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High power projectile weapons, often firing their rounds near the speed of light, are a staple of sci-fi. Under certain assumption sets these weapons are valid, I'm not interested in the science (or lack of) around how to build such a weapon, but I have wondered for a while about one particular aspect of such a weapon, detection. This excellent answer indicates that any projectile travelling near the speed of light will be reduced to a cloud of plasma by its continuous impacts with the atoms and dust of the stellar medium very rapidly. At lower relativistic speeds you'll see the same effect just a bit slower. Even if we assume that the projectiles in question are travelling slow enough to make their designated trip they'll still shed some mass to, and be lit up by the energy from, molecular impacts during the trip to their target. The glowing plasma they'll be surrounded by will make them relatively easy to detect en route and possibly defend against in some way.

Is there some material that a relativistic projectile could be sheathed in that would minimise this effect and allow such weapons to go unnoticed in transit for as long as possible?

In answering this question ignore the larger impacts with dust and micro-meteors they're going to be both rare and due to their size unavoidably highly destructive. Concentrate on the effects of the solar wind and it's mitigation, and projectile speeds of approximately 10% light speed.

Ash
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  • Unless you postulate faster than light speed detection, the standard SF answer is you detect an incoming close to light speed round (or plasma cloud) just before it hits you, so have no time to do something about it. If you want a stealthy round, use a missile that doesn't travel at relativistic speeds, then put hardware on it that makes it harder to detect. Detectable radiation from a round traveling close to light speed is not much faster than the round is (in the direction it is traveling) so you will see it before impact only if fired very long distances. – Mark Ripley Sep 16 '21 at 14:51
  • It is very important what distances are between the attacker and the target. That can mean no change needed or something special. – Trioxidane Sep 16 '21 at 15:19
  • @MarkRipley A projectile doing 0.1C is going fast enough for impacts to create significant detectable radiation and slow enough for detection to do you some good if you're on the receiving end and that was the speed specified. – Ash Sep 17 '21 at 09:48
  • @Trioxidane That is a question I've been beating my head against for a very long time now without a meaningful answer. I know for the particular setting I'm working on at the moment the computers start off good enough to fire a ballistic round across a solar system and hit an object of similar size also on a ballistic trajectory 75% of the time and they get better from there but a round isn't actually going to survive that trip at high enough speeds to be used in ship-to-ship combat, nor is another ship going to stay ballistic while being fired on. – Ash Sep 17 '21 at 10:15
  • @Trioxidane For the purposes of this question the ultimate range of the shot isn't that important I'm concerned with what you could make a slug out of, or coat it in, to make detection more difficult. – Ash Sep 17 '21 at 10:18

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Install a powerful refrigerator in the projectile. Cold refrigerant coils cover the forward surface. The heat from molecular collisions is pumped from the front to the back, where it is radiated away. If the heat generation rate from collisions matches the rate at which heat is radiated away, the projectile won't reach excessive temperatures. The radiator should direct the heat away from anyone that might see it.

Alternatively, or in addition, equip the projectile with an extremely powerful magnetic field sufficient to redirect incoming charged particles to the sides. Something like a Bussard ramjet, except you want to push the particles away instead of funneling them to the center.

All this equipment would increase the required mass of the projectile, but it's best for the projectile to have a small cross-section, to reduce the collisions. So the projectile would be shaped as a long and thin rod.

causative
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    Install a powerful refrigerator in the projectile. Building a refrigerator, with power source and all that is needed, that will survive a massive amount of acceleration that a projectile accelerating to speed of light will experience would be quite an achievement. – Bartors Sep 16 '21 at 07:04
  • @Bartors It was not specified how much time the projectile is given to accelerate. Equipment damage due to acceleration is no problem if it accelerates for at most a few Gs for weeks to years. If you were thinking it would accelerate to relativistic speeds in the barrel of a relatively short gun, you'd have to invoke magic; no solid object could survive those accelerations, and the recoil would tear apart whatever the gun was mounted on, as well as the gun itself. – causative Sep 16 '21 at 07:16
  • We'll it is specified that the projectile will be used in "space combat". And while it could technically mean solar system to solar system or galaxy to galaxy combat my understanding was of a more "dynamic" combat environment. I do agree that accelerating to relativistic speed is "short barrel" is impossible with todays material, but I have read some places about a railgun projectile surviving 60kG, way beyond what a object more complex than a solid block of metal might survive (I recall a paper where it was stated that a hi-tech tank projectile survived ~50G) – Bartors Sep 16 '21 at 07:51
  • @Bartors 60,000 Gs through a gun 1 km long would only get you to 0.01% of the speed of light. If you want to get to 0.9C at this acceleration your gun has to be 60 million km long. Now, maybe your gun is only 1 km long and accelerates instead at 600,000,000,000 Gs; this would get you to 0.9C but no material could survive it. Maybe your gun accelerates in a way that the projectile feels nothing, like gravity. Then you could send a refrigerator no problem. – causative Sep 16 '21 at 08:05
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    Another interesting fact: When radiating the heat out backwards, a tiny amount of acceleration is gained. – DarthDonut Sep 16 '21 at 10:23
  • @causative I'm not saying you don't have a point because you do but I did specify 0.1C which is still obscene but not quite so much as 0.9C. – Ash Sep 17 '21 at 09:49
  • @causative The refrigeration idea is cute but the consequences of a leak would be catastrophic to accuracy. The magnetic field is better. – Ash Sep 17 '21 at 10:22
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    @causative:

    60,000 Gs through a gun 1 km long would only get you to 0.01% of the speed of light.

    This was my point, a hardened piece of military equipment may barley survive 60kG, a refrigerator would survive way less.

     – Bartors Sep 20 '21 at 06:29
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The particles you will be impacting are of two types: charged and neutral.

Providing the bullet with a magnetic field will deflect the charged particles and leave you only to impact the neutral particles.

This will result in a lower fingerprint and a more difficult detection.

L.Dutch
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  • And solar winds are pretty much all charged particles, there will be a bow wave but it'll be too close to the projectile for a meaningful detection window. – Ash Sep 17 '21 at 10:19
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In addition to the already existing proposals of magnetic deflection and/or refrigeration, a much simpler cooling scheme can be used.

Coolants (e.g. liquid helium, nitrogen or any other good evaporative coolant, depending on the requirement of surface temperature) can be directed to the "front" of the projectile, absorb heat and be ejected from the projectile. With adequate design, this cooling system requires very little moving parts and complex, expensive machineries.

Of course the disadvantage is that the projectile gets lighter during the flight, which isn't a good idea for relativistic projectiles, but with cheaper projectiles you can always shoot more to compensate for the loss of kinetic energy.

RedMoon
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  • Evaporative cooling works because a substance steals energy from its surroundings, due to the fact it evaporates at a given threshold, so if it reaches the threshold due to random exchange of energy (movement of heat) in the system, it escapes from the system having more average energy than the rest of the system. Problem is, it requires surrounding air, in vacuum it just evaporates instantly. Another problem is, since it's on the front, it's the first thing to be heated, so it makes sense instead to have a material that when warmed up, just breaks and is left behind... – Markus von Broady Sep 20 '21 at 09:14
  • And finally, the amount of energy that could be dissipated by evaporative cooling, seems far too low compared to what Nepene Nep calculated. – Markus von Broady Sep 20 '21 at 09:15
  • @MarkusvonBroady I meant a system that deposits the heat into the coolant, like heat pipes embedded in the front of the projectile. The word "evaporative" is used because phase change is very effective at absorbing heat, and the low surface temperature required to evade heat signature detection leaves little room for the coolant to absorb heat by increasing temperature. I think what you mentioned is closer to ablative material, which operates by the same principle of phase change. – RedMoon Sep 20 '21 at 13:50
  • As for the calculation, I will try to do it myself to see if it's really viable, in the mean time, toughsf's hydrogen steamer may be worth for reference, which has the same idea (though on a different scale) – RedMoon Sep 20 '21 at 13:52
  • Each gram of liquid hydrogen stored at 5K can bring around 270J out from the system right after evaporation, which according to Nepene Nep can provide 6s of cooling. (At this rate we might as well consider solar radiation, but that's out of the scope of this discussion). For a projectile to cross a distance of 1AU, 4990 second is needed. This corresponds to 832g of liquid hydrogen. This number itself isn't bad, but considering the atrocious density of liquid hydrogen, some better coolant is needed. Nitrogen is a good candidate, but its boiling point may be too high for effective stealth. – RedMoon Sep 20 '21 at 15:27
  • Again, why do you need to "cool", "deposit heat", etc. All you need is a material on front, that heats up, and then falls off. Effectively, the amount of "cooling" you can do is just the thermal capacity of the material on the front. – Markus von Broady Sep 20 '21 at 15:46
  • because, in my opinion, a material that is resilient enough and breaks off at the right temperature (so blackbody radiation doesn't signal its presence to the entire solar system) is somewhat harder to engineer than heat pipes embedded in graphite armour that are connected to a liquid nitrogen tank – RedMoon Sep 20 '21 at 16:40
  • Hmmm... I'm thinking, take an optimal ratio of high melting point, low thermal conductivity and high thermal capacity, put it in a cone shape, the engineering really comes down to such layers of the cone so the cone shape is retained as the material melts and keeps sliding off. I perceive it as much simpler in design than basically building a fridge inside a bullet. Not only that, but I fail to see how in the void of space, transferring heat from front to back and using gas to release this heat, could release more energy than avoiding heat transfer and simply releasing heated stuff. – Markus von Broady Sep 20 '21 at 17:53
  • "high melting point" This instantly defeats the objective of stealth. For a material to melt at high temperature and shed off, it has to reach that temperature first. And before you reach a temperature of say 1000K, infrared telescope can see you on the opposite side of solar system. That's assuming the material will slide off at first. In space there's no air to blow the material off, and the force delivered by impacting stellar medium is virtually nonexistent (on the magnitude of 10^-20N per second). Anything that melts is going to stay there. – RedMoon Sep 20 '21 at 18:17
  • I said "optimal ratio". Your gas is also hot when you eject it, right? Otherwise it's cooling the object less. What's the logic I'm missing here, why it's better to keep some of the energy while transferring it to the gas, instead of eject the energy in its entirety? As for sliding off, fair, perhaps you need something that evaporates... – Markus von Broady Sep 20 '21 at 18:34
  • Hot in the sense of tens or at most <200K. Yes this is very inefficient, but stealth in space got to be inefficient if it is stealth. "why it's better to keep some of the energy while transferring it to the gas, instead of eject the energy in its entirety?" What? for whatever coolant or exit temperature you use, the total heat energy coming out is the same. The problem is that while using high-temperature coolant is very efficient, they also radiate infrared radiation that is incredibly easy to detect in space. It is therefore important to keep the surface temperature low to avoid detection. – RedMoon Sep 20 '21 at 18:40
  • You can't just take all the heat from the front and move that to some atoms to the back and eject that. Some heat will dissipate internally to the entire bullet. Sure you can use the refrigeration principles to keep the entire temperature low, but it becomes increasingly hard as you want to get the temperature lower. Meanwhile you can send a bullet with a near 0K temperature and a shield of a frozen gas, that as soon as it is heated to a temperature probably still below fridge-bullet operating temperature, it evaporates locally, removing the hottest (and so radiating) element from the bullet. – Markus von Broady Sep 20 '21 at 18:47
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They can't be stealthed.

Each collision with a proton generates (1.6726219*10^-27 kilograms) 30000000^2= 1.510^-12 joules of energy. Assume a projectile of area 100cms, and 3 projectiles per cubic centimeter. Every thirty million meters of motion, every second they'll heat up 45 joules.

20 watts is enough to detect Voyager 1 from 18 billion kilometers away. It would be enough to detect your projectile, especially with futuristic space technology.

Nepene Nep
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