Shkval, the Rocket Torpedo

This torpedo seems like something from a James Bond movie. Surprisingly though, it has been around for decades.

http://www.popsci.com/scitech/article/2004-06/supercavitating-torpedo

by John Macneill: Several challenges remain for the supercavitating torpedo, including how it will be steered underwater. Water-tunnel tests have already proven that speed can be achieved: In 1997, the Navy tested a supercavitating projectile that reached 5,082 feet per second, becoming the first underwater projectile to exceed Mach 1. Photo by John Macneill

Submarines peaked in power and relevance during the Cold War; there has since been a shift in focus to aircraft-based combat, and subs have become budget-cut victims. But subs are still prized for their ability to sneak about global waters undetected and to defend surface ships from attack. Many U.S. subs are being converted from missile launchers into delivery vehicles for special operations troops.

But the supercavitating torpedo—a rocket-propelled weapon that speeds through the water enveloped in a nearly frictionless air bubble—may render obsolete the old submarine strategy of sly maneuvering and silent running to evade the enemy. The superfast torpedo could be outfitted with conventional explosive warheads, nuclear tips or nothing at all—a 5,000-pound, 230-mph missile could do enough damage on its own. The Russians invented the concept during the Cold War, and their version of this underwater killer—dubbed the Shkval (“Squall”)—has recently been made available on the international weapons market; the United States, of course, wants a new, improved version of the original.

The hard part about building a rocket-propelled torpedo isn’t so much the propulsion as clearing a path through the ocean. Water creates speed-sapping drag; the best way to overcome that drag is to create a bubble that envelops the torpedo—a supercavity. A gas ejected uniformly and with enough force through a cavitator in the nose of the torpedo will provide such a bubble, permitting speeds of more than 200 mph and a range of up to 5 miles (traditional torpedoes have slightly longer ranges, but lumber at only 30 to 40 mph).
Though submerged, the torpedo remains essentially dry, with a frictionless surface. “That sounds easy, but doing it is extremely difficult, especially if you’re trying to steer,” says Kam Ng, program manager for the torpedo at the Office of Naval Research, which has been developing the weapon since 1997. “If your torpedo moves in a straight line, you just aim and shoot,” says Ng. “That capability already exists with
Shkval. But the U.S. vehicle will be more capable—it will turn, identify objects, and home in on the target.” (Improvements to the torpedo to make it steerable likely froze when the Soviet Union collapsed, says GlobalSecurity.org’s Pike.)

Among the greatest challenges for U.S. torpedo researchers is developing detection and homing technology that will enable the torpedo to distinguish an enemy sub from, say, a rock formation, says Ng. Also tricky is finding a way to control the gas bubble to permit those course changes. “When you turn, the bubble distorts because it is no longer symmetrical,” he says. “So you have to compensate for that by putting more bubble to one side.” This is done, Ng explains, by ejecting more gas toward the outside of the turn.

Naval officials say the high-speed torpedo will enable submarines to attack enemy subs and surface ships without giving them time to respond. The U.S. military has tested a prototype, but combat-ready versions are not expected for at least 15 years.

For the military ethusiasts, here’s a detailed analysis on how to possibly counter this threat.

http://www.military.com/forums/0,15240,93859,00.html

Iran isn’t about to give up easily in their war of words (and associated psy-ops) about their new wonder-torpedo. They allege it’s “sonar evading.” If you know very much about sonar, you have to be scratching your head and asking, What the heck is that supposed to mean? I tag it as agit-prop doubletalk — better laughed at than worried about. I justify my tagging as follows:

Sonar evasion per se, by any weapon or vessel, is achieved in one of two ways. The first is to be very quiet, so your target’s passive sonars can’t detect your presence nearby. Scratch that, since supercavitating rockets are deafeningly loud — the vacuum bubble collapses in the turbulence of their engine exhaust, providing scant sound isolation if any at all. The second is the old, familiar use of sonar layers and other underwater noise propogation effects to mask your acoustic signature from the target as you approach. But no one understands these effects better than the U.S. Navy, and not just subs but surface ships and aircraft are equipped to leave no room for a roaring rocket motor to hide. (Surface combatants can stream variable-depth sonar towed arrays, and their supporting anti-submarine helicopters can use variable-depth dipping sonar and also drop optimized patterns of sonobuoys.) Besides, in the littorals (shallow and/or near-shore waters), where near-future naval battles seem most likely to occur against enemy diesel subs or small surface craft deploying any large weapons, there often is no sonar layer — the water isn’t deep enough for one to form. With up-to-date and thorough hydrographic data in hand (including salinity variations, charted and uncharted wrecks, gas- and oil-drilling/pumping noise sources, and water transparency or lack thereof), American and allied subs would sneak in where undersea conditions maximize their own stealth, while they use the same knowledge to seek and ambush enemy subs.

(Purists will note there is a third way to achieve low observability against active pinging sonar, namely the use of anechoic — sound absorbing — coatings on the hull. But modern U.S. submarine passive sonars are able to derive the range to any high-decibel sound source instantly, obviating the need to ever go active against an inbound torpedo. The implied analogy to radar-absorbent materials on cruise missiles fails underwater.)

So much for “sonar evading.”

To best appreciate these issues, it’s important to think of the bigger picture. Superior U.S. Navy sonars (and the skilled sonar techs who use them) in any theater of conflict or combat will constantly be hunting for the slightest hint of enemy threats. And an incoming Shkval has to come from somewhere — it doesn’t materialize out of thin air. The best strategy, as always in naval warfare, is to destroy the enemy platform before it can fire its weapons effectively. Special new active and passive sonars, advanced signal processing algorithms, and console display modes so sophisticated they’re classified are intended to eke out the slightest whiff of an enemy diesel sub concealing itself amid the naturally high background noise to be found in most littoral areas. The same thing goes for enemy fast-attack craft rushing along or lurking on the surface.

In many littoral warfare scenarios an American sub won’t be operating alone, but rather as part of a joint (or combined, i.e., allied) task force that would include other subs, unmanned undersea vehicles, temporary bottom-moored listening grids, surface warships, aircraft (including recon drones), plus intermittent overpasses by surveillance satellites. Network-centric warfare is a complicated team sport. With recent and impending breakthroughs in “comms at depth and speed,” submarines are now part of that team, and the task force would literally cast a wide net to localize, track, target, and sink any threats. In the earliest, “battlespace preparation” phase of some armed showdown, a lot of attention would be paid to accounting for and neutralizing all potential Shkval-launching platforms.

The point of the discussion, so far, is that speed of one weapon, viewed alone, doesn’t determine the outcome of either a sub-on-sub dogfight or a major naval engagement. The fact that the maximum range of a Shkval or derivative, before its rocket fuel runs out, is only four or five miles, should help put in proper perspective that supercavitating torpedoes are hardly as “hellacious” as they’ve been described. (By the way, their disadvantages hold equally well if carried on enemy nuclear subs as if carried on enemy diesels.)

Perhaps one good proof of this is that active duty submariners I’ve met on subs or talked to at conferences aren’t exhibiting any panic over Shkvals. The way they describe it, the latest mod of the Improved Advanced Capability (ADCAP) Mark 48 sub-launched heavyweight torpedo remains by far their weapon of choice. They scoff at the threat that a Shkval would pose — assuming it isn’t armed with a nuclear warhead. (In that nightmarish scenario, the Shkval with its limited range would amount to a suicide weapon. And Mark 48s are nuclear-capable, if necessary, too.) In contrast to the Shkval, the latest Mark 48 is reported to have a maximum range of some 30 nautical miles. During a game of cat-and-mouse, this means the American sub can threaten anything inside an area 36 times as big as what’s covered by a 5-mile-range Shkval. That gives a very significant, classic tactical advantage: By holding open the range using the nuclear submarine’s maximum speed advantage over the diesel’s (say, 30+ knots sustained compared to 20ish in short bursts), the American vessel can “bombard” its opponent from outside the diesel’s ability to hit back with Shkvals. Furthermore, the maximum speed of a Mark 48 is reported in open sources as some 60 knots, and I suspect that the actual (classified) figure for the latest (ninth?) mod might be several knots higher than that. The American weapon is three times as fast as the enemy diesel — and is also much faster than any known nuclear sub. (One news source claimed that the U.S. Navy had failed to invest in good torpedos for years now, and that our best fish were so slow that enemy subs and ships could simply outrun them. I have never read a more incorrect statement in my entire 10-year career as a non-fiction submarine commentator.)

Granted, Iran’s rocket torpedo is three times faster than the latest Mark 48, but I’ve tried to show above that in the wholistic framework of modern naval action, out in blue water or in the littorals, a factor of three in weapon speed makes a difference in degree but most certainly not a difference in kind. Yes, better situational awareness, and faster reaction time, are at a heightened premium aboard American ships and subs in the emerging environment. It’s precisely these attributes that the Navy is striving with a will to enhance in every possible way. (The revolutionary layout of the Virginia-class control room is just one example of many in this arena of man-machine interfaces for optimal warfighting preparedness and survivability.)

But there is a difference in kind between a Shkval and a Mark 48. The Mark 48 is wire guided, allowing fire-control technicians to adjust the fish’s course for weaknesses in the original firing solution (think torpedo “Kentucky windage”), or to compensate for evasive maneuvers by the target. Technicians can also use their submarine’s powerful sensors to help discriminate between the genuine target and any decoys or noisemakers the target might launch — all assuming the wire doesn’t break. Even if the guidance wire does break, the latest Mark 48s have such capable active and passive sonars and on-board computer brains that they can search on their own, pick out their intended target from amid littoral (or other) acoustic clutter while ignoring neutral vessels nearby, and then zero in for the kill.

In contrast, most supercavitating weapons that are operational right now are pretty dumb. This is mainly because their ancestors started out as dedicated nuclear weapons, so that pinpoint accuracy wasn’t much of an issue to the engineers and commanders who built and deployed them. Speed was the design bureau’s sole object (because conventionally-powered USSR torpedoes in the late 1960s were slow). Given this dubious legacy, most high-explosive-armed Shkval-like torpedoes still rely on traveling in a straight line, with no guidance or homing whatsoever after launch. Nowadays, the firing vessel hopes that this straight line intersects the target’s track at the same moment that the target happens to be at that point on its track. This is like early World War II technology! A well-aimed spread of such weapons could definitely prove fatal to a big surface ship, say one of our supercarriers, and higher torpedo speed does make up for softness in the firing solution. But as mentioned above, the key to successful defense is to prevent the Shkval’s launching vessel from getting close enough to the carrier to begin with. This is presumably one driving force behind the U.S. Navy’s greatly stepped-up emphasis on anti-submarine warfare for surface battle groups — with enemy diesel subs receiving particular attention, their roles played by allied diesels including the leased Swedish Gotland-class sub and her crew. At this point American submariners have been engaging in anti-diesel exercises for some time; after a few initial humbling setbacks, the U.S. side collectively developed doctrine and tactics that give them a much better edge. (For a take on the relative disadvantages of modern diesels with air-independent propulsion, compared to nuclear-powered subs, see my archives for “Diesel Downside,” 13 July 2005.)

American submariners tell me that all they need to do when faced with an incoming high-explosive Shkval is make a slight change in depth (or a fast change in heading and speed), and the Shkval will go right by, its impact or laser-proximity fuse left with no reason to explode within dangerous range. It’s very beneficial to be able to move in three dimensions, even or especially in the littorals!

Of course, as with all weapons down the ages, Shkval-type technology isn’t standing still. Russia is developing a version of a supercavitating torpedo that does have some artificial intelligence and homing sensors including sonar. The problem is that for the sonar to work, the Shkval has to slow down drastically, in spurts, so it won’t be blinded by its own noise and has a chance at acquiring and reacquiring its prey to make the needed terminal course corrections. This seeming enhancement to the Shkval introduces a substantial Achilles’ heel: When moving slowly, and relying on conventional sensors to home on its target, the Shkval becomes vulnerable to all the standard evasive tactics and countermeasures with which American submariners are exceedingly well versed. The Shkval, during such an attack, also repeatedly forfeits its one apparent advantage, its speed, before having to accelerate again. Rumor has it that Moscow is trying to make a wire-guided Shkval, but trailing a wire that doesn’t snap at 300 knots, or melt in the searing heat of the rocket exhaust, or cause the vacuum bubble to collapse enough for the whole weapon to suddenly tear itself apart, seem daunting problems indeed.

Counterattacking the Shkval’s launch platform is best done with a Mark 48 or two. The answer is definitely not for our Silent Service to rush and clone the Shkval. This is another overwhelming benefit of our current conventional torpedoes: They can be launched very quietly, be programmed to run on a dog-leg course initially at slow and quiet speed to disguise their point of origin, and then attack the Shkval’s parent platform with ease — because the launch signature and trajectory of the Shkval will point right back to its own point of origin. The American submarine, in contrast to the Shkval shooter, can shoot back while retaining good stealth. With the latest integrated combat systems, if ever caught by surprise our guys can get their retaliatory fish into the water in a matter of seconds. Then using superior technology and tactics, the American sub can regain the initiative and go on the offensive. (Speed of opening accurate counter fire can be more important than transit speed of the weapon itself.)

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