"Submarines are still characterised by an aft propeller; a rear compartment for machinery space and a propeller shaft; a central reactor compartment; and a forward compartment with sonar rooms, torpedo rooms and crew berthing.
However, Tango Bravo – for Technology Breakthrough – identified five key technology barriers that could be conquered in order to slash construction and maintenance costs. These included stripping away hydraulics and developing propulsion unconstrained by a centreline shaft; stowing and launching weapons outside the hull; alternatives to spherical sonar arrays; simplifying the hull, mechanical and electrical systems (HM&E); and reducing the size of the crew through automation.
With the latter largely considered an evolutionary process, serious money has so far only been thrown at shaftless propulsion, electrical actuation and external weapons.
In May 2005, General Dynamics Electric Boat (GDEB) and DRS Technologies were granted 12-month contracts to develop shaftless propulsion technologies, while GDEB and Northrop Grumman Newport News were awarded 18-month contracts to develop and demonstrate firing an encapsulated Mk 48 heavyweight torpedo from outside the pressure hull.
The first part of the GDEB contract encompassed 18 months' work to develop a reduced ship infrastructure using electric actuation instead of hydraulic, and mechanical actuation of ship control surfaces, including the ship's rudder, stern and bow planes.
Phase 2 includes the development of the actuator's motor drive and energy storage systems, and packages the actuator for an external sea water environment, then cycles the immersed actuator under full-scale dynamic loads.
The main advantage of electrical X-planes actuation is the elimination of hydraulic system infrastructure and hydraulic-mechanical drive pressure hull penetrations.
Challenges include demonstrating high electric motor torque density to reduce size and weight, as well as ensuring control surface system reliability and emergency performance by developing an alternative to stored hydraulic accumulator energy.
Says DARPA's Jan Walker. "The challenges of having the propulsor's prime mover in the ocean environment vice inside the submarine pressure hull are a significant focus of the project."
In existing US submarines, there are two turbines for propulsion and two for power generation, but shaftless propulsion would necessitate only turbines for producing electricity, freeing up the rear compartment for other uses.
Electric motors would be placed on pods external to the hull, eliminating steam-propulsion turbines, reduction gears and propulsion shafts and providing design flexibility in the stern.
With an integrated power system - just two turbines of the propulsion scheme allowing power to be diverted to other uses - it is possible to have propulsion turbine generators and maintain the existing ship service turbine generators.
GDEB is planning to devise an integrated power system to free up energy for other uses such as futuristic weapons or recharging a large set of underwater vehicle batteries. "It is a shared resource, so there are areas where you would have to reduce the propulsion bell," explains Pete Schilke, programme manager for GDEB.
To mitigate this, the power could also be used to charge up a capacitor bank, which would discharge in a fraction of a second, reducing perturbation on the power system.
Even if an enormous amount of power was diverted for seconds to power a futuristic weapon, the amount of time power is diverted from the propulsor would not lead to a significant reduction in speed.
One concern about shaftless propulsion and external weapons is that appendages cause additional drag, so pods must be carefully designed to hydrodynamically encapsulate the machinery.
So far, breadboard testing of the motor/controller drive components and system have been completed, with water tunnel testing of the model-scale rotor assembly completed in April and the final design of the Integrated Motor Propulsor and Drive (IMPaD) signed off in June.
The actuator is immersed and pressurised in the test-stand to simulate submarine test depth to see if there are any wear characteristics, and there will be some contamination in the sea water, as the actuator would experience as water quality degrades near port.
This constitutes the biggest hydraulic load on the submarine, so Schilke says "with the goal of eliminating hydraulics, we have done a nice job at addressing the holy grail and gone a long way to prove we can do this and it is within our grasp".
GDEB has addressed the external electrical actuators on the control surface and motors to retrieve different arrays, so once internal actuators have been addressed, the team is confident it will be able to eliminate hydraulics. "It just depends on how large the actuators are, how much they weigh and how expensive they are to maintain," says Schilke.
Another target of the Tango Bravo programme is to develop an external weapon launch system to stow, communicate with and deliver an unencapsulated Mk 48 Advanced Capability torpedo from outside the pressure hull at speed and depth.
Phase 1 involved concept development, a shallow water full-scale test, and modeling that validated hydrodynamic performance and concept design requirements.
Phase 2 aims to refine this concept and manufacture a full-scale launcher test unit to demonstrate launch dynamics, signature and operational requirements.
For its part, DARPA considers the principal advantage of externally stowed weapons to be the elimination of the largest driver of internal pressure hull volume - the torpedo room.
Additionally, putting weapons into external weapon 'clips' means 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.
The launch of full-size weapons at maximum operating depth sets the company the challenge of addressing integration problems, such as 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.
Bob Work from the Center for Strategic and Budgetary Assessments tells Jane's that "while I do not want to say Tango Bravo is unimportant, it is not as big as it used to be. The current priority is getting Virginia costs down".
Work believes that budget pressures and the high non-recurring engineering and design costs involved in developing a 70 per cent cheaper submarine have worked against Tango Bravo being transitioned to a full-blown design effort for a new, cheaper SSN. Instead, he believes that those holding the purse strings will be inclined to quit while they are ahead, and take lessons learned from Tango Bravo and plough them into ensuring a more flexible payload for a cheaper Virginia class.
Redesigning submarines demands a high premium up front, but giving the Virginia class more capability could be an attractive cheaper alternative, especially once shipbuilders have got to grips with the modifications. For example, GDEB has managed to cut one million man hours from the production cycle of the second Virginia-class submarine compared to the first.
"This translates into real savings, which can be taken and ploughed into making the Virginia even more capable with new modifications. They will be getting either a cheaper submarine or a much more capable submarine for the same amount of money," Work tells Jane's.
Walker stresses that Tango Bravo's focus is to facilitate the design of a reduced-size, reduced-cost submarine that would cut acquisition and life cycle costs of any future platform, without sacrificing capability.
She explains that DARPA never planned Tango Bravo to develop an overall submarine concept, but rather to fund industry to "give design options for consideration in future submarine designs".
For example, Work notes that the USN will soon replace the Virginia's spherical array with a conformal bow array, which was one of Tango Bravo's original goals. This and other improvements will bring the price of each boat down to USD2 billion and help the USN to afford to increase the production rate from one boat per year to two.
From a shipbuilding point of view, it seems that changing a Virginia class to accommodate external weapons or shaftless propulsion would require such major modifications that "it would probably be better to develop a whole new ship design", according to Schilke. "
How does all this tie into the earlier-released RAND study on the need to begin designing the next, post Virginia, SSN? I don't know, but check out the RAND study for yourself.