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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.

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  • $\begingroup$ 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. $\endgroup$ Sep 16 at 14:51
  • $\begingroup$ It is very important what distances are between the attacker and the target. That can mean no change needed or something special. $\endgroup$
    – Trioxidane
    Sep 16 at 15:19
  • $\begingroup$ @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. $\endgroup$
    – Ash
    Sep 17 at 9:48
  • $\begingroup$ @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. $\endgroup$
    – Ash
    Sep 17 at 10:15
  • $\begingroup$ @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. $\endgroup$
    – Ash
    Sep 17 at 10:18
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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.

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  • $\begingroup$ 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. $\endgroup$
    – Ash
    Sep 17 at 10:19
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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.

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    $\begingroup$ 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. $\endgroup$
    – Bartors
    Sep 16 at 7:04
  • $\begingroup$ @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. $\endgroup$
    – causative
    Sep 16 at 7:16
  • $\begingroup$ 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) $\endgroup$
    – Bartors
    Sep 16 at 7:51
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    $\begingroup$ Another interesting fact: When radiating the heat out backwards, a tiny amount of acceleration is gained. $\endgroup$
    – DarthDonut
    Sep 16 at 10:23
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    $\begingroup$ @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. $\endgroup$
    – Bartors
    Sep 20 at 6:29
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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.

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  • $\begingroup$ 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... $\endgroup$ Sep 20 at 9:14
  • $\begingroup$ And finally, the amount of energy that could be dissipated by evaporative cooling, seems far too low compared to what Nepene Nep calculated. $\endgroup$ Sep 20 at 9:15
  • $\begingroup$ @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. $\endgroup$
    – RedMoon
    Sep 20 at 13:50
  • $\begingroup$ 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) $\endgroup$
    – RedMoon
    Sep 20 at 13:52
  • $\begingroup$ 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. $\endgroup$
    – RedMoon
    Sep 20 at 15:27
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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.

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