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Old 09-07-2016, 10:38 AM   #1
kreios
 
Join Date: Jul 2013
Default Building a Fun Spacecraft Combat Paradigm

I have, for quite some time, looked at building a (comparatively hard) setting which includes combat between spacecraft. Naturally, I want to use the Spaceships rules.

Conventional wisdom is that this results in eggshells armed with sledgehammers, missiles dominating everything, and engagements being either inconclusive (enough point defense) or deadly (not enough point defense). This is - obviously - a situation I want to avoid.

In doing so, there are a few parameters of my analysis:

- I'm not using the basic space combat system.
- I'm wildly varying between time scale and hex/range depending on my analysis.
- I'm looking at probabilities, not skill rolls. That means I'm going to use roll vs 10 as 50% missiles intercepted, for example.
- I'm assuming the existence of skill-12 targeting programs, which do the actual weapon firing. That means one computer-gunner per weapon.
- I'm looking at TL10, with only the reactionless drive being superscience. Used features are Exposed Radiators, Accelerator Tube Limits (SS7:22), Slower Industrial Systems (SS7:23), and the Telescoping Robot Arm feature (SS8:9)
- I have not yet decided which stardrive to use; my current favourite would be a jump drive which can jump from most places except lower than about GEO; and one jump being at most half a lightyear, requiring realignment. It is instantaneous, though.

I am also thinking about using Armour as Dice (Pyramid 3/34:32). Using this, nanocomposite armour looks like this (slightly simplified to remove most die adds):

Code:
SM          +5      +6      +7      +8      +9      +10     +11     +12     +13     +14     +15
Armour      2d      3d      5d      7d      10d     15d     20d     30d     40d     60d     90d





Missiles

Damage
The smallest missile that fits on a spacecraft is the 16cm missile, which weights 67kg (about 150lbs). It can mount a nuclear warhead for 4dx1,000 burn etc plus a linked 3dx1,000 ex damage, or a conventional warhead for 6dx4 (2) collision or the same without armour-piercing when firing for proximity detonation. It can also be equipped with an X-ray laser warhead (SS8:9), for a range of 300/1,000 miles, RoF of 8, and 10d(5) damage.
I'm restricting myself to this missile since it has the highest firing rate, and can still mount a 25kt nuclear warhead. Should it prove too small, I'm going to upgrade.

Nuclear Warhead
The nuclear weapon itself suffices to destroy everything up to SM+15 on a direct hit. An SM+15 spacecraft has dHP of 1,000; devoting all six systems in a location to nanocomposite gives it an US dDR of 1,800. The burn damage will bring it down to at least -1,200HP/-1.2xHP (up to -22xHP best case, meaning total vapourization); including the explosion the spacecraft ends up at -2.4xHP. This means two rolls against HT to avoid destruction, plus the component loss. In summary, a nuclear missile hitting means certain death for even the most over-engineered block of armour floating in space. Using armour as dice doesn't change that.

It looks differently when looking at nuclear proximity detonations. This decreases damage by a factor of 100. Suddenly, our SM+15 spacecraft is able to easily survive even a worst-case proximity detonation. Yay!
However, how does it look like for a more reasonable-sized spacecraft? Immunity to nuclear proximity detonations is guaranteed at 240dDR, which is five systems at SM+10, four at SM+11, three at SM+12, two at SM+13 and one at SM+15. It is sufficient to reduce a SM+5 spacecraft with two nanocomposite systems to -1xHP; it's also pretty effective against radiators. Note that a nuclear blast might also damage spacecraft in the same 10-mile hex, though a 25kT warhead will never roll high enough to do so, except for missiles.
Using armour as dice, we need to withstand 40d dDmg. This can be achieved at one system at SM+13, two at SM+11, three at SM+10 and five at SM+9.

What if we use more power? A 10 megaton nuclear warhead - at $1M per pop - fits only the biggest, 52cm, missiles. It does 9dx10,000 burn plus a linked 6dx20,000 ex. Of course, it'll still destroy everything on a contact detonation. On a proximity detonation, it does 8dx100/6dx200. That's much higher than the 25kT damage, and only an SM+15 fully-armoured section will be able to avoid damage - if the damage roll is minimal. On average, it's going to be 4,200 dDmg; meaning it's roughly as powerful as a 25kT contact detonated warhead. Using armour as dice, that's 900d to withstand, where even a fully-armoured SM+15 section only gives you 540.

Summary: You get hit directly by a nuclear warhead, and you're dead. If it's powerful enough, even a proximity detonation will kill you.


Kinetic Damage
Our 16cm missile has a dDmg of 6dx4 (2), which is multiplied by velocity. It has, according to SS3:36, 5G acceleration and 10mps dV. Actual analysis is difficult, since we don't know the closing speed at firing time. However, we can look at speeds needed to destroy certain craft.
A heavily-armoured SM+10 (3 armour systems in the section) craft has a dDR of 150, and dHP of 150. Meaning that, to ensure it's reduced to -1xHP, our 16cm missile has to travel at 15mps; to do so on average, 5mps. This seems doable. Using armour as dice, it has to inflict at least 45d damage to penetrate, which is equivalent to slightly more than 2mps; it has to travel at 14mps to ensure destruction.
Against the most heavily-armoured spacecraft (SM+15, 6 armour systems) with 1,000dHP and 1,800 DR, it has to travel at most 120mps and on average 35mps. That's much more difficult - but keep in mind that it would still allow the missile launcher of a fighter to easily kill a Class V starport. Using armour as dice, it has to travel at most 105mps.

Using proximity detonation slightly increases the required speed, but since multiple hits are generated, it might be worth it.

Summary: Being hit by a kinetic missile means that you're dead.

X-Ray laser warheads
The X-Ray laser warhead is interesting in that its damage does not scale with the missile's diameter; only RoF does. Our 16cm missile does 10d(5) d-damage, and has an RoF of 8. This 10d(5) gives it a penetration of 50 to 300dDR; a heavily-armoured SM+10 spacecraft (3 armour sections; 150dDR, 150dHP) will lose 5dHP on an average roll, and risk losing up to 30dHP. This might be repeated up to eight times per missile. With armour as dice, our SM+10 spacecraft takes just 1d of damage per hit, for an average of 3.5dDmg. This does not really scale with the missile size, however.
Why would you ever use it? Well, because it's ranged, with a range of 300/1,000 miles. That is kind of huge - it decreases time for point defense to hit, and decreases hit chance. Firing from 29 10-mile hexes away gives them a range modifier of -2, compared to one of +12 (!) for point defense.
It should also allow you to hit the centre or rear of a spacecraft by letting the missile pass the other spacecraft and then attack. This increases time spent in the point defense envelope, but might decrease armour.

Summary: Being hit by an x-ray laser warhead is definitely survivable, but you are going to take damage.


Summarizing all of the different missile types available, it seems as if the x-ray laser warhead is close to what I'd want: Hitting with one doesn't instantly obliterate every single target.
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Old 09-07-2016, 10:38 AM   #2
kreios
 
Join Date: Jul 2013
Default Re: Building a Fun Spacecraft Combat Paradigm

Defense against Missiles

There are five possible defenses against missiles: Vector change, counter-missiles, point defense, dodge, and armour.

Vector Changes
Compared to the spacecraft's reactionless engine, the missile looks pretty short-legged. What the spacecraft can try to do is accelerate laterally, trying to cause the missile to miss them. For this, I'm going to assume the missile to match the spacecraft's lateral acceleration using half its dV budget; the other half is used to (instantly; we're simplifying here) accelerate initially. Both spacecraft are at rest relative to each other; the spacecraft has a total acceleration of 1G.
The missile, closing in at 5mps, can be evaded by the spacecraft every time after 800 seconds (13 and change minutes), during which it covered a distance of only 4,000 miles. When not starting from rest, the every five mps of the launching spacecraft adds another 4,000 miles to that range. A major UV laser on an SM+10 spacecraft has a longer range than a missile launched from a 20mps velocity difference!
A larger missile gives us twice that dV; from rest, it reaches 12,800 miles. This is a longer range, and each 8mps from the launching craft gives us another 12,800 miles. We can work with that, but the result is that small missiles are actually short-range weapons (at least, using these assumptions)!

Counter-missiles
Counter missiles are interesting in that they perform better the more missiles are incoming. A nuclear-tipped counter-missile is simply going to destroy every missile in a ten-mile hex, whether there are ten or a thousand. As such, counter-missiles are useful as an oh-crap-button, and should form a complement against missile numbers that can't be stemmed by point defense. We can assume it hits every time, since the incoming missile can't expend much dV - if it does, and therefore doesn't hit the spacecraft, we've won anyway.

Point Defense
Point defense is mostly going to use the smallest VRF guns that fit, since each hit is guaranteed to destroy a missile. We're therefore looking at 30KJ UV lasers, with a range of 300/1,000 miles and a fire rate of 100 per 20-second turn. Skill modifiers are +12 for range, -1 for missile SM and up to +7 for RoF, for a total of up to +18. Obviously, that's really not needed, so we're going to split RoF between multiple missiles. Assuming skill-12 for the computer-gunners, we can fire at five missiles with each gun, for an effective skill modifier of +12 (range) +4 (RoF) - 1 (SM) -10 (multi-tasking) = +3. This means each gun has an expected 5.7 missiles killed. This, of course, assumes that we're allowing arbitrarily-small guns on there; the Spaceship Design Spreadsheet has those as Peripheral Batteries.

Now, how about those X-ray warheads? Assuming they'll close to 30 10-mile hexes, point defense suffers from a -3 in range. This changes total modifier from +18 to up to +3. Splitting RoF is still possible, but a two-missile split results in a modifier of +0, for an expected interception of only 1.5 missiles. Amusingly, not using UV lasers means that X-ray warheads can attack (with 1/2 damage) from outside of their range!

There's another way to kill missiles: Your point defense batteries can fire in non-point defense. The whole 1,000 mile range of the 30kJ lasers can be used. How many interceptions are possible depends on the range; if we assume 5mps closing speed a total of ten shots can be fired. Those have a lower hit chance; but the furthest out is still only -5, for a skill modifier of +1, i.e. 83% hit chance. This means you're expected to intercept more than 8.3 missiles, or about another six of X-ray warheads.

In summary, point defense should kill the missile fired (at once) by fourteen equally-sized missile-launcher systems, or by about five x-ray warheads-firing ones.


Dodge & Hoping it won't hit

What's the typical to-hit roll of a missile? It has an sAcc of 2 (3 for larger ones). Let's assume a large-ish spacecraft of SM+10, and a closing velocity of less than 100 mps. The roll modifier is at least +2 (sAcc) + 10 (SM) - 12 (speed) = 0, for a hit probability of 75%. This increases to 98% when using proximity detonation. Hoping it won't hit is, clearly, not a good approach.

Dodge itself rolls against pilot skill/2 + handling + dodge modifiers, at most for 6 (skill) + 0 (handling for SM+5; larger spacecraft are worse) + 1 (evasive manoeuvre) = 7, plus up to three from ECM. Best case, therefore, is half of the attacking missiles missing; without ECM it's only 16%.

Summary: You can rely on not getting hit by missiles, hoping you can dodge and hoping they won't hit. You can also detonate your reactor; that should be quicker and roughly as deadly.


Armour
As we've seen before, armour does not help against contact detonation, but does help against x-ray warheads.



Missile Conclusion
All in all, this leads us to the following results:
- A spacecraft with a single peripheral battery will be able to kill many more missiles than an equivalently-sized spacecraft can put out. Including a few missile launchers (for counter-missiles) increases protection against huge strikes.
- The only way to effectively damage a spacecraft is using x-ray warheads from a stand-off range.

This means all spacecraft will mount at least one system of peripheral batteries, for a thousand weapons half-ton weapons at SM+10. Attack spacecraft, on the other hand, mount the same number of missile launchers, launching 66kg missiles (roughly a Sidewinder or IRIS-T) at each other.
All in all, this seems like an unsatisfactory situation.
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Old 09-07-2016, 10:39 AM   #3
kreios
 
Join Date: Jul 2013
Default Re: Building a Fun Spacecraft Combat Paradigm

Kinetic Weaponry
Kinetic weapons have a comparatively low accuracy. However, they do not suffer from range modifiers. All in all, their to-hit-modifiers are relative velocity, target SM and sAcc (-8 to -6). A choice of rapid fire and proximity detonation can enhance this.
Since gun salvoes are unguided, they can only hit when fairly close to the target: At 0.5G, each 10-mile hex difference a spacecraft wants to achieve takes ten 20-second turns. Accordingly, a salvo has ten 20-second turns until it is being avoided. From rest, this gives the projectile a range of 20 ten-mile hexes or 200 miles. This is increased linearly by the velocity of the launching spacecraft: If it moves at 20mps, effective range will be 2,200 miles. Still short-ranged, but not completely so.
There is another way to use kinetic weaponry in combat: Use it to produce zones where the enemy does not dare to move out of fear to be hit by a salvo. This is more effective with guns than with missiles, since guns have ten times the shots of a missile salvo. By firing a total of ten salvoes, you're restricting your enemy from every vector they pass through; by doing so you can effectively multiply their required vector change by five. This also increases your effective range by five (more can be achieved with good shot placement against bigger craft that are less manoeuvrable). Note that this is much more effective in Gurps Spaceships since space is assumed to be two-dimensional; in three dimensions, your ten salvoes would only triple the effective range.
Once in the same hex, the gun salvo still has to hit. If we go with the scenario above and a 12cm fixed EM gun (major battery on an SM+6 craft), the relevant modifiers are +10 (SM) -7 (sAcc) -9 (relative velocity) +2 (fixed) for a total modifier of -4. Using the skill-12 gunner, only 25 percent of the shots will hit. That's why proximity detonation is so useful, bringing this up to 75%.
An alternative is to use rapid fire weapons. The same craft can mount a 3cm VRF gun, for an RoF of 100. This decreases damage from 6dx3 to 4d, but provides a +7 bonus meaning that proximity detonation is no longer necessary.

Damage is similar to missiles: A heavily-armoured SM+10 craft with a dDR of 150 and dHP of 150 requires a launch velocity for the 12cm gun of 23mps to ensure destruction, and 6mps to make it probable. That's with proximity detonation, so multiple hits are probable and will ensure destruction even earlier. For the 3cm rapid-fire weapon, it's 113mps to ensure destruction, and 32mps to make it probable. Compared to the 12cm gun, this weapon will hit an additional three times, reducing the effective velocity needed to 46 and 14mps respectively.
Using armour-as-dice, the 3cm-gun at 14mps will do 56d damage, minus the 45d dDR makes penetrating damage of 11d, for 11 to 66 (38.5mean) damage per shot. This means an average four-shot hit is sufficient to reduce the spacecraft to 0HP.

The VRF guns also have the advantage of making interception by point defense basically impossible due to volume of fire; other guns can have their shells intercepted.


Beam Weapons
Available beam weapons are Laser (both normal and UV) and Particle Beams.

Lasers and UV lasers are (2) armour-piercing, and have a longer range, while the Particle Beam is (5) and has shorter range and less accuracy.
An exemplary major battery on an SM+10 spacecraft mounts a 3GJ beam weapon. An UV laser does 3dx10 (2) dDmg with a range of 7,000/20,000 miles. A particle beam does the same damage except for the (5) armour piercing, at a range of 1,500/5,000 miles.
The UV laser can penetrates 60/210/360 dDR (min/mean/max); the particle beam 150/525/900 dDR. They penetrate 30/105/180 and 90/315/540 respectively hardened dDR. Even at its half-damage range, the particle beam penetrates more armour than the UV laser. Looking at the armour, an SM+10 design can be assumed to have a dDR of 150-200 at its front hull; an example would be the Riguang SDV (SS8:31). Against these, the UV laser has only a 5% chance to penetrate; a particle beam 95%! On average, the latter is doing 55dDmg, meaning three hits are sufficient to reduce the Riguang to 0xHP. That sounds good.
Using armour-as-dice, the 3GJ UV laser can penetrate up to 60d armour (30d hardened), the particle beam 150d (90d hardened). The former is two hardened armour systems on an SM+10 spacecraft, the latter six. As in the example above, spacecraft will be able to withstand UV lasers, but not particle beams. Against the Riguang, the 3GJ particle beam will do 15d penetrating damage, for 15-90 (mean 52.5) damage. An average of three shots are required to reduce it to 0HP. At the particle beam's 1/2d range, hit chance is 12 + 2(fixed) -7 (range) + 10 (SM) -3 (sAcc) = 14, for a hit chance of 90%. At the 1/2dmg-range, damage is only 1d-3 (since armour and damage cancel out). Hit chance is reduced to skill-11, or 62%.
The Riguang does not mount a 3GJ particle beam, but a 300MJ RF one. This decreases damage to 3dx5(5), and range to 700/2,000 miles. This reduces penetration against hardened targets to 45/158/270. However, the Rcl of 1 makes multiple hits very likely; this may be worth it. With armour-as-dice, and against itself, it never does more than 1d-3 penetrating damage, making it not worth it.


Summary: UV lasers are useful due to range, but a military craft of your size is going to be pretty much immune to it. Some space forces may choose to distribute their craft allowing you to fire on the flanks and rear of the enemy (which will not be armoured as much; you have to put your stuff somewhere!) and use the superior range of UV lasers; the other alternative is mutual annihilation with particle beams.

AKVs
Using Transhuman Space as an orientation, an AKV (i.e. a possibly kamikaze spacecraft, armed mainly with kinetic weaponry) might be useful. Assuming said AKV is SM+6 (30dHP), with five nanocomposite armour systems in the front hull (total dDR 50) plus one 3cm VRF EM gun (4d). Our target is a three-armour-systems (150dDR) SM+10 (150dHP) spacecraft.
As seen above, if the enemy spacecraft is armed with an UV laser, it should be able to kill the AKV in two or three hits at 7,000 miles, and several more shots out to 20,000 miles. A particle beam is overkill; it should destroy every AKV in just one hit yet its range is too low to be really effective.
This suggests the AKV has to attack from 20,000 miles, for which it requires a velocity of 40mps and firing a spread of ten salvoes. It is a suicidal attack, of course; yet trading one, or ten, 100t AKVs against one 10,000t enemy spacecraft should be worth it.


Summary
All in all, there are three weapons available to the enterprising spacecraft: Missiles are hard-hitting when they hit, but are intercepted in numbers too great to make sense; the alternative x-ray warheads are intercepted less often (but still in quite some numbers), but require a few warheads to cripple most craft. Kinetic weaponry is devastating yet can be intercepted; RF/VRF weapons should therefore be chosen which do less damage, but do so more consistently. For beam weapons, the choice against combat spacecraft is probably the particle beam, which has the shortest range but is ignoring most armour. AKVs carrying kinetic weaponry are good against softer targets, but die easily against specialized combat spacecraft.
Spacecraft will aim to be armoured against UV lasers (and take care to only present their front to the enemy), which leaves particle beams as the weapon of choice. Engagement ranges are up to 5,000 miles.
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Old 09-07-2016, 10:40 AM   #4
kreios
 
Join Date: Jul 2013
Default Re: Building a Fun Spacecraft Combat Paradigm

Tweaks
There's actually not many things I'd like to tweak. One of them, though, is the open-endedness of kinetic damage, which scales with velocity (it also scales worse with size than the beam weapons, but that's not a big issue for us). However, the threshhold for completely vapourizing an SM+15 spacecraft by a 12cm gun is roughly 175mps. Obviously, that's an issue when using reactionless drives. There would be a few solutions; mine would be to assume that every spacecraft mounts whipple shields. These ensure that the damage grows with the square root of the velocity, and that maximum damage applied to a spacecraft is 10x the basic dDmg of the attack. For the 12cm gun, that's be 180 to 1080 dDmg. Enough to effectively kill such an SM+15 craft, but you'd have to move at 100mps. That seems much more reasonable.
In addition, I think it's not good that spacecraft have to mount a gigantic array of 30kJ lasers. A usual SM+10 combat spacecraft would probably mount a thousand of these. Imagine the Iowa with a thousand machine guns to fight off kamikazes! This mounting is a result of the gigantic risk of collision with missiles, and that 30kJ VRF lasers give you the maximum chance to intercept them. To avoid this, I'm ruling that a) 300kJ lasers are the minimum you need to ensure destruction (30kJ destroys only one third of the missiles, 100kJ only two thirds) and that b) the missile shield rule is in effect against missiles, eliminating RoF missiles automatically when in point defense. Roughly 1/100th RoF projectiles can be killed since those are far smaller and must be vapourized.
In addition, I dislike the fixed damage of the x-ray laser warhead. Instead, look at the weapon tables and use the beam d-Dmg noted there. For example, a 16cm x-ray warhead does 3d, and a 48cm does 4dx10. Their RoF is reduced to 1/4 diameter (i.e. 16cm --> RoF4; 48cm --> RoF12). Rcl is unstated in the books, but I'll set it at 2. 16cm is the minimum diameter.
While the last two rules push missiles to x-ray lasers only, I believe that they're not useful anyways in any other role.


In summary, these rules should result in a) missiles being used as x-ray lasers, where they have to swarm defenses to be effective, b) kinetic weapons being used to deny the enemy manoeuvre space, since they have to avoid collisions, c) particle beams ruling large-spacecraft combat, d) AKVs being useful against non-combat spacecraft or during suicide missions.
However, there is still the issue of this - apparently - not having much tactics to apply. Spacecraft are easily destroyed, and I don't see much use for specific formations (unless for missile defense and to maybe allow you to hit from the flanks), nor for manoeuvres outside of closing and accelerating away. One possible change would be to abandon the reactionless drive and introduce limited dV.

That's where you come in, and one of the three reasons I've made that point (aside from forcing me to write up my thoughts, and to provide an example of how to tweak spaceships): How do you think combat tactics would look like? Are there any tweaks I've overlooked that would help me? Any errors I've made?

Thanks in advance, and congratulations for making it through the wall of text.
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Old 09-07-2016, 11:27 AM   #5
tshiggins
 
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Default Re: Building a Fun Spacecraft Combat Paradigm

I think it's safe to say that, no matter how much armor you pile on a space craft, the sheer energy produced by any particle moving at relativistic velocities will swiftly penetrate. If enough mass of particles hit, the space craft will get vaporized, no matter what.

So, that means Battlestar Galactica-style armor plating has only limited use. Yeah, it's nice to keep out hard rads from background radiation and near-misses, but a direct hit from a nuke or a strike from a ball bearing moving at relativistic velocity will punch through it.

So, what if you have a softer defense, rather than a harder one?

I can think of two possible systems, and we might want to consider both in tandem. In both cases, you use the reality of kinetic damage at relativistic velocities to help defend the space craft.

Firstly, even though you kind of resist the notion, an aegis-style point-defense is just too useful to pass up. I'd put in a combination of automatic turrets sporting high-velocity rotary cannons, and maybe some BSG-style turrets that fire AAA that puts out an ungawdly amount of shrapnel.

Secondly, I'd pull in a trick I read in one of David Brin's Uplift novels. In that one, a dolphin-piloted starship destroyed a pursuer by dumping much of the water in its habitat-tanks in a cloud behind, as it whipped through a sling-shot orbit around a gas giant. The water immediately froze into a cloud of fine ice crystals.

The vector of the pursuing craft, also in a sling-shot orbit, took it straight through the cloud of ice, and it promptly vaporized.

So, what if you do something similar? What if the space craft have Traveller-style sand-casters that put up a cloud of fine debris between the defending craft and the incomiing ordinance?

They could consist of high-albedo metallic dust that disperses lasers reasonably well, particle beams less well, and catastrophically ablates incoming ordinance of a more traditional sort.

Use the anti-missiles to stop stuff further out, AAA for what gets through that, and dust-clouds to (more or less) create useful cover.
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Old 09-07-2016, 02:17 PM   #6
kreios
 
Join Date: Jul 2013
Default Re: Building a Fun Spacecraft Combat Paradigm

Quote:
I think it's safe to say that, no matter how much armor you pile on a space craft, the sheer energy produced by any particle moving at relativistic velocities will swiftly penetrate. If enough mass of particles hit, the space craft will get vaporized, no matter what.
That's definitely true - yet that requires particles to hit at relativistic velocities. I believe that restricting usual relative velocities to less than let's say 150km/s will not (kinetic energy of 400x its mass in TNT). That's still less than a nuclear weapon, and has to actually hit.

Quote:
Firstly, even though you kind of resist the notion, an aegis-style point-defense is just too useful to pass up. I'd put in a combination of automatic turrets sporting high-velocity rotary cannons, and maybe some BSG-style turrets that fire AAA that puts out an ungawdly amount of shrapnel.
Oh, that's definitely not a something I'd resist. I should probably clarify that:
I fully expect (and want) spacecraft to mount point defense. What I do not want is them mounting what amounts to a laser rifle on an articulated mount. Instead, they should mass a few tons to be effective.

Quote:
Secondly, I'd pull in a trick I read in one of David Brin's Uplift novels. In that one, a dolphin-piloted starship destroyed a pursuer by dumping much of the water in its habitat-tanks in a cloud behind, as it whipped through a sling-shot orbit around a gas giant. The water immediately froze into a cloud of fine ice crystals.

The vector of the pursuing craft, also in a sling-shot orbit, took it straight through the cloud of ice, and it promptly vaporized.
Interesting. They'd have to have pretty different trajectories to generate that relative velocity.

Is that actually a possible tactic, both offensively and defensively?

Let's assume for a moment we want to use this tactic of sandcasters to project a "wall" of particles across one 10-mile hex, each of the grains being one gramme. To avoid a missile to penetrate this defense, we need roughly five or so particles per metre, for a total of 80,000 grains. That's a total weight of about 80kg, the equivalent to one 12cm EM Gun projectile. That's the absolute minimum; the actually required material is going to be much more since the grains won't be equally distributed, move outwards, etc. And many, many more particles against laser fire.
It's even worse when we're looking at three dimensions, where you'd need to use at least 6.4 billion grains, for 6.4 tons, to generate a ten-mile by ten-mile barrier.
Both suggests that the use is for an already-guided projectile to use this to ensure a kill. This is actually how I imagine proximity detonations to look like - the projectile disintegrates into many particles a second or so before impact, spreading into a rough cloud.

Which brings me to another idea: Use Hitting the Wrong Target rules (B:389) to check whether you hit something in the same hex, too. This would probably be bounded by 9 plus 4 for proximity detonation instead of just 9.
Thanks!
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