Thursday, 16 June 2011

Armoured Vehicle Design : Density

Density is the ratio of mass to volume or in plain language,how much stuff* there is in a particular space,the more we put in to a particular space,the greater the density.

An armoured vehicle is essentially an armoured box to which is added weapons,sensors and a drivetrain.

We are going to use this cube as a simple mathematical substitute for an armoured vehicle hull in order to explain a basic problem related to amphibious armoured vehicle design.

For the sake of simplicity we will ignore the thickness of armour and concentrate on it's weight per unit area.

If each side of this cube was 2 metres long,it would have a volume of 8 cubic metres and a surface area of 24 square metres.

If we cover that area in armour weighing 1 tonne per square metre the cube will weigh 24 tonnes and have a density of 3 tonnes per cubic metre.

If we cover that area in heavier armour weighing 2 tonnes per square metre the cube will weigh 48 tonnes and have a density of 6 tonnes per cubic metre.

The greater the weight of armour a vehicle carries,the greater it's density.

For a given weight,increasing a vehicle's internal volume or reducing it's weight of armour will reduce it's density and increase it's vulnerability.

Conversely,reducing the internal volume or increasing the weight of armour will increase the vehicle's density giving higher levels of protection.

Well protected vehicles have high densities and low density vehicles have poor protection.

This is a particular problem for amphibious vehicles.

In order to float,an amphibious vehicle must weigh less than the volume of water it displaces.

As the density of fresh water is 1 tonne per cubic metre,an amphibious armoured vehicle must have a density which is lower than this.

This is a fundamental limitation on the armour protection of an amphibious vehicle - a small amphibious vehicle can carry only very lightweight armour.

However,there is a solution to this problem.

If we were to look at a larger cube with each side being 6 metres long then each side would have an area of 36 square metres and the cube as a whole would have an area of 216 square metres and a volume of 216 cubic metres.

If we cover this larger cube in armour weighing 1 tonne per square metre it will weigh 216 tonnes and have a density of 1 tonne per cubic metre.

As this is the same as water,this armoured cube will float.

Compare this to the cube with 2 metre long sides mentioned above,even though both carry armour weighing 1 tonne per square metre,the cube with 6 metre long sides has only one third the density of the smaller cube.

This is why heavily armoured battleships are very large and also why the L.V.T 5 amphibious vehicle was so big.

The heavier the armour an amphibious vehicle carries,the larger that vehicle must be in order to float.

But very large amphibious vehicles may be too big for use on roads designed for much smaller land vehicles.

Increasing an amphibious vehicle's armour protection increases it's size and reduces it's practicality on land.

Reducing an amphibious vehicle's armour protection allows it to be smaller and more practical on land,though more vulnerable.

There is a way round this problem which entails adding additional buoyancy external to the vehicle's armour.

The most well known example of this is the collapsible floatation screen most famously used by the Duplex Drive (D.D.) tanks of the Second World War.

These screens were flexible and vulnerable to damage by both enemy fire and the action of the sea resulting in the loss of many vehicles.

Building more robust collapsible buoyancy screens would be complex,expensive and heavy.

Another alternative is to fit rigid external buoyancy boxes.

These add more dead weight and bulk than collapsible screens.

Rigid buoyancy boxes were often designed for very heavy vehicles with little inherent buoyancy and consequently were inconveniently large.

A more practical design is a composite of the three approaches mentioned above.

If a vehicle has an under armour density close to that of water,it may only require a modest amount of additional external buoyancy in order to float.

Rigid external buoyancy boxes filled with buoyant material may be added flush to the top of the vehicle as well as the sides,front and rear adding the required modest amount of additional buoyancy without being impractically large.

This composite approach allows an overall density low enough for floatation combined with an under armour density far higher than that of a vehicle with all of it's buoyancy carried under armour.

A modest amount of additional external buoyancy may be all that is needed to turn a 30 tonne medium armoured personnel carrier in to an amphibious vehicle.

*Explaining the definition of mass and weight to the lay reader is beyond the scope of this article so we will keep things simple instead,even if not technically correct.


TheRagingTory said...

Rigid Buancy Tanks could be ditched once the vehicle is beached, assuming they are only needed to drive from ship to shore.

Which takes care of some of the mobility/size issues.

How quickly this could be done, is questionable, the joints will have many tons of stress on them, so will need to be strong, but removing them, is possibly going to happen whilst under fire.

Although I suppose they may function as Alblative armour on the beach, where volume is unliklely to be an issue, and may even be an advantage.

Personaly, I think its just a bad idea.
Its worth it for proper odd balls like Hobarts Funnies, but not for normal vehicles, its just not.

Use your specials to clear a beach, then land normal kit.

S O said...

"A modest amount of additional external buoyancy may be all that is needed to turn a 30 tonne medium armoured personnel carrier in to an amphibious vehicle."

+ water tightening
+ salt water corrosion protection if meant for sea water amphibiousness
+ water pumps (necessary for longer swimming)
+ either track design suitable for propulsion in water or addition of (steerable) screws and associated power lines/shafts
+ safety equipment, procedures and training
+ protection of electric equipment against water, not just high humidity

GrandLogistics said...

Hello TheRaging Tory,

I will be doing a post on the replacement options for the Expeditionary Fighting Vehicle in the near future.
This post was just meant to explain the design conflict between protecting a vehicle and making it float,to save me putting that in to a longer post.

Droppable buoyancy tanks have been tried before,they worked but the problem is they take up a lot of space on the ship if they are supporting a high density vehicle like a tank.

My conclusion is that it is better to build more landing craft capacity in to amphibious ships than to try to make armoured vehicles float.
But I can see why the United States Marines might want a new amphibious vehicle.
It is not just memories of Tarawa.
Their current amphibious ships have limited landing craft capacity and it will take 40 years for them to be replaced.

The density issue is fundamental to explaining their options.

I am glad you mentioned Hobart's Funnies.
The United States Marines have their own funnies these days but very specialised vehicles are a problem when they are operating in small amphibious groups.

It might make some sense for them to form their gun tanks,funnies and others in to heavy assault battalions to support major surface landings.
Leaving their on ship Amphibious Readiness Groups as pure medium and light forces.


GrandLogistics said...

Hello Svenn Ortmann,

you are quite right but this post was just addressing the protection versus floatation design conflict and ways to get around that.


TheRagingTory said...

"My conclusion is that it is better to build more landing craft capacity in to amphibious ships than to try to make armoured vehicles float."

Thats basicaly what I was going for in my rambling way.

leesea said...

GL I agree with your conclusion to build more (fast) landing craft. I suggest the USN look at British PASCAT or French L-CAT ERD-A.

GrandLogistics said...

Hello Leesea,

there is a post on heavy landing ships on the way in the not too distant future which you may find interesting.

Until then,I think you might like this: