What to shoot?

We’ve explored how we launch projectiles at the target (Space Guns), now we will explore what we launch at the target.

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Point defense railguns firing tiny 10 gram bullets against a missile salvo.

The simplest projectile is a solid block of mass with a burning pyrotechnic tracer on it. But even a block of mass has several complexities.

The material of a bullet can also be varied to cause for differing effects upon hitting. However, for railguns and coilguns, launching the armature itself is more cost effective than using the armature as a sabot for another projectile. This means railgun and coilgun rounds are restricted to highly conductive and highly magnetic materials respectively.

The bullets need to be cylindrical because they are launched from a tube, but they need not be aerodynamic in space. This means any sort of shape is viable in space, not just a bullet or thin penetrator. Fat blocky bullets are a viable shape, as are launching thin plates, flat side to target.

Given that aerodynamics is not a concern, what is the ideal shape of a bullet in space?

sabot_separating
A fin stabilized Kinetic Energy Penetrator.

 

The answer is rather complicated. Certain relations are obvious though. For instance, a thinner shape applies a greater amount of pressure, as the energy is concentrated into a smaller area, so it seems like thin penetrators would be ideal.

However, there are two issues with that.

One is that Whipple Shields shatter thinner projectiles easier. Whipple Shields are often judged primarily by their critical diameter. This is the maximum diameter of projectile that they can be hit with and still successfully shatter or vaporize the projectile so that it causes no major damage. Obviously, material properties and impact velocity are both important, but projectile diameter is the main factor. Thus, thin penetrators are often not really worth the extra damage since Whipple Shields are ubiquitous.

whipple-shield
A single-stage and a multi-stage Whipple Shield after impact.

Conversely, high velocity projectiles can sometimes be too effective. Indeed, given a large coilgun, shooting extremely high mass, high velocity rounds, the bullets can often blast straight through the initial Whipple Shields, straight through the main bulkhead, through several filled propellant tanks, out the external bulkhead and Whipple Shield, and finally off into space again.

Because only the crew compartments are pressurized, a spacecraft can suffer complete penetration and still keep trucking at 100% effectiveness. Spacecrafts can even get blasted in half and both halves can still remain reasonably effective (assuming each half still has a functioning crew and powerplant). Or into thirds, or fourths, and so on.

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Cut in half but still threatening. Probably not for much longer though.

This is one reason why heavy redundancy in spacecrafts is necessary. A single lucky shot can immediately disable a ship if there is only one crew compartment.

On the other hand, one of my alpha testers went a different route. Rather than a single large capital ship with multiple redundancies, he preferred tons of tiny capital ships with zero redundancy. Either solution works, and has different pros and cons. More on this in a later post.

But back to projectiles being too effective. A projectile works best if it can penetrate the outer Whipple Shield and bulkhead, but is stopped there. That way, it can ricochet around, or if it shatters into plasma, it can inflict the most damage on the internals. In a sense, a larger area of effect is more important than simply raw damage.

Not only that, a projectile which passes straight through a ship fails to transfer much of its momentum to that ship, while a projectile that hits inelastically transfers all of its momentum. With very massive and very fast projectiles, inelastically hitting can cause tremendous torques on the impacted ship. Ships that spin out lose their carefully aligned targeting, and require precious seconds to reorient, which can mean life or death in a battle.

Or if the ship is particularly small, fast spins will splatter the crew against the inside of their crew compartments. This is actually one reason why I try to keep the crew modules near the center of ship mass, to reduce the torque on the crew in such a scenario.

hypervelocity-impact
Of course, if you don’t have enough power behind your bullets, you won’t penetrate the main bulkheads at all.

If your projectile is too powerful, then the obvious solution is to fire large, flat, plate shaped projectiles rather than thin penetrators. This reduces the pressure, and the damage area is increased. However, this can be tricky since large bores will mess with the performance characteristics of all projectile weapons.

Another solution is to split the projectile into smaller, less massive pieces right before hitting the target. Flak bursts were developed as anti-aircraft warfare, and they remain an effective way to distribute damage over a larger area of effect. A small explosive detonates the payload into a cylinder of fragments, and the detonation speed can be determined accurately (using the Gurney Equations). This allows you to have very fine grained control as to the size of this “sparse projectile”, and you can detonate it at different proximities to yield differently sized clouds of fragments.

flak hit.png
A flak barrage slices through a Laser Frigate.

Other payloads possible in Children of a Dead Earth include explosive payloads and nuclear payloads. Nuclear tends to be a very powerful but expensive option, though due to the lack of atmosphere, their damage is incredible within a few meters, and then it falls of painfully fast. Explosives are similarly restricted to very small areas of effect but are much weaker, though they are fantastically cheap.

Should your projectile have thrusters? You can put thrusters on the projectiles you launch, and this can greatly increase their accuracy, however, the mass of the rocket engine and propellant is very costly. From what I found, barrages of thousands of small “dumb” projectiles tend to win out against tens of large “smart” projectiles, though I’d be interested if someone managed to optimize them to be competitive.

Often, a laser battery can effectively point defense tens of smart rounds, but against thousands of incoming bullets, no laser battery can keep up. Generally, against point defense, either you saturate them with a storm of tiny bullets, or you launch full blown missiles with heavy armor and large delta-v stores to get through.

point defense 2.png
Railgun point defense against a salvo of armored missiles. The missiles and their exhaust plumes are too small and dark to see, so they are each highlighted by the green circle.

There is another interesting aspect of projectiles that is often overlooked. Muzzle velocity is often optimized to be as high as possible. It increases the range and the impact damage. And even if it is too damaging, increasing the area reduces the damage without sacrificing range. It seems velocity should always be maximized.

Yet the equations for damage on Whipple Shields and Bulkheads are very nonlinear, and they have very different damage responses between hypervelocity and hypovelocity impacts. Indeed, Whipple Shields lose effectiveness for low velocity impacts, as the projectiles suffer little to no break up at low velocities.

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Whipple Shields perform the worst between the Ballistic Range and the Shatter Range, which is approximately 3 km/s.

This is one particular case where conventional guns and their low muzzle velocity are actually desired. When the enemy is packing multiple and/or stuffed Whipple Shields optimized against high velocity railguns and coilguns, low velocity conventional guns tend to be the trump card. This is one case of many where drones, which usually carry conventional guns, tend to make short work of the enemy.

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39 thoughts on “What to shoot?

  1. Whipple shields can be defeated using multi-stage projectiles. The ultimate form of a multi-stage projectile is a submunition cloud that expands lengthwise near the target.

    What sorts of g-forces will electronics on guided munitions be able to withstand?

    You mentioned smart projectiles losing out to dumb projectiles en masse. Does the target accelerate? What about combination projectiles (missile busses) with a smart booster tracking the target then releasing a saturation attack of dumb masses in the terminal stage? Can projectiles be armored like mini-spaceships?

    Are you going to model missiles able to perform random high-g lateral accelerations near the target versus laser defenses with a finite target acquisition time and tracking rate?

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    1. There may be fertile ground in modern segmented-rod penetrators for tanks. Initially they were designed to maximize armor penetration, but soon after manufacturers started experimenting with getting conical and lateral post-pen frag from passive kinetic rod designs.

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    2. Laser targeting and tracking time is modeled, and missiles are modeled with a physics engine, so any and all dodging is possible. Children of a Dead Earth combat is entirely a physics engine, it’s not approximated with probabilities or anything like that. And yes, targets do accelerate to dodge. Everything dodges if possible, and often at top acceleration in any direction.

      You can also launch any sort of ship/payload that you could build in ship design, so long as it survives the acceleration. So you can launch missiles which launch missiles which launch missiles, etc. And they can all be armored as much as you like.

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      1. You can make multiple staged weapons in Children of a Dead Earth. Technically, you can attempt to launch any sort of ship you can build in ship design as a payload, so long as the payload survives the acceleration.

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        1. …Mind if I ask the details on this?
          The drones and missiles seemed to be deployed from internal bays. Would making a staged weapon require an internal bay for the second stage?

          Also would like to repeat the question on drone recovery.

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          1. Hm, right now, yes, you can simply make a launcher with negligible mass with a single ammunition round (or multiple) to make submunitions. I suppose it’s overloading the original use case, but it does work.

            Sorry about missing the drone recovery question. So many questions, it’s hard to keep up with them!
            Currently, drones don’t have any way to be recovered back into the carrier since in campaign, the use case doesn’t really come up. They can be refueled/rearmed after combat though.

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            1. Does that mean drones can be refueled and rearmed during the mission, or between missions?
              If not, (not to sound demanding, but..) would it take any significant amount of time and effort to program in the ability to recover drones mid-mission even though the use case doesn’t currently come up?
              Also. does the campaign have missions with multiple separate enemy forces? Your comment seems to imply not-if there was, the player might be able to (or have to) engage them one at a time, with enough time in-between to intercept and recover drones.

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                1. Do you see any use in actually recovering the drones, as a way to shuttle them from one battle to another, or do you feel refueling the drones is enough for that?

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                    1. So how are drones refueled and rearmed?
                      Pipes? Waldos or crew EVAing with hoses and crates of ammo?

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  2. Hardened electronics exist today… it is no biggie. And getting through whipple shields is pretty much the flak solution mentioned. Proximity-fusing on smart rounds could be messed up with simple ECM or chaff. Plenty of scope for design.

    I like that the multiple mini-capship approach works too. Not quite space fighters but more like space torpedo boats. Cinematic yet realistic.

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  3. Would be chemical explosives that important compared to the amount of kinetic energy? Or is it simply to shatter the projectile and avoid kinetic energy to be wasted as it go through the other side of the target? (Or, inversely, for fragmenting the projectile so a larger area is impacted)

    Also, can a projectile use the impact itself to trigger a nuclear reaction? It seems this could be made cheaper than a self-triggered nuke.

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    1. Chemical explosives aren’t that effective compared to kinetic energy, so the explosion is generally just to shatter the projectile into a larger area.

      Using the impact to trigger a nuclear reaction is a bad idea, because nuclear implosion needs to be very carefully controlled. If the explosive lens is triggered nanoseconds too early or too late, the nuke will not achieve prompt criticality.

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      1. But what about a gun-type fission bomb like “Little Boy”?
        Yeah, it’s dangerous as hell as a fairly simple accident can create supercriticality.
        In Little Boy the two uranium masses moved at “just” 300m/s, so if we have a projectile moving at some km/s, and if upon flattening it’s front on the enemy and crumbling-as-designed along the length (or transmitting the energy in the non-crumbling gun barrel into the enemy’s hull), the rear will smash towards the front at high speeds and cause BOOM.

        It might be a problem to make the front part survive the impact for long enough in good enough shape to get to the nuclear explosion, but then it should matter little if the uranium shatters as long as it’s dense enough in the right place.

        Alternatively, neutron reflectors and moderators may also play a role in the ‘smash together’ action.

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        1. Gun-type fission bombs are actually considered obsolete under modern nuclear weapon technology because their efficiency is so low compared to all other nuclear weapon types.

          Even if it was efficient, hitting sloped armor might fail to trigger criticality. The slope can cause the weapon to ricochet rather than collapse the two pieces correctly. Also, a Whipple Shield might shatter the initial piece, widening it out, reducing the density low enough to prevent criticality. Lots of ways for the weapon to fail when hit.

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  4. Another question. Will there be a way to control formations within a fleet? So, for example, you could organize a missile and drone fleet so the missiles go first and soak point defense while the drones go to work.

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  5. Spherical nuclear explosions have short range in space, but directional designs like Casaba-Howitzer would improve range, as well as putting more energy on the target, so smaller nukes could be used.
    Whether it would be more effective on missiles or not, it seems like a legitimate gun payload.

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    1. Yeah, Casaba-Howitzer was one particular tech I was very curious about. However, like the Orion drive, key components of it are still classified, and I couldn’t get reasonable numbers on the weapon’s performance.

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  6. So, I’d tend to expect that the mass required for gun power plants would be substantially larger than the mass of the guns and ammo themselves. In that case, would it make sense for a vessel to have different guns for high and low mass projectiles so they can use high mass rounds against highly armored targets and low mass rounds against lightly armored targets?

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  7. Just had another question. Wouldn’t flak not really penetrate Whipple shielding, since it’s essentially multiple smaller projectiles on impact?

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    1. Whipple shields are a lot more complex than ‘all or nothing’, since small projectiles at certain speeds can actually achieve penetration, while in some cases, larger projectiles might not. As a whole, though, Whipple shields perform better against flak than larger bullets, but never 100% in every case.

      It’s a tactical choice. Your flak is at reduced effectiveness until you can take out the enemy Whipple Shields, but on the flip side, the larger area of flak will damage more of the Whipple Shield. Or maybe you’ll want to go in with nukes first to flash away the shield, and so on.

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  8. You mention that you can fire anything design-able – would it be feasible to re-purpose ‘extensible radiators’ as fold out diameter-aids for your projectile? Because the idea of having a projectile umbrella outwards post-barrel into a large contact plate or series of ‘blades’ is greatly amusing, though I don’t envy the designer who has to make the machinery survive a launch intact.

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    1. While it’s possible and feasible in game, it’s not exactly going to net you much. Having a greater projectile diameter is beneficial only if the projectile has significant mass across that diameter, otherwise the Whipple Shields will simply shock the weaker parts of the projectile into plasma immediately. For instance, if you attach giant sails to your bullets, that’s not exactly going to make them more effective at all.

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  9. Somewhat related to the topic, I am curious as to why the crew compartments would remain pressurized during planned conflicts?

    Would it not be safer to depressurize the crew compartment and have the crew in space suits during combat? After combat has concluded, you could patch any holes in your compartment(s) and then re-pressurize to return to “normal” activities.

    Modern naval crews during a general quarters situation don anti-flash gear ( https://en.wikipedia.org/wiki/Anti-flash_gear ) as a pre-cautionary measure. I imagine something similar would be commonplace for the hypothetical space navy.

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    1. It is possible to do that, however, then the main problem becomes you need separately armored air tanks which are now the target of priority instead of the crew module. Essentially, it just moves the problem to a different part of the ship and costs more mass. From what I’ve found, simply having several separately pressurized crew compartments scattered about the ship is much more effective.

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        1. Pressurization inside the crew compartment is modeled to a small degree. The mass of the air is modeled (and needs to cover a potentially multi-month journey), and the mass flow rate of the air through the compartment is accounted for. Furthermore, the walls of the crew compartment and additional air storage tanks need to be able to contain the atmosphere without rupturing upon impact of weapons fire (to some degree) and engine acceleration.

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    1. Using engines as a weapon doesn’t really become viable until you have extremely powerful engines (far future torchship engines). Otherwise, most modern rockets like you see in game only are damaging at very close ranges (meters away).

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