Building a Spaceship in Dungeons & Dragons, Part 2.5

This is an update to Spaceships in D&D Parts I and II. I had hoped to give you an update to Rock to Hide Me instead, based on a paper I found on a government server. But unfortunately, after I read it, I concluded the paper in question was so mercilessly boring that I don't think I can turn it into an article that anyone would want to read. So, on to the next item on the itinerary instead!

I've made some significant modifications and updates to Spaceships in D&D Part I and II, responding to and incorporating some suggestions and criticisms by commenters on them – suggestions and criticisms more than a year old (egg on face), but better late than never. I'd particularly like to thank Brian Ballsun-Stanton, who made a number of very helpful comments. So as not to make everyone reread those articles, I will be discussing the changes separately here. Part III in the series, on mission planning, will follow, hopefully in the not too distant future. There may or may not be a Part IV, on things to do in space, depending on what I come up with.

A Big Mistake

While prepping this article, I discovered a very serious error in my geometric calculations that led to me underestimating the volume of the combustion chamber by approximately a factor of four.   Since the volume of the combustion chamber determines the thrust produced by a given blast of flame spell, this led to a significant overestimation of how much work we needed to do to get a given acceleration.   This issue has been corrected.

In the process, I also made the following diagram of our spaceship:

What can I say, I'm no artist.



Throttling System

This is something that was supposed to be in the Part II, but which I forgot to include. To recap: our propulsion system consists of a mithril combustion chamber reinforced with a lyre of building; a collection of decanters of endless water to provide matter; and an energy transformation field loaded with blast of flame to turn the matter into hot gas to serve as our exhaust. The energy transformation field is powered by a collection of spell traps loaded with first-level spells, with the traps activated by a button in the cockpit.

However, there's a problem here: there's nothing to turn the decanters off when we're not thrusting. We might actually be okay leaving them running, given that the water should eventually boil away in the vacuum of space, but that means we'll have a continuous residual thrust that we need to account for, and it means we have to worry about things clogging with water ice. In particular, our combustion chamber is only invulnerable for two thirty-minute periods a day; I'm worried about the nozzle clogging up with ice and then pressure building up inside the chamber until it ruptures. Best to have a way to turn them off when not in use.

According to the SRD, a decanter is controlled by command words. There's no actual ruling that I can find about, e.g., how close the command word needs to be spoken to the item for it to work. My guess is that the writers implicitly assumed that you would be holding the item when you spoke the command word, but there's nothing explicitly saying that you can't activate the item from a world away. Fortunately, this is easily determined via Science: during the Propulsion Research phase, measure how far away you can be from the decanter to control it, and particularly whether the mithril sphere we used in measuring the thermodynamics of our thrusting options blocks the effect. For the purpose of the rest of this article, I'm going to assume that the command word needs to be audible to the magic item with a DC 0 Listen check - which means we can't just say the command word in the cockpit to shut the decanters off. Fortunately, there are other options. Unfortunately, those options are a bit complicated, not to mention pricey.

The simplest method I've found - and please comment if you can suggest a better one - is an antimagic field. We use an energy transformation field loaded with antimagic field, and powered by 1st-level spell traps. Subcontracting the energy transformation field to Bo-Wing the Aeromancer costs 5,190 gp, and the 1st-level spell trap costs 24.05 gp, for a total of 5,214.05 gp.

Now, the geometry of this is going to be a little complicated. Our antimagic field is a 10' emanation.   An emanation is a sphere, but it's blocked by anything that blocks line of sight. In other words: if I can draw a straight line from a point to the origin of the antimagic field that doesn't pass through any physical objects, and which is 10' long or less, then that point is effected by the antimagic field. We also have two energy transformation fields, one powering our blast of flame, and one powering our antimagic field. Energy transformation fields are 40' radius spreads: a spread is like an emanation, except it can turn corners. In other words: if I can find a path from a point to the origin of the energy transformation field that is 40' long or less, where that path can bend and twist, then that point is effected by the field.

Neither of our energy transformation fields can overlap the decanters of endless water, because otherwise they'll suck up the decanters' energy and they won't work. Our antimagic field, on the other hand, must overlap the decanters. However, the antimagic field can have its origin anywhere inside the energy transformation field powering it.   By placing the origin of the antimagic field-generating energy transformation field inside a box connected to a 39'-long coil of pipe, we can position the origin of the antimagic field so that it covers the decanters but the energy transformation field does not.

Unfortunately, because antimagic field has a minimum 110 minute duration, this means we'll need to plan our course changes well ahead of time. To thrust, we set the lever controlling the spell traps at point B to off, and wait 110 minutes for the antimagic field to dissipate. At this time the decanters begin to generate water, and we play the lyre of building and set the lever controlling the spell traps at point A to on. The blast of flame field charges and fires, and we begin to thrust. When we want to stop thrusting, we set the lever controlling A to off and the lever controlling B to on. The blast of flame field stops firing, while the antimagic field field charges and fires, shutting off the decanters.

(Since antimagic field can be shut off by its caster at will, it would be a reasonable DM call to say that we can program our energy transformation field to create an antimagic field that lasts a shorter period of time, but that's definitely not supported by the rules.)

Do Decanters Count as Construction?

This is a rules interpretation issue which I neglected to raise in the original draft. Our decanters are inside the combustion chamber, which, during thrust, is full of high-temperature, high-pressure gas. The lyre of building protects the chamber walls from damage. Does it also protect the decanters? If not, they're going to be destroyed pretty quickly.

The text in the SRD says that the lyre "negates any attacks made against all inanimate construction (walls, roof, floor, and so on)". My personal ruling would be that, if we build the decanters into our chamber wall, then they are part of the construction. However, that's just a ruling. We need to add a step into our Propulsion Research section to determine if this is the case - fortunately, we already have all the materials ready to hand.

If the lyre does not protect the decanters, then we're in trouble. There are no readily-available ways to protect them that I can see, and almost all propulsion methods require decanters or some analogous magic item to serve as reaction mass. I can think of a few ways that might work, like make whole spell traps to repair damage faster than it's done, but the only way to determine if they will actually work is by trying - there's no way to determine how much damage the decanters are subject to per round from exposure to the combustion chamber from the rules as written.

Alternative Approaches to Propulsion

Several alternative propulsion methods to the decanter + blast of flame approach have been suggested. Some of them are definitely inferior to my current method, some of them we can't say without doing in-game research. Since we can't say for sure that any of them are actually superior I've decided to stick with my current approach, but I discuss them all below, and I've added appropriate sections to the Propulsion Research section.

Using decanters of endless water as a water rocket: The idea here is to use decanters on geyser mode, exploiting the thrust of the water itself without trying to heat it. The data we have is insufficient to determine what the thrust produced by a decanter is. We know that, on geyser mode, it produces 30 gallons per round, or 5 gallons per second, but we don't know what speed that's produced at. But, if we know the size of the decanter's mouth, we can calculate how fast it's moving. The SRD describes the decanter only as an “ordinary-looking flask”. If we assume the mouth of the flask is 1 inch wide, or 2.54 cm, then the mouth has an area of 5.06 square centimeters. Five gallons is the same as 18,927 cubic centimeters, so the speed of the water is 18,927 / 5.06 = 3,740 cm/sec, or 37.4 m/sec. The density of water is 1,000 kilograms per cubic meter, or 0.001 kilograms per cubic centimeter, so the decanter produces 18.9 kg/sec of water, for a thrust of 706.86 N per decanter.

Not counting the combusion chamber - which the water rocket would not need - our vehicle weighs about 30 tons, so to get 1.5 G acceleration, we'd need 450,000 N / 706.86 N = 637 decanters to replicate our current thrust. With our various cost reduction modifiers, it costs us 2,164.22 gp per decanter, for a total price of 1,378,608.14 gp... Somewhat in excess of our current approach.

Steam rockets: The basic idea here is to combine decanters of endless water with a source of heat, such as matter agitation or heat metal, to produce a steam rocket.   Unfortunately, the descriptions of these spells and effects are extremely vague about what specific temperatures they produce and so on.   For example: matter agitation effects 2 square feet of material; what is that in volume?   How hot does heat metal make the metal it effects?   And so on.

Let's talk lower estimates: what is the absolute minimum that a steam rocket could cost?  We can figure this out by ignoring the actual turn-it-into-steam parts of the rocket, and just considering the cost of the decanters.   Now, take a look at part II of this series, and specifically the section labeled Engine, and specifically specifically the equations therein.   Now, the specific heat ratio of steam is about 1.3, the exhaust temperature is 373.2 K, aka the boiling point of water, and the average molecular weight of water is 18 kg/kMol.   Plugging those in, we determine that the exhaust velocity of a steam rocket with an ideal nozzle is 1,222.27 m/sec.   Since our decanter produces 18.9 kg/sec of water, this amounts to a total thrust of 23,100.8 N per decanter.   To replicate our current thrust of 2,078,574 N, we will therefore need 90 decanters.   At a cost of 2,164.22 gp per decanter, that's 194,779.8 gp in decanters alone, before we add in the cost of the systems to actually boil the water - more than the cost of our current engine.

Nonetheless, such a system could still have a role to play.   In particular, unlike blast of flame, it's plausible that our steam rocket might not need a lyre of building to keep the combustion chamber and nozzle from evaporating - we can make it out of the same stuff they make tea kettles out of.   That means the engine could thrust continuously, instead of for two half-hour periods per day.   Also, if you're willing to accept a lower acceleration, the steam rocket can have a smaller minimum cost than a blast of flame rocket.   I think the steam rocket has a role to play as a system for getting around once you're out in space, the magical equivalent of the real world's ion drive.

Using pocket dimensions as ideal combustion chambers: This is a very cool idea, but I don't think it actually gets us anything – because we already have an ideal thrust chamber. Since our “combustion” chamber has infinite strength, thanks to the lyres of building, we can let the chamber pressure be (effectively) infinite, and at a much lower price than using this approach.   A pocket dimension could thrust all day, but I don't know of any way to buy a suitable pocket dimension for less than the cost of 48 lyres, which would also let us thrust all day.

Additional Cost Reductions

I've found some ways to eke out our budget a bit further.

First, in the original draft, I gave the astronaut the Landlord feat... And then forgot to make use of it! At 11th level, the Landlord feat gives you an allowance of 75,000 gp to be spent on a stronghold, plus one-to-one matching funds for any money spent beyond that. Unfortunately, the big expense of the ship is the engine, which is not statted as a stronghold, but this can still be applied to the rest of the ship.

Second, since writing the original, I found the Complete Cost Reduction Handbook, which has a bunch of ways to reduce the price of the items we're using. Unfortunately, many of them are from books I don't own (Bind Elemental applied to conventional magic items, membership of Dark Spire college). But we can at least use Favored in Guild (Arcane), which reduces costs by 5%, and Apprentice (Craftsman), which reduces costs by 10%.

Other Minor Corrections

I forgot to include the price of the spell trap powering the navigation system (24.05 gp).

We can't actually cast energy transformation field - I thought it was 6th level, but it's actually 7th - so we'll need to subcontract that. Each casting costs 910 gp, in addition to the material component costs that were already included.

We need to include a small closet in the cockpit to hold the energy transformation field powering the navigation system. Otherwise, the field will cover the whole cockpit, making it impossible to use other magic items or cast spells.