The suddenness of the impact disallows the armor plating
to bend on a large surface and absorb the energy of the shell,
like a pillow. The shock wave inside the armor plating has the
time to spread only on a very little surface around the impact,
so that little surface takes all the force and is violently
teared apart.
The toughness of the armor plating is of little importance
because the pressure at the point of the arrow during the
impact is so high the temperature rises and makes both arrow
and armor plating melt and vaporize away.
An arrow shaped shell is both very aerodynamic and has a
high density. It is made out of heavy metal. It is a lot less
submitted to the air resistance than a normal shell. It can be
shot further, or hit a given target with more precision.
It is lighter than a normal shell. The thin arrow shell is
located inside a lightweight cylindrical clog. When the clog
leaves the cannon it breaks away and releases the arrow. Clog
and arrow can be more lightweight than a normal shell. So the
arrow can be shot with a higher velocity.
It is very difficult to destroy or deviate an arrow like a
rocket or a normal shell can be.
The cannon tube needs not be a rifled bore. The arrow
shape keeps naturally its nose towards the front and does not
need to be rotated like a gyroscope. Rotation is important to
equilibrate the arrow assymetries yet this can be achieved by
the arrow tail.
It contains no explosives. So it is more secure to handle.
It is reasonably easy to manufacture. Every nation can use
them and make a possible opponent understand an attack could
yield the destruction of all its tanks and vehicles. (One of
the pet stores in my city sells arrow shells for hunting,
intended for conventional shotguns. It is a conventional cal. 12-70
cylindrical ammunition containing a plastic clog that separates
in two parts and releases a thin metal tube with a short
plastic arrow tail. It seems the thin metal tube flattens and spreads
when it hits the target.)
The higher the speed of an arrow shell, the better. Yet there
is a problem with conventional gunpowder. Once the powder is
lit it produces a huge quantity of gas at a temperature of
about 2000 °C. The molecules that constitute that gas
are
merely nitrogen, water and carbon dioxide. At 2000 °C
they
move at a speed of about 1 km/s. So they cannot move along the
canon tube at more than 1 km/s. Thus they cannot push the shell
at more than 1 km/s.
Several solutions exist to push a shell towards higher speeds:
Use an electromagnetic cannon. The shell is like an
electromagnet pushed away by another electromagnet. The french
army seems to prefer that system. The french military industry
has found out several ways to make the cannon be an effective
linear accelerator and solve problems like the size of the
condensators needed to hold the electrical power for the flash
of the shot. The tank no more contains gun powder: the
electricity is produced by the tank engine, consuming its
regular fuel.
Heat the gas at a temperature higher than
2000 °C. This is
done by making a monstrous electrical flash go through the gas,
using electrical condensators like above. This seems to be the
solution preferred by the US army.
Put a little powder rocket at the rear of the arrow. The
rocket is ignited once the arrow leaves the canon and
accelerates it towards a little higher speed.
Use helium gas or hydrogen gas. Their molecular weight is
8 to 16 times less than the gas molecules produced by normal
canon powder. So their speed is 3 to 4 times higher at the same
temperature. The problem with helium or hydrogen is they cannot
be produced with a high temperature by a chemical reaction.
What's more they are uneasy to store inside cryogenic
reservoirs. So they have only be used for experiment. I've
heard of an installation where a piston-shaped reservoir of
helium is suddenly compressed by the explosion of conventional
powder. Once the helium is fully compressed and heated it
pushes a shell away inside a canon tube at very high speed
(this works like a 4.5 air gun for kids, yet using helium
instead of air and explosive instead of the tough spring).
Another solution would be to inject liquid hydrogen inside a
reservoir with a high internal surface and heated at high
temperature (like a Da Vinci canon, yet using liquid hydrogen
instead of water). If the reservoir is heated electrically a
lot of shells can be shot in a few time. A variant
is to heat the gas by a strong electric discharge through the gas. Then
it can become a plasma and reach even higher speed, yet with a low
yield. The electric discharge can either be fired through the gas
reservoir, just before the shell is fired. This allows for a "slow"
discharge by batteries. The discharge also can occur at the entrance of
the canon or throughout the cannon. Then a fast discharge is needed,
through condensators.
The RAMJET. Imagine a long tube filled with an explosive
mixture of gasses. The shell has an accurately calculated
shape. It does not close the tube, the gas can flow around it.
It is launched at supersonic speed inside the tube. The front
of the shell compresses the gas and accelerates it to a speed a
few less than its own speed. The gas flows along the shell body
and is lit. The rear of the shell and the tube behave like a
rocket motor burning the gas. This pushes the shell towards a
speed of about mach 6.
Some other solutions I thought of are the following:
Use aluminum hydrur (AlH3) and liquid oxygen (02l).
Provided sufficiently less liquid oxygen is used I think all
the aluminum will burn and produce aluminum oxide powder at
very high temperature. Most of the hydrogen will become free
hydrogen gas H2 at high temperature. The aluminum oxide
powder
must stay at the back of the canon while the hydrogen gas
accelerates the shell. This is not a good method for large
scale use since a part of the aluminum oxide will be poisonous.
What's more the aluminum oxide powder is a dangerous abrasive.
Latch a big block of though solid powder at the rear of
the clog (green in the drawing below). For example plastified
nitrocellulose. It must burn
quickly during the travel of the clog inside the canon tube and
release high pressure gasses that press upon the previously
released gasses. The gasses released by the powder block
at the rear of the clog will always move at a speed much
higher than the clog speed (relative to it). A high pressure
will be maintained at the rear of the clog whatever the speed
of the clog.
Use a lengthy sandwich of lightweight clogs and canon
powder. So to say, the back of the canon will push the first
clog towards a speed approaching the maximum gas speed. That
clog will push the next following clog towards twice the
maximum gas speed, and so on. The last clog, containing the
arrow shell, will be pushed at a speed a lot higher than the
maximum gas speed. The effective final speed will for sure be a
lot less than an integer multiple of the gas speed yet it will
at least be two times higher. Perhaps the successive clogs can
be replaced by a lot of little thin sheets of metal. Their
position and orientation inside the canon tube does not matter
as long some of them have a part perpendicular to the flow.
This can be used for self-shaping canon-less shells too, the
successive clogs being replaced by successive plates.
An inside-out version of the RAMJET. A more or less long
rod (or cable lift by a balloon) made out of high explosives.
The rod goes trough the center of the shell. The shell has the
shape of a conventional rocket motor yet with a hole at its top
to let the rod trough and a plate inside against which the
bottom of the rod smashes. The explosion speed of the rod must
start from nearly zero at its base and increase up to the
maximum speed at its other end. The rod is lit at its base,
inside the rocket motor shaped shell. The gasses that are
produced push the shell along the rod because it is a rocket
motor. All along the travel of the shell the inside plate
smashes the rod material and gasses to the speed of the shell
then the gasses are expelled at the rear and accelerate the
shell. The shell "eats" the rod away and expels the explosion
gasses at its rear. The arrow can be located along the center
line of the shell, just behind the plate. If this system is
used for pacific purpose then the load should perhaps better be
located above the rocket motor, around the rod.
