The Hugoniot Elastic Limit and impact impedance interface countermeasures are all set! Part 1
The research on developing a long-range weapon for Kou had been ongoing for a long time. However, after Kou’s weaknesses were exposed, the importance of this research became much higher making it a priority.
The time goes back to when Nyanta was reassigned to rear support duty.
Aki had just revealed the concept of the new rifle. Nyanta was busy checking the data.
「It consumes a little too much energy to generate the plasma, though.」
「It can still be treated as a ballistic weapon. It only consumes energy when igniting the plasma after all. It also uses energy to power the built-in magnetic field generator for the projectile, but that’s negligible nya.」
Nyanta, after looking at the blueprint, immediately grasped the concept. It was an electrothermal gun, accelerating projectiles rapidly with plasma combustion speed. While the initial velocity depended on the explosive reaction of the plasma, and to create such plasma great quantities of energy were necessary, it was still trivial compared to the energy necessary to power a railgun.
「Yes, that’s right. I had to get some help from Asia, Astraea, and Arges to finish it.」
Nyanta and Aki’s current challenge was to create a weapon that could counter the mid-range shooting abilities of the Aberration Silhouettes. The key issue was how to improve Kou’s mid- to long-range combat performance, a range at which Kou was particularly weak.
Modifying existing firearms and refining them to suit Kou’s needs was difficult, the only effective weapons they currently had were railguns. However, Kou wasn’t fond of using railguns due to their elevated power needs, and he had even asked them not to focus on railguns if possible. In reality, both Aki and Nyanta disliked railguns so they were happy to comply.
「 Kou always charges in for close combat. So, I thought we should take Kou’s proficiency in close range to our advantage and force the enemy to come close instead. For that, we need Kou to become so dangerous at long range that the enemy won’t have other options than to close in.」
「You’ve gone full yandere again, nya. When you become like that you always end with some extreme Ideas, Aki.」
Aki had a habit of occasionally coming up with absurd weapons when she focused too much. Maybe it was just her gunsmith’s nature.
「Who are you calling yandere? With Unit 5’s mobility, keeping a distance would be easy. With this weapon, if Kou is able to keep his distance the enemy is doomed. And even if the enemy manages to come closer, Kou can simply slice them down.」
「So they are dead whatever option they choose. That’s terrifying nya!」
「They can either run away and die, or close in and risk close combat. We’ll leave it to the enemy to choose their preferred death. And this is the weapon that will force them to choose: the D-Rifle.」
「Where does the ‘D’ come from?」
「It could stand for destruction, destroy, or even detonation. Whatever you prefer. I thought about calling it the ‘Double D Rifle,’ but that sounded a bit too over-the-top, so I decided to simplify it.」
「Kou would probably love that kind of name though… Ahem, never mind nya. Anyway, what’s this projectile? It’s so brutal that it’s scary.」
Nyanta switched into serious mode. The design of the ammunition was completely different from anything that had come before.
「The key to the D-Rifle is its liquid metal rounds. Thanks to the feedback we got from the technology used on the Anti-Fortress Rifle, we’ve finally understood how to make them. The ammunition used by the D-rifle are MAHEM—Magneto Hydrodynamic Explosive Munitions—made from metallic hydrogen.」
「So, the shape of the liquid metal is altered by the magnetic field generator?」
The mechanism behind the projectile launched by an electrothermal gun involved using the electromagnetic field created by a magnetic generator to reshape the liquid metal into a spear-like form, which then pierces the target.
「The current armor materials consist of an alloy of several layers of nanomaterials with a plasma catalyst sandwiched between layers. These include 2.5-dimensional stacked nanoceramic plates made from borophene and graphene. The muscle fibers of the armor are made from metallic hydrogen as a plasma medium, and the material includes nanoceramics and high-density metallic glass. In short, it’s like having multiple layers of armor. With the armor further enhanced by Whis, a typical penetrator won’t be able to pierce through to the internal systems.」
「And this setup allows the projectile to penetrate all that at once, by using two different types of liquid metal penetrators?」
「Exactly. The density of metallic hydrogen is only twelve times that of liquid hydrogen. That’s why we use an amalgam—a mixture of two types of metallic hydrogen. This amalgam uses powders of tantalum and niobium, metals that are tough and resistant to destruction, and the core of the projectile is made from metallic hydrogen mixed with tungsten powder.」
An amalgam generally refers to an alloy that includes mercury, but it can also simply mean a mixture. By the 21st century, it had been discovered that metallic hydrogen shared similar properties to an amalgam, and Aki decided to exploit this feature.
「When the armor and the penetrator collide, both undergo plastic flow, essentially turning into pseudo-liquid metals, surpassing the Hugoniot Elastic Limit and piercing through the armor. But if the projectile is already a liquid metal spear—or rather, a lance—it can efficiently turn only the armor into a pseudo-liquid state and penetrate through it.」
「Exactly. At this point, the material of the metal is practically negligible. The real issue is the density of the material used in the projectile. Metallic hydrogen alone isn’t enough.」
「Metallic hydrogen by itself lacks mass. So, you mix in tantalum and tungsten to increase the mass, right?」
「Yes, and to add to that, the metallic hydrogen causes a detonation. This detonation blows through multilayered nanoceramics with a high Hugoniot Elastic Limit, even surpassing the plasma layer. Since the penetrator inevitably causes a detonation, it won’t just punch straight through lightly armored units like the APFSDS rounds did in the Earth era. It ensures that the target will be critically damaged rather than simply pierced.」
「The Hugoniot Elastic Limit of metal armor is at most around 4 GPa. Meanwhile, the Hugoniot Elastic Limit of the nanoceramic armor exceeds 20 GPa. The projectile takes advantage of the liquid metal properties combined with detonation and combustion to blast through it. Afterward, the plasma heat is dealt with by the second type of metallic hydrogen amalgam penetrator, which has a different mass. And the power is enhanced by the addition of kinetic energy…」
「It’s a magneto-hydrodynamic explosive munition. Both the Hugoniot Elastic Limit and the impact impedance interface countermeasures are perfectly accounted for!」
「Even a grazing hit will trigger a detonation, chipping away at the armor.」
Kinetic energy rounds, like the APFSDS, are known for making sloped armor—a feature characteristic of tanks before the Cold War—obsolete by turning the armor surface into a pseudo-liquid. To counter them is necessary to not just increase the thickness of the armor, but also to combine materials with a high Hugoniot Elastic Limit, such as ceramics, and metal armor with impact impedance interfaces to counteract penetrators. Shallow angles can help as well. Sloped armor may seem ineffective against kinetic energy rounds, but that’s not entirely true, there are cases where an APFSDS can ricochet if it hits at an extremely shallow angle, nearly parallel to the armor’s surface. Additionally, composite armor that combines materials with different slopes and hardnesses can deflect the penetrator along the shock impedance interface of the dissimilar materials, causing the penetrator to lose its trajectory and wear out the core. This is mainly employed in modular armor systems, though the drawback is that the armor must be replaced after sustaining a hit.
「In theory, using the electrothermal gun would give a muzzle velocity of 4,000 m/s—roughly Mach 12.」
「So you’re mixing metallic hydrogen with liquid oxygen as a kind of monopropellant liquid propellant? Not quite as strong as electronically excited explosives, but we can’t use that technology lightly.」
「Correct. The damage the Epona received gave Kou a strong sense of danger. But with this technology, we don’t have to fear the enemy getting their hands on it.」
Electronically excited explosives had once been used as a countermeasure against the Ate, but their destructive power was excessive against Silhouette-sized enemies. It was akin to launching a miniature nuclear bomb, and even the Epona was nearly destroyed by the recoil. Given the risks of such weapons falling into enemy hands if the unit carrying them was captured, it was necessary to limit their use to last-resort situations.
The D-Rifle may not have the power to destroy a Fortress Area’s shelter, but it was specifically designed to efficiently take down Silhouettes and tanks.
「The secret lies in adjusting the burn rates of the two types of metallic hydrogen amalgam used in the projectile.」
「And that’s where you come in, Nee-san.」
「Wait, you want me to handle the propellant adjustments for this projectile?」
「Exactly! Who else but you could handle the creation of this ammunition?」
「Astraea has already laid the groundwork, so it’s easier, but… this is still a lot of work!」
「Yes! Please, I’m counting on you!」
Aki was so excited—her twitching ears said it all—that she didn’t notice that she had just blatantly pushed the more tedious work onto her sister.
Nyanta could only smile wryly at her sister’s unreasonable request. But despite the challenge, it was work worth doing. She prided herself as a Wildcat, and creating new types of ammunition was part of her responsibility.
「Plus, we still need to fine-tune it for Unit 5. What we have now is only a prototype.」
「You’ve already made one?」
「Of course. I’m a gunsmith after all. This is my main job.」
Aki, too, took her craft seriously.
Thus began Nyanta’s trial-and-error process. First, she had to balance the weight distribution and recoil control of the D-Rifle. Next came the fine-tuning of the projectile itself.
After test-firing the new projectile with the Epona, Aki murmured in disbelief.
「Nee-san, the penetration power has increased by twenty percent. What did you do?」
「I just tweaked the dual-layer metallic hydrogen amalgam. The outer layer was optimized for armor-piercing, and I added more rhenium to the core penetrator to destroy the internal structure, nya.」
It seemed that Nyanta had adjusted the second layer of liquid metal to focus more on penetration.
「You used a really rare metal for it… So, the core penetrator uses a denser material for extra impact.」
Even on Earth, rhenium was considered a true rare metal. The annual production of tungsten at the start of the 21st Century was about 90,000 tons, and tantalum production was around 2,000 tons, but rhenium production was only about 50 tons per year—placing it among the rarest of the rare metals. On a super-Earth planet like Asia, both the content and production of rhenium were higher, but due to the Stone’s attack on the mines, its value had skyrocketed.
「This is, so to speak, a special munition meant for decisive battles. I’ve also prepared projectiles using niobium and tungsten for regular use, but their power is about ninety percent of the ones you just used. I’m also prototyping a more simpler version with only a single layer of metallic hydrogen to use against less armored enemies.」
Niobium shares similar properties with tantalum, being a ductile metal with a density close to that of iron. When designing this kind of projectile, the combination of metals in the amalgam is more critical than the caliber.
「The standard rounds you’ve designed are already more powerful than what I made! That’s more than enough!」
「Well, you know, it’s just a matter of Wildcat pride, I suppose.」
Nyanta had the mass production of the D-Rifle in mind. Affordable projectiles were crucial for this, as ammunition that couldn’t be replenished was useless on the battlefield.
「Now I just need to adjust it for Unit 5, nya.」
Nyanta stretched while saying this. It seemed she hadn’t gotten much rest. After all, she had managed to create projectiles with about ten percent more power than what Aki had originally expected.
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