I’ve been thinking about the points I made in my previous post, and it got me thinking how to simplify it even more. And, I believe I have an answer. One that reduces ~6,000 words of ruleset and changes into a single optional rule, which only requires you to do a few sums and divisions.

The Rule

With this tweak, each system now is a set of components representing 5% of the spacecraft’s total volume, rather than loaded mass. This has no effect during building, but we need to change the computation of acceleration and total dV. For this, we’ll calculate the mass factor.

Armour is now a Surface Feature

Rather than armour taking up a system slot, it now acts as a surface feature. For each hull section, decide how much and which kind of armour it mounts. Note this down as “3 Light Alloy Armour (total dDR 30)”; I’d recommend to put it above the first system for each hull section. Each armour you add this way masses as much as an vanilla Spaceships armour system, and provides as much protection. It just doesn’t take up space inside the ship.

This obviously replaces the Armour and Volume rules from Pyramid 3/34’s Alternate Spaceships article.

Compute Mass Factor

The Mass Factor determines how much more or less massive a spacecraft is compared to the default. Sum up the Mass for each system, then divide by 20 to get the spacecraft’s Mass Factor.

We treat most systems as “default density” (i.e. with a Mass of 1). We treat other systems as less dense, and for these we add a smaller mass factor:

  • Habitats and Passenger Seating: 0.2
  • Fuel Tank with Hydrogen: 0.1
  • Open Space: 0

Count each armour as 1.

The Mass Factor can also be used to compute the total mass the spacecraft actually has: Multiply the one from Spaceships by the Mass Factor.

Acceleration

During acceleration computation (SS1:35), compute the G value as normal. Then, divide it by the Mass Factor.

dV

For dV computation, we’ll modify the Fuel Task table in SS1:17. Rather than Tanks, count the Effective Tanks. That’s just the number of tanks divided by the Mass Factor. Use the number of effective tanks to compute the total dV; adjust by the fuel tank table as you’re used to doing.

There is one major change to engines: Count each hydrogen fuel tank as only 1/10th of a tank. That’s because the tank no longer has full system mass but rather 1/10th of it. You’ll have to make up on this by adding many, many more tanks. That’ll push down your mass factor, so engines will be much more efficient—but armour much less so.

Example: The Prospero-class Interplanetary Liner, from SS2:11, has 6 habitats and one open space. These give a total mass of 1.2. It also has three Light Alloy systems; we’ll replace these with three fuel tanks. That makes a total of six systems of hydrogen tanks for another 0.6; the remaining 7 systems have normal mass. We’ll give it a third of an armour system per section, for another 1 Mass. That gives a total mass of 9.8, for a mass factor of 0.49.

This makes mass 1,470t, rather than 3,000t. Acceleration is 0.04g rather than 0.02g.

Six hydrogen fuel tanks make 1.22 effective fuel tanks; there is obviously no dV increase. This gives us 30mps * 1.22 tanks = 36.6 miles per second dV. Quite some difference to the original 90mps, which comes from using hydrogen. But note how we now no longer have 15% of our spacecraft mass being hydrogen (as in the original system), but just 6%. Learning: Hydrogen is really inefficient for a given volume, and we’d probably need to up our SM by one and make about half of our volume hydrogen tanks.

Let’s use water instead (and use 7 tanks, eliminating the second fusion rocket). Total mass is now 15.2, for a mass factor of 0.76. That makes total (loaded) mass 2,280t. Acceleration per engine is 0.03g from the water reaction engine; dividing by our mass factor gives 0.04g.

For our dV, we have seven fuel tanks, for effective 7 / 0.71 = 9.2 tanks. That gives us a dV multiplier of 1.4 and makes our total dV 10mps * 9.2 tanks * 1.4 = 129mps. Not a surprise, since almost half our loaded mass is water reaction mass.

Super Optional Bonus Rule

If you’re interested in (much) more detailed performances, you can recompute mass and dV for a certain load of a ship. For this, count the following system masses differently:

  • Cargo Holds have loaded mass / mass in SS1 Mass. For example, if you load 2000t of iron ore into an SM+10 cargo hold, it will count as 2000t/500t = 4 Mass.
  • Empty fuel tanks count as 0.
  • Hangar bays count as 1/3rd if empty. If loaded, add the Mass Factor of the loaded ship to this, multiplied by 2/3rds.

Conclusion

This really simplified my volume-based spacecraft rules significantly: Rather thhan 6,000 words, it’s now worth about 800 words. And there’s only a few changes you’ll need to keep in mind while building:

  • The number of armour and passenger systems influences acceleration. If you want to keep a specific acceleration minimum, I’d recommend you fill the armour first, then decide how many habitats etc you need. Then, you’ll have all the numbers to get your acceleration before filling the remaining systems.
  • Similar for dV: Once you have the number of armour systems, you’ll know how many fuel tanks you need to get a certain dV value. Fill the rest to taste.