Hull Mass

Space Shuttle – realistic and … exiting … totally

So, I always wanted to create a system of rules or mechanics to design spaceships for roleplaying games. Interesting and yet somewhat realistic spaceships. Not as realistic as a Space Shuttle but also not as wild as a Star Destroyer from Star Wars. So I had to sit down and think about what I expect from those rules and which of my expectations were already corrupted by pop-culture and misconceptions about spaceships.

Impressive piece of cheese cake in space

One basic question is – on what do you base your design rules, if there are no equivalents to spaceships in reality? The classical way is to transfer naval ships into space. Which obviously is nonsense. On the other hand I have to keep in mind that most players will not let go of their misconceptions easily. They will probably not consider narrow and crowded white tubes to be heroic and cool spaceships. They will probably not even want to talk about them, unless they are engineers in real life by coincidence. So, the only hard evidence or knowledge about spaceship construction comes from NASA and the like, but the trick is to break just so many boundaries that it becomes interesting, without slipping into the completely fantastic.

Even if the construction of space ships is carried out in orbital shipyards with the aid of skyhooks and existing fleets of transport vessels and construction robots, the fundamental question remains about the origin of the required materials. Especially if the shipyards are in remote places like Jovian moonsfor example, the effort to transport huge amounts of construction materials from Earth, other stations or the asteroid belt to the construction site are immense. So one very important question will be, how much mass will a spaceship have in the end – this will affect it’s capability to accelerate. Very slow ships will probably work as transports, but if your setting requires fast passenger liners or military vessels, ships should be able to accelerate at rates between 0.5 and 1.2 g. This definitely limits the mass for spaceships and has direct impact on their design. Sea-going ships have to withstand the forces of water and winds, space going ships have to withstand accelerating force, maneuver stress and particle impact. Spaeships would clearly not be built like naval ships – welding tons over tons of steel together will create way too heavy ships with a rather bad integral structure.

Home sweet home…

How heavy would a ship hull actually be? The Zvezda module of the ISS was designed as habitat and working module and can support up to six crew including separate sleeping quarters for two cosmonauts at a time. It’s mass is about 18 tons (empty) and about 22.7 tons (fully loaded start weight). The dimensions are 13.1 x 4.15 m (the solar panels extend 29.7 metres), the total volume is 195 m3, the living/habitable volume for the crew is only about 45 m3. The module provides it’s own power supply and waste water recycling, has drives for limited flight maneuvers and prolonged life support. It has been reported as being overly noisy inside, too. The module weighs roughly 116 kg per cube meter of enclosed space.

All in all it comes very close to what we would call a spaceship, besides lacking a proper drive to go anywhere. Still, it is a design made for being used as a component of a space station in orbit, not an independant vessel going to Jupiter. The interior is tightly packed with science stuff and technology, leaving not much room for the crew. This would be acceptable on a warship but not bearable on a luxury cruiser.

A Typhoon-class submarine has an average density of 606.07 kg/m3. I would at least double the average density for ship hulls because they will have to be sturdier to withstand acceleration stress and will contain less lightweight tools and devices and more heavier structure and machines. For military vessels I would even consider to triple the average density, coming closer to the value of a naval vessel like the Typhoon.

[su_box style=”soft” box_color=”grey” title=”First Conclusions”]

  • Average Density (Volume/Mass) Zvezda Module: 116.41 kg/m3 (11,34 tons/Crewmember)
  • Average Density (Volume/Mass) Spaceship: 232 kg/m3 (22,68 tons/Crewmember)
  • Average Density (Volume/Mass) Military Spaceship: 349 kg/m3 (34,02 tons/Crewmember)
  • Average Density (Volume/Mass) Typhoon submarine: 606.07 kg/m3 (300 tons/Crewmember**)
  • Minimum requirements per crewmember*:
    • 96.75 m3 of enclosed space
    • 22.5 m3 of habitable space
    • 5220-7852 kg of mass

*assuming every crewmember needs full accomodations like a sleep cabin for him/herself. Many people in Tokyo have smaller homes, but they can leave them. The hull of the spaceship is the end of the world for it’s crew.
**They Typhoon reference is misleading as the hull covers more purposes than being a habitat.