SUPERSONIC SUBMARINES!
THE ULTIMATE WEAPON FOR FUTURE WAR?
Aglittering vessel is moored to the quay, ready for departure. A narrow gangway leads to a small hatch, which is barely big enough for an adult to pass through. Excited crew members are getting ready for a test voyage. They put on pressure suits with breathing equipment. Their luggage is stowed into a pressure-sealed container and placed in a separate room. It is not possible for the men to stand upright, so they move about the submarine in a hunched position. Soon, they will be the first humans to race through the ocean at speeds close to that of sound in water – around 5,436 km/h.
This scenario could come true in the near future. Speeds of 5,000+ km/h through ocean water are theoretically possible, and a team of Chinese engineers have made a crucial breakthrough, which could make the dream come true over time. At this point, only a few sketches exist of how the technology will function, but the principle i s wellknown, as it is already used i n torpedoes. The secret is supercavitation, by which a vessel moves through the ocean inside a self-generated air bubble, avoiding the effect of water drag.
WATER DRAG SLOWS SUBS DOWN
Conventional submarines cannot break speed records comparable to those of ships moving at the surface of the ocean. In spite of nuclear reactors and a streamlined design resembling the shape of a blue whale, even the most sophisticated subs move at speeds of no more than 60- 80 km/ h. The water drag makes it impossible to go any faster equipped with the technology used by engineers today. This is due to the fact that subs are entirely submerged in water, and drag affects the entire hull. A ship moving at the surface is only partly affected by water drag. Water is about 800 times as dense as air, so the water drag is some 800 times that of the air affecting a car. Moreover, the faster a conventional submarine moves, the greater the drag, as the increased speed will
In spite of nuclear reactors and a streamlined design similar to a blue whale, even the most sophisticated existing subs move no faster than 80 km/h.
enhance frictional resistance. As a result, even a small increase in top speed requires a disproportionately huge engine.
THE SECRET IS BUBBLES
If the water drag is eliminated, the sub’s top speed will suddenly be multiplied – and that is exactly what supercavitation is about. Supercavitation can be observed when a projectile is shot through water. At first, the projectile will drag an air bubble along from the surface. If the speed is sufficiently high, and the projectile is shaped in a particular way, it can continue through the water surrounded by the air bubble. The US Navy has already developed projectiles that take advantage of supercavitation, but unfortunately, the air bubble is dissolved after a few seconds, making the projectile lose speed very quickly.
If supercavitation is to continue its effect for a longer period of time – which is necessary in order for a submarine to take advantage of the principle – the air bubble must be supplied with compressed air or gas from the sub itself. That can be done at the nose of the hull via a nozzle equipped with small valves, from which gas flows in the shape of bubbles. The bubbles will soon unite and surround the entire vessel, making the submarine avoid direct contact with the surrounding water, and so remain unaffected by water drag.
CHINESE BREAKTHROUGH
During the Cold War, the Soviet Union managed to develop a torpedo known as Shkval, which can keep up the effect of supercavitation for long periods of time by emitting gas bubbles from its nose and race through the water at speeds of 370 km/h. Since then, several countries have been working on developing subs that take advantage of the same principle. However, the scientific activities are veiled in secrecy, as the development of supercavitation submarines are military matters. But Russia, China, Iran, and the US are reportedly developing supercavitation submarines. Recently, Chinese engineers published several considerable scientific breakthroughs. Although they don't want other countries to catch up with them in the development of supercavitation subs, they have published a series of scientific articles, which disclose some information as to how some of the most significant challenges to obtaining supercavitation could be overcome.
DISSOLVENTS MAINTAIN THE BUBBLE
The Chinese scientists made use of a combination of calculations and simulations, borrowing many of the principles from another wave research discipline: sound research. Moreover, they shot projectiles through liquid to see, how the supercavitation bubble develops and perishes. One of the challenges consists in producing a stable air bubble. Air bubbles in water are volatile and very difficult to control. Normally, bubbles forced out of the nose of a sub will soon lose their shape and size as a result of the pressure of the surrounding water, the frictional resistance against the hull, etc.
But the Chinese engineerss discovered that water-soluble dissolvents, which we know from ordinary dishwashing liquid, can reduce the frictional resistance of the bubbles against the hull. By spraying dissolvents through the nozzle at the nose of the sub along with the bubbles, engineers can produce a membrane around the vessel. So, the bubbles maintain their shapes for a longer time.
CHEMISTRY REPLACES RUDDER
Dissolvents have also proved to be the solution to other great challenges. At a speed of 5,000 km/h, a major challenge consists in controlling the submarine. Scientists cannot use a rudder or a guide fin to alter the course without affecting the speed and risking to destroy the air bubble
surrounding the vessel. The Russian Shkval supercavitation torpedo features guide fins, but moves at a maximum speed of only 370 km/h – and even then it is a challenge to control the torpedo. However, the Chinese engineers have discovered that they can control the sub by dosing the amount of dissolvents sprayed onto the hull. By regulating the dissolvents, the scientists can produce very slight changes in frictional resistance, manoeuvring the sub without destroying the air pocket.
But the Chinese engineers still face a number of major challenges. In order for the supercavitation bubble to be produced, the submarine must achieve a speed through the water of 75-100 km/h in only 1-2 seconds. The Shkval torpedo is shot through a torpedo tube by means of hydraulic pressure, which can send the torpedo into the water at a speed of around 100 km/h, after which the torpodo’s rocket engine takes over. A supercavitation submarine would probably have to accelerate to 75-100 km/h by means of a solid fuel rocket engine based on for instance hydrogen or sulfur, which requires explosive combustion. The Shkval torpedo is equipped with a solid fuel rocket engine, but it only has a reach of 1015 km. A rocket engine has not yet been developed that could make a supercavitation sub travel a longer distance.
So, the sub would probably need to be equipped with a two-stage engine, in which the first step is the rocket engine, that would very quickly accelerate the sub to 75-100 km/h. When supercavitation has been achieved, another engine with more adjustable combustion - a so- called throttleable engine - could take over, gradually accelerating the sub to its maximum speed.
MANY CHALLENGES REMAIN
If the scientists manage to develop a supercavitation submarine, many challenges will still remain. For instance, they must make sure that the sub will not hit any obstacles en route. At speeds of 5,000+ km/h, it would be a disaster, if the sub ran into, for instance, a 190 tonne blue whale.
The route must also be planned so the sub avoids undersea reefs such as the Midatlantic Ridge. And no matter if the sub’s route includes as few obstacles as possible, the vessel must have a navigation radius of several hundred km to be able to avoid obstacles. A safe passage would require the development of brand new types of sensors that could monitor the waters.
All these things must happen before our oceans fill with supersonic submarines. But who knows - one day they could be a reality.