Critical Dimensions, latest of my AFV design threads. Critical dimensions are those that define the overall size of the vehicle and constrain many other elements. This is a long one, and still doesn’t come close to covering it properly. I need a blog... #miltwitter #tanktwitter 
Usual disclaimer - this is Twitter, I don’t have much space and so some things are simplified or omitted for simplicity. This is a hugely complex science, I’mim just giving a flavour of the considerations inherent in AFV design. With that out the way…
The core critical dimensions are the vehicle’s overall length, width and height. Broadly speaking you want the maximum width and length you can get to maximise volume, but the lowest height to minimise presented target to the enemy
The reality is that max length and width are tightly constrained by established logistical limitations and the basic physics involved, as is minimum height, so AFV size is rather locked into an established design bracket
Vehicle width is perhaps the most important core dimension of an AFV design, having a range of knock-on effects. Height and length are also important, but most AFV are trying to be as low as possible, and length is relatively fixed by width as ill explain
Statutory limits exist for max width of road vehicles, 2.6 m in EU in accordance with Directive (EU) 2015/719. Whilst you can gain exemptions, equivalent rail widths are physical constraints of route itself - bridges, tunnels, cuttings etc and dictated by the rail gauge used
There are a lot of railway gauges and there would be an interesting thread on this topic alone (@ThinkDefence has covered it on his site) but the generally accepted limiting gauge in Europe is the sensibly named Standard Gauge @ 3.150m max load width
However most rolling stock are configured for a max width of 2.44m – the width of an ISO container. AFV wider than this overhang and/or require special wide loaders, but trackside furniture may be configured for an assumed 2.44m max width and limit route selection
In military context widths are constrained by dimensions of aircraft & landing craft loads, but these are often larger than rail limits. If you’re shooting for C-130/A400M transportability then it becomes a factor, but usually your weight negates that option before size does
Why is this important? 1st that the baseline vehicle needs to be able to be configured below this dimension for movement. If not, you cant deploy (or have significant difficulty doing so). You can see this in practice with Leopard 2 armoured skirts having to fold for rail transit
From a design perspective, if the turret remains within the width of the tracks to accommodate a manned basket, turret width plus the widths of the tracks (+ a little more for the structures between) is your max overall width possible
You can increase turret ring width up to overall width by raising the deck and projecting it over the tracks on sponsons. This can result in higher profile and flat faced hull sides though, which isnt ideal for protection, though modern arrays are
Again for a manned and large calibre turret, even with a turret ring projecting over the tracks, the fighting compartment will still be limited by the available space within the hull between the tracks
Therefore, turret size is tightly related to track width. My earlier thread (bit.ly/2W6aQIU) showed track width impacts mobility, so from outset you must consider what target dimensions & weights are & what track is needed to meet mobility reqs. That locks turret size
Or you can take the opposite approach, that you need a turret ring diameter of n to accommodate the required systems and weapon, in which case you immediately constrain track width within the overall design and consequent mobility. Tradeoffs
Why is turret ring size important? Aside the obvious desire for max space in the fighting compartment for crew, turret ring width limits the size of weapon you can fit to the vehicle
If you desire to be able to load and fire the main weapon at all angles, then turret ring diameter is critical to accommodate the weapon as it elevates, depresses and recoils (the latter at all elevations), and to provide space to reload it
There are a lot of considerations in turret design, but that will be covered in another thread. Things like location, balance, layout, forces exerted, elevation/depressiong limits and so on
In practice, owing to road/rail limitations and typical track dimensions for contemporary AFV drivetrains, turret rings are ultimately limited at around 2.5 m for a manned turret
Length then, is the next critical dimension. Overall length defines a few things including internal volume for systems, armour etc. It also defines the maximum track length
Aside the contact area and ground pressure considerations (see my separate topics), the length of track defines the steering ratio, referred to as the L/C ratio. L is length of track on ground, and C distance between track centres
The target for L/C is 1.5, and no more than 1.8 to avoid degradation of performance. With width locked in @<3.150m, track length on ground becomes locked at ~5.7m
Extending L without corresponding C increase results in L/C ratio increasing, which rapidly degrades steering performance. In extremis, this can mean the vehicle cannot conduct a neutral turn at all
Reducing L without making the vehicle narrower (C) are instability, making the vehicle prone to spinning and an increasingly unstable ride over rough terrain.
Any significant overhang at either end of the vehicle rapidly creates problems for overcoming obstacles, so the ratio of contact to overall length, called pitch ratio, is a constraining factor. This also coincidentally needs to be in the 1.5 to 1.8 range
The hull needs to remain within the bounds of the track, else approach and departure angles are impacted and obstacles may become impassable due to front or rear hull strike. In practice modest rear overhang is used, but the front is a firm limit
Height is final of the big 3 and less critical (a generalisation). AFV seek to be low to avoid presenting large silhouette and minimise centre of gravity issues (rollovers), so lowest possible is the objective, however there are factors that present a minimum possible height
If manned, tank turret must accommodate standing loader (ideally 95th percentile Scandinavian male!). Above is roof structure and top armour with commander periscopes above that. Below is floor of basket and connections with hull. Broadly, that’s minimum turret height set
In IFV it will be the height of a seated 95th percentile male with the same constraints above and below. In both cases the obvious advantage of a remote turret is clear – no turret basket means no need to accommodate that height
With unmanned turrets the height constraint will be the space required for recoil and loading operation at maximum elevation, which projects the main armament well below the turret ring in larger calibres
The minimum hull height will be the greater of two dimensions added to the dimensions of the steel hull structure itself – the height of the reclined driver, or the height of the turret basket projecting below the turret, both of which must be contained within the hull
For IFV/APC it will be height of a seated passenger in the rear or the turret basket, and other AFV will be defined by any specialist equipment transported internally, but generally, seated occupants are the tallest thing in the back
Suspension selection has an impact even at this early stage of design consideration. If you go for torsion bars, then these must run along the bottom of the hull, adding effective height to both dimensions above, typically this is about 150mm
Additional considerations would be any hull ammunition racks and the powerpack. The latter sees raised rear decks to accommodate in essentially all MBT designs, as the engine compartment is already is constrained by width and often length from rear of vehicle to turret ring
Combination then of the hull and turret (allowing for any penetration into hull of turret basket etc) + ground clearance of the running gear gives min. overall height possible. Ground clearance can be mitigated by variable suspension for transport height limitations
This fantastic graphic from Brassey’s Fighting Vehicles shows the inherent relationships between the base elements of AFV design, and how one consideration impacts or is constrained by the overall critical dimensions
Often this results in a design spiral as each change causes increases in another, and in most cases size increases, requiring more armour, increasing weight. Keeping this spiral under control is a challenge, and one of the many magical things these engineers do to earn their keep
In summation, logistic and legislative limits set your max width, which sets your max length via physics. Height is minimum possible but set by turret, weapon and passenger dimensions. Thereafter, you can actually think about what you want the vehicle to do
There is a whole lot more to say on this, but hopefully a basic flavour of some of the considerations, and useful to understand the limitations and relationships that exist in these things. /end
