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Section Leaders:
Artillery
and VT
Fuses
The
following article is based on e-mail between Jon Sowden and myself.
So, consider Jon as the author, and myself as the editor.
Jon is quite knowledgeable and did a much better job explaining
this then I ever could of. If
any of the following doesn't make sense please get back to me and I'll
clarify as necessary :) Pull up a chair, 'cos its looong... (but
hopefully not too boring) Also,
check out this link: http://192.41.43.237/man/dod-101/sys/land/bullets2.htm Note
Projectile = round = shell (they all mean basically the same
thing) Right,
before getting into the specifics of VT, I want to detour through
normal HE and some other fuses first. An HE artillery projectile looks,
and during flight works, broadly the same as a rifle bullet. It has a
flat base, tubular body that mates with the rifling in the barrel, and
a conical nose. Now
for the differences. Obviously it’s much bigger. And made of steel
rather than copper or lead. The tubular body of the HE projectile has
two copper driving bands around it which serve two functions. The first
is to impart spin to the projectile by engaging the rifling (just like
a bullet) which stabilizes the projectile in flight. Two bands are used
instead of the whole body since its cheaper, and produces less
friction. The second function is to get good obduration between the projectile
and the barrel. In basic English, this means forming a good seal
between the moving projectile and the stationary barrel. This is
important because it prevents the explosive gasses from leaking past
the projectile and resulting in a loss of speed, range, penetrating
power, and accuracy. Interesting
side note: it is usually these copper driving bands that give arty
rounds their distinctive warbling sound in flight. Occasionally you'll
hear one that sounds different, which is usually caused by part of the
band breaking away (partially, or completely falling off), resulting in
a kind of whistle on the side of the round. More
differences: The body of the projectile is hollow, and filled with a
very fast burning explosive. However, without a fuse, it is completely
safe – it will not 'just go off' without a trigger. When a projectile
does go off (we'll look at the trigger/fuse shortly) the explosive
filling rapidly expands causing massive hoop-stresses on the body. This
causes massive fracturing in the high-carbon steel (high carbon to
cause brittleness and therefore brittle fracture), and then the bits of
body rapidly expand outwards, inflicting damage and casualties. If you
can imagine a stationary round going off in mid air (ie, no
obstructions), then the fragmentation pattern would look kind of like
an expanding torus (a donut - the one with the hole in the middle).
Most of the fragments come off the straight sided tubular body, with
relatively few coming from the nose and base. Also, the size of the
fragments depends on where in the shell they come from. The ideal size
is about the size of your thumbnail - this has enough mass to carry a
good distance and still inflict worthwhile injuries, but is small
enough that you get a good amount out of each shell. Too big and the
number of fragments/shell drops and the fragment density/unit area in
the expanding donut drops too low and the chance of a man sized target
being missed by all the fragments gets too high. True, if one of the
large fragments hits him, he's screwed, but the chance of that
happening becomes unacceptably low. Fragments much smaller don't carry
very far, and don't have the kinetic energy to severely damage what
they do hit. Now,
because the amount of explosive filling to unit of surface area drops
off at the nose and tail, the fragments from these areas tends to be
larger, which is part of the reason for the donut shape. Right,
so if we take out hypothetical shell and move it so its body is vertical,
and the nose is just touching the ground, and then detonate it, you can
see that the donut shape doesn't really matter - the two zones of least
fragments - off the end of the nose and tail - are into the ground, and
straight up into the air. The expanding donut of lethal fragmentation
runs out parallel to the ground, and angling upwards as it expands. So
far, so good. {Err,
this should have been mentioned before, but I'll put it in here. There
are two mechanisms by which HE rounds kill and wound: Flash/Blast and
Fragmentation. Fragmentation is being covered, so I'll touch on
Flash/Blast here. When the explosive filler goes off it rapidly raises
the local temperature, and creates a local over-pressure zone. These
cause damage by burning, and blast damage. However, the radius at which
these effects start to fall off is quite short - about 30m? I think.
Fragmentation will cause casualties out to 250-300m from point of
detonation for a 105mm round} Now,
the bad news. Fragments travel in straight lines. And can be stopped
and deflected by pretty much anything reasonably substantial.
Especially small changes in the ground surface. Like culverts, small
ditches, railway embankments - oh, and foxholes and entrenchments.
Also, rounds don't land vertically (though mortar rounds do, which is
one of the reasons they are so well liked), so the donut isn't parallel
to the ground, but inclined. This means that the lethal pattern, when
viewed from above, looks like pattern1.bmp, rather than a circle (the
arrow is the direction the shell was moving, the + is where it landed).
Normal
rounds are fused HEPD, which stands for High Explosive Point Detonating.
(The actual fuse we use is the M739, though that isn't important)
These fuses have two settings: Quick and Delay. The idea is that the
round strikes something solid, then the fuse initiates the explosion
sequence. If you consider that the delay setting retards this by
something like 0.05seconds, you can imagine how fast the sequence is.
Anyway, quick is to get the maximum surface fragmentation, similar to
that in patter1.bmp, while delay is used to get the round to borrow
into the ground a meter or three before going off. This is used against
dug-in troops as it increases the blast effect transmitted through the
ground onto field fortifications. Here
is part two, in which we shall discuss airburst fuses, namely MTSQ
(Mechanical Time Super Quick) and VT (Variable Time, or Prox). Its
another long-un, so make sure you've comfy and have a coffee ... ;) Last
time I explained how an HE shell works, and how it gets effects against
troops in the open, why DELAY is used to get troops dug in, and why
regular HE isn't terrifically effective against troops with a modicum
of cover. The
MTSQ fuse is basically a timer that starts the detonation sequence a
set time after the round is fired. It can be set to whole seconds only,
and also has a Super Quick part to it which causes the round to
detonate just like M739 fuse if the round hits the ground. The
MTSQ fuse is used for a number of rounds which need to detonate in the
air to be effective: Illumination, BE (Base Eject) Smoke, Propaganda (I
suppose), and HE airburst. Illumination
and BE smoke use a carrier shell to hold the functional parts until the
right moment is reached, at which time the fuse detonates, the base of
the carrier shell is blown off, and the payload spills out. In the case
of Illumination this is a single para flare which drifts down in about
60 seconds (don't know the altitude). For BE smoke there are 3 smoke
canisters inside the carrier which spill out in an arc on to the ground
and start forming a smoke screen. Burst height for smoke is about 200m. The
reason HE airburst is so useful is that the fragments come down more
or less vertically, and so can reach out and touch troops who are sheltering
in shell scrapes, in ditches, behind walls, or other slight, solid
terrain obstructions. If you remember from last time, when an HE shell
explodes it produces an expanding ring of fragments. With an airburst
this is still true, but due to the momentum of the shell in its
complete state the fragments continue to move forward along the
trajectory of the shell and impact the ground in a rough circle.
Optimum height of burst (HOB) for these missions is 20m. When
adjusting an MTSQ mission the FO [Forward Observer] starts out by
adjusting with regular HE (ground burst) until the rounds land on the
target. Then the FO calls for 3 guns to fire one round fused with MTSQ,
and watches where these burst. If all burst on the ground he calls for
"Up 40", and the battery adjusts the time on the fuses to
give that correction. If one goes of in the air, and two on the ground
he calls for "Up 20" and again the battery will adjust the
fuses to give that correction. If all three go off in the air 20m up
(average), or if 2 go off in the air and one "decks out"
(hits the ground) then no further corrections are required and the FO
can go into Fire For Effect. So,
while time fused HE is effective, it also takes extra time to adjust
because of the need to fiddle around with the HOB. If a target can be
pre-recorded [pre-registered] with the data needed to fire time fuses
then a quick air burst mission can really screw up some ones day. MTSQ
fuses are still used today for several reasons. For starters they're
cheaper than Proximity fuses, which always makes the bean counters
happy. There are several technical reasons also. Because the fuse can
be set to go off anywhere along the trajectory they can be used for any
round - HE, Illumination, Smoke, etc. Also, for the same reason, the
HOB can be adjusted for different rounds, or for difficult terrain. As
you are probably well aware, VT or Proximity fuses originated in WWII
as an AA fuse to make AA fire more effective. It didn't take long
before someone realized that the same type of fuse could be very
effective when employed as an artillery fuse attached to an HE round.
An artillery VT fuse is set so that it will detonate when the radar in
the fuse detects that it is 20m above ground level. In addition there
is a Super Quick that will detonate the round if the radar fails for
some reason and the round hits the ground. In
addition there is a timer on the fuse that is set to the time of flight
for the round -5 seconds or so. The reason for this feature is funny
and tragic at the same time. During the Vietnam War several missions
were fired in mountainous terrain where the round had to clear several
ridges before reaching the target. On at least one occasion (probably
many more) friendly forces occupied one of these intermediate ridges
AND the trajectory was such that the rounds only just cleared the ridge
by less than 20m! Sure enough the fuses functioned as they were
designed to and went off as soon as they got within the 20m envelope
and showered the Friendly Forces with shrapnel. So, now VT fuses have
the safety timer which will not let the round go off before a
determined amount of time. The
big advantage of VT over MTSQ is that it always explodes 20m above
the ground, and so each round gives optimal splinter coverage over the
target area. Because there is no need to fiddle around with the HOB,
the adjustment procedure is much quicker than for an MTSQ mission. But,
as can be seen from the paragraph about the advantages of MTSQ, and the
safety issue with the VT fuse (I always feel a bit odd talking about
"safety" in respect to military equipment...), it isn't the
be all and end all to artillery fuses. Oh, there is one other point
Bullethead bought up a while ago: The radar in the VT fuses can be
jammed by an un-sporting enemy and so screw up your whole mission. Another
page from the Federation of American Scientists website: This
page has details about the 105mm rounds the US uses. From the table,
the M119A1 is the gun we use here in NZ (actually, here its called the
L119, but it's the same thing). More interestingly, the M101A1 was the
standard US divisional 105mm artillery piece. M1 is the standard ammo
type. Cheers Jon Editor
Note: Jon as you can see is very knowledgeable. I made a few minor
changes to his e-mails and cleaned up the format to get this article.
Thanks to Jon for his help and his knowledge.
Editors Note to the other Editor: And since rune seems to know squat about proper HTML formatting, I (Madmatt) had to format it further! ;-)
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