Railgun: Deep Beyond

In THS: Deep Beyond, a railgun component is introduced. This railgun is an “extremely long-barreled 155mm gun”, and has a muzzle velocity of 15km/s. It fires bursts (maximum of 2 per 100 seconds), each of which masses about ten kg. Each burst does cDAM 6dx(5+RV) damage, with an after-armour multiplier of 10 which can be reduced by point defence fire.

The gun masses 22t and uses 1,700MJ per burst. It can also be installed only when that dimension is longer than about 30 metres.

GURPS Vehicles

Now, let’s start by building this gun in 3e’s Vehicles. There are a few things we know (or at least can assume): Barrel length is extreme-length, calibre is 155mm, and power is probably normal. Using these, we can already compute weight per shot:

$$\text{WPS} = B^3 * D * P$$

where B is the bore size in mm, D is 0.0000375 (I always have to double-check whether I forgot a 0 in there), and P is 1. This gives us a weight per shot of 139lbs, or 63kg.

Check the intro again: That single shot has six times the mass of our supposed burst. Clearly, there’s something wrong with that.

I therefore spent some time collecting 3e Vehicles’ formulae into a spreadsheet to autocompute them. I had some issues there, so I redid it. And then I still had some issues with it, so I just reimplemented everything in python. Should’ve done that before.

Now, a 155mm extreme-length normal-powered fast autoloader, according to 3e Vehicles, does 744d damage (vs 3,000d damage in Deep Beyond), masses 20t (vs 22t), draws 22,000MJ per space turn (vs 3,400MJ) and fires one 63kg projectile every four seconds (26 projectiles per 100s THS space turn). It has an accuracy of 22 in 3e terms, and 11 in 4e.

Muzzle Velocity

Barrel length, by the way, is more than 100 calibres (Vehicles, p. 99 sidebar). That’s more than 15 metres, which meshes with the minimum requirement of 30 metres.

Unfortunately, Vehicles doesn’t give us a muzzle velocity. Fortunately, Douglas Cole apparently wrote an article in pyramid for exactly that. It contains a fairly complicated formula which is supposed to mesh well with GURPS damage values and gives you damage given projectile cross-section and muzzle energy. Inverting that formula gives me a muzzle energy of 164kJ. This meshes quite nicely with the power draw of 220kW (850kJ per shot) for an efficiency of 20%.

Muzzle velocity depends on how much of the WPS is actually the projectile. If there’s a conductive sabot involved, it’s definitely less than the WPS. I’m going to use Douglas Cole’s rule of thumb to “Use about 2/3 of the WPS to get the projectile weight”, the same for conventional weaponry. This gives a muzzle velocity of 2,800m/s.

Other Ammunition

What other ways of increasing that muzzle velocity are there? A saboted round with a heavy core - Vehicles gives us APDSDU (Armour-piercing discarding-sabot depleted uranium; it doesn’t make much sense in space to add fins for APFSDSDU). That gives 1.66x kinetic damage plus an armour divisor of 3. That increases total penetration to 3500d (compare this to 3000d from Deep Beyond). It also divides WPS by 1.5.

What does this mean? We have two effects in there, which we can both quantify: The effect from using a sabot, and the effect from a heavier core. Luckily, there’s both APDS and APDU rounds. Where a normal AP round does KE(2) damage, APDS does 1.66xKE(2), and APDU does 1.33xKE(3). So, the material density influences the armour-piercing factor, while the sabot effectively increases damage by decreasing cross-section.

Indeed, it is this 1.66x factor which we’re mostly concerned with. This can be achieved in two ways: Either by decreasing the cross-section (more energy per area) or by increasing the velocity (more energy in general).

Looking at the Pyramid article again, there are two relevant formulae: The penetration damage is linearly dependent on $KE^{0.52}$ and on $Xsect^{0.157}$. The wounding, though (assuming that’s similar for spacecraft and people and ignoring tumbling for now) is proportional to momentum and cross-section, i.e. proportional to projectile mass, projectile velocity, and cross-section.

Kinetic energy has to remain identical (we’re not going to suddenly produce more energy). But even if I assume that damage isn’t actually computed as for normal wounding and just use kinetic energy, velocity is only increased to 4km/s.

Use in Combat

According to 4e’s Spaceships rules, sAcc assumes an Acc 18 weapon, dedicated targeting, and a vehicle targeting system for a total of +30, exactly correcting for the -30 range modifier at 100 miles.

The railgun above, with its acc of 11, would therefore have -7 to hit at even the relatively short range of 100 miles (short, in THS’ space combat system). That’s close, by the way, to 4e Spaceship’s EM guns.

However, this is for an unguided projectile. Using Vehicles missile rules, we can build a “missile” (actually a guided projectile) which can be fired from that railgun. This missile has a brilliant imaging infrared guidance, mounts a big warhead, and masses a total of 42kg. Of this, 20.3kg is warhead, 21.1kg motor, and guidance accounts for less than a kilo.

Its burn endurance (at 2G) is 276s, for 5.5km/s dV¹. If we assume that this is just spent on correcting for enemy movement, it will give us a guided range from rest of 830 kilometres at full acceleration; if we just have to correct for high-efficiency thrust from an AKV it’s about four times that. Also, closing speed increases effective distance: In last post’s scenario, the AKVs could fire at the SDV within 82,000 kilometres and still have guidance during the final attack run! The SDV in turn could fire at 35,000 kilometres.

But it still takes 220MW to fire, far more than the heavy laser for only slightly more damage. This is rather inefficient. However, there’s an alternative: A low-powered 155mm railgun only needs 27MW to accelerate its 32kg projectile to 2km/s. That one does CDAM 1.12d(3)xV (round down during play, i.e. unless you reach 9km/s, it’s just 1d(3)xV) with a from-rest V of 2. It has 3.45km/s dV. In fact, I’m going to treat it as 2.45km/s (245 burn points) and boost V to 3 with a post-firing acceleration phase.

In Space Combat

A low-powered 155mm railgun component is 5t and 0.5 spaces. It takes 27MW to fire two bursts per turn. That’s 1,350MJ per burst (66 bursts per battery space).

Damage is cDAM 1.12d(3) x V, where V is in closing speed in km/s. At rest, V is 3. Alternatively, you can use a bursting charge, in which case you lose the armour-piercing modifier, but gain +4 to hit².

Going back to THS space combat rules, the railgun can fire two bursts of 10 rounds each per 100s-turn. If still in guided range, roll against Gunner (Railgun) to hit, at no range modifiers. If a critical success, double damage. A normal success means that you multiply post-armour damage by 10. Each point by which you fail reduces the post-armour modifier by 1.

There are two ways to avoid being hit: You can either run the burst out of deltaV. Every burn point the target spends has to be spent by each burst currently on the way, too. Once a burst reaches 0 BP, it is eliminated from play. This can be an effective tactic against all bursts currently on the way (an SDV has about 20 times a burst’s dV) but will fail under constant fire.

Alternatively, you can use laser point defence. Using the rules from Deep Beyond, each laser eliminates one burst on a successful gunner roll. On a failure, it reduces post-armour multiplier by 10 - margin of failure.

Each space of such railgun ammunition provides 35 bursts, and masses 11.5t.

A smaller Railgun

A 5-ton railgun is arguably fairly big, and we might want to mount something smaller onto AKVs, for example. A 40mm weapon masses 1.3t (0.12 spaces), fires up to 100 projectiles per minute at 4km/s, and uses 14MW of power per turn (700MJ per burst, 125 bursts per battery space).

Damage is cDAM 0.3d(3)xV (V from rest is 4), with after-damage modifier of 50. As usual, this is reduced by point defence; use the same rules as above except that you multiply all numbers by 5, i.e. each point defence laser can intercept five projectiles instead of one.

Railgun Tower

At the moment, a railgun is simply a fixed-mount weapon. But, like laser weaponry, you should be able to put it in a turreted mount.

As a very simple approximation, assume that the turret has the same armour as the main spacecraft. It also takes its volume away from that spacecraft - this represents that we “stretch” the armour further. This is, admittedly, a simplification. Such a turret has then an additional 20% volume (for the rotation gear). It is also equipped with a universal mount, enabling it to fire directly upwards. Both of these factors conspire to 1.5x mass and 1.8x volume. I’ll increase the latter to 2x to represent a larger relative surface mass.

This means a low-powered 155t railgun masses 5t and takes up 0.5 spaces when not turret mounted, and 7.5t and 1 space in a turret. For the 40mm railgun it’s 1.95t and 0.25 spaces.

Summary

In summary, this completely replaces the THS Deep Beyond railgun. Compared to a heavy laser, which takes roughly the same power to fire, it has less penetration (10d from rest vs 20d) and slightly less injury (33d from rest vs 40d). Once you figure in relative speed, though, it becomes deadly.

I’ll be testing that soon.