Thunder Spear

The Thunder Spear (雷槍) is a technology specifically developed to fight against the Armored Titan. Referred to as "iron rods" by Eren Yeager, their official name is Thunder Spears, due to their spear-like shape and the thunderous sound they make when used.

Mechanics
The Thunder Spears are based on research records that had been preserved in secret by some inside the Military Police Brigade, despite their orders to wipe out any sign of technological innovation. This technology was adapted to design a new special weapon for use against the Armored Titan. It works in a similar fashion as the anti-personnel vertical maneuvering equipment's guns and hooks, but with a strong spear loaded with explosives designed to shatter armor.

These weapons work as a two-stage rocket: an initial charge launches the projectile by pressing a special trigger in the handle of the device. The projectile, connected by a string to the main engine, is launched with enough power to penetrate hardened Titan skin. When that string is cut, a safety pin is pulled and the second charge is activated. This causes a tremendous explosion which makes the projectile penetrate even more into the target. The time taken for the second charge to fuse and cause explosion is near instant, but nevertheless enough for users of the Vertical maneuvering equipment to move away safely.

Usually one was carried on each arm, but up to four can be carried with proper modification to the vertical maneuver gear, making eight in total.

Usage and limitations
The Thunder Spears are a weapon which enables a human team to efficiently launch a surprise attack against the Armored Titan. However, this weapon plays the role of a double-edged sword, as it is a danger to its user as well as the Titan. So much so, that it is only useful in a specific type of situation where after use, immediate retreat to close structures via vertical equipment is possible. Without a proper surrounding, the user is easily liable to get caught in the explosion. While it was originally designed to kill the Armored Titan, it's extremely damaging against other types of Titans. A single Thunder Spear is powerful enough to sever the Female Titan's arm.

Later, they were used as anti-Titan and anti-fortification weapons against the War Hammer Titan, and the Anti-Titan artillery that the Marley military acquired after their war with the Mid-East Allied Forces. The Thunder Spears gain a further use in restraining those with the Power of the Titans. This is done by placing a primed spear inside the captive while wiring the fuse around their neck.

Training
In preparation for defeating the Armored Titan, Squad Levi did not have much time to train, but Hange carefully placed their troops so that they would be in an optimal position to attack. It is a maneuver that requires three successive attacks: the first to blind the target, the second to break the protective shell, and the third one to kill.

Return to Shiganshina arc
Hange Zoë has a team of engineers develop the Thunder Spears, using technology uncovered following the coup d'état. Designed as a weapon to combat the Armored Titan, Hange displays the performance of the spear to Survey Corps soldiers.

In the Battle of Shiganshina District, the members of Fourth Squad and the Special Operations Squad surround the Armored Titan in its battle with the Attack Titan, barraging it with explosives and blowing through its armor to detonate its nape. The attack destroys Reiner Braun's head from the jaw upwards, though he survives by transferring his consciousness.

Later in the battle, part of the Special Operations Squad combat the Colossus Titan. While Jean Kirstein, Sasha Blouse, and Connie Springer attempt to divert its attention, Mikasa Ackerman uses Thunder Spears to attempt to hit its nape from behind. However, the transparency of the plan leads to the attack being countered with a burst of steam. The shrapnel hits Mikasa, creating a shallow wound.

This part of Squad Levi later uses their four remaining Thunder Spears to combat Reiner, following his return to the conflict. Mikasa uses the first spear to break the Armored Titan's leg. Sasha and Connie try to use their spears to break Reiner's jaw, though one spear misses. Hange returns to the battle, using their spear to blow the second hinge of the Titan's jaw, allowing Mikasa to fire the final spear through its mouth, blowing Reiner directly out of his Titan's nape.

Marley arc
Four years later in the Raid on Liberio, the Survey Corps employ Thunder Spears to counter Marleyan personnel, as part of their urban warfare tactic. The Thunder Spears are used in conjunction with the corps modified anti-personnel vertical maneuvering equipment, to destroy anti-Titan artillery set up as defense in the internment zone. Mikasa assists Eren Yeager against the War Hammer Titan, firing numerous Thunder Spears at the Titan at once, to sufficiently destroy its nape. When this is not enough, due to the mechanics of the War Hammer Titan, Mikasa distracts the Titan by engaging it using her Thunder Spears. She later tries to destroy its pilot's crystal, though the explosion proves unsuccessful.

The spears see continued usage against the Warriors throughout the battle, being used to defend Eren from the Jaw Titan, and later to assist in the destruction of the Cart Titan's Panzer Unit.

War for Paradis arc
After the Raid on Liberio, the Survey Corps would bring a number of Thunder Spears with them to the Titan Forest where Zeke Yeager is being detained. Later, Levi uses several of these to defeat the Beast Titan, using them to destroy the Titan's hardened nape. After dragging Zeke out of the forest, Levi places the tip of another spear inside Zeke's stomach. By wrapping the spear's fuse around his neck, Zeke is stopped from being able to move, lest his insides detonate. Despite this, Zeke goes on to detonate the Thunder Spear, catching Levi in the resulting blast.

Later, as the Wall Titans arrive to Odiha, Hange buys time so that the flying boat can be repaired. Using two Thunder Spears, Hange is able to strike down two of the Wall Titans.

Trivia

 * The Thunder Spears were loosely inspired by rocket fireworks that Hajime Isayama would play with when he was a child.

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