Shakunetsu Hadoken

The Shakunetsu Hadoken (灼熱波動拳) is the name of a special type of Hadoken introduced in the Street Fighter II series. It is used by Ryu (as well as his evil side), Ken and Akuma.

Origin
The original inspiration for the Shakunetsu Hadouken was a rarely appearing easter egg in Street Fighter II where fireballs would randomly be red rather than their usual blue color. This bewildered many players, who speculated on what causes red hadoukens to appear and if they were more powerful than regular blue ones. Capcom eventually made the red fireball official by adding in the Shakunetsu variation to Ryu's moveset.

Description
Executed by performing a half-circle forward or backward motion, depending on the character, and pressing punch, the Shakunetsu Hadoken is imbued with thermal energy that sets it on fire.

Ryu
Ryu's version is considerably less damaging than Akuma's, but Ryu does not charge when using it, thus making his version faster to pull off than Akuma's (about the speed of a regular Hadoken charge). It hits 1 to 3 times, and in certain games such as the Alpha series, it can knock a foe down at close range.

In Street Fighter III and upto Street Fighter IV, his version of the move does not appear directly, instead functioning as his two-hit EX Hadoken.

Akuma
Akuma glows red and charges slightly before releasing it. The projectile hits 1-3 times, depending on the strength of the punch button; this also impacts recovery, with light punch recovering the fastest, and heavy punch hitting the most. Akuma's EX Special version hits three times, recovers as fast as the light punch version, and does some more damage. If executed with medium or heavy punch, it will knock down an opponent.

Akuma does not have this move in the Marvel vs. Capcom series. In the Street Fighter IV series, the alternate form of Akuma known as Oni has the Gorai Hadoken, a projectile imbued with electricity instead of thermal energy; its gameplay purpose remains largely the same as Akuma's Shakunetsu Hadoken.

Evil Ryu
In the Alpha series, Evil Ryu's version is similar to Ryu's: he does not charge when using it, and only hits 1 time. In Super Street Fighter IV, it retains its "charge" property, and functions largely the same as Akuma's, including the EX Special version.

Ken
In Street Fighter V, Ryu has no access to the Shakunetsu Hadoken. Instead, more fittingly, it is found in Ken's arsenal. Activating his V-Trigger, Heat Rush changes all of his regular Hadokens into the fiery Shakunetsu Hadoken. As in the Alpha series, the move causes a knockdown irrespective of the distance it hits at.

Tactics
Prior to Street Fighter IV, Akuma and Evil Ryu's versions could be nullified by another single-hit projectile, limiting the move's usefulness to 'chipping' an opponent on wake-up or forcing them to parry it (thrice) in the same scenario (Street Fighter III only). However, in the Street Fighter IV series, every hit of a projectile counts, allowing the heavy punch version to beat out regular and EX projectiles and still hit. Though this gives Akuma an option to win 'projectile wars' without using any meter, it requires him to preempt the opponent's moves, and is more often used for its chip damage.

In Street Fighter V, Ken's version causes a knockdown on hit without being needed to 'charged' like Akuma's; thus combining the best properties of Ryu's and Akuma's versions. This is obviously offset by the inherit time limit on his V-Trigger. The knockdown effect of this move helps immensely in maintaining Ken's already excellent offensive pressure while his V-Trigger is active.

In Super Smash Bros. for 3DS and Wii U, Ryu can unleash a Shakunetsu Hadoken by performing a half-circle motion and pressing the Special button. It deals the same damage as a regular input Hadoken, but with notable differences; it hits multiple times (four in total, 2% damage each), does more shield damage, and knocks the target back further, at the cost of having significantly lower priority, to the point where an opponent can destroy it with almost any move without suffering damage or hitstun. Thus, it is considered to be best used defensively when compared to a normal Hadoken.

Screenshots
灼熱波動拳