Questions: Spinal Mounts

[DivineWrath]

I have some questions regarding spinal mounts.

Spinal mounts get size reduction as their tech level rises. Normally, a particle spinal mount is 3500 tons per increment with the max tonnage of 28000. That gives 8 increments (max damage of 8DD). However, with 3 additional TLs, a railgun spinal mount shrinks to 2800 tons per increment. From the looks of it, it seems that it increases the number of increments to 10 (max damage of 10DD). Is this how it works?

How many crewmen are required for spinal mounts? Bay weapons may use more crewmen depending on size. Does the same apply for spinal mounts? I can't find any information on it.

 

[allanimal]

I have some questions regarding spinal mounts.

Spinal mounts get size reduction as their tech level rises. Normally, a particle spinal mount is 3500 tons per increment with the max tonnage of 28000. That gives 8 increments (max damage of 8DD). However, with 3 additional TLs, a railgun spinal mount shrinks to 2800 tons per increment. From the looks of it, it seems that it increases the number of increments to 10 (max damage of 10DD). Is this how it works?

How many crewmen are required for spinal mounts? Bay weapons may use more crewmen depending on size. Does the same apply for spinal mounts? I can't find any information on it.

I think you have it right about the sizes.

As for crew, I can't quote a page number, but the spreadsheet I have worked up for 2e ship construction has 1 gunner per 100 tons of spinal mount. I didn't just make that up, so it must be somewhere in the book...

 

[Condottiere]

It's a loophole; you could also argue that with an early prototype of the maximum size, if you incorporated that.

The intent appears to be keep spinal mounts at distinct but arbitrary levels of destructive effect.

As for personnel, one per hundred tonnes, plus supervision.

 

[AndrewW]

How many crewmen are required for spinal mounts? Bay weapons may use more crewmen depending on size. Does the same apply for spinal mounts? I can't find any information on it.

1 per 100 tons of spinal mount weaponry

 

[DivineWrath]

Thanks for the replies. I have more questions.

The Railgun Spinal Mount has capacity for 5 ammo. Given that it takes 1 ship combat round to reload a weapon, it means that this spinal mount will be reloading close to twice an hour. Is there no special loading mechanism to take care of that? I mean, I don't see a bunch of gunners being physically able to move a single dt 20 round by themselves. If it has the heavy hardware needed to move such an object, I think it should be able to treat an ammo storage as a large magazine.

Damage? Multiplied by a 1000!? It didn't hit me at first, but now I realize that a spinal mount ship isn't likely to survive an average attack by a spinal mount that is possible for its weight class. A ship with a 3500 ton railgun spinal mount will need to be 7000 tons, so it will have 2800 hull points. The average damage from its spinal mount would be 3500 (3.5 * 1000). Even if you factor in armor, lets say 10, it'll still be 2800 hull damage. Half of the dice rolls will be enough to take it out completely, and the other half will be enough to deal crippling amounts of damage.

I'll do some more number crunching because spinal mounts seem to be overwhelmingly powerful.

 

[steve98052]

The Railgun Spinal Mount has capacity for 5 ammo. Given that it takes 1 ship combat round to reload a weapon, it means that this spinal mount will be reloading close to twice an hour. Is there no special loading mechanism to take care of that? I mean, I don't see a bunch of gunners being physically able to move a single dt 20 round by themselves. If it has the heavy hardware needed to move such an object, I think it should be able to treat an ammo storage as a large magazine.

I would assume that the capacity of five rounds means five rounds before reloading is required. If I were designing a ship that could fire more rounds before reloading, I'd just add 20 dtons per additional shot at the same cost per dton as the rest of the spinal mount, unless someone finds rules that say otherwise.

Damage? Multiplied by a 1000!? It didn't hit me at first, but now I realize that a spinal mount ship isn't likely to survive an average attack by a spinal mount that is possible for its weight class. A ship with a 3500 ton railgun spinal mount will need to be 7000 tons, so it will have 2800 hull points. The average damage from its spinal mount would be 3500 (3.5 * 1000). Even if you factor in armor, lets say 10, it'll still be 2800 hull damage. Half of the dice rolls will be enough to take it out completely, and the other half will be enough to deal crippling amounts of damage.

Here are some assumptions and some math:

I don't know how fast a railgun spinal mount throws its rounds. But for the sake of discussion, let's assume it's the same as a VRF Gauss Gun, 4500 meters per second.

I don't know the density of the 20 dton railgun rounds. For the sake of discussion, I'll use a few different figures: A. Their mass is 20 tons, but stored in a 20 dton space, whatever their actual density. B. Their mass is the same as 20 dtons of water, or 280 tons. C. Their mass is the same as 20 dtons of iron, or 2240 tons.

I don't know the shape of a railgun round. In case A, let's arbitrarily assume they're 1 meter in diameter, and as long as they need to be. In cases B and C, let's assume they're 5 meters in diameter, which makes them 14 meters long.

I don't know the dimensions of the railgun spinal mount. Let's assume that they're 15 meters in diameter, which makes the 3500 dton base model 277 meters long. (I was going to use this to calculate the acceleration required to reach the example velocity, but I'm not sure that matters.)

So, what's the kinetic energy of the railgun spinal round? Mass times velocity squared.
A. 20 000 kg × (4500 m/s)^2 = 405 billion kg (m/s)^2 = 405 GJ = 97 tons of TNT equivalent, all of which is applied to the hull of the target and the space behind it, until it blows through the other side.
B. 280 000 kg × (4500 m/s)^2 = 5670 billion kg (m/s)^2 = 5.67 TJ = 1355 tons of TNT equivalent.
C. 2240 000 kg × (4500 m/s)^2 = 45.36 trillion kg (m/s)^2 = 45.36 TJ = 10.84 kilotons of TNT equivalent, a bit less than the 15 kiloton Hiroshima bomb and the 20 kiloton Nagasaki bomb.

What if we bump up the velocity to 10 km/s, and use just the least massive round?
20 000 kg × (10 000 m/s)^2 = 2 trillion kg (m/s)^2 = 2 TJ = 478 tons of TNT equivalent, comparable to the US backpack nuclear demolitions bomb.

A million dton ship is 14 million cubic meters. That's roughly equal to a 241 meter cube or a 299 meter diameter sphere.

Based on figures from http://nuclearsecrecy.com/nukemap, a 100 ton TNT equivalent nuclear bomb on a surface target produces a shallow crater of 10 meters inside radius, 20 meters lip radius, a 30 meter fireball radius, a 100 meter 20-psi radius (destroys heavily built concrete buildings), a 230 meter thermal radiation radius (usually fatal burns), and a 560 meter 500-rem radius (fatal radiation). A kinetic weapon distributes its energy differently. It would produce a much deeper crater, less thermal radiation (which would be be blocked by ship compartmentalization outside the immediate area of devastation), and no radiation unless it caused a power plant fusion spill, but without expending energy as radiation, more energy would be available to cause structural damage. The 100 meter 20-psi radius would demolish most of the million-dton ship if it were made of reinforced concrete, but such a ship would probably be weaker between bulkheads and tougher through bulkheads. One thing that might reduce damage is the possibility that the round would punch completely through the ship, carrying much of its kinetic energy off into space with it.

So, even the smallest example weapon would be devastating, essentially destroying everything from one side of the target ship to the other, out to the nearest compartmentalization at least. More armor would likely make the weapon more damaging, because less energy would escape when the round blows through the other side of the target.

The catch to the weapon is that its chances to hit are (presumably) lower than a particle accelerator or meson gun, unless the range is really short.

 

[phavoc]

Unless it's firing from a magazine that needs to be ejected, common sense tells you that as long as their is magazine space set aside a ship should be able to fire it's railgun continuously until it runs out of ammunition. As soon as one round is loaded a spare round should be placed into the ready-magazine. If they could do that with WW1 battleships they surely can do the same with a 52nd century weapon too?

 

[AnotherDilbert]

I don't know how fast a railgun spinal mount throws its rounds. But for the sake of discussion, let's assume it's the same as a VRF Gauss Gun, 4500 meters per second.

That is much slower than most spacecraft. You can't really hope to hit anything mobile with something that slow... To hit something at Medium (up to 10 000 km) range I would require a speed of something like 1000 km/s.

You might fire lots of small slow projectiles in a large cloud in the hope that some of them might hit the target before it wanders off from the target area. Even so, at less than 30 km/s it will not reach the target in a single round.

So, what's the kinetic energy of the railgun spinal round? Mass times velocity squared.
A. 20 000 kg × (4500 m/s)^2 = 405 billion kg (m/s)^2 = 405 GJ = 97 tons of TNT equivalent, all of which is applied to the hull of the target and the space behind it, until it blows through the other side.

Note: E = mv²/2, doesn't change the conclusion.

 

[Condottiere]

It's either near relativistic speeds or maybe twenty five kay klicks in five minutes.

The balls aren't packed with explosives, and you don't need a combustible propellant, so a crane plus brackets, so that the bullets don't move around.

 

[steve98052]

I don't know how fast a railgun spinal mount throws its rounds. But for the sake of discussion, let's assume it's the same as a VRF Gauss Gun, 4500 meters per second.

That is much slower than most spacecraft. You can't really hope to hit anything mobile with something that slow... To hit something at Medium (up to 10 000 km) range I would require a speed of something like 1000 km/s.

Good point. One ton of mass (never mind 20, or 280, or 2240) at 1000 km/s, with the corrected kinetic energy formula below, is 500 TJ, or 120 kilotons TNT equivalent. Ouch.

Note: E = mv²/2, doesn't change the conclusion.

Oops! I knew that, but for some reason messed that up.

It's either near relativistic speeds or maybe twenty five kay klicks in five minutes.

What qualifies as "near relativistic"? One percent of the speed of light, perhaps? A 45 gram golf ball at 3000 km/s is about 0.2 TJ, or about 48 tons of TNT. Bump it up to ten percent of the speed of light and it's 20 TJ, or 4.8 kilotons of TNT. Ouch.

---

Another point is that however much energy is delivered to the target must be applied to it must be supplied to the mass driver, and any energy lost in the process must be removed from the weapon so that it doesn't melt itself. How many power points are used by the weapon? (That's mostly a rhetorical question, but I don't have the table handy.) How much energy is in a power point?

Even if Mongoose doesn't define a power point in real units, we can figure out the order of magnitude from other editions. I know that Mega Traveller uses real units at least in some places, and that GURPS Traveller uses real units throughout, and it uses real physics wherever possible, and tries to make fictional physics reasonably consistent with real physics as well as possible.

 

[Reynard]

There was a comic series called ALBEDO that featured a bit of hard science space combat. The ships didn't use energy weapons but instead fired high speed projectiles like a railgun. The stories made note the projectile still couldn't hit well for the speed alone. What made the projectile work was they were autonomous drones with sensors and small reaction thrusters to correct for the target's defensive maneuvers. Maybe this is also the case?

 

[AnotherDilbert]

What made the projectile work was they were autonomous drones with sensors and small reaction thrusters to correct for the target's defensive maneuvers. Maybe this is also the case?

AKA missiles?

 

[AnotherDilbert]

Another point is that however much energy is delivered to the target must be applied to it must be supplied to the mass driver, and any energy lost in the process must be removed from the weapon so that it doesn't melt itself. How many power points are used by the weapon? (That's mostly a rhetorical question, but I don't have the table handy.) How much energy is in a power point?

Undefined. We can guesstimate based on earlier editions. E.g. a TL15 power plant produces 20 Power per dT ≈ 14 m³. In CT & MT a 1 dT TL15 fusion power plant produces 250 MW. That would make 1 Power = 12.5 MW, let's call it roughly 10 MW to make it simple.

A minimal railgun spinal consumes 500 Power ≈ 5000 MW. During a 6 minute round that is 5000 MW × 360 s = 1.8 TJ. If we assume it is less than 100% efficient it can only deliver max 1 TJ muzzle energy. As you have calculated heavy rounds will not be very fast.


I tend to assume that the rounds are precision machined to very exact dimensions to be able to withstand the astounding acceleration in the railgun and be very, very round in order to be accelerated in a straight line. Even minute imperfections would accelerate the round off-bore, and miss the target. A small bump would make the round unusable. So I assume the 20 dT "ammunition" is mostly very sturdy packaging, perhaps keeping the rounds contained in a magnetic field with no physical contact.

 

[Reynard]

"AKA missiles?"

Actually no, a smart slug. A missile has propellant to accelerate the projectile to a target. Steve Gallacci's Autonomous Combat Vehicles are a chunk of metal fired to tens or hundreds of kilometers a second then coast to a target but have a brain, sensors and thrusters to make small changes in direct flight path. It sounds like a reasonable description how a Traveller spinal mount railgun might work.

 

[DivineWrath]

I did some number crunching, some thought experiments, and actual ship designing. Here are some of my thoughts:

Build bigger ships. Spinal mounts can easily cripple a ship of its weight class, if not out right one shot it. There is a limit to how big a spinal mount can go (unless you are using super laser). Try to build your ships significantly bigger so it can take a hit and still keep going.

Build smaller. Spinal mounts have trouble shooting smaller targets. If you build small, the spinal mount will suffer a penalty to hit you, or maybe can't target you at all. Mind you, there are some disadvantages for building small...

Just because you can build a big spinal mounts doesn't mean you should. You can build spinal mounts to be about 50% of your ship's tonnage. However, you shouldn't forget that you need to fit other ship parts on a ship. Jump fuel for instance will need 10%, 20%, 30%, etc of the hull's total tonnage. If jump fuel is competing with a spinal mount taking up 50% of the hull, you are not going to have a ship that can jump very far. And this is before you add in armor, other weapons, and other ship modules.

 

[AnotherDilbert]

Spinal mounts can easily cripple a ship of its weight class, if not out right one shot it.

Sorry, I do not see it. Can you provide an example?

A battleship with J-3, max manoeuvre, and max armour has a payload of about 30% (depending on TL). Carrying a maximum spinal without any defences or secondary armaments will not even remotely one-shot itself.

Example: TL15, 110 kT, J-3, M-9, Armour 15, leaves a payload of ~35 kT. You can carry a 10DD meson or particle spinal (and very little else). You have 80666 Hull. Particle does average 10DD ≈ 10 × 3.5 × 1000 × 0.55 ≈ 19250 damage. It takes ~4 hits for the ship to kill itself (and you won't hit every time).

Just because you can build a big spinal mounts doesn't mean you should. You can build spinal mounts to be about 50% of your ship's tonnage. However, you shouldn't forget that you need to fit other ship parts on a ship. Jump fuel for instance will need 10%, 20%, 30%, etc of the hull's total tonnage. If jump fuel is competing with a spinal mount taking up 50% of the hull, you are not going to have a ship that can jump very far. And this is before you add in armor, other weapons, and other ship modules.

I quite agree. In the example above you might carry a 8DD spinal and leave some space for defences.

 

[DivineWrath]

Spinal mounts can easily cripple a ship of its weight class, if not out right one shot it.

Sorry, I do not see it. Can you provide an example?

A battleship with J-3, max manoeuvre, and max armour has a payload of about 30% (depending on TL). Carrying a maximum spinal without any defences or secondary armaments will not even remotely one-shot itself.

Example: TL15, 110 kT, J-3, M-9, Armour 15, leaves a payload of ~35 kT. You can carry a 10DD meson or particle spinal (and very little else). You have 80666 Hull. Particle does average 10DD ≈ 10 × 3.5 × 1000 × 0.55 ≈ 19250 damage. It takes ~4 hits for the ship to kill itself (and you won't hit every time).

Well, one of my recent designs was using a 21000 dt railgun spinal mount (6DD) on a 42000 dt space craft. It is a small as you can get while getting the biggest railgun spinal mount you can use. It has 25410 hull point (after factoring in reinforcement and close structure) and its average damage is 6 * 3.5 * 1000 = 21000. Average damage is pretty close to being able to one shot itself.

I included armor of 10 (I was designing a TL 10 ship), so the damage would be reduce to 16800. 2 average hits and it'll be dead.

Also, your example ship is different from my example. It has significantly more hull than it needs for the spinal mount. My example ship is the largest railgun spinal mount possible, on the small ship that can fit it possible.

I'll go punch in some numbers for ships of other spinal mounts. Maybe there was something I missed.

 

[AnotherDilbert]

Well, one of my recent designs was using a 21000 dt railgun spinal mount (6DD) on a 42000 dt space craft. It is a small as you can get while getting the biggest railgun spinal mount you can use. It has 25410 hull point (after factoring in reinforcement and close structure) and its average damage is 6 * 3.5 * 1000 = 21000. Average damage is pretty close to being able to one shot itself.

Ah, TL10, that is J-1, basically a battle rider, they are a bit iffy. It's different at TL11+.

With only Short range and only laser turrets for defence you are very vulnerable to missiles...

Instead of the spinal you could have 50 large torpedo bays, launching 1500 torpedoes per round. With Multi-Warhead Nukes that is average 49500 damage after armour with auto-hit. Even 200 laser turrets would only destroy ~400 torpedoes, still auto-killing itself with a single salvo from Distant range.

 

[Condottiere]

The difference in damage for railgun spinal mounts is muzzle velocity, since the ball nearing is both the same size and cost, whether a five hundred tonne mass driver, or a twenty thousand tonne railgun.

Though you have to ask yourself, what material costs two hundred kay schmuckers for two hundred eighty cubic metres, when a similar dollop excavated from a nickel iron asteroid is between ten to a hundred thousand schmuckers, depending on how you cost it.

I'd use a 8.12 metre diameter ping pong ball.

 

[DivineWrath]

Ah, TL10, that is J-1, basically a battle rider, they are a bit iffy. It's different at TL11+.

Battle rider. Whats that? I'm not that familiar with Traveller lore.

With only Short range and only laser turrets for defence you are very vulnerable to missiles...

Instead of the spinal you could have 50 large torpedo bays, launching 1500 torpedoes per round. With Multi-Warhead Nukes that is average 49500 damage after armour with auto-hit. Even 200 laser turrets would only destroy ~400 torpedoes, still auto-killing itself with a single salvo from Distant range.

Hmm... Is there a good defense against this? I'm having fun toying with different designs for the sake of making them.