Re: Facing was: Well, too interesting

Adding a few comments to Alan Brain's post:

>OO is the expert on exterior ballistics :

More *in*terior (propellant charges, rocket engines etc.) than exterior,

though I do both. The terminal ballistics set some of the parameters I
have 
to play with so I need to stay reasonably up-to-date with them too, but 
they're not my particular area at work.

>but I've done a little study on effective armour thickness of vehicles.

>The problem is that to say that "Vehicle X has thickness Y mm at a
slope 
>of Z degrees" is a gross simplification. Depending upon the angle of 
>incidence of the vehicle, the type of terrain it's going through, the 
>range and the type of ammunition being fired, you can get anything from

>100cm at 85 degrees through to 2cm at 0 degrees (ie episcope etc).

Some vehicles are even more extreme in this respect; eg. the upper
glacis 
plate on the Abrams is (all numbers IIRC, but in the right neighbourhood

:-/ ) about 2" thick... but since it is mounted at an 85-degree slope
from 
the vertical, giving it an effective *horisontal* thickness of ~24".
(Not 
to mention that the odds of hitting the nearly horisontal upper glacis 
instead of the *much* better protected turret front when shooting at an 
Abrams at your own level of altitude aren't very good, of course; OTOH, 
when shooting at it from above the upper glacis is a rather bigger
target 
than the turret front.)

>Hit one place, you have to go through metres of armour - hit 2 cm away,
and
>you can jam a turret ring with even a rifle bullet. And a rifle bullet
that
>hits the open muzzle of a tank gun while the breech is open can KO
anything.

Very much so, even though the odds for this happening are quite low.
Which 
is why things like DS2's "BOOM" chits are mostly a good idea (though the

exact number of times they should crop up can be discussed <g>), and
also 
why I don't always have too much sympathy with those who complain about
SG2 
(or DS-FMA)-style level-5 armour (rolling D12x5 for its armour score)
being 
damaged by a Size/1 Impact D6 weapon if the weapon happens to roll a "6"

and the armour die rolls an "1". It does happen occasionally... not very

often, but it happens.

>What tank designers do is play the percentages: they skimp on armour in
areas
>normally covered by terrain, they skimp on sides, top and rear. Normal
AP
>shells that hit the top of tanks do so at extreme angles of incidence,
over
>85 degrees. Long-rod penetrators basically don't even do that. If you
divide
>the silhouette of a tank into 100 eqaul-sized areas, you may find
something
>like 5 having 2 metres+ armour, 5 having 1.5 metres, 85 having 1 metre,
4 
>having
>5cm, and 1 having 2cm.

And of course the game designers play (or at least *should* play; not
all 
do) the percentages the other way: when firing at the above vehicle a
freak 
lucky penetration roll from a weak weapon represents hitting one of
those 6 
weak areas, while a very poor one from a heavy tank-killer could
represent 
hitting one of the exceptionally tough areas (or a faulty HEAT round, or

any other of a myriad things that could've happened). Merely looking at 
nominal armour values and saying that "this weapon has an armour 
penetration of 500 mm RHA, so it can never hurt that vehicle which has 
armour equal to 600mm RHA" is quite misleading.

>[...] HEAT rounds which if they're exactly right have metres of 
>penetration, but slightly wrong and it's at best 20 cm. (No, they're
not 
>plasma jets, the copper lining becomes a slug that moves at about 5 
>km/sec, though I'm sure OO can give more details).

The slug Alan mentions here causes at best the after-armour effects, 
though. Most of the actual armour penetration is done by the front part
of 
the jet (while the jet actually consists of solid metal under extremely 
high tensions it behaves as a fluid, so we call it a "jet" nevertheless
- 
but this does *not* turn it into a plasma... <g>) which tends to move at

10+ km/sec instead in modern HEAT warheads. Not that is in any way
relevant 
for how to model the things in a game, of course <g>

>Have a look at the WRG rules for AP - they took actual combat results, 
>then came up with the simplest method that resembled the reality. They 
>seem overly simplistic, but the results they give are more accurate
than 
>more complex systems.

Yep. The WRG AP rules *seem* overly simplistic, so many players spurned 
them because "rules this simple can't possibly be *real*istic"... :-(

Later,

Oerjan
oerjan.ohlson@telia.com

"Life is like a sewer.
  What you get out of it, depends on what you put into it."
-Hen3ry