Exploring Efficient Weapons in Sci-Fi Settings

[Rive]

I'd be curious what aspect of nuclear flak wouldn't work. I would have assumed that the addition of a pile of energy would at least divert the masses from their trajectory.

Try to calculate the time necessary to cross the few hundred meter diameter of the effective range of a decent nuke in space at relativistic speed.

Unless you have a projectable gavity field, it's just hopeless: just like without some FTL detection system.
We tend to forget that while it is cool that the ship goes with 0.1c, it also means that the whole universe is rushing toward the ship with the same speed... Don't try this in an (artifical) asteroid field

 

[GTOM]

Kinetic energy is vector quantity, so I agree its not going to stop it, that is not the goal, the idea is to give it a gentle nudge normal to the direction of travel where even a fractional change in trajectory over large distances in space would make it completely miss the intended target.

More plausible is that a relativistic object may not spend enough time in the fireball of a nuke to impart any significant Ke on it even if it is normal to its travel.

Without air, the pressure on the projectile isn't much. Ok vaporized material make some push. Still with really high speed, even shrapnels can seriously damage the ship, even a redirected bomb can fire like a shotgun.

 

[DaveC426913]

the idea is to give it a gentle nudge normal to the direction of travel where even a fractional change in trajectory over large distances in space would make it completely miss the intended target.

No - that isn't the idea.

You're not aiming a single compact object at your target - you're essentially throwing the asteroidal equivalent of a police spike strip in the path of a high velocity target.

No objects are "aimed" - and no specific objects can be knocked out of the way - there's too many, and their paths are too chaotic.

Furthermore: you don't need a lot of mass, the mass can be a cloud of pebbles. Their delta V when they pass through the target will make Swiss cheese out of it.

 

[member 656954]

Man space reality sucks.

Trying to invent new physics that includes QM / GR /SR but adds the big lies over the top is hard and very few writers bother because (as you appear to be finding) it gets in the way of the writing. Also, it is generally unnecessary. As a wonderful example of why you don't need to do it, consider Richard Morgan's sequence in Broken Angels where Takeshi Kovacs unEarth's the hidden Martian portal that gives us even more impossible Martian tech / physics. It is the awe and fear it triggers in even the Envoy-hardened Kovacs that catches you as a reader, rather than a "WTF, this makes no sense."

My advice to authors falling towards the impossible physics storytelling event horizon is to step back and figure out the true imperative. Very few sci-fi novels fall apart because of mumbo jumbo physics. Most of them fall apart because the characters are lackluster, the plot is ridiculous, or the prose is crap.

 

[essenmein]

No - that isn't the idea.

You're not aiming a single compact object at your target - you're essentially throwing the asteroidal equivalent of a police spike strip in the path of a high velocity target.View attachment 255378

No objects are "aimed" - and no specific objects can be knocked out of the way - there's too many, and their paths are too chaotic.

Furthermore: you don't need a lot of mass, the mass can be a cloud of pebbles. Their delta V when they pass through the target will make Swiss cheese out of it.

Oh I know exactly what you mean, the police strip is a great analogy. So for the that spike strip, eyeballing it 90% of the spikes don't matter, only the 5-10 spikes that will hit each tire matter. I would assume the area project tile sheet (and it would end up sheet like as it travels and spread out) might cover is many times larger than the effective blast radius of a reasonable nuke in vacuum. Since once moving the rocks or what ever are going in a straight line, you only need to nudge the ones that are coming for you, the rest might matter to someone else, but not you!

 

[Klystron]

Not to beat that poor horse to death, but...

Always consider your intended audience. Judging by popular entertainment most writers, actors, directors and probably the audience have no experience with actual weapons and little basis to judge limits and effects.

@Tghu Verd is correct: concentrate on story, plot, and realistic character development. Let the science, weapons and tech serve those goals of good story telling. The audience will appreciate the world you create through strong characters with minimum explanatory material.

 

[DaveC426913]

So for the that spike strip, eyeballing it 90% of the spikes don't matter, only the 5-10 spikes that will hit each tire matter.

Then you miss the point. Traveling at high speed, careening down the road - all the spikes matter - you don't have any idea which ones you'll hit, let alone try shoot them with your gun. You can't pick and choose where you'll go, or where they'll go, or anything. And it's all over before you can react with any useful action.Note, also, that small masses the size of pebbles will be impossible to image. Never mind figuring out what trajectory they're on. It's cheating to just make up "magical" sensory capability.

 

[GTOM]

Then you miss the point. Traveling at high speed, careening down the road - all the spikes matter - you don't have any idea which ones you'll hit, let alone try shoot them with your gun. You can't pick and choose where you'll go, or where they'll go, or anything. And it's all over before you can react with any useful action.Note, also, that small masses the size of pebbles will be impossible to image. Never mind figuring out what trajectory they're on. It's cheating to just make up "magical" sensory capability.

A good radar and lidar and supercomputer can pretty much calculate trajectories.
Nukes can knock off missile electronics. Against remaining dangerous objects, one need focused energy.

 

[member 656954]

A good radar and lidar and supercomputer can pretty much calculate trajectories.
Nukes can knock off missile electronics. Against remaining dangerous objects, one need focused energy.

If you are talking about rubble then that's not guaranteed. Small, fast moving objects are hard to track in space, especially if they are non-reflective, and incoming rubble would likely number in the tens of thousands of tiny objects.

And there is a limit to how much energy you can splash around with a beam weapon. You need to vaporize the incoming rubble, and that will take a sophisticated 'focused energy' system that can target thousands of incoming objects at once and transfer gigawatts of energy. Iridium, for example, has an enthalpy of vaporization of 604 kJ/mol so flinging a ton of iridium pebbles at an incoming ship makes for a hard-to-destroy weapon in the real world.

In a sci-fi world, it's likely some pebbles will get through to cause just enough damage for the hero to fight on with diminished capability. This allows the hero to display their wits, and guts, and superior "just because", so they prevail despite horrendous odds against success, winning the [pick your option] battle/ war/ glory/ fame/ fortune and moving to the next chapter for another nail-biting rinse and repeat

 

[Rive]

A good radar and lidar and supercomputer can pretty much calculate trajectories.

The requirements (response time and the power of the radar beam) for this part are a bit underestimated.

 

[GTOM]

If you are talking about rubble then that's not guaranteed. Small, fast moving objects are hard to track in space, especially if they are non-reflective, and incoming rubble would likely number in the tens of thousands of tiny objects.

And there is a limit to how much energy you can splash around with a beam weapon. You need to vaporize the incoming rubble, and that will take a sophisticated 'focused energy' system that can target thousands of incoming objects at once and transfer gigawatts of energy. Iridium, for example, has an enthalpy of vaporization of 604 kJ/mol so flinging a ton of iridium pebbles at an incoming ship makes for a hard-to-destroy weapon in the real world.

In a sci-fi world, it's likely some pebbles will get through to cause just enough damage for the hero to fight on with diminished capability. This allows the hero to display their wits, and guts, and superior "just because", so they prevail despite horrendous odds against success, winning the [pick your option] battle/ war/ glory/ fame/ fortune and moving to the next chapter for another nail-biting rinse and repeat

Well, two things.

One: realistic engines can sustain miliGs order of acceleration with high exhaust velocity. So i calculate with closing speeds on the order of hundreds of km/s, not relativistic speeds, more time to calculate and vaporize.
(Interstellar travel is done with hyperspace in my story. Cheat is more acceptable to me, than find reasons for interstellar war with sublight travel.)

Two: as far as i know a shield in front of the ship is pretty efficient against micrometeors.
I can also imagine a self repair nano, or liquid armor against debris.

 

[member 656954]

So i calculate with closing speeds on the order of hundreds of km/s, not relativistic speeds, more time to calculate and vaporize.

I've no worries with that in a story, @GTOM, but it is not real world physics or electronics. Work out the energy you need to impart to vaporize a common metal like iron (enthalpy of 349.6) and if there are thousands of pieces of rubble coming at your ship, it's a non-trivial problem to solve. Your beam weapon is less effective at a distance and 100's km/s speed means closing times measured in seconds by the time you can find and track that many small items.

Two: as far as i know a shield in front of the ship is pretty efficient against micrometeors.

It would be worth doing some calculations on the kinetic energy of rubble traveling at 100's km/s to test this because it takes a significant shield to absorb such impacts. At 100 km/s, a kilogram imparts 5,000,000,000 Joules. That's like a ton of TNT going off against your shield.

I can also imagine a self repair nano, or liquid armor against debris.

Not sure how a liquid would work, as that would boil away in the vacuum of space. Nano does whatever you need of course, it's a legitimate get out of jail card for story purposes

 

[GTOM]

I've no worries with that in a story, @GTOM, but it is not real world physics or electronics. Work out the energy you need to impart to vaporize a common metal like iron (enthalpy of 349.6) and if there are thousands of pieces of rubble coming at your ship, it's a non-trivial problem to solve. Your beam weapon is less effective at a distance and 100's km/s speed means closing times measured in seconds by the time you can find and track that many small items.
It would be worth doing some calculations on the kinetic energy of rubble traveling at 100's km/s to test this because it takes a significant shield to absorb such impacts. At 100 km/s, a kilogram imparts 5,000,000,000 Joules. That's like a ton of TNT going off against your shield.
Not sure how a liquid would work, as that would boil away in the vacuum of space. Nano does whatever you need of course, it's a legitimate get out of jail card for story purposes

More like a dozen pieces of debris. The others miss.

 

[member 656954]

More like a dozen pieces of debris. The others miss.

I fear we're getting to the point of a Thomas Aquinas reductio ad absurdum discussion, but how big are the ships you are imagining? I'd assumed dimensions measured in kilometers, but now I'm thinking they are smaller. In that case, over the distances involved, you are right it might be only dozens. However, that makes it less likely the ship can pack enough real-physics beam weapon firepower to vaporize the rubble or have sufficiently discriminating sensors to detect the rubble in the first place.

 

[GTOM]

I fear we're getting to the point of a Thomas Aquinas reductio ad absurdum discussion, but how big are the ships you are imagining? I'd assumed dimensions measured in kilometers, but now I'm thinking they are smaller. In that case, over the distances involved, you are right it might be only dozens. However, that makes it less likely the ship can pack enough real-physics beam weapon firepower to vaporize the rubble or have sufficiently discriminating sensors to detect the rubble in the first place.

I think the length of a battleship can be a km, but not the cross-section.

 

[DaveC426913]

I think the length of a battleship can be a km, but not the cross-section.

So now you need enough sensors, computers and weapons and fuel onboard your ship to locate, track and fire upon all those incoming rocks that track within that a half square km or so cross section. Which means you need a bigger ship than if you didn't need those things...

 

[GTOM]

So now you need enough sensors, computers and weapons and fuel onboard your ship to locate, track and fire upon all those incoming rocks that track within that a half square km or so cross section. Which means you need a bigger ship than if you didn't need those things...

Actually i think about an acre cross section.
The main laser is pretty fine to illuminate debris. Sensors don't have to be all onboard.
Well, an advanced defence system is even better.

 

[member 656954]

Actually i think about an acre cross section

Are you assuming the rubble is coming head on? Because it needn't be and ideally your foe would be aiming for the largest cross section of your ship.

 

[GTOM]

Are you assuming the rubble is coming head on? Because it needn't be and ideally your foe would be aiming for the largest cross section of your ship.

Not if you face them. Although i also think, if kinetics play an important role, that favors lots of smaller ships instead of big capital ships.

 

[DaveC426913]

Not if you face them.

Again, this is predicated on the assumption that
1] you have sensors that can pick up marble-sized objects at enough range to do something about it, and/or
2] there's only one attacking position. If the attacker can flank the defender then the defender will come up short on "faces".

 

[GTOM]

Again, this is predicated on the assumption that
1] you have sensors that can pick up marble-sized objects at enough range to do something about it, and/or
2] there's only one attacking position. If the attacker can flank the defender then the defender will come up short on "faces".

If the enemy has superior numbers, that is bad. Otherwise each ship face one attacker.
Each can have recon probes too. Maybe motherships even fighters, so they can target the butt of incoming missiles, and turn them into shrapnel very far away.

 

[member 656954]

Not if you face them.

Adding to @DaveC426913's observation, you are assuming that you can face them. I'm not sure that you are adequately visualizing how 3D warfare might happen, @GTOM. You can be readily bracketed by rubble coming from many directions in space, and if you are close to planets or large moons, then their gravity wells can be used to fling rubble at you from over the horizon. And again, these can be really small fragments, traveling very fast. They will be very hard to spot!

But I am still not sure if we are discussing the real-world physics of space warfare, or the "whatever goes" science-fiction physics of stories? They really are different beasts and the OP noted "hardish" sci-fi was their goal, so I've been applying that.

 

[GTOM]

Adding to @DaveC426913's observation, you are assuming that you can face them. I'm not sure that you are adequately visualizing how 3D warfare might happen, @GTOM. You can be readily bracketed by rubble coming from many directions in space, and if you are close to planets or large moons, then their gravity wells can be used to fling rubble at you from over the horizon. And again, these can be really small fragments, traveling very fast. They will be very hard to spot!

But I am still not sure if we are discussing the real-world physics of space warfare, or the "whatever goes" science-fiction physics of stories? They really are different beasts and the OP noted "hardish" sci-fi was their goal, so I've been applying that.

1. Realistic engines can maintain miliGs of acceleration for long time.
If you randomly throw rubble, the ship only has to face 1-2 of them at most. The main laser is great for illumination, fleet formation and additional recon/fighter craft can solve the rest.

2. If you fire guided missiles their thrusters are pretty bright, that makes it easy to estimate trajectory (they can't maintain acceleration for long time without nuclear power source) and deploy countermeasures, face toward them.

With the miliG order of sustainable acceleration, closing speeds are a few hundred km/s in deep space and much less in orbital combat.

 

[trurle]

1. Realistic engines can maintain miliGs of acceleration for long time.
If you randomly throw rubble, the ship only has to face 1-2 of them at most. The main laser is great for illumination, fleet formation and additional recon/fighter craft can solve the rest.

2. If you fire guided missiles their thrusters are pretty bright, that makes it easy to estimate trajectory (they can't maintain acceleration for long time without nuclear power source) and deploy countermeasures, face toward them.

With the miliG order of sustainable acceleration, closing speeds are a few hundred km/s in deep space and much less in orbital combat.

The problem should be evaluated actually as two separate problems
1) Defense against coasting threats.
*only active sensors with Detection_range=power^0.25 rule (laser/lidar, radar) are useful, therefore point defense only
*tradeoffs between defender energy budget on (maneuverability)/(point defense), (attacking swarm density)/(penetration capability of each element of swarm)
2) Defense against maneuvering threats
* Passive sensors can be relied upon, therefore area defense instead of point defense.
* Larger attack vehicles are generally not vulnerable to point defense (especially if point defense is laser-based)
* Tradeoff is between interceptor/attacker maneurability ratio, (interceptor range)/(interceptors amount)

Generally i expect category (1) threats to be more common in space-faring society. All you need for attack of type (1) is to dump enough mass of loosely constrained sort on right trajectory, and this capability is available for any spaceship. Category (2) threats are likely to be custom-tailored attack vehicles against specific (highly mobile) targets.

 

[GTOM]

Recon part: with advanced recon probes, you can pretty much decrease the beam scattering problem.

If the ship only maintain miliG s acceleration, i consider it mobile.

Laser focusing mirrors are vulnerable, so i think even a fighter squadron can use its lasers efficiently against bigger ships.
Of course with big distance the bigger ship still has the advantage, but it is possible to reduce distance in orbital combat.

 

[trurle]

Recon part: with advanced recon probes, you can pretty much decrease the beam scattering problem.

Sensor picket methodology. Well, it is not a very efficient protection. Because volley aimed at main ship can take out sensor pickets too. And pickets itself to be efficient must be outside of point defense range of mothership, therefore vulnerable to wider range of threats.

 

[essenmein]

With regard to the sensor topic and ships reaching relativistic speeds to be able to launch piles of rocks at people. If you can fly at those velocities, you'd better hope you have good sensor tech, because a stationary rock/rubble shield would be just as effective against you as the weapon you plan on wielding.