Mr. Henry is a League member and is Treasurer of the Capitol Chapter. He is a naval architect and retired from the Naval Sea Systems Command in 1999 after 35 years of working in early stage submarine design and submari11e-related R&D management. His last position was as Head of Submarine Preliminary Design and as Principal Naval Architect for the Virginia class.
It sometimes appears that everyone inside the Beltway is a submarine designer-expert witnesses testifying before Congress (and members of Congress too), senior DOD and Navy personnel, journalists and novelists, scientists and engineers working in submarine-related R&D, and others. They have, over the years, voiced their opinions (loudly or quietly) about what the next US Navy attack submarine ought to look like or the effects of introducing new hardware into an already existing submarine design. Unfortunately, with very few exceptions, these well-intervention people do not understand the intricacies of submarine design and many of their conclusions regarding the ship design impacts of their ideas are erroneous.
In fact, there are very few people who do understand the intricacies of submarine design and those who understand them best are the naval architects who, at one time or another, have performed studies related to designing a new submarine and/or evaluating the total-ship impacts of major design changes to existing designs. In the United States, this group probably numbers fewer than three dozen people, more than half of whom are retired or working in other areas of ship design or in other fields. (Designers of submersibles are not included in this count.)
Examples of proposed submarine design changes include:
- Engineers, seeking R&D funds, propose new components for an existing submarine’s engine room that are considerably lighter than the existing components. The engineers, who do understand that to add “new weight” to a submarine requires taking some “old weight” out, state that this will permit carrying additional weapons without increasing the ship’s size.
- A proposal is made to significantly reduce the crew of a new design submarine. With significantly less space required for its crew, the ship would be smaller and less expensive than an otherwise equivalent ship with a typical complement.
- Proposals are made to move weapons or propulsion machinery components outside of the pressure hull in a revised version of an existing submarine design. With less volume needed for these functions, the pressure hull and, perhaps, the total ship, can be made smaller.
- R&D personnel state that adding their new component to an existing submarine will greatly increase its effectiveness and, since the component only weighs X tons, it will have little effect on the ship.
Are the postulated ship design impacts of these proposals accurate? The answer is, It depends! Yes, it depends on the naval architectural attributes of the specific submarine design being addressed. What might be true for one submarine may not be true for another. To explain the naval architectural attributes that so greatly influence the impact of design changes, we’ll start with a very short course on early stage submarine design.
Process for Performing Submarine Concept Studies
Early stage submarine design encompasses a broad range of design activities, from rough order of magnitude (ROM) studies, involving one or two people for up to a few weeks, to preliminary design, involving a large team of people for many months. This article focuses on design studies performed to an intermediate level of detail, commonly (and interchangeably) called concept studies, concept designs, and feasibility studies, hereinafter called concept studies.
The primary products of a submarine concept study are ship characteristics (displacement, length, draft, speed, etc.), arrangement drawings, and a weight report. They are performed to a level of detail enabling:
- definition of ship characteristics for use in operational effectiveness analyses
- calculation of weight groups for use in cost analyses
- accurate trade-off studies
- While sometimes useful, design studies performed to a lesser degree of detail (ROM studies) will not meet these requirements.
Beginning with a set of requirements developed by OPNA V [which come in the form of ship characteristics (such as depth, speed, and number of torpedoes) and specific payloads (weapon systems, sonars, etc.)], the conduct of a concept study entails four major steps: arrangements, volumetrics, weights, and ship balance.
Submarine arrangements is a graphical process (performed with paper and pencil or computer graphics) wherein the ship geometry is selected (hull diameter, pressure and outer hull configuration, major compartments, bow and stem shape, etc.) and the compartments are arranged to satisfy the design requirements in the mini-mum ship length. While submarine design is extensively computational, there is still a significant degree of art in arrangements.
The initial step for a new concept study is to select an appropriate overall ship geometry that will be driven by the ship design requirements, for example, reserve buoyancy (single or double hull), single or twin screw, number and types of weapon launchers (SSN vs. SSBN), etc. Unlike in surface ship design, submarine beam (hull diameter) is typically selected very early in the design process based on a number of factors with the minimum diameter of propulsion spaces (usually determined by the selected reactor in nuclear powered ships) and the number of platform decks in the operations compartment among the most important. With hull diameter selected, the arrangement process primarily involves establishing compartment lengths to accommodate the required systems. The process yields a total minimum ship stack-up length satisfying various system-related geometrical constraints. The resulting arrangement drawing is extensively used in subsequent volume and weight calculations. Since the hull thickness and frame dimensions affect the arrangement of components, during this step the pressure hull is designed, using an appropriate hull material, to attain the required operating depth.
In this step, surface and submerged displacement, and their centers of buoyancy, are calculated from the arrangement drawings. The everbuoyant volume of the submarine defines the surface displacement. Adding the net blowable main ballast to the surface displacement yields the submerged displacement.
The largest component of surface displacement is the pressure hull. Since it is typically defined by simple shapes, its volume and center of buoyancy are readily obtained. Hence, most of the effort required to calculate surface displacement involves those portions of the submarine external to the pressure hull: non-pressure hull structure, external components (air bottles, VLS tubes), recesses, and appendages (sail, control surfaces, propeller).
The KM (metacenter), used to determine surfaced stability, and the surface trim and navigational draft are also calculated using the results of the volumetric analysis.
In this most time-consuming aspect of the submarine concept study process, weights and their centers of gravity are estimated for approximately 160 weight groups to obtain the total Al weight, similar to the lightship weight for surface ships. The weights and centers of various loads (diesel fuel, ammunition, provisions, etc.) are also calculated. Accuracy is essential to avoid potential large inaccuracies in ship size estimates and to enable valid tradeoff studies.
The primary sources of information for weight estimates are:
- historical data, found in weight reports for similar submarines, which is appropriately modified to suit the requirements of the current design
- calculation of weight from the design, for example, pressure hull weight from hull scantlings
- direct input of weight data from cognizant system engineers, such as the weight of a diesel generator or combat system electronics component
Vertical and longitudinal centers of gravity are determined from the arrangement drawing.
The weight estimate includes sufficient margin to assure that the ship can accommodate weight increases occurring during design development and ship construction and to permit improvements to be incorporated into the ship during its service life without having to modify the ship’s geometry.
With displacement and weight determined, along with their respective longitudinal and vertical centers of buoyancy and gravity, the naval architect can balance the ship. This is also referred to as finding the lead solution because the computations determine the amount and proper location of lead ballast in the design. Since balance determines the submarine naval architectural attributes that so greatly influence the impacts of design changes, this step is more fully described. The process includes:
- Satisfying Archimedes principle: Weight must equal displacement for neutral buoyancy.
If the displacement is greater than the weight, the ship is volume limited -the arrangement determines minimum ship size. Lead ballast is added to increase the total weight until it equals the displacement and there are no design iterations involving changes to the hull geometry. Volume limited ships have excess margin. Thus, there is essentially no space within the pressure hull to add new components but heavy items not requiring internal space might be accommodated by removing margin lead. Spending large sums of money (on exotic light-weight materials, for example) to reduce weight has no benefit since saved weight will be replaced with lead ballast.
If the weight exceeds the displacement, the ship is weight limited and the displacement must be increased. This is usually accomplished by lengthening the pressure hull which adds both displacement and weight, but more of the former. With additional pressure hull volume, the main ballast tanks must also be increased in size to maintain the desired percentage of reserve buoyancy, further impacting weight and displacement. As a result, weights and volumes must be recalculated and the ship rebalanced. Weight limited ships have excess volume but this is usually spread around selected portions of the ship (perhaps providing improved access for maintenance) rather than concentrating it in empty compartments and, thus, providing a temptation to install even more equipment and worsening the weight limited situation. Weight savings are highly beneficial since they result in ship size reductions.
Achieving longitudinal balance: For the submerged ship, the longitudinal centers of buoyancy (LCB) and gravity (LCG) must be at the same longitudinal location for the ship to float at an even keel. [Envision a playground seesaw with children attempting to keep it level with their feet off the ground. The combined center of gravity of the seesaw and children (LCG) must be directly over the supporting fulcrum (LCB).]
Providing sufficient stability: The measures of surfaced and submerged stability, GM and BG, must meet minimum values to be certain that the submarine floats upright and has satisfactory submerged maneuvering characteristics.
The LCB and LCG are rarely aligned. To adjust the location of the overall LCG, trim lead is placed in either the bow or stern, but usually in the bow to compensate for very heavy propulsion and other components aft. If BG and/or GM do not meet stability requirements, the overall vertical center of gravity (VCG) is lowered by placing stability lead low in the ship. When trim or stability lead are added, weight increases with little or no change in displacement and the ship must be rebalanced. If the ship is weight limited, the pressure hull and main ballast tanks are lengthened to obtain more buoyancy and the volumes and weights are recalculated. If the ship is volume limited, some of the excess margin is reallocated as trim or stability lead. (A volume limited ship could become weight limited if sufficient trim or stability lead are added.)
The process of adding ship length to obtain a match between weight and displacement (with the required GM, BG, and alignment of the LCB and LCG) requires recomputing displacements and weights (and their centers of gravity) for each iteration. The number of iterations required to attain sufficiently close agreement between weight and displacement seldom exceeds three or four.
Submarine concept studies, for which there is no previously existing baseline design (starting with a “blank piece of paper”), take approximately four to six man-months to complete and are typically accomplished by a team of two or three naval architects using a variety of computer-aided design tools and an extensive library of submarine design data. Roughly half of the total effort will be expended estimating the weights. Utilizing effective short-cut methods for recomputing volumes and weights, the final balancing process takes very little time.
So, to conclude this submarine design course, we see that submarine designs must be balanced and that they may be weight or volume limited and may have trim and/or stability lead. These are the naval architectural attributes that so greatly influence the impact of incorporating design changes.
Design Changes During Early Stage Design
During early stage design of a ~ submarine many variant, trade-off, or impact studies are typically conducted to answer a multitude of What if? questions; for example:
- What if the candidate design carried more (or less) weapons and/or weapon launchers?
- What if sonar system B was used instead of system A?
- What if a stronger hull material was used -how much deeper could the ship operate without changing ship size?
Each variant study defines the total ship effect or impact of a specific design change- its effect on ship size and other characteristics.
Ultimately, all design changes entail material objects, having volume and weight, that are added to and/or removed from the baseline design, affecting total ship volume and weight, the function of various ship systems, operational characteristics, mission effectiveness, and life-cycle cost. By also detennining the effects of the change on ship performance and cost, fully educated decisions can be made by higher authority as to whether or not to incorporate the specific design feature in the next baseline design iteration and, ultimately, in the final ship design.
Submarines in early stage design, existing only on paper (or computer files), can grow or shrink without impacting program plans or existing platforms and can be thought of as n1bber ships. The baseline design (the one to be modified) meets all design criteria such as a certain percent margin or reserve buoyancy. When introducing changes (the variants), the design criteria are typically held constant, which may require increasing or decreasing the length of one or more of the pressure hull compartments and the main ballast tanks to balance the variant design.
In variant studies, the feature being varied must be fully identi-fied in the baseline design; a variant study to detennine the impact of changing feature X cannot be performed unless feature X can be separately identified (weight, space, services, etc.) in the baseline and the ship’s naval architectural constraints must be known. Hence, new submarine designs, upon which a number of variant studies are intended to be conducted, must be performed to, at least, the previously discussed concept study level of detail. Quickly con-ducted ROM studies, including those using computer synthesis models, are rarely adequate for this purpose.
Design Changes After Early Stage Design
While there are instances of ships being lengthened during construction due to excessive weight growth or equipment substitu-tions, hull geometry changes are highly undesirable for ships in later stages of design (although still on paper!), under construction, or in service. To avoid geometry changes, the installation of new compo-nents may require shifting or removing existing components to make space and/or removal of margin lead to compensate for increased weights.
A later, lengthened or otherwise modified flight of submarines may, for administrative purposes, be considered as members of the same class as the original versions. However, with new equipment installed and additional buoyancy, they may be operationally more capable and different in some of their naval architectural attributes too.
For ships in service, some initial design criteria may no longer be met. For example, incorporation of new components will consume future growth margin or reserve buoyancy may no longer be the standard percent because of changes to the main ballast tanks. Variant studies of this type usually determine whether it is practica-bly feasible to introduce the design change. More specifically, they answer the question, Can the design change be made without so adversely affecting the ship design that it has negative margin (the submarine is not neutrally buoyant), has insufficient reserve buoyancy, does not meet stability criteria, has insufficient electrical power or air conditioning, etc.?
Applying the Lesson
Let’s return to the proposals listed early in this article and examine some of the naval architectural implications.
- Use lighter components in an engine room to permit new weight to be added (in the form ofadditional weapons) without changing ship size.
This submarine’s torpedo room is forward and has no room for additional weapons. Unless new weapon launchers and stowage structure are installed aft (not very likely), newly available weight in the stem would not help the weapon load situation. If the baseline ship had aft trim lead, the saved weight aft would have to be replaced to retain ship balance and, hence, saving weight with no other purpose in mind has no benefit. If the baseline ship had forward trim lead, the saved weight aft would permit reduction of the forward trim lead and future growth margin would increase by the sum of the two savings.
- Significantly reduce the crew and reduce ship size by the amount of the reduced crew space required.
Low density spaces (especially crew spaces) help float high density spaces (such as machinery and weapon spaces). If the ship is volume limited, the ship size can be reduced by, approximately, the crew space reduction. However, ifthe ship is weight limited, the resulting ship size reduction will be relatively small. (Such a change could cause a volume limited ship to become weight limited.)
- Move weapons or propulsion machinery outside of the pressure hull to reduce the size of the pressure hull in a revised version of an existing submarine design.
External components have weight but relatively little buoyancy and, unless a large amount of internal weight can be saved, rather than become smaller, the pressure hull may have to become larger to float the external components and new associated non-pressure hull structure. (Typhoon and Oscar class submarines, the two largest submarine designs ever built, feature external weapons!)
The belief among some proponents that external weapons can be accommodated at less cost (in terms of ship size) than internal weapons may stem from their limited knowledge of the naval architectural details of adding twelve VLS tubes in later LOS ANGELES (SSN 688) Class ships without increasing ship size, a major selling point of the SSN 688 VLS program as it most likely would not have been approved otherwise. (While ship length and compartment dimensions did, in fact, not change, surface displace-ment actually increased and submerged displacement decreased by small amounts due to changes within the forward main ballast tanks.)
However, this free (in terms of ship size) capability enhancement came about as a result of very fortuitous circumstances. Early LOS ANGELES Class ships had excess reserve buoyancy forward due to main ballast tanks design changes occurring during later stages of design. Further, these submarines carried considerable future growth margin, forward trim lead, and had single hull bow structure. If any one of these four naval architectural characteristics had been otherwise, installing VLS would have entailed lengthening the ship. Installation of VLS in the earlier STURGEON Class would have required either completely redesigning the front end of the ship (for external VLS) or adding a new pressure hull section aft of the operations compartment (internal VLS).
- Adding a new component, which 011/y weighs X tons, will have little effect on the submarine design, certainly not requiring a modification to the hull geometry. (For simplicity, we ‘II assume that the component’s volume is not an issue.)
– Looking at longitudinal balance, let’s assume that the ship has forward trim lead.
If the X-ton component is installed forward, remove X tons of forward trim lead; future growth lead is unaffected. If the X-ton component is installed amidships, remove X tons of future growth lead; trim lead is unaffected. If the X-ton component is installed aft, add X tons of forward trim lead, and remove 2X tons offuture growth lead to satisfy Archimedes.
– Looking at transverse stability, let’s assume that the baseline ship has stability lead.
If the X-ton component is installed near the keel, remove X tons of stability lead; future growth lead is unaffected. If the X-ton component is installed near the hull axis, remove X tons of future growth lead; stability lead is unaffected. If the X-ton component is installed near the main deck, add X tons of stability lead and remove 2X tons of future growth lead. If the X-ton component is installed near top of the sail, add 2X tons of stability lead and remove 3X tons of future growth lead.
So, ignoring displacement effects, adding weight to a submarine (without also changing the hull) can impact future growth margin anywhere from no impact at all to as much as three times the added weight. This may be trivial for submarines with adequate future growth margin but for later ships of some classes, where multiple improvements have been incorporated, the future growth margin may be so low as to preclude adding any significant weight.
Hopefully, my readers now understand the naval architectural attributes upon which the ship impacts of design changes depend and, also, that these attributes can only be determined by performing the appropriate naval architectural analyses, including an evaluation of the ship’s arrangement, volume, and weight, and balancing the design.
As has been explained, the ship impact of incorporating a change into one submarine design is not necessarily indicative of the impact ofincorporating the same change into another design or even into the same ship but in a different manner. So, while I encourage innova-tive concepts, please be aware that the ship design impacts of those innovations are not likely to be plainly evident.
When a seemingly worthwhile submarine modification is identified, before spending large amounts of time and money on development, do the naval architecture-determine the total ship impact of the idea. It may be good, bad, or indifferent. However, even when the ship design results are unfavorable, a fuller under-standing, by everyone involved, of the reasons why, may lead to a rethinking of the proposed modification that can turn it into a concept worth pursuing. NAVSEA and the submarine shipyards have the capabilities to do these studies-take advantage of them!
The United States is in danger of losing its capability to do the naval architectural analyses described above. Its most experienced submarine naval architects are aging and retiring (there is one experienced early stage submarine designer remaining in NA VSEA) and, with the lack of new early stage design studies, few are being trained to take their place. Submarine design skills are different from surface ship design skills-surface combatant designers cannot be expected to successfully design a submarine. Even if a new submarine acquisition program is not in the Navy’s near or mid-term plans, the continuing conduct ofadvanced submarine concept studies and technology assessment studies (to determine the total ship impact of installing the products of ongoing and planned R&D) would be sufficient to exercise and maintain submarine design skills and to train new submarine designers while providing valuable information in support of the overall submarine program.
CAPT Richard S. Garvey, USN(Ret)
CAPT Alfred F. Kennedy, USN(Ret)
Mr. Howard Langerman
RADM Albert G. Mumma, USN(Ret)
CAPT Richard H. Scales, USN(Ret)