RogerBW's Blog

Why no anti-submarine Osprey? 07 November 2020

When the V-22 Osprey was still in development, one of the major roles it was expected to fill was that of submarine hunting. But this never happened, to the point that it's now largely been forgotten that it was ever contemplated. Why?

Well, why was it wanted? The answer is dipping sonar. A sub-hunting helicopter can hover and lower a sonar transducer into the sea, send out a ping or two and listen to the response, then reel it up, move to a new site, and do the same thing again. But it is intrinsically limited by the transit speed of a helicopter (it would have been replacing the original LAMPS Mk III, on the SH-60B platform, making 146 knots); a number of naval officers I was in correspondence with were very much looking forward to having a dipping sonar that could be moved around a task group rather faster than the helo could manage.

So why didn't it happen? I speculate. A folded SH-60B is 19.7m long by about 3.1m wide (and 5.2m tall); a V-22 can be packed down to 17.5m by 5.6m (and 5.5m tall). That's about 60% more hangar footprint and 80% more width; a frigate that had hangarage for two SH-60s might only be able to fit a single SV-22. (And adding hangar space to an existing ship is hard.)

Well, all right. But you could still fly them off a carrier, right? Yes, but at that point you're committing to developments and updates to two separate ASW helicopters – three, if you also keep the SH-2 for the older frigates – and that starts to bite you in the wallet. So it had better be a really good platform.

One potential problem is that the V-22 is restricted in rotor size: its two 12m-diameter rotors can't be any bigger or they'd collide with the fuselage, so its 19 tons of typical combat mass are hanging off 211m² of rotor disc, 92kg/m². The SH-60 hangs only 8 tons off a single 16m rotor with roughly the same total area, for 38kg/m². That means that a hovering V-22 produces substantially more downwash than the helicopter. Would that produce more hash on the sonar if it were being used near the water surface, enough to make a difference to shallow-water operations? I don't think there are any public data on this, but it's certainly worth considering.

An extra hundred knots and more of speed over the SH-60 wouldn't hurt, though it would be nice to have more; and you could hang a lot of sensor gear on this large aircraft, and maybe even more than the two torpedoes the SH-60 can haul.

Torpedo carriage could be tricky to arrange, but the V-22 as built can be filled with 24 paratroopers who then charge out of it in flight or even fast-rope from the hover, so maintaining balance with a shifting load ought not to be a problem. (This page about a model kit shows one possible way of arranging it, not to mention proving that I didn't just make this up.) Spare weight could go to air-dropped sonobuoys. But unlike the helos you can't have forward-firing (as opposed to dropped) weapons under the wings (because the rotors will sometimes be turned forward), the ground clearance isn't high enough to hang anything substantial under the belly, and there's no provision for a ventral hatch.

So I think in the end what did for the SV-22 is primarily the size of the aircraft, but that was combined with various other advantages not being quite as advantageous as had been hoped.

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