# Relativistic Interstellar Combat

This question is heavily motivated by the details of my story, but it has a lot of free parameters to work with. I invite you to think creatively!

Setting:

You are in command of a constant acceleration rocket, capable of interstellar travel and your mission is to hop from one system to another 12 ly away as rapidly as possible. Your mission lies in this other system and it is absolutely imperative that you arrive there first, before anyone else.

The great thing about your ship is that you have unlimited delta-v. You can maintain constant thrust or acceleration indefinitely, without losing mass.

Two weeks after you depart, another ship begins the same journey as you. It is essentially the same 'make and model' ship as your own; however, for reasons unclear to you, it is capable of much greater acceleration. While your ship's acceleration max's out at 12 gees, the other ship accelerates at a constant breakneck 26 gees. (To simplify my calculations, a 'gee' here is $$10$$ $$ms^{-2}$$.)

If not stopped, the other ship will reach the other system before you, with a bit over 2 weeks to spare.

Plot Details:

• This is disastrous--remember, for 100% success, you need to arrive first--but it does not create an all-or-nothing scenario. Even if you fail in stopping the other ship surpassing you, you have something of a mission to salvage. (Basically, no suicidal action such as a full-on collision is acceptable.)

• You have something of an advantage in that the other ship is strictly pacifist and initially ignorant of the possibility that you might attack it. Left alone, the other ship is comfortable coming within 10 million kilometers of your volume, however, once it learns that you're hostile it will actively avoid you and create countermeasures where it sees fit. It will not attack you or take any retaliatory action. You know these things from the outset.

• There is no dialogue between you and the other ship, so no possible negotiation to turn it around or coaxing it nearer. Ultimately, the other ship must be destroyed.

• In your planning, you should consider both what you would do, as commander of your own ship, and what the other ship (with your very same capabilities, minus the whole berserker acceleration thing) might employ as a countermeasure (if it is capable of producing countermeasures after your first action).

• The other ship will not employ countermeasures until your behavior deviates from the norm, e.g., it observes your trajectory changing unexpectedly, or it observes the radar reflections of many relativistic objects deployed and scattered toward it, and so on. For example, a countermeasure might include intelligence gathering, e.g., deploying arrays of remote telescopes behind itself to observe your activities in detail and make predictions, or a counteractive effort, e.g., shooting at your missiles or the relativistic objects you left along its path.

(Keep in mind, the other ship has essentially the same capabilities as you.)

• Your ship is a cone 6000 m long and 500 m wide, massing 1 trillion kg. You have two constant thrust rocket engines, each capable of applying a maximum 6 trillion Newtons of force. This gives your 1 trillion kg ship a maximum acceleration of 12 gees ($$120$$ $$m/s^2$$). Conveniently, this acceleration is also your ship's structural limit (your ship will break apart if you accelerate any faster than this). For make-up, assume the hull and innards of your ship (and consequently the other ship) have a material density of $$2.0$$ $$gcm^{-3}$$ and specific heat capacity of $$12.0$$ $$Jg^{-1}K^{-1}$$.

• Your rocket engines mass around 10 million kg apiece and are special in that they can only be instructed remotely; they are black boxes that cannot be opened or tampered with. Doing so unleashes a multigigaton explosion. An engine alone can withstand 40 gees acceleration before succumbing to structural stress. The exhaust of an engine is a stew of hot and initially dense matter: excited protons, electrons, and tau neutrinos--not to mention light ranging from radio to gamma-rays.

• Your ship has great industrial capacity and carries millions of tons of raw materials (assume 100 million tons of anything). You can mass-produce structures of any composition (gold, carbon nanotubes, diamond, etc.), and rather quickly. Any technology that we (us modern-day people) can hypothesize (with our modern physics) but lack the industrial capacity to manufacture is now on the table for your use with stopping this other advancing ship. (No Clarketech.) Along with your material resources, you have 800 kg of solid antilithium at your disposal.

Initial conditions:

• You begin your journey at an unnerving 10 gees acceleration. After two weeks (relative to the origin star system), the second ship sets off.

• From this point, given the information known to you about the other ship and given your ship's manufacturing constraints, you're free to act as you see fit to complete the mission.

What's at the Destination?:

• At the destination, you will need to slow down enough to orbit the star (from relativistic velocities, this is basically the same as coming full-stop). You need at least one of your black box rocket engines and at least 50% of your ship's mass. (How you go about losing more than half your ship, I haven't got a clue...) You may expend all of your resources destroying the other ship if necessary, including all the antilithium.

Further Considerations:

• The living occupants of both ships are more or less immune to the acceleration stresses, but for good measure, assume the maximum stress they can survive extensively is 30 gees. While the other ship is accelerating breakneckedly, the majority of your ship's substrate cannot sustain acceleration greater than 12 gees without tearing like putty.

It may be helpful to keep in mind the arena. 12 lys is a great distance and small changes in direction have enormous effects on where in the destination system you arrive (if you arrive in it at all). Both ships will therefore tend to lie in some cylinder of space between the two star systems. The other ship will be advancing from behind, however, considerable time will pass (years) before it surpasses your ship. For example, at the half-way point, where your ship rotates 180 degrees and decelerates, the other ship will be about one week ahead of you (if not destroyed). Depending on what you do (accelerate, decelerate, whatever), this point of passing changes.

It may also be helpful to consider the relativistic effects, effects that take place seemingly right off the bat. One month into flight, your speed will be greater than 40% $$c$$, while one month into the other ship's flight, it is already moving greater than 60% $$c$$. For help with calculating relativistic variables of each ship, I find that this site and a graphing tool helps.

## The heart of the question is: What is the best way to engage and destroy the other approaching ship?

By best, I mean the method that gets the job done with the least expended resources and the least risk of mission failure. For the sake of giving 'destruction' a measure, any hit that affects more than 5% of the other ship's mass can be said to have destroyed it (e.g., vaporizing >5%).

One tactic I can see (and that I've seen in sci-fi before) is to litter space around the predicted future path of the oncoming ship with debris. As I alluded earlier, an economic approach might be to manufacture hundreds or thousands of large-spanning, thin light sails and scatter them at random behind yourself, then push them up to speed with lasers into the airspace of the other ship. Because the relativistic energies are so great, collisions would unleash catastrophic energies. A warhead of, say, 1 kg of antimatter, could, under relativistic circumstances, release more energy from its sheer kinetic energy than from the detonation of the explosive itself.

Another tactic might make use of your intelligence surrounding the other ship, which is that it is pacifist and initially ignorant of the possibility you might attack it. Perhaps your plan takes effect when the other ship is nearest and most vulnerable. Maybe you find that some form of a focused beam or laser may be powerful enough at 10 million km distant to vaporize a significant portion of the other ship's hull.

• 1. What is your mission at the destination? Does it require that you stop and get out and walk around or just get there? 2. Are the occupants of your ship and the fast pacifist ship robots immune to acceleration? Oct 30, 2019 at 19:25
• @Willk Good question, I'll edit in the details in a bit, but yes and yes, there will be some walking around (the mission itself should only take a few days to ensure success), and everything is more or less undamaged by high acceleration up to the given constraints. For the fast pacifists, assume they can go no higher than 30 gees.
– BMF
Oct 30, 2019 at 19:31

You fire a kinetic kill vehicle with enough fuel and acceleration much greater to that of your opponent.

A KKV is basically a weight with fuel and engines in all directions that is designed to detect and intercept another projectile, usually a nuclear warhead.

This vehicle tracks the immense signature of the opposing ship, and simply puts itself between the ship and its current direction. The kinetic kill vehicle is as stealthy as it can at first.

The opposing ship will move at a certain velocity (say 100.000m/s, random number) when it detects the kill vehicle's exhausts. It will try to avoid the KKV and will use "ordinary" thrusters at first. The opposing ship will gain a sideways direction that will avoid the KKV while continuing to accelerate passed 100.000m/s. The KKV can match the thrusters and moves sideways as well, putting it again in the path. At first small changes in direction will get the opposing ship out of the way easily, but it will also mean it'll need go go around the KKV by a large distance which takes time. Still it passes the first one and puts itself back on track to the destination. The later the opposing ship detects the KKV the more thrust he needs sideways and the more off-target he'll be.

Then it detects a second KKV (or better yet a group of KKV's). You are ahead of your opponent, any KKV with equal acceleration as your opponent will always be between the ship and the destination before the opposing ship can get there. So at some point the opposing ship will have to pass your KKV's to catch up to you, and pass closely to it. The opposing ship cannot try and turn itself and suddenly boost out of the way to pass closely to the KKV as the KKV has at least the same acceleration and can anticipate the much larger targets direction. Additionally a group of KKV's could spread out over a distance so that any direction they try to escape too will be covered by a KKV.

The target has a massive velocity and the KKV only has to wait for the opposing ship to keep accelerating to get the kinetic force differences that would make the collision between ship and KKV a massive fireworks in the early part of the trip. You dont need to use KKV's for the later part of the trip but if you somehow do the KKV's you launched will have half the velocity of your opponent has as he passes that point. So even then the opposing ship can only catch up to you by passing the KKV's especially if they decelerate beforehand.

• I like this analysis. Plus, the KKV don't have to worry about squishing the puny humans when it maneuvers. Even better, if it's structural integrity is able to survive stronger tensions than the target ship, it can easily outmaneuver it to land it's payload. Oct 30, 2019 at 23:38
• @laancelot "... it can easily outmaneuver it to land it's payload." That's why I like this answer. If you've got a bunch of these things, perhaps thousands and thousands (easily manufactured from 100 million tons of raw materials lol), you can put the advancing ship in quite a pinch.
– BMF
Oct 31, 2019 at 0:41
• @BMF In the "Hyperion Cantos", author Dan Simmons has some relativistic combat ships fight one another. Basically the crew dies every time, but good captains plan strategies ahead and let the computers do the actual fighting (and then resurrect the crew). Some of these fights are really clever. Oct 31, 2019 at 0:46
• I've considered KKV's as well, but my conclusion was that they aren't even worth enough to be mentioned as an option. There are two reason for these differ conclusions. The first one is about the stealthyness of the KKV's and the secound one is about the reactions of the pacifist captain. I'll add an analysis of KKV's to my answer to explain this. Oct 31, 2019 at 8:30
• @TheDyingOfLight imagine the release point of the KKV's as a 2D plane the opposing ship has to pass. As long as the KKV has the same acceleration as the opposing ship that ship needs to stop accelerating forwards and accelerate sideways full tilt in order to get enough sideways movement to avoid the KKV, even though due to its size it probably wont get its bulk out of the way before the KKV reacts. So best case scenario the opposing ship can power out of the way not accelerating forwards during that time and going off-course. Ignoring options like higher acceleration and multiple group KKV's. Oct 31, 2019 at 8:57

I've got a number of ideas and this list presents them in the order I would consider them as the commander. I'll however take the liberty of changing one of your conditions and redesign the vessels a bit. You assumed rightly that impacts at relativistic velocities will be devastating and that littering the enemy's path with debris will kill them. However the universe has already littered space for you. Interstellar dust and gasses are a huge issue at some point. Sure matter based impact shields will help you for a while, but they'll be be a destroyed by abrasion and the faster you go, the faster they are gone. You need a non-matter based dust protection system. The easiest solution would be to use point defense lasers to vaporise any dust grain or rock in your path and then to use larger lasers to ionised the gases. Then you use tremendously powerful magnetic fields to deflect, not stop as that would take ten times the energy, the gas away from your ship. An interesting side effect of this is that you are no longer limited by damage to the sacrificial shield, but by the equilibrium of the drag of the gas and the power of your thrusters. Your ships will reach a terminal velocity at some point.

Laser Strike Against Protection System

This would be the quickest and cleanest solution to end them. 10 Million km is within the striking range of an array of RBoD's (ravaging beams of death). Striking a target 33 light-seconds away and dealing significant damage might be hard for a single x-ray or gamma laser, but you got years to build an array of free electron lasers or Wakefield plasma accelerators. Their path is predictable, so you can't miss the first shot. Their evasion attempt will take time and your laser strike will have messed up their protection system and sensors. Abrasion will instantly start to take a toll on them. If you are lucky a random dust flake or impacting gas will finish it quickly.

Since I don't like gambling, I would follow up my laser strike with more RBoD fire and a barrage of laser propelled lightsails. Maybe timing the attack in a manner that a rock you already know about, but they don't hits them milliseconds after you disabled their defenses could also work in my favor.

What the enemy commander can do? Nothing once my hostile intentions become clear. Preparation would be key. Don't bring one vessel, bring several. Surprise attacks work poorly against such an armada. Or he could just not moronic ally fly into what is known in this universe as my possible weapons range.

Hack Their Ship

You said there's no communication. That doesn't mean that they won't read the messages I send them. This is a long shot by all means, but one that can pay off greatly. An AI-virus withe the instruction "open all airlocks" works wonders on uncooperative spacefarers. After they all suffocated or where killed of by their ship's systems turning hostile, you can board the craft and take it over. You might actually be able to take the enemy crew as prisoners.

The enemy commander could simply keep up strict IT security. But if current behavior of people is any indication, writing into the mail that you're an Nigerian Prince, who wants to give the enemy captain his inheritance and trying 12345 as master password might be worth a shot.

Other Options

I've got a number of other ideas. Stealth missiles (check out the ToughSF blogspot (just google it) for the stealth in space is possible series before repeating the mantra of "there ain't no stealth in space"), restructuring you ship to increase integrity and loose mass or using the exhaust beam to fry them would be other options. I just think that the two I presented in detail are the best ones.

Kinetic Kill Vehicles

Another answer suggested KKV's as a weapon to use against the other ship. I originally considered KKV's myself, but have thrown them out as they are not as useful as one might first think. To make a meaningful analysis of KKV's we need to know their time to target. Beeing generous, I' ll assume we deploy them at the closest approach, but note that this isn't tactical sound. We want to deploy them as long as they are behind us, making the distance much greater.

$$100G = 74.5 min$$ $$1000G = 23 min$$ $$10000G = 7.5 min$$ $$100000G = 2.4 min$$ $$1000000G = 45 sek$$

While especially the later times look decent, the thermodynamics of such rocket drives are dubious at best. Its either going to be some form of Clarke-Tech, technology so advanced that it can be considered magic, or an antimatter based nuclear pulsed propulsion concept. Fuel, structural and especially radiator mass will impose strict limits on KKV-design. Don't expect a space torpedo, expect a white hot pusher plate, magnets for the magnetic field (the actual pusher plate), huge fuel tanks even with antimatter and a football field sized, red-hot radiator. This KKV's is going to be about as stealthy as the sun in the sky. Even assuming a Clarke-tech drive with the magic ability to ignore thermodynamics, we are talking about an object about as stealthy as your average radio tower. The impact of interstellar hydrogen gas will create this strong radio signal. This is why I propose a laser strike against the point defense and indirectly against the sensor system.

The other differce in assumptions is that the KKV's are apperantly not beeing touched by the pacifist captain. I'd just assume that the KKV's are just eaten up by the dust and rock defence laser, which are essentially a highly sophisticated point defense system. Even assuming that the pacifist vessel doesn't have other weapon system or even blueprints for them (which isn't a forgone conclusion, as the very reason why they might turned pacifist could be that they have such horrible weapon technology that any fighting among themselves is prevented by a mutually assured destruction doctrine and that noone dares messing with their culture because they have a reputation for beeing so good at war that they don't have to fight any more (just some interesting twists for OP)), weaponising any advanced technology is trivial. Just to name a few options for KKV-defense.

• expand the point defense laser grid: More lasers kill more KKV's faster.

• counter missile missiles: Probably the first stage of the defence; the incoming KKV's will be limited in their manoeuvring capability by their own inertia, so the defensive missiles can be lighter and have weaker drives. Even if the KKV's are armored, the shock waves from the impact on the front will all meet up at the rear of the KKV, destroying engines and fuel tanks. In the end mass economics give the pacifists the win.

• Casaba Howitzers: These things are amazing for point defense purposes. A wide angle particle beam like this can take out dozens of KKV's in one strike. It trivial to build those with advanced technology.

• Use the Engine: The torchdrives of the rocket is itself a powerful weapon. Op mentioned electrons and protons coming out at what I presume to be a decent fraction of the speed of light. This is a death ray. Just turn the rocket around, my calculations show that the pacifist captain has ample time to do so and vaporise the attacking KKV's. The Pacifists got superior acceleration, so breaking for a few days is not an issue.

I hope that this analysis shows why KKV's will be useless in this scenario.

• If the other ship was holding station with you or moving slightly faster or slower, I think you'd be right in that they're infeasible. But the other ship is moving decent fractions of the speed of light relative to you. I think the idea and lure behind KKVs is that you can scatter a bunch of them in the predicted path of the other ship and make them slightly adjust course as the scenario evolves, putting the other ship into a pinch. Relative to you, each of those KKVs have hardly any kinetic energy, but to the other ship, any one could utterly obliterate.
– BMF
Oct 31, 2019 at 18:05
• If you run the numbers, after 1 month of flight time the other ship has about 20% c on you. I'm not sure what a car-sized thing or perhaps baseball-sized thing could do to a 6 km long structure at 20% c, but it probably isn't pretty.
– BMF
Oct 31, 2019 at 18:07
• But you're certainly right about the point defenses. The other ship has the same manufacturing capabilities as you. If you can launch a bunch of intercepting KKVs, then so can it as countermeasure.
– BMF
Oct 31, 2019 at 18:12
• @Demigan I've addressed the issue of stealth at relativistic velocities already. They are going to be as stealthy as a radio tower at best, unless you shut of the ships engine and send them down in the gas-shadow. Mirrors for blinding lasers just don't work. Ablative matials I combination with laser fire crates a reaction drive, pushing the KKV's out of the way. This is what they would want to do. Any laser the KKV's could mount will be overpowered by the ships lasers, as their energy budget, focusing mirrors and redundent systems will be superior. Nov 1, 2019 at 7:03
• If you supercooled the KKV's as you jettisoned it and gave it a velocity that crosses the pacifists path as it passes then it doesnt even need to use its engines until the pacifist detects it and tries to outmaneuver it, meaning it could function like the hydrogen steamer and be very hard to detect until the last moment. The 12G engines are at the back... can they be used properly to steer the pacifist ship? The ship is MASSIVE and requires a ton of energy to turn. Nov 1, 2019 at 7:53

Option 1: Stop your engines, pretend they have malfunctioned, beacon a SOS on an intercept path with the other ship. When it comes close enough to rescue you, turn on your engines and reduce it to scrap metal.

Option 2: When the other ship is reasonably close, shoot as many unguided masses in various close trajectories on an intercept course with the target (unguided because if they are under propulsion, they will be detectable from longer distances). Something will either hit it, or they will force it to change trajectory enough by altering its acceleration vector to slow it down (significantly, at such speeds). If needed rinse and repeat until you reach the system.

Option 3: Do nothing until the ship is reasonably close behind you, then simply fire off anything and everything that you have at it and its path. Chances are its maneuverability will not fare well under the stresses of relativistic velocity so this is the most certain-kill option.

None of the above have a 100% hit rate, but obviously nothing does.

• Option 1 won't work at all. For plot reasons, there is no dialogue between the ships, and "no coaxing them nearer." Your other suggestions look like viable approaches.
– BMF
Oct 31, 2019 at 17:47
• To nitpick, option 1 doesn't absolutely require communication between the ships, just a way to broadcast "I'm in deep shit" in a way that the other ship can intercept and understand it (it is likely that in societies engaged in deep space travel, such "codes" will be commonplace). But yeah, obviously it's not the best idea since the other guy could simply ignore you. In a slightly variable scenario where you might not have access to anything that could be used as a projectile / weapon, that might be your only choice. Oct 31, 2019 at 18:17
• Stresses of relativistic velocities? What? Nov 1, 2019 at 6:41
• Also, the leading guy has a big advantage here. Even if the trailing guy shoots stuff forward, they have to catch up first, meaning much more time for the target to dodge. Whereas if something is running behind you and you shoot stuff at it, the velocities of the two objects are effectively combined, meaning you have much less time to dodge (and much more to fear from a collision, although since we're talking relativistic speeds anyway, not sure how relevant "much more" is) Nov 1, 2019 at 14:36
• In fact, given all the constraints placed and the fact that the chasing guy has no idea you want to murder him, you can just allow him to get close enough while taking no action (assuming you are on a similar course, and why would you not since you are doing the same route and are not - yet - enemies), then just use the railgun you conveniently manufactured to shoot him down. Nov 1, 2019 at 14:43