Why are Harpoon missile launchers mounted at a 90-degree perpendicular angle on Western naval ships?

Actually the majority of US Navy missile launchers are vertical, mounted flush with the deck. Here’s a pic of an Arleigh Burke’s forward deck. That array of metal hatches behind the gun are 32 Vertical Launch System missile tubes. Arleigh Burkes have a total of 90 or 92 vertical tubes, depending on when they were last overhauled. Older ships often have a mix of launcher types added over the years, but for new ships it’s all VLS. I assume the one you saw was an older model where Harpoons were added after she was built. The Navy (and our other services) did that a lot. Once Harpoons and Tomahawks demonstrated how versatile and useful they were, they started appearing everywhere.

But no matter how the launcher’s oriented, all current US missiles can fire right now and then go wherever they’re needed, making U-turns as necessary. One of the features of the Aegis radar system is that the missile just has to go up, then the Aegis radar guides it to its target.

The Slava cruiser you mentioned actually has a mix of two different large launchers. The forward-facing tubes are for SS-N-12 missiles, but aft of the superstructure they have a group of vertical launch tubes for SA-N-6 missiles. See the two photos here. The vertical-launched SA-N-6s can definitely engage enemies in any direction. I don’t know whether the SS-N-12s can, but they’re the only real anti-ship weapon Slavas have, so I think it’s likely they can also make mid-air U-turns. Otherwise, from behind Slavas would be practically defenseless against other ships. I doubt the Soviets would have left such a hole in a capital ship’s defenses.

Incidentally, the biggest advantage of the US Navy VLS system is flexibility. Before, pretty much every missile had a custom launcher. But VLS missiles are all interchangeable. So each ship leaves port with a custom mix of missile types matching her expected mission. Anti-aircraft, anti-sub, anti-ship, ship-to-surface, recon drones, even anti-satellite missiles and more, all from the same launcher. Mix and match to suit today’s needs.

By mounting them at 90 degree angles, the exhaust will go over the side of the ship instead of being aimed at the superstructure, which obviously has advantages.

Also, keep in mind that the range of these missiles is rather large, and flight time is measured in minutes at the minimum. So having to turn the ship to fire isn’t really THAT much of an issue, you won’t do snapshop engagements with them (at that point, you’d be in gun range already).

Besides, the mount points facing forward are more useful for systems that DO need to do fast snapshot engagements, namely RAM and CIWS… because when an enemy missile is inbound you indeed don’t have time to turn the ship around, and turning broadside toward an incoming threat would be a bit stupid 🙂

The Harpoon missile programming system is set up to fly 90 degrees off the ships axis of travel and then fly it’s pre-programmed flight path. They could have been programmed for a different angle, but setting them for 90 degrees allowed for installation on all sorts of different ships because a beam angle could be cleared with the launcher from some location.