Navy and Industry Pursuing New Power and Propulsion Methods

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Published on: 05/23/2010
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Navy and Industry Pursuing New Power and Propulsion Methods

By Edward Lundquist

Alion Science and Technology

Advanced Materials, Manufacturing and Testing IAC, Rome, NY


The AMMTIAC Quarterly, Volume 4, Number 1
Powering the Future Force
New Power & Energy Technologies for the Warfighter

The Navy is testing new concepts in power generation, conversion,
and distribution to make ships more efficient, economic,
and combat-effective. Ships being developed in both the near
term and long term will have a variety of newly designed
propulsion systems depending on their size, mission, and ship
characteristics. This article discusses some key technologies on
the horizon.

ALL-ELECTRIC INTEGRATED PROPULSION

An integrated power system (IPS) is an all-electric architecture,
providing electric power to the total ship with an integrated
plant. IPS enables a ship’s electrical loads, such as pumps and lighting, to be
powered from the same electrical source as the
propulsion system (e.g., electric drive), eliminating
the need for separate power generation capabilities
for these loads.  To meet the increased power demands for new
sea-based weapon systems, next-generation surface
combatants, such as the DDG 1000
Zumwalt-class
of guided missile destroyers (see Figure 1), will feature
all-electric propulsion and an entirely new way of
distributing power for propulsion, ship service, and combat
capability.
All-Electric Propulsion is a promising technology for
both naval and commercial marine applications. On the DDG
1000, power will be generated by two large gas turbine generators
and two smaller ones. By using efficient power management,
power is available to handle all of the electric loads throughout
the ship, including potential future power-hungry weapons such
as rail guns or directed energy weapons.

The combat value of an electric ship goes well beyond weapon
capability and capacity. There are significant efficiencies and
redundancies. At full power, DDG 1000 will achieve speeds up
to 30 knots. If one of the main turbines is lost, the plant can
be isolated and still achieve 27 knots. Since a warship usually
cruises at reduced power once it has arrived on station, normal
station-keeping can be accommodated with the two small
turbines to save fuel and reduce radiated noise. The power previously
trapped in the propulsion train can now be directed to
enhance combat capability and mission flexibility. At lower
speeds,
Zumwalt has a surplus of power that can be made available
as needed. Further advantages include the elimination of maintenance-
intensive and hightemperature auxiliary steam systems, reduced noise
and vibration, and better fuel efficiency.
Among the major advantages of electric drive for naval ships is that the prime movers, whether gas
turbines or diesels, do not need to be located in a central machinery space or
mechanically connected to the propeller shaft as with
traditional propulsion systems.
Instead, the engines can be located anywhere
in the ship, distributed throughout the hull, and connected to
generators to supply power. This power can be fed to a central
bus that can be used for propulsion.
An all-electric integrated propulsion system enables more
design flexibility in terms of engine placement. For example,
the engines can be placed in the bow, stern, or even in the
superstructure for smaller engines. One of the advantages of
distributed power in a warship is survivability. If an engine incurs
damage or is incapacitated in one part of the ship, that part of
the distribution system can be isolated while power can still be
generated and distributed throughout the rest of the system. The
DDG 1000 will be powered by Rolls-Royce MT30 gas turbines,
which is based upon the Rolls-Royce “Trent” engine that powers
the Boeing 777 airliner. The aviation version of the engine has a
demonstrated reliability of 99.98%. The ‘marinized’ version of
the MT30 has 80% commonality with the Trent 800 but is
shock-mounted and has different blade coatings for operation in
a saltwater environment. This engine is also serving today aboard
the new Littoral Combat Ship
USS Freedom
(LCS 1). Zumwalt
will also have a smaller gas turbine, the Rolls-Royce 4500.
DDG 1000 power generators produce 4,160 volts alternating
current (AC), which is rectified to direct current (DC) that
allows ship service power distribution to be tailored to the ship’s
needs. There are three primary advantages to DC. First, DC uses
solid state power conversion that supplies loads which are converted
back to AC and is a cleaner way to supply power.
Secondly, many of the combat systems’ loads are DC. Finally, it
enables power to be shared and auctioned. DC enables uninterrupted
power even in the occurrence of a casualty.
The DDG 1000 will employ fixed pitch propellers.
Controllable pitch propellers and their associated complex
hydraulics are not required since the motor, and thus the shaft,
can be electrically reversed. But novel approaches to propulsion
are being considered for future combatants.
Other new naval ships are also adopting integrated electric
power systems. The next-generation CVN 21 aircraft carrier,
the
USS Gerald Ford (see Figure 2), will have a newly designed
nuclear power plant and all-electric systems and propulsion.
The next amphibious assault ship, the
USS Makin Island (LHA
6), will feature a combined gas turbine and electric propulsion
system.
The surface combatant IPS propulsion engineering development
model (EDM) for DDG 1000 is being tested at the Land-
Based Test Site (LBTS) at the Ships Systems Engineering
Station in Philadelphia. The test site has been used to evaluate
different configurations and motors. The test program validates
key system metrics such as torque, speed and power output, and
specific fuel consumption for the various configurations.
The Navy has tested the 18-megawatt (MW) advanced induction
motor (AIM), which will be the baseline for DDG 1000,
produced by Alstom at the LBTS. This is essentially the same
system installed on the Royal Navy’s new Type 45 destroyer, the

HMS Daring
, which has just been commissioned. The IPS
features Integrated Fight through Power (IFTP), a fully automated
DC Zonal Electric Distribution System (DC ZEDS) that
provides flexible, reliable, high quality power to all shipboard
loads. Other configurations are also being tested. The IPS system
is fully automated with little operator intrusion. The testing at
the LBTS will validate that the DDG 1000 IPS will automatically
take appropriate corrective action if there is a malfunction or
casualty without the input of an operator.
Engineers at the LBTS have also tested a 36-megawatt permanent
magnet motor (PMM). PMM has greater power density
than the AIM and may be used in future ships.
Many studies were performed on different combinations of
gas turbines. The purpose was to avoid development of new
gas turbines that were not qualified and in service or on their way
into service.
Although there are advantages to distributing the power system
throughout a warship hull, the size and weight of the various
components has usually necessitated keeping the propulsion
equipment low in the ship for stability reasons. The DDG 1000
engineering plant layout is relatively conventional because of the
air intake, exhaust, and drive arrangement.
DRS Technologies and General Atomics Electromagnetic
Systems are developing a hybrid electric drive which permits a
smaller service gas turbine to power a permanent magnet motor
that can power the ship at slow or “loiter” speeds. Using a smaller
turbine can result in significant fuel savings. Furthermore, the
motor can be reversed to function as a generator when propulsion
gas turbines are online.
Overall, integrated electric drive offers ship designers and
operators a plant flexibility that does not exist with mechanical
drive systems. However, trade studies must be used to select the
appropriate power and propulsion system for each ship.
There are some ships with partial electric drive or hybrid electric
drive mechanical drive systems. These include the operational
Type 23 frigates; the European Multi-Mission Frigates
(FREMM), a joint program between France and Italy, which are
now in construction for France, Italy, Morocco and Greece; and
the amphibious assault ship
USS Makin Island
(LHD 8), now
undergoing trials.
Despite the advantages, there are not a lot of electric drive
warships in service. The new generation of electric ships has yet
to prove themselves. The DDG 1000, Royal Navy Type 45, and
T-AKE propositioning ships are examples of all-electric warships,
but they are still in the design phase, under construction, or
just entering service. Even though there is significant interest in
electric drive systems, there are only a relatively small number of
ships actually under construction and in operation.

SUPERCONDUCTING MOTORS

American Superconductor and Northrop Grumman have
recently tested a 36.5-megawatt high-temperature superconductor
(HTS) ship propulsion motor at the LBTS. The motor uses
HTS wire that can carry 150 times more power than copper
wire used in more conventional motors. The advantage is more
compact propulsion systems which have greater power density.
Superconducting wire can carry more current and generate
higher magnetic fields in very small areas and thus can result in
a significantly smaller motor. In other words, more power is
available from smaller, lighter motors. That means Navy ships
can carry more fuel and munitions and have more room for
crew’s quarters and weapon systems.
General Atomics’ (GA) superconducting DC homopolar
motor for propulsion applications is small and light compared
to traditional and superconducting AC motor systems. This
motor uses low-temperature supercooling that employs gaseous
helium to maintain the superconducting wire within the motor
at 5 Kelvin, which is almost absolute zero. Since some materials
are much better conductors at very cold temperatures, and with
virtually no electrical resistance supercooled conductors make
for much more efficient motors. A comparable high-temperature
supercooled system operates between 40 and 75 Kelvin,
depending upon the technology chosen. Refrigeration at higher
temperatures is easier, but the high-temperature superconducting
material is not as easy to produce and is much more expensive
than the superconducting niobium-titanium wire in the
low-temperature motor. Niobium-titanium wire is the most
widely used and available superconducting wire in world-wide
commercial applications.
GA has built a 5,000 horse-power (HP) motor which is
4.5 feet in diameter. This technology is slender, light, and fuelefficient
and can be more readily adapted to propulsion pod
applications.
Additionally, while superconducting AC motors have similar
costs to the superconducting DC motor, there is no need for
power inverters and the associated electronics to switch DC to AC.

Propulsion Pods

Most marine motor applications are located within the hull and
coupled to a shaft to turn a propeller or waterjet impeller. Electric
power can also be used for propellers or waterjets but can also
power propulsion pods, which can be located outside the hull.
Pods provide better maneuverability to ships entering and
leaving port or maintaining a precise station. With a significant
amount of propulsion equipment located outside the hull, more
room is available inside the ship for other purposes. Also, the signatures
could be mitigated if the propulsion system was isolated
inside the hull.
Cruise ship pod systems, such as “Mermaid” from RRAB (a
joint venture with Rolls-Royce AB and Alstom) and ABB’s
“Azipod” systems, can rotate 360 degrees and eliminate the need
for rudder assemblies.With a pod, the motor is in the pod, while
an azimuthing thruster has the motor located in the hull. The
Royal Navy’s
Echo-class of survey vessels uses electric azimuthing
thrusters. Pods were considered for
Zumwalt-class ships but
ruled out because of their size.
The US Navy has used Small Water Plane Area Twin Hull
(SWATH) ships for research and surveillance. These catamarans
have long and slender motors and other propulsion
equipment located in the submerged cylindrical buoyant hull
sections, but prime movers can be mounted above the waterline.
ThyssenKrupp’s Nordseewerke has built the SWATH
research vessel
Planet for the German Federal Office of
Defense Technology and Procurement.
Planet will assess new
propulsion technologies and evaluate the sea keeping characteristics
of the SWATH hull form. Its electric propulsion
enables it to test mine detection and undersea warfare systems
and countermeasures.
Siemens in Germany is finding improved power availability
and system responsiveness with high-temperature superconductors
for podded waterjets applications. Siemens is also
developing fuel cell technology for ship propulsion.
Waterjets

While not a new form of propulsion, waterjets have not been
used on larger ships until recently. They present some clear
advantages for warships. Waterjets deliver rapid acceleration and
can sustain high speeds. Waterjet-powered ships are extremely
maneuverable and can stop quickly. They offer simplicity. The
flow is constant in a single direction. Engine loading is constant,
regardless of vessel speed, and waterjets do not overload the
engines. There may be no need for a gearbox. Astern propulsion
is applied by means of deflectors that divert the jetstream forward.
Precise station keeping can be maintained with waterjets.
There are many advantages of waterjets. The most prominent
advantage is the shallow draft of the system. Waterjets do not
have appendages (such as propellers, shafts and struts, or rudders)
that extend below the waterline. This minimizes the risk of
damaging the propulsion gear from grounding or from hitting a
submerged object, and it also reduces the maintenance requirements.
As a result the boats can operate close to the shoreline,
land on a beach for deployment of troops or equipment, or even
run over submerged logs or sandbars without damaging the
propulsion equipment. In addition, floating debris (such as
ropes, nets, or weeds) does not pose much of a risk to the system
particularly at high speed. Even though these items may be
drawn into the jet unit at slow speeds, they are unlikely to cause
damage and can easily be removed.
Waterjets are reliable. Like propeller-driven ships, there is still
a shaft but it turns the pump impeller at a constant speed as
compared to a much larger propeller. Drive shafts, gear boxes,
and engines receive less stress, thus prolonging their service lives.
The entire propulsion system requires less maintenance.
Waterjets are more efficient at higher speeds, particularly in
multiple drive installations such as catamarans. With no underwater
appendages, there is no increase in hull resistance as speed
increases or more drives are added. Efficient operation can also
be achieved over a broader range of speeds compared to propellers.
Waterjets cannot overload an engine due to excess boat
weight, towing, or extreme seas because they operate independently
of the body of water under a boat.
A fast vessel needs a relatively higher amount of power than a
slow vessel, and waterjets can provide a relatively large amount of
power despite their relatively small size. Conventional propulsors
would require relatively large propeller diameters.
A clean hull design, free of appendages, delivers greater speed.
Drag resistance increases significantly as ship speed increases.
Therefore, the absence of appendages becomes increasingly
important as ship speed requirements increase.
The Office of Naval Research (ONR) uses an experimental
130-foot-long craft called the Advanced Electric Ship
Demonstrator (AESD) to test various waterjet-based propulsion
configurations at the Navy’s Acoustic Research Detachment at
Lake Pend Oreille, Idaho. ONR engineers achieved improved
efficiency and maneuverability with a smaller, lighter propulsion
system while reducing noise at the same time. Named
Sea Jet
(see
Figure 3), the craft is essentially a quarter-scale model of the
DDG-1000 destroyer. It has been used to test an AWJ-21 underwater
discharge waterjet from Rolls-Royce Naval Marine, Inc.,
to validate better propulsive efficiency, reduced acoustic signature,
less drag, and better speed as well as improved maneuverability
for future surface combatants by eliminating rudders,
shafts, and propeller struts.

Sea Jet
has also been employed to demonstrate the General
Dynamics Electric Boat RIMJET propulsor, which is a podded
system that features a permanent magnet motor to power a
propeller in the rim, rather than the hub, of the pod. The system
uses sea water for coolant, which eliminates the typical
elaborate cooling system consisting of pumps, piping, and heat
exchangers.
ONR has also developed an Advanced Hull Form Inshore
Demonstrator (APHID) which is testing a complete electric
podded propulsion system. The Rim-Driven Propulsor Pod
(RPD) uses a Pulse-Width Modulated (PWM) motor drive system
mounted on the Hybrid Small Waterplane Area Craft
(HYSWAC). Called
Sea Flyer, the HYSWAC is built from a
modified Navy Surface Effect Ship and uses a Vericor TF-40 gas
turbine prime mover. Sea Flyer features an underwater lifting
body ship that combines the high-speed capabilities of a hydrofoil
and the rough-water stability of a small waterplane area twin
hull (SWATH), so it delivers higher speed and improved stability
over comparably sized vessels.
Cost can be an initial disadvantage of waterjets. They are
expensive to purchase and maintain. Waterjets are made from
costly stainless steel, which is more expensive than other propulsors
that are typically made from copper alloys. However, waterjet
lifecycle costs are relatively lower. Waterjets are less prone to
impact damage, and reduced engine stress results in less engine
maintenance and longer engine life.
The Littoral Combat Ships (LCS) will employ waterjets.
Waterjets were chosen for LCS to provide high speeds in shallow
waters, where the LCS will operate to combat asymmetric antiaccess
threats in the littoral regions of the world. Two variants
of LCS are being built. Lockheed Martin has delivered the

USS Freedom
(see Figure 4), a semi-planing monohull design
built at Marinette Marine in Wisconsin. General Dynamics is
building a trimaran, the
USS Independence, at Austal USA in
Mobile, Alabama. Both will have diesels and gas turbines, and
both will employ waterjets. The General Dynamics LCS has four
steering and reversing waterjets, while the Lockheed Martin LCS
has two steering and reversing and two booster jets. Both ships
displace about 3,000 tons and up to 4,000 tons fully loaded.
This will make the two LCS combatants the largest naval waterjet-
powered warships.
While the two versions have taken different naval architectural
approaches to the mission, both “seaframes” will carry mission
modules that can be reconfigured to adapt to each ship’s combat
mission assignment.

USS Freedom
is powered by two Rolls-Royce MT30 36 MW
gas turbines and two Fairbanks Morse Colt-Pielstick 16PA6B
STC diesels. The seaframe is based on the Fincantieri-built,
Donald Blount-designed high-speed yacht
Destriero, which
holds the record for the fastest transatlantic crossing (60 knots).
The 378-foot
Freedom has a steel hull with aluminum superstructure.
The two 36 MW gas turbines and two diesel engines
power four large Rolls-Royce Kamewa waterjets. Four Isotta
Fraschini Model V1708 ship service diesel generator sets provide
auxiliary power.

USS Independence
, the slender stabilized trimaran monohull
built by the General Dynamics team, has an overall length of
418 feet, maximum beam of 93 feet, and full load displacement
of 2,637 tons. The seaframe is based on Austal’s design for the
Benchijigua Express passenger and car ferry. Two General
Electric LM2500 22 MW gas turbines and two MTU
20V8000M90 9100 kW diesel engines are the prime movers,
powering four large steering and reversing Wärtsilä-Lips 2 X
LJ160E and 2 X LJ150E waterjets. With all propulsion flat out,
the Wärtsilä-Lips waterjets together expel roughly 27,000 gallons
of seawater per second exiting from the jet nozzles at a speed
around 90 mph. The trimaran variant built by General
Dynamics will also have a retractable azimuth thruster.

CONCLUSION

One design is not optimum for all situations. Cruise ships with
large portions of their itineraries at low power benefit from electric
drive. Fast ferries, which go to full throttle as soon as they
clear the breakwater and remain at full throttle until they reach
the next port, would be at a disadvantage with electric drive.
There are advantages to a mechanical drive system. Mechanical
drive systems are more efficient compared to electric drive systems
in terms of their ability to transmit energy from the prime
mover to the propulsor. For example, the mechanical drive is
estimated to transmit approximately 98% of the energy from the
prime mover output shaft to the propulsor. The electric drive is
estimated to transmit between 91% and 93%.

ACKNOWLEDGEMENTS

The author would like to thank Mike Worley, Vice President
of Naval Marine Programs for Rolls-Royce North America;
Mike Collins, former Program Manager for Integrated Power
Systems with Program Executive Office–Ships (PEO Ships);
Read Tuddenham, General Electric’s Manager of Integrated
Propulsion Systems and New Applications; Michael Reed,
Senior Vice President for Advanced Technology with General
Atomics Group; Tony Kean of HamiltonJet, Christchurch, New
Zealand; and Marit Holmlund-Sund of Wärtsilä.
Captain Edward H. Lundquist
, US Navy (Ret.), is a Senior Science Advisor with Alion Science and Technology, Washington, DC. He is a senior-level communications professional with more than 24 years of public affairs, public relations, and corporate communications experience in military, private association, and corporate service. During his 24-year naval career, Mr. Lundquist qualified as a Surface Warfare Officer and later served as a Public Affairs Officer. He retired from active duty in 2000. He currently supports the Director for Surface Warfare on the staff of the Chief of Naval Operations. Lundquist currently is member of the executive committee for the Surface Navy Association, and serves as vice president of the Greater Washington Chapter. He is an Accredited Business Communicator (ABC) and the vice chair of the International Association of Business Communicators Accreditation Council. Lundquist is a graduate of Marquette University in Milwaukee,Wisconsin and holds a master’s degree in journalism and public affairs from the American University inWashington, DC. He writes frequently for publications including Armed Forces Journal, Unmanned Systems, Naval Forces, Warships International, Maritime Reporter,
and others.

http://wstiac.alionscience.com/pdf/WQV9N1_ART04.pdf 

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