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Combat Vehicle Design

The operating concept drives system requirements: personnel capacity, level of protection, amount of firepower, necessary mobility, ease of transport, and others. Prioritization of, and balance between, these requirements results either in design tradeoffs leading to a successful vehicle or in an unacceptable system. The operating concept drives the selection and prioritization of system requirements for combat vehicles. When the operating concept is disconnected from the operating environment, as occurred with both ASM and FCS, there is a high probability that system requirements will not deliver an effective system.

There are two other parameters that affect the ability to procure a combat vehicle that derive from requirements: cost and schedule. Cost is the biggest driver; an unaffordable but perfect system provides no capability to the force. Affordability was a primary or secondary factor in the demise of ASM, FCS, and GCV, both at the individual vehicle level and the Army’s combat vehicle fleet portfolio level. Controlling cost is paramount to delivering capability.

System requirements break down into design specifications that suggest the specific technical solutions to be used, but the requirements are vehicle capabilities, not technical solutions. Mismatch between these two can have negative second order effects. Some design parameters, such as size and weight, derive from required capabilities, but might also be design constraints; for example, if a system has air or rail transport requirements that limit allowable size or weight.

Prioritization of system requirements, protection, lethality, mobility, deployability, and affordability, drives design. However, some parameters carry significantly more importance than others regardless of requirements prioritization. Volume under-armor–the amount of protected personnel and component space, at what level of protection–is the most significant design driver in combat vehicle design. The number of personnel, both crew and passengers, along with mobility components and other under armor mission equipment, sets the initial vehicle envelope. The level of protection those personnel require from which threats under what conditions, (from small arms to tank rounds, from RPGs from the front or from attacks from below), determine the range of design approaches applied around that volume. Since fully reliable, active protection systems are not yet fielded and cannot defend against all threats, that protection drives passive armor weight that sets an initial weight for the vehicle.

Protecting the personnel volume requires tradeoffs. Protecting from underbelly attacks, for example, favors a heavy vehicle with the maximum possible standoff above the threat, to dissipate as much energy as possible before the blast affects the personnel inside. This requires interior space that allows personnel and seats to move in a blast without hitting the vehicle roof or receiving injuries from floor deformation. Underbelly protection requires a larger personnel volume, thus a large vehicle, which is higher off the ground and presents a larger, easier target to hit for direct fire attacks.

Protecting against direct fire attacks requires a smaller vehicle profile to avoid detection, engagement, and contact, which requires smaller vehicle volume placed close to the ground. However, if the vehicle is hit, greater volume of armor to prevent penetration of the crew compartment by deflecting, disrupting, or defeating the attack, is required, which drives a larger, heavier armored volume. Using active protection to reduce vehicle mass increases vulnerability to underbelly attacks.

Under armor personnel volume and the resulting vehicle weight class, compared to the system’s mobility requirements, drives decisions on powertrain and suspension such as, wheeled or tracked, engine or transmission size, and others. A combat vehicle requires a large ground contact patch to reduce ground pressure for movement on soft and wet soils, which favors tracked systems in heavier vehicles. However, the larger contact area makes high-speed road movement more difficult. Wheeled systems with their smaller contact areas tend to be more efficient for long- range, high-speed, operational-level movement. Turning radius and obstacle climb and clearance requirements typically favor tracked systems, but are dependent on final vehicle design; it is possible to achieve small turning radius or greater obstacle performance with wheeled systems given appropriate tradeoffs. Current technology drives vehicles above the 20 to 30-ton range to tracked solutions if off-road, soft soil mobility is important.

Placing powertrains under armor in combat vehicles prevents loss of the system to a mobility kill. This means that the under armor envelope must expand to include the powertrain, thus increasing vehicle size and weight. To keep vehicle size to a minimum, combat vehicle powertrains seek to be power dense, offering the most power for the smallest volume.13 This results in the technical challenge to cool a tightly packed high output engine in a heavily armored space, where access to cooling air and points to radiate heat are restricted. This challenge means that commercial engines and transmissions are poor solutions. Military combat vehicles have made-to-order, low volume, and expensive powertrains.

Once powertrains are in place, the combat vehicle is armored, protected, and moving Soldiers at some required mobility level. Other capabilities, such as weapons, sensors, communications, and situational awareness are added. Lethality systems require a structure level that supports the weapons system and mitigates recoil. These systems receive some level of protection, (from equivalent to the personnel space down to no protection), and require protected volume for ammunition and fire control. The volume of ammunition and level of protection required for the weapon and associated components adds to volume under armor, which drives increased size and weight. Additional size and weight increases automotive and suspension requirements.

Sensor performance to enable lethality requires some armored volume (long-range performance, particularly through obscurants, drives large sight apertures, meaning larger sensors) and sufficient electrical power capacity to power the sensors. Network and communications equipment, situational awareness systems and displays, chemical, biological, radiological, and nuclear (CBRN) defensive systems, and other required subsystems have space and weight requirements that add to vehicle volume under armor, and bring electrical power requirements to operate.

Turret configurations create significant size and weight tradeoffs. There are three basic turret configurations: manned turrets, where the turret crew rotates with the weapons system; unmanned turrets, which operate remotely but maintain some level of armored protection for the weapon and may allow some access or operation by personnel from within the vehicle; and remote weapons stations, which are typically unprotected and inaccessible from the vehicle interior.

• Manned turrets drive large structural and protection requirements and consume protected volume, which translates to increased vehicle size and weight. However, manned turrets facilitate crew observation as well as operation of the weapons system.
• Unmanned turrets have reduced weight and integration burdens compared to manned turrets and can usually sacrifice some protection, but are more complicated to access, operate, and repair.
• Remote weapons stations typically sacrifice protection for the weapons systems and usually require an exposed operator to reload or service the weapon; they have the lowest structural and weight impact because there is no crew to protect. Remote weapons stations tend to have degraded performance compared to manned or unmanned turrets for large weapons systems due to lower mass available to handle firing loads even assuming implementation of comparable fire control and stabilization solutions

Other design attributes affect combat vehicle design differently. Deployability and transportability requirements are initial design constraints. Though the Army moves large formations by sea where vehicle weight is not a major deployability driver, setting limits on vehicle, size, and weight affects the ability to deploy those systems by air and rail and affect tactical mobility. If a vehicle must be sized to deploy by air in a specific aircraft, this sets a maximum weight and volume that the vehicle cannot exceed.15 A vehicle whose size grows behind a certain point challenges rail movement in some parts of the world due to tunnel clearances; road clearance under bridges while riding on vehicle transporters, and movement across bridges provide additional size and weight constraints.

While vehicles under 20-tons weight can access over 97% of the world’s road networks, that number falls to 81% to 97% for 30 to 40- ton range vehicles, and 81% to 95% for 60- ton weight class vehicles, and so on as size and weight increase. Size and weight also affect tactical mobility as they limit maneuverability in streets and alleyways in urban environments, and drive the technical choices in propulsion and mobility systems needed for off-road mobility.

Current combat vehicles have grown 25 to 30% in weight and power over their service lifetimes. Future growth requirements in space, weight, electrical and automotive power, or cooling add to the size-weight-volume-power challenge. In this case, components must be overdesigned to handle future weight growth; automotive systems must have power margin, and interior room most allow for future space growth. This overdesign to allow for future capability increases the initial size and weight of the vehicle.