Back in the mid-1950s naval architects thinking about ideas from the aerospace world wondered if the ALBACORE could “fly underwater” just like an airplane?
An aircraft flies in the fluid medium or the atmosphere with virtually no constraints in the vertical or horizontal dimensions. Its only problem is that it needs wings to help support it in such a thin fluid as air.
Submarines fly in the fluid medium or water with no constraints in the horizontal dimension but with very serious limits in the vertical dimension due to the very high pressure or water as one goes deeper into the ocean. Crush depths or only 4-6 ship lengths contrast quite markedly with the 10-15 mile vertical space for the aircraft world. Nonetheless, one can continue the analogy somewhat more accurately by comparing the modern submarine with the airship or dirigible that was so popular before WW II.
Neither submarines nor airships require wings to support themselves in flight. Their displacement of water and air, respectively, take care of this little detail, which makes life much easier when one wants to slow down or stop and talk things over. Airplanes don’t stop very well in mid-air.
Indeed, for the early ALBACORE experiments with single man control, a number of ALBACORE crewmen were sent down to NAS Lakehurst. Here they practiced flying the ZPG 2 and ZPG 3W airships to become familiar with the control responses that were expected with the new, slippery body of revolution hull.
Although the original HOLLAND submarine was nearly perfect, streamlined, underwater body-ofrevolution, its ideal shape was to change very soon. The reality of operating on the surface forced the addition of a “conning tower” to prevent flooding of the captain’s hatch from wave motion and to support the periscope necessary for performing its stealth mission underwater. Unfortunately, these early submarines were only surface ships that were able to go underwater for very short periods of time due to the lack of a suitable powerplant. If nuclear power were available in 1900, I am sure that modern submarines would have a very different look today.
Nonetheless, with the arrival of the “GUPPY” submarines, a fairing was installed over the periscope/snorkle/antenna protuberances and the “conning tower” gave way to the sail or fairwater. Quite inadvertently, the modern attack sub was now saddled with a wing that would severely limit its maneuvering ability in the horizontal plane (because it wasn’t balanced by a wing sticking out from the bottom), and soon lead to the discovery or the roll/yaw coupling phenomenon known as “the snap-roll.”
This writer first heard about this instability in 1959 while working in the development of a Navy airship as a “flying wind tunnel.” Since the ALBACORE was the first submarine to explore the other side of 30 knots, it did not take long for rumors to surface about the “submariner’s J.C. maneuver,” where the crew nearly found itself hanging upside down from their seat-belts after attempting a high-speed, 30-degree rudder turn.
It makes sense, of course. What does one suppose would happen to a fighter plane in a turn if it lost one of its two half wings? So how can one expect a submersible to fly in the horizontal dimension with only half a wing?
The ALBACORE engineers and crew worked for several years to solve this basic limitation to horizontal maneuvering, and there were many possibilities. For the time being, limiting the degrees of rudder used in a high-speed turn was used. But how can one avoid a hostile torpedo or sonar contact on a potential enemy sub at 2-3,000 yards if limited to modest, large-radius turns? One does not have much time to avoid or reposition to make ready for a quick counter-attack and getaway.
Recall the famous ALBACORE demonstration in 1956 — with Admiral Arleigh Burke and Admiral Mountbatten on board — while being hard-pressed by a friendly destroyer off the Florida Keys. When the destroyer thought she was almost on top of the submarine, the ALBACORE suddenly heeled over into a 180 deg. turn. With a skilled pilot using their single-man controls in cooperation with an aggressive captain, the ALBACORE quickly broke the pursuing destroyer’s active sonar contact and disappeared into the azure-blue depths of its natural habitat.
Today. however, we are losing the luxury of being able to detect the Russian at over 20,000 yards. With quiet submarines now proliferating all over the world, the underwater melee or “underwater dogfight” is becoming increasingly likely. There is no question the Russians have been working on the problem. Look at the ever smaller, blended sails on their subs. It is already acknowledged that many can dive deeper and go faster than we can. The ADCAP torpedo is not enough, we must learn to outmaneuver them in the horizontal plane.
For any untracked vehicle entering a turn, it is necessary that the outward centrifugal force generated by the mass of the vehicle be offset by an inward force generated by the vehicle itself, Figure 1. An aircraft simply rolls its wings over to allow the horizontal component of the wing lifting force to counter the outward centrifugal force of the weight of the aircraft. An airship or a submarine, without lift generating wings, has a problem. The only sizeable surface available to these large, buoyant bodies of revolution is their hull — which is basically a total disaster with it’s inability to generate large side-forces for tight turning in air or water. Nonetheless, let us examine how it works.
Most of us have observed, on a rainy day at any airport, the peculiar wing-tip vortices twisting up and trailing behind aircraft — Boeing 747’s, etc., –. These are the result of the high-pressure air underneath a wing sliding around and equalizing pressure with the low-pressure air above the wing. These vortices are visual evidence of a wing’s lifting ability, and are the only evidence of the term “circulation,” used by aerodynamicists to describe the phenomenon of lift forces in fluid flow. When you see such a vortex, whether it be on an airplane, the rear wings on Indy race cars, or even off your hand in the bathtub, you are seeing “circulation” and a lift or side force in the fluid medium.
To visualize these vortices on an airship or submarine body of revolution, simply eliminate the wing between the two wing-tips — leaving only the two, half-circular tips — which when joined together, form a body of revolution. If this is now inclined to the air or water flow, the two vortices will still be present. This indicates that there is “circulation” available. with the resultant side-force facing leeward perpendicular to the body between the two vortices on the lee side per Figure 2.
Unfortunately, the two wing-tip shapes clamped together to create the hull form of a modern submarine now generate theoretically weaker “circulation” than when they had a long thin wing between them. As a result, the side force available to allow a submarine to turn is only sufficient to allow turns in, perhaps, 5-6 lengths. The presence of the sail, by happenstance, improves this turning performance considerably, if one is brave enough to deal with the resultant roll/yaw coupling problems.
It is important for submariners to have a complete understanding of how essential these vortices are to the opportunities available in high-speed maneuvering “flight.” Referring to the photographs of the SKIPJACK flow visualization studies (ref: January 1988 SUBMARINE REVIEW, pg. 48), one can clearly see the strong influence of the sail as it pulls the upper bow vortex out of its place along the hull and up into the wake of the sail itself. The resultant crossflows and separated hull-flow decrease the vortex-generated side force and push its center-of-pressure toward the stern. The initial problem of a 30-40 degree snap-roll is now hugely complicated by a pitch-up of the bow and further increase in depth as the rear half of the submarine “squats” down. All this because of the rude displacement of the very powerful twin vortices createq by the yawing hull at speed.
Since the vortex itself consists of a swirling mass of water rotating inward towards the center of the hull — similar to the rotation of the top of a 20 foot high surfer’s wave, but with much more energy –, it will also be a potential source of turbulent noi~e. This sound energy must be contained and minimized for quiet rapid maneuvers in the lateral or horizontal plane. Happily, the twin vortices are huge drag generators which will produce a marked slowing of the submarine whenever they appear, thus discouraging a possible inverted spin.
I It is clear that the best solution to such a problem is to keep the sail s~ructure upright in any high-speed turn and not allow it to influence the vortex patterns from the bow. This powerful vortex structure must be left alone to seek its normal position on the lee side of the hull with no interference from any hull protuberances forward of the sternplanes.
Part II of this article in the next issue will discuss possible retrofit solutions for all 637 and 688 class ships which should allow them to make 180 degree turns, fully upright with no roll angle at 30kts. This retrofit should also allow single-man control without the pitch/roll/yaw coupling difficulties and undue drama that require a Chinese fire drill to handle today.
Henry E. Payne III