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With relatively current technology (i.e. no force shields, etc) what are ways to protect a spaceship (or at least mitigate the amount of damage) against hypervelocity kinetic projectiles? I'm a retired HS history teacher with a limited knowledge of science, so, be kind...

I'm thinking:

  • whipple shields of various materials & thickness
  • redundancy of systems
  • tanks/baffles of water (or some liquid)as armor
  • tanks/baffles of an expanding foam to seal leaks
  • a combination whipple shield with a layer of ERA underneath

Looking forward to answers, speculations, observations, and convaluations (I made that up).

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    $\begingroup$ Depending on how hyper is the hypervelocity (please edit and specify), you may have to live with the idea that there is nothing in the way of armor to protect a ship against a hypervelocity kinetic projectile. Look for example at modern surface naval ships, which are basically armorless, and instead of armor rely on the ability to detect and suppress enemy ships sufficiently well so that the losses are sufficiently low as to be acceptable. (And Whipple shields are intended to offer protection against tiny objects, not more than a few grams, moving at reasonable orbital speeds.) $\endgroup$
    – AlexP
    Dec 7, 2023 at 15:55
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    $\begingroup$ For future reference: (a) The help center prohibits giving us your answers and expecting more. (b) Per the tour, Stack Exchange is not a discussion forum. (c) "Opinion Based" is literally a reason to close questions, so asking for speculation suggests you have an expectation outside of SE's rules. (d) This question has been asked many times on this site and likely should be closed as a duplicate ([1], [2], (*Continued*) $\endgroup$
    – JBH
    Dec 7, 2023 at 16:42
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    $\begingroup$ @JBH those linked questions of yours are very much not duplicates. Given that the OP is asking about kinetic protection, questions about rayguns, nuclear explosions, antimatter, energy shields, near-lightspeed travel and general armor are not necessarily very helpful. $\endgroup$ Dec 7, 2023 at 17:02
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    $\begingroup$ How much pre-impact can the projectiles be detected? Is it like a laser, where you don't know they've even fired at you until you're hit, or do you have time to deploy countermeasures? Is the defending ship sitting in place, or do the defenses have to move with you? Are you trying to avoid getting hit, or just trying to improve damage resistance? $\endgroup$ Dec 7, 2023 at 17:14
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    $\begingroup$ You should check out some clips of "The Expanse" for believable defence of space ships. If it's a "torpedo", shoot it down, if it's a long range kinetic weapon (like the Roci's rail gun) jink when you detect it being fire. And if it's a close range rapid fire kinetic weapon, you're getting filled with holes. Make sure everyone has a suit on! :-) $\endgroup$ Dec 8, 2023 at 11:21

9 Answers 9

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I'm sure we've had questions on this before, but a quick search wasn't turning up anything particularly good, so here we are. You could have a quick read of my answer to Ideal materials for the outer layer of a whipple shield as a sort of warm-up.

You might also reasonably read up on (or indeed play!) a game called Children of a Dead Earth, which is intended to be a plausible simulation of space combat, and they've done a reasonable job of it. They've got a load of interesting blog posts on the subject, explaining the reasoning behind their decisions.

You might also want to think about what "hypervelocity" means. Wikipedia suggests over three kilometers per second, but regular modern day ASATs will hit with closing velocities of more than twice that and slightly more exotic things like nuclear-driven projectiles could easily hit twenty or thirty times that velocity with cold-war era technology. The Plumbob Pascal B nuclear test accelerated a 900kg metal disc to over 60 km/s, and in the face of weaponry like that the only practical defense is to not be in the way.

(You may also find references, here and elsewhere, to Matterbeam's take on nuclear-driven projectiles on theire blog Tough SF. It has some interesting ideas and useful references, but the extrapolation is frankly implausible, so take their conclusions with a couple of tonnes of salt accelerated to .2% of lightspeed. There's some much more sober and grounded discussion by Fenstermacher in The effects of nuclear test‐ban regimes on third‐generation‐weapon innovation)


  • whipple shields of various materials & thickness
  • redundancy of systems

This is "table stakes"... if you can't manage this sort of thing, then you risk being mission-killed by accident by regular old space debris, whether human made or otherwise.

Your spacecraft must be At Least This Tough just to be a useful spacecraft, let alone a warship.

  • tanks/baffles of water (or some liquid)as armor

In the face of hypervelocity projectiles, the intermolecular bonding that makes solid things solid simply isn't strong enough. Impacts with things moving at several kilometres per second are modelled as jets of liquid colliding and splashing off each other. Modern shaped-charge weapon research gives us a handy equation (A Jet Penetration Model Incorporating Effects of Compressibility and Target Strength calls it the Hill-Mott-Pack equation, but other names are associated with it too including Birkhoff and Tarantello, but I digress) and it looks like this:

$$P_d = \ell \sqrt{\rho_j \over \rho_t}$$

Where $P_d$ is the depth of penetration, $\ell$ is the length of the penetrator, $\rho_j$ is the density of the penetrator (with j-for-jet) and $\rho_t$ is the density of the target.

The critical thing to be aware of there is the density of the target, and water just isn't that dense, and whilst there are denser liquids they can't really compete with the density of solids because that's how physics works. You can have a thicker tank of water to make up for it, but that just increases your moment of inertia and makes variouys kinds of manoever more difficult. A nice hard dense solid material makes for better armor, and as a secondary benefit there's no risk of it all leaking out of a hole in a tank.

Water or ice does have many other uses on a spacecraft, including as a coolant, heat sink and particle radiation shield, so there will probably be plenty of it about... it isn't great as projectile armor by itself.

  • tanks/baffles of an expanding foam to seal leaks

Well, I guess, but you should be able to work OK even if all your air leaked out. If the crew can't survive in a vacuum by themselves, they can always wear spacesuits, and probably should at least wear some kind of pressure suit in case of incidents.

  • a combination whipple shield with a layer of ERA underneath

ERA exists to combat a specific threat to tanks in cold-war-era conflicts. There are various countermeasures against it even now, and I'm not at all certain it will be of much use in space combat which is a very different environment involving potentially much faster projectiles in potentially much larger quantities.

That's not to say it is useless, but I'd be skeptical.

What I think you'll end up with is long, pointy spacecraft making use of sloped armor and minimising their cross section, using layered and spaced shielding made of a range of different materials to help break up incoming projectiles and disperse and absorb the impact.

And then what will happen is that the ridiculous overkill associated with space-based hypervelocity weapons will absolutely wreck it more or less as soon as a hit is scored. Parallels might be drawn with modern warships, the designers of which are well aware of the striking power of antiship weapons and don't waste time and mass on a load of armor which won't help much.

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  • $\begingroup$ Yep. Modern warships are weapons platforms designed to last long enough to fire all their ordinance and then allowing (most) of the crew to abandon ship to be picked up if their side wins. Space warships will be similar. $\endgroup$
    – Dale M
    Dec 9, 2023 at 11:40
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Evasion

Space is big, and space battles could plausibly occur between ships located thousands or even millions of km apart. The first defense is not to get hit, and being very far from the enemy makes that easier. You may have several seconds or minutes to get out of the way.

To know how much time is available to evade, we need to know how fast a projectile you're being shot with. Hypervelocity projectiles on Earth, such as the Navy railgun, might have a muzzle velocity of up to 5 km/s. This is actually not very fast by solar system standards. Meteoroids strike Earth typically at 20 km/s. The relative speed between your ship and your enemy's ship is likely higher than the muzzle velocity of the enemy's railgun relative to his ship.

Sometimes you hear about the Casaba-Howitzer idea for producing hypervelocity projectiles. The notion you sometimes hear is that you could set off a small nuke and have it propel a solid bullet at 100 km/s. However, more realistically, the initially solid bullet would be vaporized by the nuke into a plume of material that spreads out at 100 km/s in a cone. The effect if the plume hits your ship would be very different from a solid projectile, and less damaging; it would dump energy broadly across your hull, heating it up and vaporizing the outer layers of hull. It wouldn't punch a hole. So we won't worry about Casaba-Howitzer any further here. It's not a solid projectile and it would take different measures to defend against.

So, realistically, we might say you are faced with only 30 km/s projectiles, with most of that speed being due to the relative velocities between your ship and the enemy's ship, and not due to the muzzle velocity of the enemy's railgun. This means that if you are 1000 km apart when the enemy shoots, you have around 30 seconds to evade. (If you're a million km apart, you have nine hours to evade - the enemy would likely not even bother with kinetic projectiles at such ranges!)

Evasion can be divided into "eyes-open" evasion, and "random walk" evasion. You would use "eyes-open" evasion if you know the trajectory of the enemy's projectile so you can get out of the specific target area. This could potentially be done using a radar telescope. You can certainly use radar to detect that a projectile is incoming (although the enemy can also cover their projectile in angular radar-reflective surfaces like a stealth bomber), but determining specifically where it is aimed down to a few meters is a much bigger ask, although possible in theory.

For random walk evasion, you thrust in random directions at specific intervals, to maximize the enemy's uncertainty in your future position. This is what bombers did in WW2 to avoid flak. This is generally a good idea to do any time you are in hostile space, because you won't always be aware of the specific projectile or bomb before it hits you, if it is cold, small, and stealthy to radar.

A ship that is good at evasion should:

  • Have a small cross-section presented to the enemy, and be capable of thrusting sideways while presenting the small cross-section
  • Have a high thrust-to-mass ratio, not weighed down by too much heavy armor or weapons
  • Have good "eyes" (telescopes) and "reflexes" (computers) for eyes-open evasion when possible

Armor

A 1.2 cm aluminum sphere impacted an aluminum plate at 6.8 km/s, leaving a crater 5.3 cm deep:

The amount of armor you need certainly depends on the size of projectile you're being shot with. There is a trade-off between the weight of armor, which lets you tank bigger hits, and the maneuverability of your craft, which lets you avoid hits entirely.

When a projectile hits your armor, it has two effects that can be considered separately. The first effect is that it penetrates some amount based on its density and length. The second effect is that it dumps its energy into the armor, causing an explosion and crater.

As Starfish Prime helpfully mentioned, at such high speeds both the projectile and the armor act like fluids; the molecular bonds are too weak compared to the force of impact to be very important. I'll go ahead and steal his formula for penetration depth:

$$P_d = \ell \sqrt{\rho_j \over \rho_t}$$

where $P_d$ is the depth of penetration, $\ell$ is the length of the penetrator, $\rho_j$ is the density of the penetrator (with j-for-jet) and $\rho_t$ is the density of the target.

Note: this depth of penetration is before the crater is formed. The crater will be usually several times deeper than this formula gives. More on that later. But we do want to minimize this initial penetration depth according to this formula, because it affects the crater depth too.

We can suppose that the enemy is shooting you with an iron projectile, because he's using a railgun. Iron has a density of 7.8 g/cm^3.

Now, in space mass is king; the more your spacecraft masses, the slower it can accelerate and the more fuel you need. So you want the most protection for the least mass. Are you better off with dense armor, or an equal mass of less-dense armor? Based on this formula, surprisingly, to minimize penetration you are better off with less-dense armor!

Let's do an example. The enemy projectile is 1m long and your armor is also 1m of iron. The density of projectile and armor are the same, so the penetration depth is 1m, and the projectile gets through.

On the other hand, you could use 2.9 meters of aluminum armor for the same mass as 1m of iron armor. With aluminum, the penetration depth is 1m * sqrt(2.9) = 1.7 meters. The projectile only gets a little over half-way through!

Whipple shields are used in practice currently, and a scaled-up Whipple shield is probably a better option than solid steel or aluminum. Current Whipple shields are designed against micrometeorites (up to 1 cm size), but the same principles will apply to larger projectiles if the Whipple shield was just bigger and thicker. The principle is that the projectile hits a thin layer of armor, which it penetrates completely but disperses as it does so. It is allowed to spread out further over a gap of empty space before the next layer of armor. Then when it hits the next layer of armor, it may penetrate that too, but it is dispersed even more. After a few layers it is dispersed enough that it no longer penetrates.

Note that a Whipple shield, with all the empty space between the plates, has a low overall density. That tracks, when you consider the penetration depth equation.

That's all to minimize penetration. The other factor to worry about is the energy of impact, and the crater. The crater volume is proportional to the impact energy. Intuitively, that's because to make a crater that size you have to break a lot of the molecular bonds in that volume of armor, which takes energy per bond broken. This means the crater depth is proportional to the cube root of the impact energy.

A 100 kg projectile at 30 km/s has the energy in about 10 tons of TNT. To minimize the crater that makes, you want strong materials that take a lot of energy to vaporize. Whipple shields are made of steel, which fits the bill. You want a (giant) Whipple shield made of several thick layers of steel with gaps in between them.

If you plan on getting hit more than once, and survive the first hit, you may want to weld new plates over the holes in the armor. However, the enemy might not bother shooting you with a weapon that doesn't penetrate your armor on the first hit. It's less efficient to hit you with ten smaller rounds that don't penetrate but just damage your armor, than to hit you with one round 10x the size that goes through and destroys your ship.

Thus, we might say that what the armor really does, rather than actually protect you if you're hit, is make the enemy pack a bigger gun and heavier ammunition, which slows him down, reduces the number of times he can shoot, and reduces the other armaments he can carry. Actually getting hit with a solid object from the enemy is probably game over.

Your idea of a water tank is interesting for the purpose of healing the armor. It is not as dense as steel, which makes it favorable for the penetration depth, and if the enemy blows a hole in water, the hole just fills up on its own. However, water takes much much less energy to vaporize than steel. The first good hit and much of your water tank will be steam. Also, water lacks any tensile strength, which does matter.

Active interception

The idea here is that you send out your own projectile to meet the projectile a few km in front of your ship. The two projectiles annihilate each other. Your ship may be hit with some chunks from the explosion, but it would be much less damaging than if the enemy projectile directly hit your armor.

Two basic ways to do this. One is, you track the incoming projectile and when it's close enough to be sure of its trajectory, you shoot it with your own railgun. This is tricky but it has the advantage that the momentum of the incoming projectile can be partially canceled by your own, so less of the incoming projectile proceeds to hit you.

The other way is, you have a rocket drone loitering a few km in front of your ship. The drone masses about as much as the incoming projectile, and maneuvers to be exactly in front of it so the projectile hits the drone. This may be easier to accomplish than shooting the incoming projectile with your railgun, but the net momentum from the collision is still towards your ship so you may be hit with more debris, though it will at least be spread out more.

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  • $\begingroup$ I'd add energy weapons to the mix but if and only if the enemy is using 'smart' projectiles i.e. kinetic projectiles fitted with senors and terminal guidance packages as well as some (even if limited) form of course correction like simple chemical or gas thrusters. The idea being to cripple or blind the projectile not destroy it since at heart its still just a solid piece of metal anyway. $\endgroup$
    – Mon
    Dec 20, 2023 at 2:07
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The Sponge Approach

Realistically there is very little to stop high velocity rounds.

enter image description here

It's age old problem faced by the armed services in the battle between armour and weapons.

In The Expanse, everyone puts on suits and and puts the ship in vacuum so there is no explosive decompression. The ship has redundant systems and everything is made to try to prevent permanent damage from projectiles pass through the ship.

When all said and done, you patch the holes and restore the atmosphere. The ships are built to try and receive as little damage as possible. You want as much kinetic energy to stay with the bullet and not blow massive holes in the ship.

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I have always been a fan of ice. It's easy to come by, space is cold anyway. It works well as an ablative armor either stand alone or as a type of pykrete. Can be used as fuel radiation shielding and life support resource, a large mass of ice makes a very useful heatsink.

A very helpful resource you may already be aware of. Atomic Rockets website.

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    $\begingroup$ Space ain't necessarily that cold. Witness the ready availability of ice in the outer solar system, compared to the rather drier and rockier inner solar system. The Jovian and Galilean satellites have a hojillion tonnes of the stuff, but its a long walk there from Earth. $\endgroup$ Dec 7, 2023 at 16:41
  • $\begingroup$ @StarfishPrime Fair enough point taken, can we say if you can find a permanent shadow you can likely find ice? $\endgroup$
    – Gillgamesh
    Dec 7, 2023 at 17:24
  • $\begingroup$ Couldn't a hypervelocity projectile hitting ice produce a steam explosion? $\endgroup$
    – DKNguyen
    Dec 8, 2023 at 6:01
  • $\begingroup$ @DKNguyen some probably but I doubt it would be significant in the equation. Although a cloud of steam might be helpful in dispersing energy from any incoming laser weapons. $\endgroup$
    – Gillgamesh
    Dec 8, 2023 at 13:36
  • $\begingroup$ @Gillgamesh I guess as long as the ice was just exposed on the hull rather than contained inside an outer hull it wouldn't cause too much damage. $\endgroup$
    – DKNguyen
    Dec 8, 2023 at 15:22
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In Rendezvous with Rama, Clark solved this by having Rama extrude miles and miles of fuzzy cable. This wasn't quite a Whipple shield because the cables had a bit of intelligence and could wrap incoming projectiles in a cocoon. In the book, dumb-ass humans shot nukes at it, but I'm sure it could be adapted.

In Children of Ruin, Adrian Tchaikovsky comes up with a less conventional method. It's difficult for an entire ship to get out of the way of missiles, but the protagonist's ship isn't rigid. It's bags of spiderweb connected by actuators. When shot at, it changes shape to move its vital parts out of the way without having to move the entire ship.

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  • $\begingroup$ I like the second idea. "Build your ships not as one solid thing, but as connected parts where the connections can create air-gaps (well, vacuum-gaps) to allow projectiles to pass through". Combine with the (mentioned in another answer) highly realistic Expanse method "accept that you will be hit, build your ship and equip your crew so both can probably survive as many hits as possible" and we might have a quite workable concept! $\endgroup$
    – Syndic
    Dec 8, 2023 at 7:20
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    $\begingroup$ The connectors could even be in constant motion when not actively making a ship module dodge a projectile - the enemy would have to aim at a mass of connected nodes that constantly shift, but are at all times mostly empty space. The response would likely be to attempt saturation attacks (just fire more projectiles so they can't all miss / be dodged) $\endgroup$
    – Syndic
    Dec 8, 2023 at 7:22
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    $\begingroup$ @Syndic large fuzzy things are somewhat more vulnerable to crude weapons. Focussing nukes is hard (and what you may have read about casaba howitzers is marketing, to some extent) but building big nukes is straightfoward. Small, dense objects are probably safer to be in, unless you can manage Rama-scale ultratech, at which point you probably have a different flavor of weaponry to deal with. $\endgroup$ Dec 8, 2023 at 10:06
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    $\begingroup$ @Syndic, I believe that was the antagonist's response, yes. It's a good read, I recommend it. $\endgroup$ Dec 8, 2023 at 16:22
  • $\begingroup$ Also, I don't believe that these two would work well combined. Grabbing projectiles would make the craft spin, making it harder to predict what would be in the path of future projectiles. I think that Clark was suggesting that Rama was adapting a meteor shield. Also, Clark's description of how nukes would work in space suggested that they'd have the same concussive force as in an atmosphere. $\endgroup$ Dec 8, 2023 at 16:59
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A LASER or similar "electromagnetic energy beam" [EM] weapon could be useful against kinetic energy weapons in several simultaneous ways.
Using technology that will almost certainly be available in any scenario where people (or other variably sentient beings) are firing kinetic weapons at spaceships. Modes of interaction include

  • Destruction of any electronics or other control logic. "LASERS" in this context will operate in the deep XRAY / Gamma Ray area. Shielding is extremely difficult and beam impact on any part of the projectile will generate secondary emissions. Loss of control makes subsequent trajectory prediction trivial and evasion trivial at ranges liable to be possible. "Bathing" most control systems in a somewhat diffuse Gamma Ray LASER beam is liable to produce "useful" results. Values for "somewhat diffuse" TBD.

  • Melting or vapourisation of some or all of the projectile. Energies of hundreds of kilojoules per kg for liquefaction and under 10 MJ/kg for vapourisation are likely for likely materials of choice (eg see figures for iron and tungsten below). Projectile masses are unknown, but to impart 10 km/s velocity change to a 1kg mass requires 5 MJ of energy. Smaller masses require proportionately less.

  • A significant capability against "dumb" projectiles, or those rendered brainless by control system incapacitation, is course deflection. Back of brain (for want of an envelope) suggests that at 6 km/sec we can achieve in the order of 100 m of diversion, per second of flight per 10 kW.s of impingement. Adjust time and energy and mass to suit desired scenario. Over ranges of 10's or 100's of km, a far less energetic beam can achieve deflections liable to be adequate to deflect dumb or now dead projectiles adequately. Realistic figures could probably be derived, but work calls.

EM beams travel at 300,000 km/second. Reflections off the target (passive, active, choose wavelength) gives returns at 150,000 km/s or about 6 uS/km, and more importantly, vastly faster than any except relativistic hyper velocity matter based weapons. Add to that that the faster a matter based weapon travels the worse it changes course, then a E.M. beam or even particle beam (electrons, various ions, ... ) are going to easily target it.

Energy delivered depends on LASER energy, distance, focus, more.
If desired and interceptor carrying an EM weapon can be launched towards the incoming threat, reducing the distance. More so with time.

Nuclear explosion pumped "One Time" [!!!} LASERS have been suggested, with multiple simultaneous targets being engaged.

Continuous operation high powered nuclear-reactor powered LASER systems have been extensively investigated.

The technology is already on our doorstep (nuclear and other). By the time you get to space battles it will be well available.


Nuclear Pumped LASERS

Wikpedia

NASA 1979. 174 pages

Book Springer Introduction to Nuclear-Pumped Lasers

Wikipedia Project Excalibur

Project Excalibur was a Lawrence Livermore National Laboratory (LLNL) Cold War–era research program to develop an X-ray laser system as a ballistic missile defense (BMD) for the United States. The concept involved packing large numbers of expendable X-ray lasers around a nuclear device, which would orbit in space. During an attack, the device would be detonated, with the X-rays released focused by each laser to destroy multiple incoming target missiles. Because the system would be deployed above the Earth's atmosphere, the X-rays could reach missiles thousands of kilometers away, providing protection over a wide area.

Continuous operation, LASER powered:

Sandia FALCON Reactor-Pumped Laser Description & Program Overview*

1974-1991 Project. 9 pages

FALCON Reactor-Pumped Laser Program The FALCON (Fission Activated Laser CONcept) reactor-pumped laser program at Sandia National Laboratories is examining the feasibility of high-power systems pumped directly by the energy from a nuclear reactor. In this concept we use the highly energetic fission fragments from neutron induced fission to excite a large volume laser medium. This technology has the potential to scale to extremely large optical power outputs in a primarily self-powered device. A laser system of this type could also be relatively compact and capable of long run times without refueling. ...

In summary, FALCON is a nuclear reactor pumped CW laser system that can be scaled to very high powers (100 MW or more) with essentially near-term technologies. Issues that are currently being addressed include the detailed optical system design and the achievable beam quality. Longer term issues involve mission analysis, total system cost, and the difficulties associated with the use of nuclear technology.


Some smallest reactor size figurings


Added:

causative said:

It takes a lot of energy to vaporize iron, around 7.7e6 J/kg, and focusing a laser to a small spot size at distances of thousands of km is extremely challenging. Then, lasers are only 0.01% to 30% efficient, so you're dumping lots of waste heat into your own ship when you fire it. Ask yourself what characteristics of projectile the laser could stop - how fast a projectile, how far away, and how massive.

@causative Indeed. It took us a lot of effort to perfect this defence :-). We lost quite a few target ships in the process.

Total vapourisation of 1kg of iron takes about 8 MW.s of energy, as you say. Tungsten is 'only' about 4.4 MJ/kg. We found we need far less than that per kg to adequately ruin a 1 kg projectile's day. Heat of fusion is "only" around 280 kJ/kg (Tungsten around 193 kJ/kg). Melted blobs of iron (or Tungsten) incoming are not desirable, but far better than the initial 'warhead'. However, ablation and vapourisation are not the primary mechanisms. Our beam is in the deep Xray/Gamma ray region. It takes very few photons to wreak serious havoc on any control systems yet encountered. Individual photons penetrate "very nicely".

But a major factor is path diversion. Having destroyed any guidance system is a bonus. At 6 km/sec we can achieve in the order of 100 m of diversion, per second of flight per 10 kW.s of impingement. Adjust time and energy and mass to suit desired scenario.

[[Figures are back of envelope with lots of assumptions and may be very roughly correct. E&OE. Corrections welcome :-) ].


Note to would be editors: spellings are NZ antipodean Kings English where apposite. Alteration for alterations sake would not be considered cricket :-).

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    $\begingroup$ ...because the OP asked about "relatively current technology", and much as I love scifi lasers, the current state of the art is kinda rubbish and the near-term prospects aren't great. Nuke-pumped lasers are great for mission-killing missiles and RVs, but dumb KKVs are a much tougher nut to crack. Reactor pumped lasers require big reactors to drive them, which require massive heatsinks, and neither play well with near-term spacecraft design. $\endgroup$ Dec 8, 2023 at 12:03
  • $\begingroup$ @StarfishPrime Comment got lost :-(. Briefly. See added 9 page Sandia link (missed in error) and reactor size figurings (very rough). A useful reactor would be launchable now with our available and soon to be available launchers. Viability depends on threat level and craft size and ... . Despite the challenges I suspect that it and other LASER or EM based weapons are a good competitor for most effective defence system- AND not a bad offensive one either. $\endgroup$ Dec 8, 2023 at 12:30
  • $\begingroup$ @StarfishPrime Depending on the method of propulsion the spaceship uses. I know you are familiar with the orion battleship, in NIvins Footfall the Orion variant "Michael" utilised its atomic pulse drive to pump it's X-Ray lasers. Granted, whoknows how much was done on paper vs real world testing that was actually carried out in and around Excalibur. $\endgroup$
    – Gillgamesh
    Dec 8, 2023 at 13:29
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    $\begingroup$ It takes a lot of energy to vaporize iron, around 7.7e6 J/kg, and focusing a laser to a small spot size at distances of thousands of km is exremely challenging. Then, lasers are only 0.01% to 30% efficient, so you're dumping lots of waste heat into your own ship when you fire it. Ask yourself what characteristics of projectile the laser could stop - how fast a projectile, how far away, and how massive. Then ask yourself, why not just dodge it? For a single shot nuke pumped laser, even if it would work (probably wouldn't), if you had that mass in armor instead you could stop many projectiles. $\endgroup$
    – causative
    Dec 8, 2023 at 14:26
  • $\begingroup$ @causative Useful points. See addition at end. $\endgroup$ Dec 9, 2023 at 6:03
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In Neal Stephenson's Anathem, the spaceship is protected by a huge net of gravel which can be positioned.

Certainly not easy on your delta-v expenditure, but at least somewhat effective.

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Hmm... how about something more exotic? A big chunky ship is a prime target, and as the other answers point out, super-fast space junk just tends to vaporize whatever it comes in contact with so there's little defense against it. If our big ship is hit, it's toast.

So how about we make it really hard to hit? My idea: have a "hive" ship. Lots of tiny little ships that come together to form one big "hive" for travelling/whatever, but when danger is close, they split up into a swarm of miniships and disperse. Your hypervelocity projectiles might get lucky enough to hit one or two of them, but that won't affect the whole swarm much.

Attacking is done similarly via many remotely- or AI-controlled drones. Again, you can hit one or two with a projectile, but you can't take out a swarm that way.

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It's been mentioned already, but the Expanse does a lot right (considering that it's not super far in the future and tech is pretty close to what we have). Look at modern day helicopters like the Apache helicopter, or the tactics used by Hind pilots in the Soviet-Afghan war, as well as jet fighter dogfighting. Flares/chaff to actively hit any large explosive projectiles, but against Flak, you have to accept that it's ripping through you.

Enormous amounts of redundancy, first of all. The pilots themselves have their own oxygen systems and wear body armor, allowing them to keep going. In the world of space combat, this could be doing what the Expanse does, having depressurized ships and crew in space suits.

RCS thrusters are extremely important, as is managing huge range. It might not be as cool as in the movies, but combat would not take place in spitting distance. If the enemy has high-velocity projectiles and decent targeting systems, these fights would be happening from tens, if not hundreds of ship lengths away, so that one could maneuver around streams of fire. In the Expanse, roughly 10 kilometres was referred to as close quarters at one point, because it meant that they were close enough that the enemy couldn't outmaneuver direct kinetic shots from a railgun (as opposed to missiles/torpedoes which lock on and track the enemy).

Speaking of avoiding shots, electronic warfare would be hugely important, as scattering enemy targeting systems prevents them from firing on you. The U.S. Navy and Marine Corps fly the "E/A-18G Growler", an F/A-18E with it's gun replaced with a massive electronic warfare suite. They fly alongside F/A-18s in sorties to protect them, because simply having someone around to mess with enemy radar and SAM systems makes an enormous difference in protecting your planes. Similarly, stealth ships or simply ships that are hard to lock onto would have an advantage.

Because maneuvering would be important, you presumably want your ships to not be super heavy. Like real-world Apaches, this would mean armour plating on the engine and avionics, but nothing else.

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