To take full advantage of the all-electric design's flexibility and power density, the Navy should consider a direct-current power grid with super-conducting, direct-current homopolar generators and propulsion motors.
The advantages of an integrated power system in a warship have been proclaimed in a variety of engineering and professional journals, and the mandate that DD-21 would be an all-electric ship seems to indicate the Navy's commitment to the concept. However, every technology has limitations, and sometimes its anticipated benefits can be devalued by external influences.
There are few better environments for exposing frailties than battle. The avenues of violence extant during combat and the demands of either deflecting or surviving that violence are too complex to admit of a single "magic bullet." Certainly, innovation must be pursued—stagnation is the path to defeat—but the decision to change fundamentally the way warships are designed, such as all-electric drive, must rest solidly on an intimate understanding and balanced consideration of both the enhancements and the potential tactical vulnerabilities inherent in the change. In particular, the limitations imposed by the realities of war at sea require sober study, otherwise our warriors will be put at undue risk.
An integrated power system can enhance combat survivability. Power sources can be distributed throughout the ship, eliminating a key vulnerability to underwater damage, namely, the propulsion shafting and the shaft alleys. So, how best to capitalize on this inherent advantage? Obviously, the smaller and more power dense the power generators and propulsion motors are, the easier it is to site them optimally, but size and power density are not the only considerations. A ship's power systems also must be robust, affording multiple levels of redundancy, especially in delivering electrical power to combat systems. They must be instantly and simply controllable. Finally, they must have sufficient capacity to support modernization as the threat evolves and escalates. There are several options under consideration that may satisfy all these requirements, but one is particularly interesting—a direct current (DC) power grid with superconducting, DC homopolar (SCDCHP) generators, and propulsion motors.
Why Direct Current?
Warships operate at variable speeds, and the necessary variations can be abruptly imposed, such as when executing evasive maneuvers. The ability to change motor, and therefore propeller, speed quickly and easily is a tactical imperative. Obviously, the all-electric ship will have electric motors for propulsion, and the first major choice to make is between direct and alternating current (AC). The speed of an AC motor is controlled by changing the frequency of the power supplied. The speed of a DC motor is controlled by changing the voltage. Voltage regulation, generally, requires significantly fewer solidstate components. For example, the Navy's Integrated Power System test bed at Naval Ship's Systems Engineering Station, Philadelphia, employs a 4,160-volt, 60-Hz, three-phase AC generator supplying a 25,000-horsepower AC induction motor. It takes about 17 tons of power electronics to convert the main distribution AC to DC and then to the variable, reversible low-frequency AC required to match motor speed to the best speed for the screw. The alternative is to use noisier and equally heavy reduction and reversing gears, markedly reducing the expected benefits of electric drive.
Because the DC motor has some significant advantages in terms of speed control, a DC main power bus and DC generators make sense. They not only allow you to avoid the high power AC to DC conversion, but they also can improve continuity of power markedly. In an AC system, it sometimes is necessary to operate two generators with their outputs connected to the same bus, termed "in parallel." This requires precisely matching their output voltages and frequencies, but that alone is not enough. For a three-phase system—the Navy standard—a single generator develops three sinusoidal voltage outputs, 1200 out of phase with each other.
Assume the phases are called A, B, and C. Since we use 60-Hz power, phase C reaches a peak 1/180th of a second after phase B peaks, which occurs 1/180th of a second after phase A peaks. When two generators are placed in parallel, they must be "in phase," meaning the output of both generators must be absolutely in synchronism and peaking at exactly the same time. If these conditions are not met, extraordinary currents are created, large enough to lift a generator off its foundations and propel it through the engineroom.
Connecting the outputs of two DC sources to the same bus is simpler, because only the output voltages must be matched. There are fairly straightforward circuits that automatically pick between two or more sources to supply the bus. The result is a passive, instantly available backup source of power that can be thought of as a very large uninterruptible power supply.
The implications for tactical systems, all of which use computers, are clear. The electronics see stable, reliable power, even if one source drops off the line unexpectedly. The captain's ability to both deflect and inflict violence is not held hostage to a single power source, the situation extant in virtually all of today's warships.
Why Superconducting?
Superconductivity—the tendency for the electrical resistance of certain materials to drop to zero at temperatures approaching absolute zero (around -460 deg F)—no longer is confined to laboratories. In years past, liquefied gas filled cryogenic systems were needed to attain temperatures low enough for the phenomenon to occur, which is impractical for shipboard applications, but recent advances have changed things dramatically. The Navy has developed a prototype seagoing, seven-foot-diameter superconducting magnet producing a field sufficient for a 40,000-horsepower motor, and its temperature is maintained by one of two installed cryo-coolers, each weighing less than 200 pounds. The cryo-coolers not only make shipboard applications possible, but they also make them highly attractive, for two key reasons. First, since there essentially is no electrical resistance, there essentially are no resistive heating losses. That makes the machines more efficient, which either saves fuel or permits the ship to go farther on a gallon of gas. Second, since the machines do not have to contend with the damaging effects of the heating, they can be made much smaller and still produce the same power.
The Navy's integrated power system uses a multipole induction motor to represent the main engine. Rated at 25,000 horsepower and weighing 117 tons, its volume is nearly 2,400 cubic feet. A 40,000-horsepower superconducting DC homopolar (SCDCHP) motor would weigh 33 tons and occupy 1250 cubic feet. So, it delivers 60% more horsepower at one-third the weight and half the volume of the multipole motor. The homopolar machine architecture has one more significant advantage. When operating as a motor, the most efficient speed closely matches the optimal speed for a propeller (120-140 RPM), and when operating as a generator the most efficient speed (20,000-40,000 RPM) closely matches the best speed of a gas turbine engine. This eliminates the need for reduction gears.
Certainly, there may be other promising approaches, but the SCDCHP configuration trades on commercially proven technology, allowing designers to focus on what truly matters: maximizing the warship's capacity to wreak decisive destruction on the enemy. They can and should take full advantage of the all-electric design's flexibility and power density to make more of the ship's volume available for fuel, magazines, combat command-and-control systems, better battle damage mitigation buffers, or a combination of all the above.
Present Challenges and Future Promises
For the foreseeable future, there likely will not be entanglements of large fleets such as at Trafalgar, Tsushima, or Leyte Gulf. However, we cannot let that fact inflict on us the euphoric myopia that attended the demise of the Soviet Union. The threat at sea is real and getting worse, and it is distinctly unconventional, making the danger more ominous, insidious and difficult to counter. Consider that, despite not having fought a true war in about 30 years, five ships have sustained damage from malicious violence: the Stark (FFG-31), Samuel B. Roberts (FFG-58), Princeton (CG-59), Tripoli (LPH-10), and Cole (DDG-67). Consider also the pattern: one ship struck from the air, three from underwater, one on the surface by a suicide bombing, all in the littorals. Interestingly, none sustained a ballistic missile attack, nor did they see chemical or biological agents. Are there lessons here for the Navy's next generation of warships? Probably, but they must be evaluated in concert with at least one other key factor-the direction and pace of technical innovation in weaponry.
Weapons themselves are becoming harder targets. Two-hundred-knot rocket-propelled torpedoes and hypersonic missiles with radar cross sections that make a bumble bee seem gigantic exist and are proliferating. Recent revelations on the transfer of missile-guidance technology to some of our potential adversaries ought to evaporate any smugness IS regarding the poor quality of the arrows we may have to face at some point. Cheap mines with sophisticated triggering schemes and cases that are devilishly difficult to detect are readily available. Small, well-organized terrorist groups are as difficult to find and eradicate as cockroaches in a Florida kitchen.
So, if the current trend in maritime strategy continues, ships will be spending more time in the most dangerous operating environment, the threat will be unconventional and asymmetric, and the more traditional tools of war—torpedoes, mines, and missiles—will be harder to detect and counter. All-electric ship technology cannot blunt every one of these swords, but in at least two cases it may be the only practical counter.
Developments in missile technology may soon make our present self-defense systems obsolete. Low-observable techniques mean missiles are closer to the ship when detected. Advanced propulsion and guidance systems make them faster and harder to hit. The physics limitations imposed on counterweapon kinematics, on proper warhead detonation timing with weapon/counterweapon closure rates that could exceed Mach 12, and on warhead blast field expansion rates following detonation easily could render impotent rocket-propelled, explosive warhead counterweapons. A directed energy weapon may be the only solution.
Similar conclusions can be drawn for undersea threats. A 200-knot torpedo provides little time to detect, localize, classify, attack, and kill the threat. Contrary to Chinese claims regarding an EP-3 and a jet fighter, it is nearly impossible for a slower, less maneuverable object to effect (or avoid) a destructive intercept with a faster, more agile one. Again, intercept kinematics and arming and fusing constraints suggest that an undersea directed-energy weapon may be the only possible defense.
As their name would indicate, directed-energy weapons are voracious consumers of power, so if their use in a warship is to be practicable, the ship must have a large reserve of generating capacity to supply them, and this is the forte of the integrated all-- electric design. Unlike a conventional propulsion plant, where more than two-thirds of the total energy available converted from the fuel is irretrievably dedicated solely to propulsion, the integrated design makes every bit of that capacity available to do whatever the commanding officer requires to best fight his ship. He can use the power for sprint speed, or he can dedicate virtually all of it to combat purposes. This approach is the key enabler for what could turn out to be the most important change in naval weaponry since Dahlgren's breach loading, rifled gun.
There Is No Panacea
Clearly, the all-electric ship brings a revolutionary dimension to warship design and to the way in which war at sea is prosecuted. But just as clearly, it cannot solve all of the challenges. Unfortunately, in this regard, the "marketing" of the concept has been deficient. In particular, the integrated power system has been advertised as a means to reduce warship crew size to absurdly, dangerously low levels.
While it is true that an integrated power system presents opportunities to reduce the number of sailors required to run the ship during normal steaming operations, normal steaming is not the primary determinant for crew size. That determinant is battle; inflicting damage on the enemy and controlling the damage he will attempt to inflict on you. It is the second part of that equation—damage control—that most often has been addressed with "ostrich engineering," looking away from the real challenges and/or explaining them away with infusions of information technology.
Since 1970, every catastrophe sustained by a frigate, destroyer, or cruiser has required the full attention of every surviving member of the crew to save the ship and, in most cases, dedicated assistance from ships in company. The Belknap (CG-26), Bordelon (DD-881), Stark, Conyngham (DDG-17), Samuel B. Roberts, Princeton, Cole—all but two of these ships were smaller than DD-21's projected displacement. All had at least two and one half times DD-21's advertised complement. None was immediately lost, although two later were struck from service. At best, one life was lost; at worst, 18% of the crew perished. In most cases, at least 25% of the crew was momentarily disabled with injuries. In several of these ships, fire-fighting, dewatering, and communications systems were disabled, either through heat, flooding, physical damage, or loss of electrical power, dramatically complicating the task of overcoming the damage.
These lessons regarding the level of effort required to combat serious damage are not new. One need only scan action summaries for Iron Bottom Sound, Midway, or Okinawa. Handling fire hoses requires a certain number of sailors per hose. In many cases, the intensity of the fires limits a team's direct attack of the blaze to less than a minute, so multiply the number of sailors on a pair of hose teams by the number of teams needed to sustain the effort. Similar analyses will reveal an appropriate number of sailors for control of fluid system damage and preservation of stability and watertight integrity.
What is new is the insistence that automated systems will reduce these requirements. Quite simply, information technology has not lived up to the sizzle promised, nor can it overcome certain physical realities of battle damage. The Yorktown (CG-48) was left dead in the water as a result of failures in her Smart Ship suite. Automated damage control features in the Cole failed. Automatic watertight closures work only if power is available, and if the structural alignments are not severely disrupted. Power failures are a given, thus a ship without sound-powered phones is literally deaf and dumb. Automatic sprinklers and fire main alignments also depend on electrical power. Further, flanged connections and brazed joints in those systems have failed and will fail under the intense heat of a fire at sea. The same vulnerabilities also can be ascribed to fuel system piping.
Building an all-electric ship with an integrated power system and a heavy dose of automated controls is within the realm of possibility, and a fleet of such ships could reshape war at sea to our Navy's decisive advantage. But the focus of the design process must be on combat capability—not accounting. After all, it is our warriors who will take the ship into harm's way. The accountants will continue to sit safely and comfortably at their desks ashore, protected by those warriors.
Captain Vining, a 1973 graduate of the U.S. Naval Academy and a nuclear-trained surface warfare officer, retired in 1999 after 26 years of service. He served in nuclear-powered cruisers and carriers, completed a command tour in a Knox (FF-1025)-class frigate, and was assigned as Assistant Chief of Staff for Surface Warfare Programs at Commander, Operational Test and Evaluation Force. A licensed engineer, he now works at Noesis, Inc., a small engineering firm providing support to the Naval Sea Systems Command and the Office of Naval Research.