Rocket propelled torpedoes and submarines which can travel at supersonic speeds underwater? Prototypes of the concept already existed prior to 1990 AD, developed by the Russian military. Related technologies could also be used to double the top speeds of circa 2000 AD hydrofoil surface craft and enable a new generation of underwater mine-killing guns.
The core of the new technology is super-cavitation, or a more extreme version of the phenomena of cavitation long associated with underwater propellors. In cavitation water pressures upon certain fast moving solid surfaces will drop so much that the water near them will vaporize, forming bubbles, which has long been a source of wear, inefficiency, noise, and other problems for marine vessels and pumps. Cavitation can occur in air too, but the lower densities compared to water greatly reduce its impact there.
In super-cavitation the phenomena is purposely amplified to absurd proportions to transform it from a problem to a quantum leap in underwater movement.
A very nearly flat-nosed object traveling at least 180 kmph could transition to a state of super-cavitation where almost its entire bulk exists in a bubble, thereby relieving it from the normal high drag of underwater travel. In practice, some venting of the rocket propulsion system's exhaust out the nose has been necessary in prototypes to enlarge the bubble to encase the entire object until sufficient speed is attained. Enormous strength is also necessary in the nose materials.
500 kmph torpedoes existed in the early 90s which required ejection at high speed from a submarine to enable rocket firing afterwards. It may be that future subs may contain something like linear accelerators to perform such ejections.
By 1997 USAmerica possessed super-cavitating underwater bullets achieving close to 5400 kmph, but of very limited range (12 meter underwater ranges are being discussed for mine-kills in 2000 AD).
Future SC torpedoes may use aluminum fueled rockets. SC submarines would likely use nuclear power plants.
One problem with super-cavitation-based propulsion may be environmental. It may produce such a horrendous acoustic din underwater as to damage sealife, such as dolphins and whales. Engineers may eventually find a way to tune the acoustics to something less injurious however. In the meantime, parties at war likely would care little about sealife and use the technology without regard for same. In peacetime, perhaps only limited testing would be performed with such devices.
Another problem is manueverability. SC transport tends to be a straight-line deal. It could be difficult to avoid high speed collisions with obstacles, via present-day SC technology. But it may be that selectively increasing the number and size of drag surfaces presented by an object will remedy that (while reducing maximum potential speeds). Also note that increased sensitivity and range of object detection technologies and the already existing Global Positioning System could do much to lessen the need for changing a straightline course. If a ship or whale could be detected several hundred km away a SC vessel might be able to slow down to change course then speed up again.
Over the long term it appears that a super-cavitation revolution in submarine propulsion would lead to underwater craft tending to be more like aircraft. That is, large SC craft might usually consist only of commercial freighters, with military SC craft tending to be the size of circa 2000 AD military combat aircraft or smaller. Indeed, combine the limits of SC with the restrictions on high-G launches with human crews, the better reaction times (and greater efficiency and zero pilot risk) of computers, and it would seem virtually all future SC military craft would be relatively small, unmanned, robot devices. But they might possess a large, slower mothership performing a role similar to that of 20th century aircraft carriers for the smaller watercraft.
But super-cavitation technology might also be applied to aircraft, giving them brief stints of hypersonic speed when required, either for orbital insertion or escape from nearby threats. Note that SC transport requires a shape much like that of a rocket. Which adds all the same costs and limitations of a vessel like the space shuttle to such a program, where some more typical aircraft functionality is also desired in the same craft.
Again, relatively small, unmanned robot aircraft can offer a remedy. The bulk and complexity normally required to fit a human crew can instead be used to make the aircraft a transformer of sorts, reshaping itself to perform either as a runway launching and landing, slow moving, fuel sipping surveillance craft in one instance, and a screamingly fast, fuel gulping, hyper-sonic rocket in another. There might also be an intermediate stage of supersonic craft for still more flexibility in the asset.
At some point a certain type of such transformers might also boast one or more adaptive conversions for underwater travel. Such combined sea and air capable combat robotic craft could be a terrifying weapon beginning at some point in the 21st century.
Keep in mind that concepts such as super cavitation might actually be applied more quickly and easily in the field of aerodynamics than hydrodynamics, for a variety of reasons. Thus, we might expect the earliest examples of super cavitation technology used in war to appear roughly simultaneously in the air and water, with perhaps aircraft and air weapons eventually making at least as much use of the phenomena as water systems (if not more).
-- New Scientist: Faster than a speeding bullet by Duncan Graham-Rowe, From New Scientist magazine, 22 July 2000 [original URL, now broken, was http://www.newscientist.com/features/features_224813.html]
Money and technological development in regards to military force in the developed nations have focused on relatively small elite elements, and this shows in the navy/air force equivalents of dozens of small submersible carriers which may surface to use hydro foils and/or hovercraft technologies for fast transit, and offer the capacity to field a small armada of both robotic boats and aircraft capable of surveillance, espionage, and/or direct combat. The robotic boats include both surface and undersea craft, often capable of landing and establishing beach heads all on their own. The robotic aircraft include everything from cruise missiles to small fighter craft and reconnaisance craft-- all unmanned. Many of these subsidiary craft also act as the escort force for their mothership, defending against any perceived threat.
Robots could begin shouldering the biggest combat burdens within only five years.
-- Robots To Fight Wars In The Future- UK Expert By Bill Rosato, Reuters/Yahoo! News Science Headlines, September 16 1999
-- Northrop Wins Navy Contract New Ships to Be Smaller, Stealthier ["http://www.washingtonpost.com/wp-dyn/articles/A5236-2002Apr29.html"] By Renae Merle; Washington Post; April 30, 2002; Page A01
Not all small carriers are sea-based. A few are airborne, though these tend to host a higher proportion of subsidiary craft not meant to return from their missions (disposeable warcraft), compared to the submersible carriers, which enjoy a bit more protection, stealth, and bulk than their airborne brethren.