Monday, November 19, 2018

Robots as Spaceships: A Simple Gunbot

What do you know? The army is working on one too!
Last post we talked about why we're undertaking this project, and what our starting point is. Our next step is to build the simplest war-bot we can imagine. The Gunbot Mark I is an SM+0 vehicle that carries a weapon into battle on treads. Its essentially a miniature tank with an AI on board. We will start building it with spaceships, and discuss the obstacles as they come up.

The Hull

Hull size is our first challenge: Spaceships doesn't list hulls of SM+0. Even Spaceships 4 stops at SM+4. We have rules for continuing the table up in the side box "Scaling Up Spacecraft", so we will reverse those rules to scale spacecraft down. Our hull weighs in at 1/10th of a ton (200 lbs), is 2 yards long, has a dST of 3, and a Hnd/SR of 2/2. We will work with an streamlined hull, as we don't anticipate high speed chases or flying through the air (or space).

On a related note, the cost for very small spaceship systems is not listed, but they almost all (with a few exceptions) scale with the weight of the hull. So dropping a 0 from the cost for every 2 SM will give us numbers we can honestly say came from the book. This number scaling is our first tweak to the rules.

Its worth noting at this point that spaceships does Hull Size by mass, and each system will weigh 1/20th of the total, or 10 lbs. 

Moving Around

For our motive system, we will use the tracked drivetrain from pyramid 34 (page 7). This system will require at least one power point. Batteries feel most appropriate for a human-sized gun bot. The closest to batteries in spaceships are fuel cells, listed under "Power Plant, Chemical" (page 20).  Adding a single power cell will give us a move of 1 and a max speed of 10. Our drive-train can use more than one power so we'll add two more power cells, as many as it can use. Now our move is 3/30.

The Gunbot's handling and stability on the ground are determined by its drive-train, not its hull size. Unlike free-floating spaceships, the handling and stability rating of tracked drivetrains don't display a clear pattern. For this first pass, we'll assume that the total of the two is always 2, and handling increases by 1 every 3 SM. This gives us a handling of -1 and a stability of 3. So that's our second tweak.


A gun bot is not a gun-bot without its gun! Installing a major battery (Spaceships 1 page 26) as a turret fits our description of a miniature best. Once again, we need to extrapolate the weapon size. Fortunately, the spaceships conventional warhead damage table (page 68) continues for a ways beyond the battery tables. We drop 5 steps down from a 10 cm gun (A major battery on a SM+5 hull) to a 4cm gun. The number of shots included with the weapon follows the size/range table, so we can extrapolate the number of shots included the the 10 lb weapon system. Dropping 5 categories for the +5 size modifier difference gives us a mere 10 shots (70, 50, 30, 20, 15, and finally 10). We will certainly want more ammunition. The Gun and Missile Ammunition Table (Spaceships 1 page 47) lists 4cm ammo as weighing 1/800th of a ton, or 2.5 lbs per shot.  We set aside two cargo spaces for the shots. That's 20 lbs of cargo space, so we can store 8 additional shots.

The damage for a 4cm gun is 6d dDamage, either as a proximity detonation or with (2) armor divisor. The actual damage type isn't given. For proximity detonations its almost certainly crushing explosive. For the shells with the damage divisor, damage type is either crushing explosive or huge piercing. The space accuracy of the weapon is -10.

We're going to include the above tweaks in that first scaling rule we made. These numbers required looking things up and extending tables down, but the tables were all pretty easy to extend, and you have to look them up any time you use spaceships.

... and Armor

We're limited to TL8 for weapons and armor, so our armor systems will be Metallic Laminate. We'll use 6 armor systems, two on each section, to keep things simple. Pyramid 34 has options for increasing the effect of large amounts of armor and for armoring by facing. Both options are appropriate for this bot, but we'll ignore them for now in the name of simplicity. Extrapolating the Metallic Laminate Armor table (Spaceships 1 page 12) down to SM+0 tells us each system gives 1/2 dDR, or 5 DR (note that armor DR values use the size/range table).

Onboard Computer

All (steerable) vehicles built with spaceships require a Control Room System. Our Control Room (Spaceships 14) won't have space for any passengers, but it won't need any! Our onboard AI will direct and run our gunbot. More troublesome is that an extrapolated SM+0 control room at TL10 provides a complexity 1 computer. That's a tiny computer even by TL8 standards, let alone TL10. We're going to need something a lot more powerful than that. The control room includes sensors and controls for the motive system, so we need to keep it, but we'll need to include a bigger computer. Its got a comm/sensor array level of -1.

So we build our first custom system: an onboard computer. We set aside 1 system for this, which gives us 10 lbs to work with. At TL10 a 5 lb computer has complexity 7 and costs $1,000. This is good enough to install a non-volitional AI with IQ 10, and we don't really want anything more out of a hired gun. The AI program costs another $1,000, assuming normal prices by complexity. An AI built to be a warrior probably ought to cost more than one built as an architect, but this is just the first iteration, so we'll keep things simple. The completed system costs $2000 and adds a computer with complexity 7.

So our third tweak was allowing gear from ultra tech to be incorporated into the "spaceship". We were able to stay within the weight of the system we needed, and the price was easy to calculate. We are making a fourth tweak by allowing the computer to use the control room to steer the gunbot. This is easy to justify: the control room's mass is mostly sensors and steering systems.

Finishing Up

Our little gunbot has 15 systems. 6 armor systems, 1 weapon system, 2 cargo systems dedicated to ammunition, 1 tracked drive train system, 3 fuel cell systems, 1 control system, and 1 custom system representing a built in computer. We have five systems remaining. There's a lot we could do with them. We could upgrade the power train and make it faster. We could add on more armor. We could mount additional sensors or weapons. But the simplest thing to do is add more cargo space for ammunition: 18 shots is not a particularly impressive number. The 5 additional cargo systems raise the number of shots to 38.

Vehicle Statistics

The Gunbot Mark I is a SM+0 vehicle with the basic following statistics.
10Gunbot Mk I3-1/3123/300.10.035+001360$3,700
The range is calculated by figuring how far the bot can go with move 30 for 6 hours (that's how long the cells last). The occupancy is 0, because you really can't get anything bigger than a rabbit in the vehicle, and the bot either drives itself, or is remotely piloted. Now, we aren't ever going to use this in spaceships combat, so lets convert it away from decimal scale numbers.

10Gunbot MK I30-1/3123/300.10.035+0010360$3,700

Its sensors have a -1 modifier. This means that its perception checks will be at an effective -1. Also of interest is that over half of the final cost is the custom system that installs the computer and the AI. The components from spaceships cost a mere $1,700. The ammunition also costs a bundle: a full load costs $3,500, almost as much as the robot itself!

Conversion to "Ordinary Combat"

The box on spaceships page 66 "Weapons in Ordinary Combat" lets us convert the gun to the combat scale of humans, which includes most PC's. Our 6d dDamage becomes 6dx10 damage, this damage is either an explosion or (2) pi++. Its accuracy of -10 becomes an accuracy of +8 when firing at paltry human ranges. However, it can only fire every 20 seconds.

If we wish to make it fire once per second, we could make the gun very rapid fire. This drops its caliber down to 1 cm. We don't have stats for that, but the table is pretty consistent about halving the caliber halving the damage, which means we're looking at 15d of human-scale damage per shot, and a ROF of 1.

We should also note that these shells are guided, and ignore range penalties (but not speed penalties).

And with those wild numbers, lets move to the analysis of the Gunbot Mk. I


There is a lot to be analyzed here, and we will do a fuller analysis in another post, but my initial impression is that this feels like its been built with GURPS: spaceships. The oddities that consistently plague spaceships haven't gone away, despite this craft being neither in space nor scaled up. We have a very cheap machine with incredible offensive capability and whose armor feels a touch lacking. Is this what we expected? Not really, but we shouldn't be surprised.

On the other hand, we've got a working Gunbot. The system will let us build what we need, and that's far more than we had before. While I stepped us through this slowly, we could actually assemble a vehicle much faster. The system lets us build a lot of different vehicles, and build them quickly and consistently.

Four tweaks let us do this. We extrapolated the cost, dDR, caliber size, comm and sensor array bonus,  dST,  and weight of  an SM+0 spaceship. We made a much trickier extrapolation of handling and stability for an SM+0 tracked drive-train. We allowed ourselves to add gear from Ultra-tech to add features to the robot that spaceships lacked. And we allowed an AI to use the control room instead of requiring humans in chairs.

If we wanted to, we could play out an entire campaign with just these tweaks. We'd be able to predict what robots could do, and we'd have very few judgement calls to make. But that's not what we're going to do. We're going to tweak this system to make the numbers closer to what we think they should be. We hope to end up with an excellent system instead of one that's "Good Enough". I look forward to talking about the tweaks in my next post.


  1. Something odd seems to be going on with the ammo weights. You've got 10 shots in the one (10 lb) Major Battery system, along with the gun, but only 4 per dedicated 10 lb cargo space.

    Is Spaceships scaling the Battery ammunition at a different rate to the ammo weight?

    1. You are absolutely correct that something odd is going on, and I talk a little about this in the analysis post, which came out today. I tried to keep commentary in this post to a minimum for the sake of length, but yes, the ammunition weights are probably the most extreme discrepancy in this build.

      Spaceships doesn't scale its calibers at the same rate as its lengths. While a Hull may go from 50 to 70 to 100 in two SM steps, the gun caliber only goes from 14 to 16 to 20. It takes calibers twice as long to double its single dimension. Spaceships tries to compensate for this by adding more ammunition to larger spaceships. So at SM+0 we've decreased our ships length by 10 times but our guns caliber by only about 3 times. We have very few rounds included automatically, but the gun-to-hull mismatch is so great that the weight of those few rounds exceeds the weight of the entire system.

      I'll probably analyze this a bit more in its own post, though for the Robots as spaceships project we're likely to just use the weapons provided in High-Tech and Ultra-Tech.