Whirlwind

Fun fact: in the real world, Steel Rain refers to rocket submunitions.

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The Whirlwind Artillery Tank is the standard artillery tank used by the Space Marines.

It starts life as a Rhino chassis, like most Space Marine vehicles, but differs in that its carrying capacity is sacrificed to hold ammunition for the Whirlwind Multiple Missile Launcher on top. It is used to counter hordes of enemies, which the Marines don't deal too well with on their own, and to take out specific targets, such as anti-air emplacements and gun-towers. If your chapter is more... Liberal in their application of firepower, it can also be used in hostage situations.

Helios

The default Helios-pattern launcher (named for the Forge World which manufactures it) fires anti-infantry missiles, giving the Space Marines anti-horde support. They don't use this tank in the way that most Imperial Guard armies use their artillery; while IG artillery supplies sustained bombardment of static positions (enemy forts, armies, cities, etc.), the Whirlwind fires a quick barrage of devastating firepower to support Space Marines who need it, and then moves on to the next trouble spot. The launcher has 2 types of missiles. The first, the imposing Vengeance missile is like a baby basilisk cannon at st5 ap4 meaning it will lol straight through any GEQ army but will just bounce off a MEQ. Its little brother, incendiary castellan, is weaker, the same stats as a frag missile, but has the ignores cover special rule. Perfect for taking out those pesky Tau pathfinders.

While there aren't rules any more them anymore, the launchers can also fire "standard" castellan rounds. These missiles are weighted to burrow into the ground, leaving long sensors trailing from the top. This allows the whirlwind to fire minefields against an approaching foe.

The Land Raider Helios takes its name from having this type of launcher.

On the tabletop, whirlwind hyperioses are generally best fielded in units. Taking a squad of three grants Shred and Ignores Cover to the entire group and ruining the day of anything short of a monstrous creature. Adding in a spotting land speeder gives the group unlimited range and re-rolls to hit.

Hyperios

Produced by Forge World, the Hyperios-pattern launcher uses a different targeting system and fires anti-air rockets (after attempts to use anti-air rockets in the Helios launcher proved "disastrous").

Rules are printed in various Imperial Armour (most recent IA2:2nd) and Horus Heresy books by Forgeworld. Unlike the Stalker or Hunter this has both skyfire and interceptor, meaning it can take a shot when that pesky enemy flyer arrives on the table and it has the "Heat Seeking" rule, which basically makes it twin-linked against aircraft for added accuracy.

Unfortunately those are the Hyperios only advantages as it's basically still only a anti-air krak missile so it has negligible ability to take out a flying vehicle when used unsupported. The Stalker at least fills the air with dakka much like a Hydra does, while the Hunter has added range, Armourbane, AP2 and that crazy homing rule that means it never misses which makes it quite threatening.

It also costs 35-40 points more than the other two AA tanks that marines get, so basically the Hyperios should be avoided and proxied as a normal whirlwind.

That said... it's the only AA TANK that Space Wolves, Dark Angels and (current) Blood Angels get access to, since Vanilla marines keep all the cool toys to themselves.

Scorpius

The Scorpius pattern is also made by Forge World for the Horus Heresy line but was added as a "Relic" option for 40k armies. It stores S8 AP3 Blast missiles in rapid-firing rotary drums, and can fire 1+1D3 at a time if it sits still, which it should do since it is still a barrage vehicle and can fire over intervening terrain. This design is leagues better then the other designs as it can smash open light vehicles and rip straight through MEQ's due to its superior AP value and ability to fire multiple barrages at once.

The design was commissioned by the Space Wolves to help them take xenos cities, and was intended to fight on the front lines, so it has armor plating like the Predator Tank, which makes it beefier than the other patterns.