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Context: It is set 50 000 years from "now" in no-FTL interstellar setting. The idea is that active defences and armor of spacecrafts have obscenely advanced to the point that any projectile has to get on the collision course both quickly enough to not get obliterated on it's way and reach at least dozens of terajoules of kinetic energy to have a chance to get through.
This is the official reasoning for the dominant tactic being fleets of billion ton spacecrafts meeting in short 50+ km/s fly-bys and exchanging rapid close range fire of relativistic (1% - 10% of light speed) projectiles, while being protected by 30+ metres of armor of unspecified composition.

So, the question is what kind of engines and maneuvering thrusters could survive this? Even though rule-of-cool definitely plays an important role here, and the 50 000 years of technological advancement is very good at plugging most holes, I'm still trying to keep to real physics, or at least not stray away from it too much, which means I'm very hesitant to just handwave it with some sci-fi magic or technobabble.
Options like chemical, fission, fusion and definitely antimatter propulsion don't seem like things that would do well once someone starts shooting up their exhaust.
I have been thinking of Orion drive maneuvering thrusters, in this case it could just be a re-purposed coilgun for the nuclear shaped charge and a well shaped piece of what is already it's standard armor, but while a bit more durable, I don't really trust the reliability of this given that debris from "shot down" projectiles and fragmented armor will be flying all around the place and could destroy the nuke used for the drive or knock it off course.
Orion drive: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

So the end goal is allowing these ships to reliably maneuver in the conditions described above, for example it has to at least survive an impact of a cloud of debris (up to 5 mm) moving at 10% of light speed (edited: previous requirement was surviving a solid 50 terajoule projectile) while turned on and moving the ship, and such debris impact should not keep it from continuing to do so. At the same time it must be based on known physics, nothing highly speculative.

(Also please be patient with any grammar issues, I'm not a native English speaker)

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    $\begingroup$ 50 thousand years is a long long time. powered flight is only 120 years old, I'd expect FTL by then or extinction $\endgroup$
    – Kilisi
    Commented Aug 30, 2023 at 23:59
  • $\begingroup$ Hi and welcome Kugelblitz, this is an interesting problem for your world. But do please review the site guidelines and try to narrow the scope of your question to a specific problem related to your designs, since the question posed is essentially asking for braistorming ideas. Answers here are just that: they are answers, not opinions or ideas. Brainstorming isn’t allowed in questions. Please narrow it down to an answerable question. Note that it may be closed until that is done. $\endgroup$
    – Vogon Poet
    Commented Aug 31, 2023 at 1:17
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    $\begingroup$ @Vesper space rocks don't really seem better than the purpose-built missiles and projectiles which they can easily counter outside close range, in this context an asteroid would be easy to avoid or pulverize. Not a bad point though, the ships themselves are already built to do close fly-bys, so ramming or using an armored kamikaze spacecraft would work if deemed to be cost-effective $\endgroup$
    – Kugelblitz
    Commented Aug 31, 2023 at 7:48
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    $\begingroup$ maybe I am having a "senior moment," but aren't the weapons as described essentially engines? A full broadside of weapons that each can accelerate something to 10%C at the aperture sounds like an engine. $\endgroup$
    – Yorik
    Commented Aug 31, 2023 at 20:06
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    $\begingroup$ Hi Kugelblitz, welcome to Worldbuilding! Another thing: you're asking how engines would survive, and also what engines could survive. I think this is still too broad (and VogonPoet's first comment is still valid). $\endgroup$
    – Joachim
    Commented Sep 3, 2023 at 9:04

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Sometimes the simplest answer is the best. If you don't need to worry too much about mass, just have the main power plant buried deep in the body of the ship with a lot of redundant nozzles.

In battle shutter or otherwise shield your primary thruster (you're going to be maneuvering mostly) and using an array of smaller redundant thrusters. If they're damaged beyond repair? Jetsetton them

All that tubing also essentially acts as spaced armour. I'd even go as far as suggesting integrating the nozzles used into the armour as an outer layer away from the main ship, like slat armour or that can be replaced as entire units. Think those cubes in cubes machinists do to show off.

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You have giant, heavy battleships doing very short flybys of each other at relativistic speeds with very short engagement times, evasion will be pretty much not a thing. So just cover up any exhausts or engines with movable armor plates before entering into close combat and become a flying brick.

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  • $\begingroup$ To add, initiate a rollover maneuver while flying by, so that the engines are always directed away from the rival fleet. If possible of course, but if the fleets' ranks mix up, any collision might well obliterate both of them. $\endgroup$
    – Vesper
    Commented Aug 31, 2023 at 7:22
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Reactionless drive is one option: "A reactionless drive is a hypothetical device producing motion without the exhaust of a propellant."

Because there is no need for exhaust, the drive can be positioned anywhere, and armored heavily from every side.

There is already one potential reactionless drive about to be tested.

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    $\begingroup$ Should be mentioned that reactionless drives are not proven to work and by a conventional understanding of physics they should not work. $\endgroup$
    – causative
    Commented Aug 31, 2023 at 4:53
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    $\begingroup$ Is that link about an EM-Drive? That thing worked by having its outside wires interact with Earth's magnetic field. $\endgroup$
    – Vesper
    Commented Aug 31, 2023 at 6:42
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    $\begingroup$ @Vesper No, it is based on the idea of quantized inertia. It is said to defy laws of motion, but let's be realistic: conventional understanding of inertia is just as insufficient as our understanding of gravity. We are aware of the phenomenona, we know how to utilize them in conventional sense, but physics cannot actually explain it. $\endgroup$ Commented Aug 31, 2023 at 13:00
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    $\begingroup$ @NuclearHoagie While it possible that you are right, I myself cannot see any reason why operation would suffer from an armor. An important feature of a reactionless drive is that it can be completely enclosed, so why it would matter if it is enclosed inside thick armor? Re having an idea how it actually looks like, one is already peer reviewed: arc.aiaa.org/doi/10.2514/1.B36120 $\endgroup$ Commented Aug 31, 2023 at 17:20
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    $\begingroup$ One drawback of reactionless drives is they instantly move you 1 tier down on the Sliding Scale of Realism, as well as on the Scale of Sci-Fi Hardness if you prefer that one. You can no longer be Star Trek (itself pretty soft), but you can still be Star Wars. $\endgroup$
    – Therac
    Commented Sep 1, 2023 at 19:51
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Tiny holes and long shafts - ion engines

The amount of time in the future gives some leniency. You can then opt for a certain form of ion or plasma engine, even if it might operate differently at such times.

The idea is simple. If you throw something away from you, you'll get an equal thrust the other way. The harder you throw, the more energy is also used to propel you away.

Ion engines use this concept for 'efficient engines'. You simply throw miniscule particles as hard as you can in the opposite direction you want to go. Using methods that generate/contain more energy than chemical bonds you have ample energy in small packages, with a small resource to fire away. You can pair the insane power of nuclear (fusion) with a cloud of gas you can fire away. This setup allows for long shafts for the exhaust, with small openings in several directions for the direction. To make things more complicated but feasible on such large spaceships you can have (a) particle accelerator ring(s). In these rings the particles are accelerated, adding more and more energy, to eventually be released through the right exhaust tubes for thrust.

In current circumstances this wouldn't work. Any form of ion thruster is incredibly weak. They can accelerate spaceships over time to great speeds better than any other methods, but would be unfeasible for any maneuvering of battle spaceships. Not to mention that it is very hard to control particles exiting a particle accelerator ring at the right time. This heavily relies on the technological advances in your story, allowing enough thrust and accuracy to do so. As it isn't sure this would work, it will only loosely be based on something that is used in real life.

The result

Your exhausts are protected by being small and numerous, allowing for acceleration into any direction, redundancy of exhaust ports, as well as being closed when not firing at that time. Even if they are open, the exhaust tunnels are long enough that it's difficult to damage anything important, let alone the particle accelerator ring 'engine'. You could also argue that any debris that might block it can be burned away thanks to the particles in many cases, allowing the exhausts to unblock themselves during a battle if the damage isn't too great.

If I can paint a visual picture, instead of huge engine exhausts you can have sleek ships. Their hull seemingly one piece until it is required to move in the opposite direction. Many tiny holes open up, showing small flames of thrust. Not that ion engines would produce that effect, but the rule of cool allows it. From a distance it could give a certain colour to a part of the ship as it moves and manoeuvres, as well as give a plausible way to move in literally any direction or maneuver in any way.

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Well timed nukes

They use orion drives, dropping nukes behind them which will explode and push them in a particular direction. The nuke tube opens for a fraction of a second, releases the nuke, and closes, so enemies can't fire on them.

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You leave them outside of the battlespace. By detaching them after you are up to speed, sending them in a parallel trajector to the combat-run. After traversing the battlesphere, the two reunite for the deacceleration and go-around push. If the combat capsule does not survive the engagement, the engine either returns to base or goes on a suicide mission.

The capsule has all the weapons, small "jump" thrusters and gyroscopes to change direction fast.

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    $\begingroup$ I don't think that is a clever solution. Once the combat cubes have no propulsion there is no reason to fight them. If the enemy just breaks contact and escorts them outside effective combat range the maneuver elements can't reattach and the combat cubes will eventually run out of supplies, drifting dead in space, for example. $\endgroup$
    – lidar
    Commented Aug 31, 2023 at 12:54
  • $\begingroup$ ? Speed never decreases in space. So they remain at combat speed. And they have all the weapons needed. Add manouver thrusters, and you are set. $\endgroup$
    – Pica
    Commented Aug 31, 2023 at 13:31
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    $\begingroup$ Yes, but they are on a trajectory somewhere in the middle of nowhere flying nowhere. Unless the combat cubes are already brought onto a trajectory hitting important installations there is no need to engage them and you can just starve them out. Even if you have to fight the combat cubes, something small and fast with a few weapons can take out the maneuver elements and even if they manage to win the big engagement they will still starve. $\endgroup$
    – lidar
    Commented Aug 31, 2023 at 17:03
  • $\begingroup$ Mass makes you slow.. ditching mass before an engagement is very meaningfull. Acceleration in a actual space combat time frame is meaning less. You can not change it meaningfull without pulping yourself. So yes.. you have lines that go through a volume. Spewing venom and eating poison. Longrange engines are useless ballast in a engagement with real physics. $\endgroup$
    – Pica
    Commented Sep 1, 2023 at 9:42
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    $\begingroup$ Still better then to hang a part of the carrier to every fighter, that does them no good, but slow them down. And the carrier is the long time, longrange mobile one. It just bad in a dog fight.The carrier could even provide longrange support fire which needs much energy. $\endgroup$
    – Pica
    Commented Sep 4, 2023 at 11:09
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The drives are made mostly of magnetic field and are empty.

Your space ships are propelled by large magnetic confinement fusion drives. Their chambers and nozzles both are made of magnetic fields since no regular matter can contain nuclear fusion plasma anyways. In space there is no need to contain vacuum, so no hull is needed. Of course the drive needs fuel injection system, sensors and magnetic coils (however a significant portion of magnetic field is created by the plasma vortex itself). But those systems are:

  1. mounted on a grid frame around the engine
  2. relatively small and hard to hit while most incoming projectiles will simply fly trough the contained plasma into the space.
  3. made redundant and replaceable.

The engines handwaved in a such way may even be considered the most durable part of the ship.

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Throwing it out as an option not necessarily a recommendation. As far as RCS or directional thrust protection.....Don't...

Just assume anything outside of the armor and shielding is going to get fried. Pitch roll and yaw can be induced with flywheels/gyroes, then just use your main thrust for any course correction.

Of course this is for battle conditions. Perhaps another configuration / method of maneuvering can be implemented while under normal operation.

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Frame Challenge: Armor does not lead to bigger ships

There is a logical issue in your assumption that better armor leads to bigger ships. In fact, better armor should lead to smaller ships because of the linear-cube law. As you increase the size of a ship, the effectiveness of armor goes down very quickly. Think of it like this. Imaging you have a ship that is 10m long and has 1cm of armor; so, you decided to make a bigger ship with thicker armor. To do this you need to make everything bigger. Bigger engines to push the added weight, bigger power sources for your bigger engines, bigger crew spaces for the added maintenance needs... so when all is said in done, you scale the whole thing up and now it is 100m long with 10cm thick armor.

But here is the thing: in order to make your armor 10x as thick, you've increased the size, mass, and cost of your ship to be 1000x as much as the smaller ship. This is because the science of armor penetration is all about focusing your energy into as small of a cross section as possible; so, when your 100m long ship gets hit by a shell that is only 10cm wide, only 2e-7% of your actual armor gets to do anything about trying to stop the shot whereas the 10m ship is contributing 2e-4% of its armor to block the same attack. This relationship of size to armor is why we still have armored tanks and attack helicopters, but not large armored naval ships.

Focus on active defense like shields and point-defense weapons

If a 10m ship has a shield that can block a 1e15J of total energy before draining it's power supply, then you must overcome 100% of the ship's shield before you can harm it. So, if you scale this ship up to 100m, then you can block 1e18J before taking damage. This creates a linear relationship between a ship's size/mass/cost and its defensibility. Furthermore, by blocking all damage until the shield fails means that even a slightly smaller ship is at a big tactical disadvantage encouraging the constant development of larger and larger ships.

Likewise, if a 10m ship has enough interceptor missiles to shoot down 4 torpedoes, then a 100m ships can carry enough to shoot down 64 torpedoes using the same % of its allotted mass.

Active defense also means that there is no practical difference between getting targeted in the engine vs anywhere else on the ship because all hits are blocked until the defense fails. Since it sounds like your setting already makes use of active defense, you should focus on these, because these will encourage bigger ships in a way that armor can not.

Soak hits using redundancy instead of armor

Maybe you don't want active defenses to be 100% effective. Instead of making your ships armored, make them able to afford more hull hits. One advantage of a bigger ship is that it's much harder to design a weapon that can damage the whole thing; so, use that to your advantage. Instead of turning your ships into solid bricks of heavy armor with systems jammed in as tightly together as possible to make them all fit behind the armor, make them big, low density, and highly redundant so that when they do take a hit, the attacker over-pernitrates and only destroys a small portion of the ship, and any systems that are knocked out have 10 backups else where in the ship to keep it going. So making it mesh like, spindly, or mostly hollow will increase the number of hits it can survive whereas armor increases how strong of a hit it takes, but will reduce the number of shots it will survive.

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Your splash-back will be plasma and radiation, not debris

With the energy levels of impacts you are talking about, there will not be 5mm debris flying back at you. Any debris that absorbed little enough energy to remain solid will not have enough momentum to reach you before you are well out of harms way. Instead the only parts of tour target you need to be worried about are those that are turned into a cloud of plasma and radiation. Because you are dealing with such long distances, the plasma and radiation with spread out a LOT by the time it reaches you making them barely more dense than conditions you could normally encounter in space. It would be more like flying through a solar flare than what you think of as a debris field.

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  • $\begingroup$ Good point about shields beying more effective, but the main premise is just confusing. For example battleships have grown to effectively carry better armor and firepower to punch through such armor, and stopped only after the nature of naval combat changed drastically with lighter anti-armor options and with aircraft, guided munition and submarines allowing hitting without getting hit. Context matters here, which is why I'm open to feedback on the context section of my post which is supposed to justify such close range armored combat. $\endgroup$
    – Kugelblitz
    Commented Sep 2, 2023 at 9:00
  • $\begingroup$ Tanks carry armor to defeat lesser weapons systems, they don't fare too well against the weapons of their peers. Ships no longer carry much armor because they're only facing the weapons of their peers and near misses aren't common enough to warrant armoring against them. $\endgroup$ Commented Sep 3, 2023 at 2:37
  • $\begingroup$ @LorenPechtel Yes, it takes a peer sized weapon to kill a tank, but it does not take a peer sized weapon to take out a ship. A 20 ton fighter bomber can breach a 10,000 ton warship, but a 2 ton armored fighting vehicle generally can't pack the firepower to breach a 60 ton main battle tank despite being much closer in weight class to the tank than the bomber to a warship. $\endgroup$
    – Nosajimiki
    Commented Sep 5, 2023 at 20:23
  • $\begingroup$ @Nosajimiki Ships are only facing weapons that can kill them even with armor. Thus armor is just a hindrance. Tanks face lots of weapons that can't kill them, armor is very useful. $\endgroup$ Commented Sep 6, 2023 at 1:27
  • $\begingroup$ @LorenPechtel Ships face smaller threats too: bombers, submarines, cruise missiles, etc. Tanks have armor because armoring a tank usually works on smaller/cheaper threats. Ships don't have armor because armor usually does not work on smaller/cheaper threats, and this difference has to do with the diminishing return on armor as you get bigger. $\endgroup$
    – Nosajimiki
    Commented Sep 6, 2023 at 13:32
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This seems like a non issue, if your engines are electric.

To figure out how much electricity an engine uses to accelerate a spacecraft, you can use this formula

I = (m*a) / (V*E)

and the symbols represent the following:

I - current (amperes)
m - mass of ship (kg)
a - acceleration of ship
V - voltage of power (i'm using 11kV as my example power)
E - efficiency of engines (as a percent, between 0 and 1)

plugging this in for a 1,000,000,000 ton ship @ 11.4 KV and 1 m/s2 of acceleration and 100% efficient, we get an electrical current of 82.47MA,that's Mega-Amps.

using ampere's law, we can calculate the magnetic field using

B = uNI/L

B - Strength of magnetic field (tesla)
u - Permeability of free space
N - number of complete loops are made around the thrust axis
I - current (amps) L - length of engine bay.

Plugging in a 500 meter long engine bay, and 1000 turns (depending on your setting you may need way more or way less), and assuming the bay is mostly empty space (permeability close to 1), this gives us 200 Tesla. this is about the strength of 130 MRI machines, or about 2 Helmholtz coils.

with a bit of engineering, one can reasonably push the magnetic field to the outside of the engines where the effect seen in the video (the loss of kenetic energy in conductive objects) will stop debris. of course, you can tweak all of these numbers to suit your needs. maybe 1 m/s2 isn't quick enough, or there are less coils in the engine bay, but the fact remains that magnetic fields are incredibly useful at stopping metallic debris such as meteors and certain projectiles.

The weakness of such a system is that the magnetic field is only active while the engines are on, and so for an engine to be off is for an engine to be unshielded.

Hopefully this is a good foundation to work off of in your writings!

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    $\begingroup$ I have downvoted this answer because the physics are false. Amperes equal ampere seconds per meter? Conservation of momentum, energy, and charge are really more of a suggestion? $\endgroup$
    – g s
    Commented Aug 31, 2023 at 2:55
  • $\begingroup$ The video demonstrates a very slow object compared to anything zipping around in space that is worth considering. Meteors will just disintegrate with the diamagnetic parts shredding anything in their path, likely similar to projectiles, they can even design the tip of the rod to detach if the middle part got caught. $\endgroup$
    – Martheen
    Commented Aug 31, 2023 at 7:57

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