Sci-fi Weapons, Vehicles and Equipment

The fun thing about matter moving at 99% of the speed of light is that, at such velocities, space is no longer "empty". The interstellar medium is incredibly sparse, but to relativistic matter it's as dense as a planetary atmosphere, slowing it down over dozens to thousands of light-years. Telescopic images of relativistic jets show them blooming out into vast bubbles as they encounter this material. It's awe-inducing. We've even detected such bubbles around the Milky Way.

Relativistic spaceships will have to contend with this problem if we ever figure out how to build them. Once you get above 10% of the speed of light or so, impacts with interstellar hydrogen and dust will generate enough heat and radiation to fricassee you.

The highest-energy particles we know of are cosmic rays, which can be as tiny as individual protons, but accelerated to unbelievable velocities by highly energetic events. We haven't even figured out where all of them come from, since in principle anything going that fast shouldn't be able to travel billions of light-years without hitting something.

Edited by Fighteer on Apr 14th 2024 at 10:15:07 AM

Well, we are pretty certain that most of the very high energy ones come from outside of the Milky Way. Which is a problem for physics because they should lose energy during collisions with the cosmic microwave background radiation.

Now there us evidence that a lot of them are filtered out by the CMB - but I am not sure if they are still too common for the theory.

I kinda would assume that if we ever manage to make an Warp/Alcubierre drive, we'd need both to limit for how long it can operate to keep the Hawking Radiation from frying everyone inside and spend a shitload of computing power to plot the safest route possible.

For relativistic speeds, even colliding with atoms can create issues, like inducing nuclear fusion thanks to the kinetic energy being high enough to fuse atoms you colide with.

I think warping space would protect you from the interstellar medium, since technically you aren't traveling in it.

I have my doubts that warp bubbles/Alcubierre drives produce significant amounts of Hawking radiation. If we assume that their Hawking radiation scales the same way as a black hole's - not a sure thing as at least in wormholes, the mass-radius relation is completely different from black holes - a warp bubble/Alcubierre drive with a radius of 100m would have a Hawking temperature of two microkelvins. Uneven areas in the bubble would tend to emit gravitational waves ("ringdown"), possibly smoothing them out.

It would depend mostly on distance I guess. If you are dealing with speeds bellow the light speed this might not be a major issue, but it would be also a slower travel method.

If you are dealing with something that can go at Light Years/Day speeds, then the interstellar medium might start to distort the bubble. IIRC there is also the bubble shooting up a gama ray stream whenever the bubble bursts.

It would depend on how the whole thing is set up.

The curvature of the Alcubierre warp bubble might generate sufficient horizon (aka Unruh) radiation to fry whatever is inside it, but it's very difficult to discuss the technical aspects of an entirely hypothetical technology. I also don't really understand what happens to things that impinge on the leading edge of the bubble. It seems that it should create a black-hole-like event horizon if the bubble itself is moving FTL, and that would be extraordinarily weird.

We can play with theoretical physics all we want, but until someone comes up with a way to generate negative mass-energy on a sufficient scale, warp drives are not going to be a thing. This is a centuries-out technology if it's even possible.

What that means for fiction is that you can give your warp fields whatever properties you want, including the ability to ignore drag from the interstellar medium.

Edited by Fighteer on Apr 14th 2024 at 1:06:24 PM

A nuclear shaped charge does not necessarily need to vaporize entirely. The projectile can also be shaped such that it becomes an explosively forged penetrator. In that case, you are firing a projectile at appreciable fractions of the speed of light, provided you have a big enough bomb.

But i think this is also where the problem obviously comes from: the purpose of a gun barrel is to focus the energy of the explosion in a single direction, in order to push the projectile forward. A casaba howizer or nuclear forged penetrator doesn't have a barrel. the explosion itself shapes the projectile. The reason you can't simply build a bigger gun is that the barrel becomes incapable of withstanding the energies involved in firing it. The losses incurred by a shaped explosive are vastly less than the losses from trying to build a big enough gun to fit it.

Electromagnetic weapons like coil and railguns serve to bypass such problems, since a neither contains explosions and so is subject to other problems. In the case of railguns, extreme friction with the barrel, and with coilguns it's the losses thanks to induction.

Keep in mind that a warp drive is properly a wave not a bubble. A bubble is causally disconnected from the cosmos and more akin to a pocket universe than a transportation vehicle.

A wave is defined by compression and expansion elements. If the peak compression of the Alcubierre wave is at the front — which it has to be — and the wave is traveling faster than light, then the curvature at the leading edge must generate an event horizon. Space inside is FTL with respect to space outside. There's no way to avoid this.

Something really awful must happen there.

Edited by Fighteer on Apr 14th 2024 at 3:08:33 PM

I seem to recall a nuclear test that put a steel plug into orbit but I can't recall the name of the test.

That's Operation Plumbbob, which I mentioned here. And it probably didn't make it to space.

To add, in order to send something to orbit, you need to apply force twice. The first time to get it off the ground, and the second time so its trajectory does not fall back to Earth. Unless you accelerate the object to escape velocity with a single push (highly difficult without breaking your projectile), or you started at a high point, you can't send something to orbit with only one push.

Edited by minseok42 on Apr 14th 2024 at 6:51:13 AM

Well, if you can accelerate enough you can get it to escape velocity. That's about 11.2 km/s, straight up or sideways. Of course, if you start at that velocity from the surface, the atmospheric drag will vaporize whatever you're trying to launch, such as a manhole cover.

Edited by Fighteer on Apr 14th 2024 at 10:15:46 AM

Yeah, best estimates of the nuclear manhole cover is that if it wasn't vaporised by air friction it was still travelling at solar escape velocity when it cleared the atmosphere.

Thats a lot of work that IF is carrying.

It most likley vaporised from air friction.

Edited by Imca on Apr 15th 2024 at 8:05:01 PM

Considering that this glorified manhole cover was at the muzzle of essentially a nuclear driven steam gun, I'm pretty sure the plug was shattered from the acceleration as much as it was vaporized from atmospheric friction.

Which brings me to my next point, what kind of Unobtainium are we making the bullet out of and can it be used as armor?

Well the magic space metal better be responsive to magnetic fields otherwise you aren’t accelerating it via railgun, particle beam/accelerator, or using it standalone without alloying or jacketing it in normal ferrous metals.

Also some things are better used as bullets rather than armor. Depleted uranium and tungsten are this way. Great armor penetrators, not so great as armor themselves. Usable but other stuff has better resilience or weight or cost or what have you.

Which raises some interesting questions about the M1 Abrams and it's DU armor package. Still, a superstrong armor material could be used to jacket a super dense material for a more effective penetrator; so while a good bullet material might not make good armor, good armor material can be a key component in a good bullet.

But yes, electrokinetics are better than explosion-driven guns. For one, you aren't limited by how fast sound travels in burning propellant. Another aspect is that you aren't limited by the laws of pneumatic pressure. Bullets need a nice flat base for the gas to push against and the thinner the bullet the less surface area you've got to push against. A coil gun does not care what shape the bullet is as long as it fits in the barrel. You could shoot a 1x100mm flechette at mach 5 if you've got the barrel length and electric charge.

Honestly, I think that's where firearms are heading. Thinner bullets moving faster and faster. Don't worry about stopping power, just shoot the target again and again until he drops.

Overpenetration is a factor. You don't want to hit the five people behind the guy you're shooting, especially if there might be civilians around.

Anyway, as projectile velocity increases, air resistance becomes the dominant factor in useful range and power. The thinner a projectile is, the less drag it faces, but the more susceptible it is to crosswinds and shear forces.

Without active propulsion and guidance, there are absolute limits to range and accuracy in an atmosphere.

Edited by Fighteer on Apr 15th 2024 at 10:29:34 AM

Even beyond the friendly fire problem over penetration is bad because that's all energy you didnt impart into the target.

^ Which is why it’s a bad idea for tanks to engage targets like trucks or M113 APC’s or a BMP-2 with APFSDS ammunition.

It just runs right through for relatively low damage if it doesn’t hit a critical spot like a fuel tank or munitions rack. They have a lot more “empty space” relative to a tank.

So it’s a waste.

If you're overpenetrating you can either dial back the power (coilgun) or switch to smaller rounds. Even if the damage is minor you can compensate with a higher rate of fire. Think of the 5.7mm. Sure, it's a tiny bullet but being tiny means you can store 20 in the same space as 15 9x19mm rouds.

[A quick reminder that we're discussing sci-fi weaponry, not modern weaponry. There is some overlap, but this thread is not for military porn.]

Speaking very broadly, a weapon system intended for certain tasks (antipersonnel, for example) should be engineered to accomplish that task and not other tasks as well. There is no such thing as a "universal" bullet. The 30 mm cannon on a fighter aircraft is meant to kill other aircraft, not people or tanks. It can be used on people, but much less efficiently than a .30 cal machine gun (or whatever).

Counterexample: Star Trek: The Next Generation proposed that the phaser arrays on the Enterprise D were capable of dividing or concentrating their firepower according to the size and durability of the target.

I envisioned something similar in one of my (unpublished) sci-fi world-building projects. A dreadnought is equipped with laser array panels that can configure themselves to spam weak shots at smaller ships and fire concentrated beams at larger ones.

Edited by Fighteer on Apr 15th 2024 at 1:33:08 PM

Tell that to the A-10 .

That said “universal” ammunition in the sense of engages the widest array of targets possible does exist as a concept.

It’s a big reason why modern armies still can’t fully break away from .30 caliber rifles and machine guns. It allows a wider range of targets and uses than 5.56 or 5.45.

Similarly 30mm is on the lower end of the range of cannon fire acceptable for use against a wide variety of targets from soft skinned humans to aircraft to hardened tanks and more.

This does translate into sci fi as well such as the Kinetic Weapons Are Just Better trope.

Need to destroy an alien ship? Launch a tungsten rod at it.

Need to destroy an extinction causing asteroid? Fling a tungsten rod at it.

Need to destroy a city overrun with alien zombies and hopelessly unable to be reclaimed any other way? Drop a tungsten rod on it from orbit.

Universal ammunition all of it. The All-Solving Hammer indeed. If the work in question can swing it, it might even be the same weapon for all three of those.

Edited by MajorTom on Apr 15th 2024 at 1:23:13 AM