I had originally done a page about ship weapons in The Expanse, but I recently reread books 1-6 and realized how biased it was by the TV show. As cool as the Gatling gun PDCs of the TV show look, they're unrealistic in many ways. And I picked up several examples in the book that are ignored/changed in the TV show to make it visually cooler. For example, Holden mentions in Leviathan Wakes that PDC rounds go several thousand meters per second, which is several times the velocity of a modern high velocity autocannon round. Granted, it's the future and they could have higher velocity cannons, but railguns make more sense. When transferring ammunition from the MCRN ships in Caliban's War, Holden muses that a 20,000 round crate of PDC rounds weighs over 500 kilos or about 25 grams/one ounce per projectile. And actually, it would be less, since the crate itself would be pretty heavy to hold half a ton of projectiles. In Cibolla Burn, Havelock is on the hull of the Rocinante and the PDC mount is described as "…a single thick barrel… The hole on the end was a black dot small enough that Havelock could have blocked it with the tip of his ungloved pinky finger." As pretty as watching the gatling PDCs in the show is, for my game, I'm trying to be more by the book. Also the TV show wiki describes the PDC guns as being 40mm, which is very large for a gatling style autocannon.
So it's more reasonable that the PDCs fire very small, super high velocity railgun rounds. After all, how much of a projectile do you need to disable a missile enough to keep it from hitting? As for PDCs being used in an anti-ship mode, that's fine if you're shooting up unarmored freighters and possibly even lightly armored escort vessels at very close range. Even at long ranges though, you have to remember the ships are probably moving much faster than the rounds and the impact of the round is going to be colossal.
Expanse Ship Weapons
Railgun
Spinal/Keel mount for light warships. x4 Newtech modifier. D6W Ship scale damage. 1 ton, 80,000 credits. Rounds weigh 10kg, cost 80 credits per, ignore 2W of armor. Unit consists of weapon with a 100 round magazine and battery pack.
Total unit- 3 tons, 80,000 credits. 8,000 credits for full magazine. (I need to work on it. This doesn't come out particularly massive built with the Serenity rules)
Okay, here's my error: the spinal railgun is not medium weight weapon; it's a shipkiller. Two or three hits wrecks a destroyer. So it needs to be bigger. 10 tons, x4 Newtech, d12 damage, ignores 4 points of armor, 800,000 credits (that may be too much). Projectile weight would be normally 100 kilos plus 100 kilos for power systems. But see speed discussion below.
Light Plasma Torpedo
Usual torpedo for frigates and destroyer class ships, sometimes used on smaller light cruisers. D10W Ship scale damage Plasma warhead, ignores 4W worth of armor. 1 ton, 2,500 credits. Launcher weighs 1 ton, 100 credits.
Light Standard Torpedo
High explosive warhead. D4W Ship scale damage. 1 ton, 2,050 credits.
Medium Plasma Torpedo
Usual torpedo for most light cruisers and heavy cruisers. D12W Ship damage scale Plasma warhead, ignores 5W worth of armor. 2 tons, 5,000 credits. Launcher weighs 2 tons, 200 credits.
Basic PDC Cannon
x4, 400cr for weapon, d4 vehicle damage, 50 kilos.
New Weapons
I feel something needs to be added to the list of weapons. So, for my Expanse canon, I will add some more options. Epstein drive powered torpedoes are the preferred weapons of real warships, but pirates and such need smaller, cheaper and more easily concealed weapons. Conventional cannons firing shells may seem odd in the Expanse setting, but when your target is a slow freighter, even cannon rounds can hit your target. Also, simple chemical rocket propelled missiles are another, perhaps more reasonable, alternative. With the level of fabrication technology available, making simple unguided rockets or even basic guided missiles is possible. Chem fueled projectiles are cheap, difficult to detect and hard for PDCs to intercept. At close range, a salvo of rockets can rip apart a freighter or even a light warship. A multiple rocket launcher could be concealed in an airlock or cargo bay and exposed for firing at the last moment when the target ship is too close to take defensive measures.
Weapon lasers possibly exist as infantry heavy weapons or vehicle mounted versions on tanks and such, but that's total supposition. Considering the extreme ranges of space combat, lasers may simply not be effective as ship weapons. I would expect them to be good point defense weapons, but the authors went with projectile weapons instead.
In Leviathan Wakes, there are automatic defense lasers on Thoth station during the OPA boarding action. The troops had anti-laser gas grenades, suggesting laser weapons are common enough that even the poorly equipped Belter assault troops had the gas grenades. They mention that the docking tube on the Rocinante is armored against lasers and such. Perhaps lasers are so power hungry they are only used in fixed installations where they can run off of massive power plants. Then there could be lasers on capital ships. I may have to sit down with the Striker miniature rules from Traveller and compare railguns and lasers for power efficiency. And maybe tweak the Striker stats to use as a guideline.
Here's a Striker comparison.
TL 9 weapons, 1 ton approx.
Beam Laser- 15mw, 0.99 tons, 60KCr. Effective range 7.5km, 37.5 pen. Long range 15km, 18.8 pen. Extreme range 75km, 7.5 pen
8cm HyperV Mass Driver- 7mw/shot, 0.99 tons, 199KCr*. Effective range 2.5km, 21.8 pen. Long range 3.5km, 15.4 pen. Extreme range 5.5km, 10.9 pen.
*Minimum cost is 100KCr, that needs to be modified.
So, a few thoughts. First, the price for the mass driver is out of whack. A battle rifle (7.62mm NATO equivalent) costs 1,000cr, a gauss rifle costs 1,500cr and a laser rifle costs 3,500cr. So the mass driver should cost less than the laser. Second, the laser has much better range. But, in space, the mass driver's projectile doesn't slow down, so its effective range penetration becomes its penetration at all ranges. Lastly, there's the question of fire control. The general rule for space is multiply weapon ranges by 1000. If we apply the same rule to fire control, then at TL9 in space we get
| Unit | Effective | Long | Extreme |
|---|---|---|---|
| Laser | 7500 | 15000 | 75000 |
| Mass Driver | 2500 | 3500 | 5500 |
| Fire Control | 2500 | 3500 | 5000 |
So, limited by the fire control, the laser is always at effective range, regardless of the fire control's accuracy, with a penetration of 37.5 and the mass driver is also always at effective range (because the projectile doesn't slow) for a penetration of 21.8. So the laser is still better, but may be more expensive.
Even the power consumption doesn't matter, because the beam laser can engage two targets per "shot" using 15Mw, and the mass driver can fire twice. But the laser has a to hit bonus as well.
Assuming lasers are not acceptable and we're using railguns, the next big issue is projectile velocity and time to target. Taking Holden's quote, we'll assume 5,000 meters/second or 5 kilometers/second which means looking at the ship combat rules that railguns are effective out to 100 kilometers it would be a 20 second flight time giving the target too much time to evade. So, for an anti-ship gun like the Rocinante's spinal gun or a dreadnought's turret guns, their velocity must be significantly higher. If we reduce the projectile weight by a factor of 20, we could in theory raise the velocity by the same amount giving us a projectile velocity of 100 kilometers per second. That would give a one second flight time to 100 kilometer range, which is better (this only works in space; shooting a projectile at those speeds would cause it to melt from atmospheric friction). Since kinetic energy is based on the square of speed, this drastically changes the impact energy. A 100 kilogram projectile at 5 km/second delivers 1250 megajoules. If we lower the projectile mass by a factor of 20 (5kg) and in theory raise the velocity by a factor of 20 (100km/second), we get 25,000 megajoules!! It probably doesn't scale exactly, I would imagine there's an efficiency drop off as you try to raise your velocity (The round could be going so fast, it leaves the barrel before it gets the full acceleration advantage. But, the rail guns on the ships are pretty big. Light railguns are supposedly over 30 meters long and super heavy guns can be over 100 meters long. The Laconia supplied railguns are supposed to have velocities of 30-50 km/second, which would give us a two to three second flight time to maximum range). Let's say there's a factor of five efficiency loss, so to get a projectile going 20 times as fast, it weighs 100 times less, making my above gun design fire a 1kg projectile. A 1kg round at 100km/second gives an impact energy of 5,000 megajoules, so we're not super out of line, I guess. The Expanse wiki says the Rocinante's railgun fired two pound projectiles, so there we are at the 1 kilogram projectile weight.
EDIT- A thought I had later is if you can build effective laser weapons, their huge advantage over railguns is their (literal) light speed time to target. At 5,000 kilometers, a laser takes 0.02 seconds to reach the target. A railgun would take 50 seconds to get there, meaning the target has plenty of time to dodge. At 100 kilometers, it's practically instantaneous (.0003 seconds).
Torpedo Tactics
Another thing I have a problem with in the show is the Rocinante's near invulnerability to torpedoes. She does have SIX PDC turrets, which is more than some heavier ships, but only two missile launchers (one of the earlier episodes shows her as having four tubes, later episodes have her doing what looks like a two shot rapid fire from each tube for four torps per salvo. The book says two tubes, twenty torpedoes in her magazines).
Looking at the game, they say you can't dodge torpedoes fired at long range because they are going too fast, you have to shoot them down. So maybe it would be fair to say torpedoes can't be shot down at close range, but they can be dodged? In that case, the Rocinante class ships as torpedo bombers explains their large PDC armament; they have to defend themselves against torpedo attacks from capital ships while they charge in to close range to launch torpedoes from so close that the target's PDCs are ineffective, and a dreadnought wouldn't be agile enough to dodge torpedoes. They do that in one episode of the show, where Holden fires a salvo of torpedoes to explode and blind the target's sensors so they can get close enough to fire torpedoes at point blank range. Personally, I would make her have more tubes so she can fire more torpedoes in one salvo. At high closing speeds, you're going to have a limited window to shoot; being able to throw more shots at a time is more important than multiple salvoes, especially when your chance of surviving multiple attack runs is low.
A side note on the Roci's invulnerability is also the fact that they may have been lower grade torpedoes. Often times in good science fiction, a point is made that the sensors and countermeasures aboard a missile are much smaller and less effective than the full sized units aboard a ship. In many instances, ship to ship missiles are either directly controlled by their launching ship or receive targeting updates to help them get closer to their targets. I think David Weber coined the term "penaids" as a contraction of "penetration aids"; a catch all phrase for any gear mounted on a missile to help it penetrate a target's defenses. It may simply be a matter of quality. The missiles used by belter renegades may simply not be as advanced as state of the art Earth and Mars made missiles with weaker or non-existent penaids. In the video above fighting the Zmeya and when the Dewalt drops about 20 torpedoes from its external racks to attack the Serio Mal, the massive salvoes of torpedoes have two possible disadvantages: One, as mentioned above, they may be lower quality. Lower quality missiles may not have the ability to perform evasive maneuvers to help throw off PDC fire or jammers to interfere with PDC fire control, and their sensors may take longer to lock onto a target for their final attack run, also making them easier targets. Two, if the firing ship is providing any kind of guidance support to the torpedoes, having to provide it to many torpedoes may overload the launching ship's fire control, preventing it from providing the best possible assistance to the torpedoes, again making them easier targets for PDCs.
