-light anyway. 0 MP empty mass is therefore reasonable.
One new introduction is the split between rocket fuels based on hydrogen and those that are not. Hydrogen has a much lower density but far better ISP; see Reaction Engine, Chemical for details.
| Fuel | Loaded MP |
|---|---|
| Water | 1.2 |
| Hydrogen | 0.1 |
| Coolant | 1 |
| Ionizable Reaction Mass (e.g. Argon) | 0.7 |
| Rock Dust | 2.3 |
| Rocket Fuel (Hydrogen) | 0.3 |
| Rocket Fuel (Storable) | 1.2 |
| Jet Fuel | 1 |
| HEDM rocket fuel | 1 |
| Nuclear Pellets | 1 |
| Uranium-Saltwater | 1.2 |
| Nuclear Bomb Pulse Units | 1 |
| Matter/Antimatter | ? |
Habitat
Habitats are less dense than usual systems, and therefore have fewer cabins per system than in the mass-based system. They have 1/10th MP loaded or empty mass.
Unless, and this is the complicated part, you have cargo space replacing some of your cabins. That complicates things. For this, note the volume converted to cargo (each cabin gives 10m³, so you can also use the number of cabins and multiply it by 10), then divide that by the m³ a cargo component gives at your SM. Divide that by 2 to get the expected load mass.
In the table below, read fractional cabins as n cabins per m components; at SM+7, you need five components to make up one cabin. This does mean that smaller spacecraft will be made up of mostly cabins. Usually, you’ll use the Cost ($) row for cost. This includes the full life-support version (air, climate, water).
Passenger seats are integrated here, as are the Control Room’s control stations (i.e. a bridge or cockpit). For a full passenger seat system, use the number of seats in the Seats row. For fewer passenger seats, replace each cabin with 10 seats.
Life Support We assume full life support for these systems. If you only need 24 hours, use the cost of one SM smaller (i.e. an SM+10 passenger seat system with 24 hours of life support which seats 500 people costs $300K rather than $1M for full life support).
For a cockpit, use passenger seats: 2 systems for SM+4, 1 seat for SM+5, 5 seats for SM+6. Everything bigger will probably want a bridge; treat that as a Specialized Room for Habitats: 1 Cabin can be traded for a Small Bridge, which can support 4 people. 5 cabins are a medium bridge (20 people), 10 cabins a large bridge (40 people). This will probably be the maximum you’ll see.
Open Spaces are now a habitat type. Just treat it as a bigger Establishment; at a capacity for 100 people, this is equivalent to 5 establishments. Therefore, treat it as 10 cabin-equivalents.
| SM | +4 | +5 | +6 | +7 | +8 | +9 | +10 | +11 | +12 | +13 | +14 | +15 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Cabins | - | 1/6 | 1/2 | 3/2 | 5 | 15 | 50 | 150 | 500 | 1,500 | 5,000 | 15,000 |
| Seats | 1/2 | 3/2 | 5 | 15 | 50 | 150 | 500 | 1500 | 5000 | 15K | 50K | 150K |
| Cost ($) | 1K | 3K | 10K | 30K | 100K | 300K | 1M | 3M | 10M | 30M | 100M | 300M |
💡 Designer Notes: Habitats
Habitat volumes are based on THS (and Vehicles), which give a space of 500 cubic feet (14m³) and a mass of 1t. That gives it a density of 70kg/m³; roughly 1/10th of our 800kg/m³ base density. Therefore, we give it a density of 1/10th MP.
At 14m³ per cabin, we can fit 45 cabins into SM+10 (625m³/component). If we remove a bit of cubage from this for access points and common facility, the typical cabin might be 10m³—2 metres high, and 2.5x2 metres area. Fitting four bunks into this works: Each has a height of a metre, and a width of one. You stack two of them on each side and still have half a metre of shared access space. That’s very, very cramped; indeed it reminds me of the bunks from Das Boot. As intended, I suppose.
Ten bunk rooms would have another 40m³ for common areas and access; suppose half of that are available for useful rooms. 10m³ (same floor plan as a cabin) will plausibly give you a shower, two toilets, and a sink; another 10m³ gives you a very small kitchen. Cozy.
The same floor plan sits four people on a bridge; that’s almost exactly four Roomy Crew Station from Vehicles:75 (1.15m³) with Bridge Access Space (3x volume), for 13.8m³.
Even loaded, a full bunk room adds about 400kg of people. That’s only 0.025 MP; I’ll accordingly ignore loaded mass for passengers.
Hangar Bay
A hangar bay has an empty mass of 0.2 MP. It can load one spacecraft of three SM smaller, 3 of four SM smaller, etc. Loaded mass depends on the carried spacecraft: Take the mean of their MPs, then divide by 30. If they are different sizes, you have to take the weighted mean.
Example: A hangar system on an SM+10 spacecraft has room for one SM+7 spacecraft. We can also fill it with 10 SM+5 AKVs. If these are 60 MP each (i.e. are 3x as massive as a “normal” spacecraft their size since they lack habitats or long-term endurance), load mass is 2. Add it to the 0.2 MP empty mass and you get a loaded mass of 2.2.
💡 Designer Notes: A hangar bay, in Spaceships, loses 2/5th of its mass to machinery, airlock systems, and so forth. It is 1/20th of the size of the mounting spacecraft (so 5%). A full spacecraft three SM smaller is 1/30th the mass (so 3.3%). If I use 1/3rd loss to machinery, it makes the numbers much nicer: A single hangar system can mount one spacecraft of 3 SM smaller, or triple that for 4 SM smaller, 10x for 5 SM smaller etc.
To get loaded mass, remember that while a component has a density of 800kg/m³, it’s also only 1/20th of a spacecraft. I did this to simplify most math. That means a spacecraft of 20 total MP is only 1 MP when treated as a component. Additionally, they only fill 2/3rds of the hangar, so the total factor you have to divide by is 20 * 2/3 = 30.
Open Space
This is removed as a separate system. See Habitat.
Passenger Seating
This is removed as a separate system. See Habitat.
Reaction Engines
Engine statistics look a bit different now. First of all, we refer to TWR rather than the acceleration it grants you. Different TWR per TL are separated with a slash. We mainly note ISP for computing the actual performance later on—if you build rockets, you actually do have to use the rocket equation.
However, the entry in dV per Tank can give you a rough estimate on how much dV a single tank gives you. This assumes a single tank, with the rest of the spacecraft being 19 MP empty. As in Spaceships (and the real world) you gain much more from more fuel percentage-wise: Having half your spacecraft (so 10 systems) being fuel tanks gives you about 12-15x as much dV—or 18x if you use Hydrogen. Using 15 systems boosts it by about 20x, with hydrogen being almost 30x. Many engines can use water as an alternative to hydrogen. These have lower ISP but much higher densities and therefore dV per tank—they are marked with a “W” in the dV per Tank column.
Now, this list is pretty gigantic, but in most settings, only a few engines are going to be relevant. Contemporary? Chemical rockets and ion drives are it. Have a Fusion Torch? That obsoletes the Fusion Pulse Drive, for example.
| Engine | TWR | ISP (s) | Fuel | dV per Tank (km/s) |
|---|---|---|---|---|
| Chemical Rocket | 200 | 400 | Storable | 0.25 |
| HydroLOX Rocket | 80 | 500 | HydroLOX | 0.1 |
| HEDM | 40 | 1500 | HEDM | 0.75 |
| Ion Drive | 0.01 | 9000 | Argon | 3.25 |
| Mass Driver | 0.2 | 900 | Rock Dust | 1 |
| Nuclear-Thermal Rocket | 2/4/10 | 900/900/1350 | Hydrogen | 0.05/0.05/0.07 |
| W: 0.18/0.18/0.3 | ||||
| Nuclear Light Bulb | 0.2/1 | 2400 | Hydrogen | 0.13 |
| Nuclear Saltwater Rocket | 40 | 7500 | Uranium-Saltwater | 4.6 |
| Orion Drive | 40 | 6/9/12/24K | Nuclear Bomb Pulse Units | 3/5.5/6/12 |
| Fusion Pulse Drive | 0.4/1 | 15K/30K/120K | Nuclear Pellets | 7.5/15/60 |
| Advanced Fusion Pulse Drive | 0.1 | 60K/300K/1M | Nuclear Pellets | 30/150/500 |
| Super Fusion Pulse Drive | 400/2000 | 1M | Nuclear Pellets | 500 |
| Fusion Rocket | 0.1 | 36K/180K/540K/ 1350K | Hydrogen | 2/10/30/70 |
| W: 7.5/37/110/275 | ||||
| Fusion Torch | 10 | 45K/135K/450K | Hydrogen | 2.5/7/23 |
| W: 9/27.5/90 | ||||
| Super Fusion Torch | 1000 | 1350K | Hydrogen | 7; W: 275 |
| Antimatter-Thermal | 2/4/8 | 5400 | AM-Boosted Hydrogen | 0.3; W: 1.1 |
| Antimatter-Plasma | 0.2 | 360K/1M | AM-Boosted Hydrogen | 19/52.5 |
| W: 75/200 | ||||
| Antimatter-Plasma Torch | 20 | 360K/1M | AM-Boosted Hydrogen | 19/52.5 |
| W: 75/200 | ||||
| Super Antimatter Plasma Torch | 2000 | 1M | AM-Boosted Hydrogen | 52.5 |
| W: 200 | ||||
| Antimatter Pion | 0.1 | 10M | Matter/Antimatter | 5125 |
| Antimatter Pion Torch | 2 | 10M | Matter/Antimatter | 5125 |
| Total Conversion Torch | 20 | 300M | Anything | W: 180,000 |
| Super Conversion Torch | 1000 | 300M | Anything | W: 180,000 |
💡 Designer Notes: TWR rather than acceleration is mostly because mass now varies. Since all engines mass 1 MP, we can just divide TWR by the total MP of the spacecraft to get acceleration in g. If you want to compute this yourself, multiply the acceleration number in Spaceships by 20 to get TWR. For example, a Fusion Torch gives 0.5g acceleration. That gives it a TWR of 10.
To get dV per tank, just multiply the given dV in Spaceships by 1.5 to get dV in km/s. For computing ISP, we note that dV = ISP * g * ln(starting/final mass). That means, assuming you have 1 tank, it’s dV = ISP * 10 * ln((19 + density)/19), i.e. ISP = dV * 2,000. (because dV is given in km/s).
The dV per 1/20th mass assumes 1/20th of the mass of the spacecraft is made up of fuel.
Other Engines
For other engines, we mostly need their TWR. Multiply the original number by 20 to get this or, for your convenience, see the table below.
| Engine | TWR |
|---|---|
| Jet Engine | 20 |
| Reactionless Engines: | |
| - Rotary | 2 |
| - Standard | 10/20 |
| - Hot | 20/40 |
| - Super | 1000/2000 |
| - Subwarp | 10,000 |
| Space Sails: | |
| - Lightsail | 0.002 |
| - Magsail | 0.02 |
Smaller Systems
As introduced in SS7 and SS8, we can also have smaller systems. Use the statistics for 1 SM smaller. MP is the average over all systems.
Upper Stage
An upper stage has no empty mass (but it’ll never actually come to that since lower stages don’t exist without upper ones). For a normal upper stage (front hull, so 6 systems), take the total mass of the upper stage and multiply it by 3/10. For a small upper stage (2 systems), divide the total mass by 10.
Weapons
Weapons have the standard density of 1 MP. I’m using a different scaling than the Conditional Damage from Spaceships post. See below for details. For reference, a spacecraft has SM + 11 robustness (with Heavy structure); it needs 3 MP to armour its front to SM + 11 Armour Potential with standard Carbon Composite armour, 10 MP for the sides. Using better armour reduces this, to up to 1 and 3 MP respectively for Nanocomposite (but you pay 30x as much for this).
For now, though: A major battery (1 weapon per component) does SM + 10 WP. This is modified by the type of weapon: Anti-Particle or Plasma beams get +2 WP, Graviton beams -2. You can change the size of your weapon: You can replace one weapon with three of a smaller size (spinal, major, medium, secondary, tertiary) for -1 wound potential. Rapid or Very Rapid fire reduces WP by -2 and -4 respectively.
Kinetic weapons, in another departure from the original Spaceships, do the same damage as a beam weapon at their size, though EM guns and grav guns get effectively +2/+4 to WP due to their relative velocity from rest. All can be shot down, though.
Missiles get a +4 bonus to WP due to their size.
For kinetic weapons, rather than dealing with converting kinetic weapons and their ammunition weights (at SM+5, a missile launcher is two-thirds ammunition; at SM+14, 10%), we’ll just leave them as-is.
💡 Designer Notes: Weapon and Armour Scaling
With the default numbers from my post, it becomes difficult to actually kill a spacecraft: Let’s say an SM+10 spacecraft mounts 3 major batteries. Each of these, if they are lasers, do 16 WP (-2). It will have 21 Robustness, i.e. the unarmoured spacecraft will take 64 hits to get to the first death check. Armouring it even slightly (0.5 MP to the front, 2 MP to the sides, all hardened) gives 16 Armour Protection and makes it completely immune to laser fire. The same applies to all sizes since we scale everything by SM. Increases in weapons penetration can be counteracted with more expensive armour or more armour.
One mitigating factor is the component damage rules (SS1:61). Penetrating damage of at least 10% of total HP means a disabled system. That’s Severity - 6, i.e. where wounds begin to accumulate. Normally, we’d need 128 hits for a first death check—except even at ten hits, we can expect most systems to be disabled. Having this as a target number means we get spacecraft that get dramatically shot to pieces with components failing (and being repaired by PCs!) rather than destroyed spacecraft. It also fits with naval battleship combat, where you’d expect a battleship to be combat ineffective (not sunk!) after about 20 hits.
If we want a major battery to cause at least Severity - 6 then, for an SM+10 spacecraft, we need it to do 15 post-penetration wound potential. This then depends on armour; if we use a laser with (-2) armour penetration, 1 MP of carbon composite from the front (18 Armour Potential) protects fully against 18 WP, makes 19 WP do 16 penetration (Severity-5), makes 20 WP do 18 penetration (Severity-3), and 21 WP do 20 penetration (Severity-1). I’ll set the WP for a beam weapon to SM + 10, and disallow hardening on armour.
That means you’ll need 3 MP of carbon-composite armour to be immune to a major laser battery from the front, and 7 to be immune from the sides. It’s less with better armour: Diamondoid reduces this to 1 and 3 MP respectively. However, at that TL you can get x-ray lasers with (-4) armour penetration and so indeed need 2 and 5 MP respectively. This is prohibitive.
5. Statistics
The main difference is that we have multiple masses. For all, to compute the actual masses, take the MP and multiply them with the Base Mass Per Component for that SM. We refer to them as follows:
- Empty Mass: The mass of just the spacecraft without any fuel or load.
- Payload: The mass of all payload, for example ammunition, cargo, carried craft, or fuel not useable by the craft’s engines. This can change in game, but it’s good to keep it at a certain value for statistics.
- Fuel: The usable fuel for the spacecraft.
- Dry Mass: Empty Mass + Payload.
- Loaded Mass: Empty Mass + Payload + Fuel
- Combat Mass: Empty Mass + Payload + Fuel / 2. This assumes some fuel has been used before.
Example: An SM+10 spacecraft with 14 MP empty mass and 21 MP loaded mass. At SM+10, the Base Mass Per Component is 500t. Empty mass is 7,000t; loaded mass is 10,500t.
To compute acceleration, sum up the TWR of all engines in use. Divide it by the Payload MP and the Loaded MP to get loaded and empty accelerations. Use Combat MP to get average MP.
Example: With two Fusion Torches (TWR of 10, so a total of 20), we get 20/14 = 1.4g for empty and 20/21 = 0.95g for loaded. Acceleration varies between 0.95 and 1.4g, depending on how much reaction mass has been used. Assume that the spacecraft has 1 MP of payload; combat MP would then be 14 (empty) + 1 (non-fuel) + 6 (fuel) / 2 = 18. That gives 1.1g of acceleration.
dV is more complicated. The best way of computing it is just to use the rocket equation: dV (km/s) = ISP (s) * g (m/s²) * LN(Loaded MP / Dry MP) / 1000. If you don’t have a calculator ready for the natural logarithm, approximate it as follows: dV (km/s) = ISP(s) * 10 * Fuel MP / Loaded MP * correction. Correction takes the percentage of fuel into account: If you have at least the percentage of the first column, use the correction factor from the second column.
| Fuel Percentage at Least | Correction Factor |
|---|---|
| 0.15 | 1.1 |
| 0.3 | 1.2 |
| 0.4 | 1.3 |
| 0.5 | 1.4 |
| 0.6 | 1.5 |
| 0.65 | 1.6 |
| 0.7 | 1.7 |
| 0.75 | 1.8 |
| 0.8 | 2.0 |
| 0.85 | 2.25 |
| 0.9 | 2.5 |
| 0.95 | 3.0 |
| 0.98 | 4.0 |
Example: Take a spacecraft with an initial water fusion torch (15,000s ISP). It has a non-fueled mass of 15 MP, and has 5 tanks with water (1.2 MP each, so 6 MP total). Exact computation is 15,000 * 10 * LN(21 / 15) / 1000 = 15,000 * 10 * 0.336 / 1000 = 50km/s. The approximation gives us a fuel percentage of 28% and a correction factor of almost 1.2. 15,000 * 10 * 6 / 21 * 1.2 = 51km/s. Close enough!
And that’s done.