Tango Bravo: breaking down barriers in submarine design
JANE'S NAVY INTERNATIONAL - APRIL 01, 2007
Tango Bravo: breaking down barriers in submarine design
The US Navy and Defense Advanced Research Projects Agency are seeking
to rationalise submarine platform infrastructure requirements in order
to reduce the cost and increase the flexibility of future designs.
In 1951 the US Congress gave the green light to the construction of
the world's first nuclear-powered submarine. Just 18 months later, USS
Nautilus became the first commissioned nuclear-powered vessel in the
US Navy (USN).
That was more than half a century ago. Yet in the intervening period,
the fundamental precepts underpinning nuclear submarine design have
changed remarkably little, with successive generations of boat still
characterised by an aft propulsor; a rear compartment accommodating
machinery spaces and a long propeller shaft; a central reactor
compartment; and a forward compartment with sonar rooms, torpedo rooms
and crew berthing. However, the USD97 million Tango Bravo programme is
looking to change all that. Launched in early 2005, its goal is to
identify techniques and
technologies to overcome identified Technology Barriers (hence 'Tango
Bravo') and thereby achieve the twin goals of reducing cost while at
the same time improving operability.
Led by the Defense Advanced Research Projects Agency (DARPA) the
central research and development organisation for the US Department of
Defense the four-year technology demonstration programme has in effect
sent the submarine engineering and manufacturing community back to the
drawing board to completely rethink the way submarines are conceived
and built. This is a demanding objective but a necessary one if the US
submarine force is to maintain its target force levels. Nuclear
submarines are, ton-for-ton, the most ex-pensive assets in the USN
fleet. While current build plans see one Virginia-class nuclear attack
submarine (SSN) entering service each year through 2010, ramping up to
two boats per year by 2012, the USD2 billion-plus unit production cost
has cast a long shadow over the projected 30-boat buy. It is
acknowledged that Tango Bravo will not solve near-term quandaries.
Rather, its goal is to rationalise platform systems engineering while
matching Virginia-class capabilities, including submerged speed and
depth, and affording the potential to accommodate an enhanced payload.
The origins of Tango Bravo can be traced to a DARPA-led Submarine
Design
Study completed in 2004 (which was itself informed in part by the
Defense Science Board Task Force's 1998 report 'Submarine of the
Future'). DARPA's conclusions demanded a radical 'clean ***' rethink
of the way in which
submarines are designed. Five technology barriers were identified:
propulsion not constrained by a centreline shaft; externally stowed
and launched weapons; alternatives to the existing spherical sonar
array; technologies that simplify the submarine hull, mechanical and
electrical systems (HM&E); and automation to reduce crewing. Tango
Bravo was charged to find solutions to the tyrannies that these
barriers imposed.
In effect, the idea was to blow apart the existing paradigms for
submarines design, and thereby drive down the size and cost of
submarines to about half that of a Virginia-class boat. It is thought
unlikely that Tango Bravo will lead to quantifiably smaller boats, but
breaking the constraints imposed by
legacy engineering solutions should free -up significant space
(allowing for greater payload) and reduce production and through-life
costs (for example, using electrical actuators instead of hydraulics).
Industry partners
Under Tango Bravo, DARPA and the USN gave a team of industry partners
a set of challenges to develop and demonstrate technologies for
shaftless propulsion, external weapon storage and launch, and electric
actuation of
ship control surfaces for reduced ship infrastructure. In May 2005,
General Dynamics Electric Boat and DRS Technologies were each granted
12-month contracts to develop shaftless propulsion technologies, and
Electric Boat and Northrop Grumman Newport News were awarded 18-month
contracts to develop and demonstrate the ability to fire an
encapsulated Mk 48 heavyweight torpedo from outside the submarine's
pressure hull. Electric Boat was given additional contracts over 18
months to rationalise HM&E infrastructure by using an external
electrical actuator for the boat's control surfaces rather than
hydraulic systems. After competing the initial phase, Electric Boat
won the contract for electrical actuation and shaftless propulsion,
and Northrop Grumman for external weapons (the latter receiving a
USD12.7 million contract to continue its work in February 2007). With
the first phase of the electrical -actuation effort complete, DARPA
informed Electric Boat as JNI was going to press - that it will be
funding Phase 2 of the X-Planes external electric actuator technology
initiative. Electric Boat told Jane's that the USD3.2 million, 18-
month Phase 2 work will "take us a big step closer to the overarching
goal of ship electrification and elimination of the hydraulic system".
The three-part programme will include designing and building equipment
to test the Phase 1 actuator at test depth pressures; designing and
building a controller that is similar to what would be used on a ship;
and designing and building a lithium ion battery to provide backup
power to the electric actuator, offering the same kind of redundancy
now provided by hydraulic accumulators used in existing steering and
diving systems. The advantages of internal actuators have been proven
in the torpedo rooms of Vir-ginia-class submarines, but if they can be
successfully applied in external applications which are the biggest
users of hydraulic systems they can inch ever closer to completely
eliminating onboard hydraulics. As much of the continuing work is
proprietary to the contractors, DARPA now responsible for the
programme remains tight-lipped about Tango Bravo. However, spokeswoman
Jan Walker told Jane's at the US Navy League's Sea-Air-Space
exposition in 2006 that Tango Bravo is a "technological demonstration,
and if successful the technology would be available to the USN.
However, Tango Bravo is not about designing new submarines, but
developing ways to combat the identified technology barriers". While
many of the design characteristics laid down with Nautilus survive to
this day, there have been two key changes, according to Bob Work from
the Center for Strategic and Budgetary Assessments. First, to improve
sonar performance, the navy placed a large spherical array in the bow
of the boat, far away from the machinery spaces and propeller. "This
required the navy to move the torpedo tubes, which were in the bow,"
explains Work. "The solution was a central torpedo room, back from the
bow of the submarine, with two tubes on either side of the ship that
angle out from the ships's centreline". This arrangement turned out to
have tactical advantages. "In an age of wire-guided torpedoes, a
submarine commander could fire a torpedo at a submarine from the tubes
on the side of the boat away from the target, using the bulk of the
submarine to help dampen any tell-tale noises of the firing," says
Work. However, having a large spherical bow array and a large central
torpedo room demanded that SSNs be bigger. One of the earlier Tango
Bravo targets was to replace the large bow spherical array with hull-
adaptable sonar arrays that could be 'knitted' into the skin of the
submarine. However, investigations into the application of large-scale
conformal sonar arrays have been postponed indefinitely. The second
key change since Nautilus was the US Navy's subsequent emphasis on
achieving a high degree of submarine quietening, as opposed to early
Cold War-era Soviet submarines, which emphasised high speed and deep-
diving
capabilities. "The navy's quest for an acoustical underwater advantage
led to its adoption of the British invention of placing machinery on
floating 'rafts' that were isolated from the hull," Work explains.
"This kept machinery noise from being transferred directly from the
hull into the water. The navy also
developed skewed propellers, which greatly reduced their noise tonals,
and later podded propulsors." Long shaftlines and complex reduction
gears currently take up valuable room
in submarines. However, this driver for internal volume could be
significantly curtailed by installing and integrating electric power
and propulsion systems using electric motors such as a permanent
magnet motor (PMM) or a superconducting electric motor, and installing
propulsion pods external to the hull. In 2005, contracts were awarded
to Electric Boat and DRS Technologies, with the latter bidding
technology evolved from the 36 MW PMM previously developed for the
DDG-1000 destroyer programme. DRS argued that its motor afforded
commonality between subsurface and surface assets, so DDG-1000 and
submarines could have common power conditioning and propulsion
building blocks. However, the follow-on research contract ultimately
went to Electric Boat. The company is now looking to make the
technology for this concept work, building a series of small and large-
scale demonstrators for its design.
Shaftless propulsion
In existing submarines there are two turbines for propulsion and two
for power generation. Shaftless propulsion would necessitate only
turbines for producing electricity, freeing up the rear compartment
for other uses. Pete Schilke, programme manager for Tango Bravo at
Electric Boat, tells Jane's that "our shaftless propulsion stern
concept calls for electric motors on pods external to the hull. The
use of electric motors in lieu of the legacy submarine mechanical
drive scheme would not only eliminate steam-propulsion turbines, the
reduction gear, the propulsion shaft and its attendant support
systems, it would also provide tremendous design and operational
flexibility in the stern of the submarine, both inside and outside the
pressure hull". Also, this type of propulsion scheme enables a large
amount of electric power to be diverted towards non-propulsion uses,
such as future high-energy weapons, without a significant impact on
the speed of the submarine. Electric Boat is also keen to point out
that removing reduction gears could afford flexibility to incorporate
innovative payloads as envisioned by the company's submarine Concept
Formulation (CONFORM) group. These include building a stern docking
mechanism where smaller manned submersibles could 'mate' with the
submarine, or an alternative to the problem-ridden Advanced SEAL
Delivery System to land special forces ashore. However, the submarine
platform and payload concept development is not funded under the
company's DARPA contract for Tango Bravo. PMM technology could be
applied to any motor within the boat, internally or
externally. According to Electric Boat, such engines are low
maintenance, inherently simple, and the robustness and reliability of
the core technology has already been demonstrated in diverse
applications. The replacement of hydraulic actuation systems one of
the most expensive pieces of technology on board with electric
actuators is motivated by the desire to drive down ship-acquisition
and life-cycle costs. "The main benefit of electric actuation is that
it enables the elimination of the ship service hydraulic powerplant
and attendant supply and return distribution systems," says Schilke.
"This elimination is aimed at reductions in acquisition as well as
life-cycle costs, as the legacy hydraulic systems, though very
reliable, are expensive to build and maintain." Susceptible to
developing oil leaks and with components that need to be rebuilt,
hydraulics are a substantial maintenance driver, so if they could be
replaced with electrical actuators, the crew needed for maintenance
could be significantly reduced. John W Biederka, director of submarine
concept formulation at Electric Boat, tells Jane's that the crew has
been factored in from the very beginning. "When contemplating the
maintenance workload we reviewed the number of hours in a week that
crew members need to check over the equipment, and watched this to
make sure we don't overload the crew." In fact, electrical actuation
could lead to drastically reduced crew sizes. Biederka adds: "The
smaller we can make the ship, the better for powering, and potentially
electrical actuation could drive manpower out of the ships." In the
first phase of Electric Boat's electrical actuation effort, the team
worked to develop the necessary torque requirements and built an
electrical actuator "the biggest of its kind in the world that we know
of, which is a great achievement in itself", according to Biederka.
All torque
High torque density is typically better for submarine applications, as
it is easier to arrange smaller components in the limited confines of
a submarine's hull, and actuator size tends to decrease as torque
density increases. Biederka adds: "We demonstrated that Electric Boat
can develop
the necessary torque requirements, and have not got any suggestions
that this technology is out of reach. Now we need to understand how
the system can operate in the submarine environment, taking into
account factors such as signatures, and the effect of sea water, while
meeting the requirements
for stealth." Indeed, he sees the benefits of breaking through this
technology barrier as manifold, and not limited to infrastructure
reduction and cost saving. "This technology could also be applicable
to surface ships and there may be significant life-cycle savings
associated with electrical actuation, but we are too early on in the
project to determine this yet." So far, there are no plans for full-
scale shaftless demonstrations. However, Electric Boat has proposed
conducting at-sea testing of external actuators in the 2008-09
timeframe. Another tyranny of classic submarine design is the need for
a large internal weapons compartment. Under Tango Bravo, Northrop
Grumman Newport News has been tasked with looking for ways in which
submarines can carry torpedoes externally, akin to weapons stored in
the bomb bay of an aircraft. The vice-president of Northrop Grumman's
submarine programme, Becky Stewart, tells Jane's that "we are used to
existing paradigms, but the cost of building the submarines is
something which is forcing all organisations like DARPA, the USN and
the private-sector ship design and construction yards to be more
innovative". Stewart adds that "having an opportunity to stretch our
imaginations is exciting, and we are encouraged that the navy is
pressing the envelope and exploring new opportunities. "This is a very
good example of how the navy and the shipyards are working together to
focus on opportunities to cut costs; it is a great partnership."
External weapons
The external-weapons concept seeks to eliminate the internal torpedo
room from within the submarine pressure hull by moving the launchers
outside, obviating the need for complex handling systems and freeing
up space for different payloads. Furthermore, putting weapons into
external weapon 'clips' could offer further benefits: there would be
no need to carefully align the angled torpedo tubes; noisy torpedo
tube doors would disappear; and complex underwater firing techniques
would no longer be necessary. Northrop Grumman's
is the prime contractor for the work, and in February 2007 the company
was awarded further DARPA and USN funding to continue Phase II
development and testing. The Northrop Grumman-led team including Penn
State Applied Research Laboratory, Special Devices, Naval Undersea
million, 18-month contract in June 2005 to compete in Phase I of the
project. Phase I saw the design and build of a full-scale prototype of
a modular launcher concept. The team successfully launched a full-
scale Mk 48 torpedo at shallow depth to demonstrate the concept,
winning the Phase I competition. Under Phase II, Northrop Grumman will
spend the next 18 months continuing to develop the full-scale modular
launcher and performing additional full-scale testing. Chief engineer
Charlie Butler says that this new phase tasks the team with launching
the full-size weapon at maximum operating depth, and to "further
explore integration into a future platform". Furthermore, Northrop
Grumman must show the ability to stow, communicate with and launch a
torpedo from outside the hull at the speeds and depths of the
operating environment, to prove the method works. One of the main, and
most obvious, challenges the team has come across is overcoming the
lack of access to the weapon when it is on the outside of the pressure
hull, and the logistics of loading and servicing the weapons. In
current submarines, when the weapons are inside the hull, operators
have access to the torpedoes to troubleshoot any problems with the
weapon. However, if the weapons are moved to the outside of the hull,
the design must be robust enough to address potential failure modes of
launch and pre-launch. According to Stewart, the current paradigm is
to have multiple weapon reloads on a submarine, but "now the question
is how many weapons and launchers we can carry, which brings us back
to the ship design and integration issue".
Pin-point alignments
The current torpedo room and launcher construction process is
labour-intensive and expensive, with large openings to the external
environment that need pin-point alignments carefully maintained so
that weapons can be successfully loaded and launched. Further concerns
centre around potential environmental hazards, such as leaking toxic
fluids into
the ocean, and hydrodynamic considerations.
study methods by which to expel the weapon, we need to make sure that
we have a total system solution addressing all concerns and not just
the ejection piece".
Future weapons development is another major cost driver that is not
part of Tango Bravo, so Stewart sees Northrop Grumman's challenge as
designing the external launch capability "as much as possible around
the existing weapon
as opposed to doing a major modification of the weapon".
However, the first step is to design the capability to launch, and the
second step is how to integrate into the host platform. As Stewart
says, "under Tango Bravo, Northrop Grumman is not designing the
external weapons stow and launch as part of a new submarine design
just yet - we are just exploring the technology required to allow
stowing and launching Mk 48 ADCAP heavyweight torpedoes from the
outside of the hull. "There still remains considerable engineering to
address integration of external weapons into a future platform".
According to Northrop Grumman, a final Phase II launch demonstration,
testing at maximum operating depths, is scheduled for mid-2008.
Striving for silence
The drive towards ever quieter and increasingly undetectable
submarines has led, until now, to incrementally larger hulls.
Displacing 4,800 tonnes, the Sturgeon class (SSN-637) of the mid-1970s
was larger than the 4,300-tonne Permit class from the late 1960s,
while the
mid-1970s, was larger than the Sturgeon class at 6,927 tonnes. The
third and last of the Seawolf class (SSN-21), USS Jimmy Carter,
displaced 12,139 tonnes, with the other two Seawolf boats weighing in
at 9,137 tons significantly larger than the
However, entering the scene in 2003, the
broke the mould by achieving the same silencing as the Seawolf class
despite being smaller alerting the USN, DARPA and industry partners
that quiet did not necessarily mean large. The major drawback was,
nevertheless, that quiet did seem to mean expensive, with the
class costing USD2.5 billion per boat. "Ton-for-ton, nuclear attack
submarines remain the most expensive ships the
Work from the Center for Strategic and Budgetary Assessments. "In
essence, the Tango Bravo programme was an effort to see what it would
take to get a submarine with the same capabilities of the
but at 60-70 per cent of the cost. This generally was accepted as
meaning the submarine needs to be even smaller than the
only way to do this would be to break the US SSN design paradigm." If
Tango Bravo is successful in eliminating bulky hydraulics and
reduction gears, some commentators speculate this will have an impact
on reducing the acoustical signature of the submarine, but this is far
from proven at this stage.
Northrop Grumman's team has designed, built and tested a full-scale
prototype of a modular launcher concept for future submarines to carry
and launch Mk 48 torpedoes outside of the pressure hull. Key
challenges include how to access, load and service the weapons.
(Northrop Grumman
The torpedo room inside the Virginia-class attack submarine USS Texas.
While this is an evolution upon previous generations of submarines,
Tango Bravo is trying to find ways to stow and launch torpedoes from
outside the hull, increasing space inside. (USN)
The world's first nuclear-powered submarine, USS Nautilus, arrives at
Naval Submarine Base New London, Connecticut, in 1986, where it is now
a museum exhibit. (US DoD)
According to General Dynamics Electric Boat, X-Plane actuators present
the best opportunity to reduce submarine manufacture costs. Because X-
Stern has greater control authority than a conventional cruciform
stern, the company
hopes to be able to eliminate costly and complex hydraulics associated
with all planes, freeing up space in the bow for additional payload
volume. (General Dynamics Electric Boat)
The team at General Dynamics Electric Boat is working to produce a
shaftless propulsion concept, whereby legacy hydraulics such as
turbines, gears and shafts are stripped out of the stern and replaced
by electric pods on the outside of the pressure hull to provide
propulsion. This frees up valuable space inside the submarine's
pressure hull for alternative payloads. (General Dynamics Electric Boat
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