Starships Measured in Tons

Someone once asked me why I measure ship sizes in my space operas in tons of displacement. Well, here’s the short answer: I didn’t like any of the other measurements.

NimitzFor the long answer, let’s look at how some water vessels are measured. On the small end, they’re measured by length. We have the occasional 20-foot sloop for tooling around the lake. The America’s cup is a race between boats in the 18-30 meter range. Bigger ships, however, are rated by their weight. Technically, they’re measured by the amount of water they displace, but they only push away their own weight in water. (And no, it doesn’t matter whether that’s freshwater or saltwater, because they still displace the same weight.) The USS Nimitz measures just over 100,000 tons displacement. A common large oil tanker design measures over 400,000 tons displacement. Note that these are metric tons, i.e. 1000 kilograms.

Airplanes are measured in wingspan, though they are also often measured by their weight. This is mostly useful when looking at their maximum thrust for determining their flight characteristics. For example many modern fighter jets have more thrust than weight, allowing them to do a vertical climb at maximum thrust. Rockets are similarly measured by weight, but also typically by launch height. The Saturn V that carried Apollo 11 to the moon was 363 feet high and weighed in at 6.2 million pounds (or about 120 metric tons).

Cars are typically measured by weight and internal engine size. Again, it’s to give you some idea of their performance characteristics.

These are all useful measurements for the vehicles in question. But what about starships, particularly cargo-carrying merchant ships?

ContainerShipsMass seemed a likely candidate since out there in the free-fall vacuum of space, rules like F=ma are king, but for a cargo ship, mass is going to change all the time. That’s reflected in that those ocean-going cargo ships are often given two displacements: an empty one, and a maximum load one. Of course, no merchant ship ever wants to travel empty, but I found myself looking for a constant, and the only constant is volume.

I confess that some it came from my background playing the Traveller RPG back in the 80s and 90s. All of those ships were given in terms of tons displacement. Of course, for them it was not as convenient as tons of water where one cubic meter equals one metric ton. No, it was tons of the liquid hydrogen ships used as fuel, and that is not nearly as dense as water. One ton of liquid hydrogen takes up about fourteen cubic meters, so for you old Traveller fans, that “100 ton” scout vessel was closer to 1400 tons when comparing it to ocean-going ships.

So between Traveller and Earth-based ocean vessels, I found myself thinking in terms of volume based on tons of water displacement. With the 1 ton = 1 cubic-meter equivalence, I could have just as easily said cubic meters, but in writing it out, “tons displacement” just sounded better to my ear. Maybe that’s a lame reason, but there it is.

But as long as we’re talking mass, how much mass do these things actually have? Well, it is going to vary – one of the very reasons I didn’t use mass – but by and large, I would say that a starship is typically going to have less mass than the equivalent amount of water. In other words, put them in the ocean, and they’ll likely float.

I say that partly because ocean-going ships float, and I don’t imagine the interiors of starships being fundamentally different than ocean going vessels. There’s plenty of crew space, crawlways to various bits of machinery, and just plenty of open space around pipes and wires to be able to work on them when necessary. In short, they’re mostly air.

And the cargo? Well, with rare exceptions, it’s going to be mostly air as well. Let’s think about some heavy manufactured goods like a car. The storage space for my sedan is about 11 cubic meters, and that’s fairly tight. Its mass is about 1900kg, giving it only 17% the density of water. Again it’s mostly air. The only reason cars sink is that nothing but the tires are air-tight. How about a box of cereal? Mostly air. Clothing? Between the folds, the padding, and the interweave spacing, again mostly air. If you factor in the packing material around your fragile electronics, again… mostly air.

RawMetalsAbout the only time you’re going to cross over the density of water is when you’re shipping raw materials, and even then, you’ve got to be selective. Check out this table of common densities, particularly the metric columns on the right. Only those over 1000 are denser than water, things like cement, iron ore, and steel chips. The heaviest, lead oxide, is not quite two and a half times as dense as water. If you’re really going to weigh these guys down, you’ve got to go with more exotic stuff like uranium or iridium.

So, even loaded down, these starships are going to mass about a third to half the equivalent amount of water. They’ll still want to know their mass – especially those guys who travel to/from ground-based starports – but it’s going to fluctuate enough that I just didn’t find it to be a useful term for describing the size of a ship.

So anyway, that’s how I opted for displacement tons rather than mass or length for describing my starships.

My N-man Starship

How many people do you need to run a starship? I see stories where it’s a crew of hundreds, while others manage with just one. It’s not that either is wrong. I think it simply depends on the rules of the story’s universe and the purpose of the ship.

At one end, I think about the one-man ships of Larry Niven or Jack McDevitt. These typically have a fair amount of computer automation. McDevitt’s ships in particular have an AI who is perfectly capable of taking the ship through all its maneuvers and activities, leaving the “pilot” as little more than a bossy passenger.

Even taking a more active hand, the single crewman usually only has to be alert and on duty for key transitions such as sub-light maneuvering thrusts or transitions into and out of the FTL-drive of choice. As long as nothing else goes wrong, this one crewman has a lot of time to kill. Then again, if something does go wrong, he has to be the one-man repair crew, and in many cases, his options are limited to sending out a distress call for a rescue ship.

At the other end we have giant warships like the Enterprise or the Galactica. They seem to have less computer automation, so they require more people spread around the ship pushing the right buttons at the right time. They also have extra functions that those one-man ships do not, ranging from combat to exploration, so they need extra crew to deal with those things. And as the button pushers and red-shirts add up, you need more officers for command and control.

Furthermore, a lot more can break on a warship than on a small passenger ship. In fact, warships frequently seek out situations where things break spectacularly. No longer is one lonely crewman replacing a leaky fuel line. Instead, it’s a team of thirty repairing a hull breach and welding the engine mounts back into place.

But what about the in-between cases?

One of the reasons I really enjoyed Nathan Lowell’s Solar Clipper series (start with Quarter Share) was that he paid attention to all the boring little details of keeping a ship up and running. From his books (and some of the ones I’m working on), I’ve realized that in addition to the obvious jobs of sitting in the captain’s chair and locking phasers on target, there are three main things that occupy the bulk of the crew: standing watch, doing maintenance, and sleeping.

Standing watch is probably the most boring thing you can imagine, because you’re essentially waiting around all day for something to go wrong. This looks like a prime candidate for computer automation. After all, the computer can wait 24/7 for something to happen, and it doesn’t need a chair. Still, it’s important to have an actual person there, because when something does go wrong – and sooner or later, it will – then you want to have a live body there, paying attention, and ready to take action. There are quite a few things that could wait five or ten minutes for you to wake up and get dressed, but the matter/anti-matter injection valves probably can’t wait.

Maintenance is almost the opposite. You’re not waiting for anything to go wrong. You’re fixing it or replacing it before it can. The environmental team is changing out the CO2 scrubbers, and the engineers are realigning the polarity on… well, you know how those engineers can be about polarity. Some maintenance is hard to do when you’re underway, but if the ship has any kind of redundant systems, you can be sure that they’ll be falling back to them on occasion both as a test and for a chance to do maintenance on the primary system.

And sleep? Sleep is kind of a placeholder for all the drawbacks to those lazy organic crews. They keep wanting to sleep, and that’s on top of wanting to eat food several times a day. I figure about the hardest you can push someone is twelve-hour shifts, seven days a week. We’re not talking about heavy physical labor in the cotton field, but keeping alert for twelve hours is a challenge. You’d be a lot better off with eight-hour shifts and enough crew to allow other downtime. Toss in a galley, maybe a small gym or some recreation, and the crew to manage all that. Pretty soon your little eight-man ship is ballooning up to twenty or more.

I screwed up on this in my first book Beneath the Sky, in that the merchant ship Jinley is crewed by only four or five, but I never delved into the day-to-day shipboard life in that story. In the upcoming Ships of my Fathers and Debts of my Fathers, I thought about it a lot more and concluded that a merchant ship that size really should have six or seven crew: two navigators, two engineers, an environmental specialist, a cook, and a captain who can hopefully jump in to fill any of those slots in a pinch. That’s largely because the rules of this particular universe makes FTL a hands-on task, dealing with the shifting tachyon winds and managing the ephemeral sails that grab that wind. Twelve-hour shifts are a bitch, but bigger ships with more crew provide an easier life, with more downtime, better rested crews, and more redundancy.

So, before you head out on that long solo flight, give some thought to who is going to fix the toilet when you’re laid up with flu. Do you have a robot helper? A first officer in cryo-sleep? The 800-number for deep space Roto-Router?