When we talk about blasting something
into space, most of the time you would
think they will be using a rocket but
what about using a giant gun.... sounds
crazy but back in the 1960s a joint
project between the U.S and Canada used
just such a giant gun to fire objects
into space to test their re-entry
characteristics, so could you actually
use a gun to put a satellite or even a
spacecraft into orbit.
Now the idea of using a giant gun to
fire objects into orbit might seem sound
in principle, you only need to send the
payload itself without having to carry
the massive amount of fuel like a rocket
and in theory launchers could be done in
much less time and at much lower costs
but there are several problems and the
first of which was shown over 300 years
ago by Isaac Newton. Newton's cannon was
a thought experiment in which a cannon
is fired horizontally from a very high
mountain, so high in fact that it would
be above the atmosphere, this was to
demonstrate how gravity is universal and
if you got the speed of a cannonball
just right it would end up in orbit
around the earth. Newton himself showed
that just like any other parabolic,
hyperbolic or elliptical orbit, if you
accelerate an object during the orbit
you will always return to the point
where the acceleration took place. As our
gun is on the surface of the earth then
the starting point or the firing of a
gun is where the acceleration will take
place and this is where our projectile,
assuming it's a dumb unguided shell will
return within one orbit. Basically it
will crash back to earth where it was
fired from. The only way to avoid this is
either to fire it with such a high
velocity that the projectile can break free
of Earth's gravity completely and end up
in deep space or once it's reached a
suitable altitude it can correct its
course itself and enter a stable orbit
like a rocket does. Setup in 1961 project
HARP or the high altitude research
project was a U.S. Canadian cooperation
headed up by Gerald bull that I
mentioned earlier. This was to use a
large gun to fire projectiles to the
edge of space and study
reentry ballistics in the race to
develop intercontinental ballistic
missiles and was cheaper and easier than
using either hypersonic wind tunnels or
the rather unreliable rockets of the time.
The HARP guns were made from 16 inch x
US Navy guns that were modified by
connecting two barrels together to
create a total length of 41 meters, one
was located in Barbados any the other in
Yuma, Arizona. The key feature of these
tests was that the gun was oversized and
the projectile, a Martlet rocket which
was held in a Sabot was undersized.
This allowed the test to have the high
speed acceleration to withstand the huge
amount of g-forces during the firing of
the gun, which at its peak reached 25,000 G
when they pumped out the air in the
barrel and then sealed it. The sensitive
electronics were potted into a sand and
resin mix which proved to be highly
effective in protecting them from the
forces of the launch. During tests in
1966 the Yuma based gun fired a Martlet
rocket around a 180kg
to a height of 180 kilometers
around 590,000 ft
easily into what is considered
space which officially starts at a
hundred kilometres and is a record which
still stands today. Although this might
seem like a lot, the velocity was only
around 2.1 kilometers per second
that's still a fraction of the 7.8 km/s
required to reach low Earth orbit.
Conventional guns relying upon an
explosive charge can only ever reach
around two kilometers per second as our
projector can't accelerate faster than
two pressure wave from the exploding charge
that's pushing it along the barrel. In
the 1980's the Lawrence Livermore
Laboratory as part of Ronald Reagan's
Star Wars initiative to shoot down
incoming missiles conducted tests on
what they called the sharp or the Super
HARP gun. This used a two-stage
propulsion technique
in an L-shaped barrel. Firstly in a
chamber at right angles and at the
bottom of the main barrel a methane air
mix is ignited, this pushes a pump piston
to compress hydrogen which then
propels the projectile along the main
barrel. As hydrogen is the lightest gas
it can achieve the high speeds and in
tests small projectiles reached 10 km/s
enough to achieve orbit. Now unlike
Newton's cannon which was above the
atmosphere any practical space gun like
the project HARP ones would have to be
on the ground and therefore of a
projectile would have to be accelerated
through the densest part of the
atmosphere at hypersonic speeds to have
enough velocity to reach an altitude
suitable for orbital insertion and this
also brings about the problem heat. Now
the faster an object goes through the
air the more the air is compressed on
its leading edges and the more the air
heats up to a point where it can be well
over a thousand degrees C and cause
damage to the projectile itself. As the
altitude increases the air becomes less
dense and the heat buildup becomes less
till you get to space where it stops
altogether. Whereas a rocket starts off
slowly in the dense atmosphere and
builds up speed through the thinning
atmosphere to space, a gun accelerates
the object to its full speed during the
length of the barrel and right into the
dense atmosphere where the air
compression and heat will be at its
greatest. Now before the Soviets put
Sputnik satellite into orbit there was a
rumor that the US had accidentally
launched an object into space during an
atomic bomb test. During the Pascal B
nuclear test of 1957 a 100 millimeter
thick, 900 kilogram armoured steel end
cap was used to seal the 152 meter deep
test shaft and it was welded into place
to see if it could contain the
radioactive material. When the bomb was
detonated at the bottom of a shaft the
end cap was blasting upwards for what
was thought to be around 66 km/h
and a 150,000 miles
an hour. Now this was actually expected
and as part of a test high-speed cameras
were trained on the cap but it was only
ever caught on one frame of film so
exact figures could not be calculated
but it was said to be traveling like a
bat and probably out of hell.
Some believe but it became the first
man-made object to be blasted into orbit
but Dr. Robert Brownlee who worked on
the test believed that it was vaporized
by the intense compression heating of
traveling through the atmosphere at such
a great velocity but either way the cap
was never seen again. Now an oversized
manhole cover is not the ideal shape for
a projectile and it could be expected to
burn up but with an aerodynamic shape
and special heat shielding the heat
issue could be overcome. So we can fix
the heating problem but due to the speed
there is also another related issue in
that the gun's projectile has an
acceleration that's somewhere around
20,000 g that would turn any delicate
satellite into scrap metal and humans
into jam but that didn't stop people
from using the idea in fiction or
reality. Jules Verne used the idea of a
space gun in his sci-fi novel of 1865
"From the earth to the moon" to fire a
manned projectile to the moon. Although
many saw it as a work of pure fiction
some of the calculations which Verne did
considering there was no empirical data
to go on at the time turned out to be
surprisingly accurate but the length of
the barrel would be the major problem
and would have to be incredibly long if
humans were to be passengers inside. In
order to launch humans into orbit and
onto the moon like in Verne story, the
maximum acceleration you'd want to
subject your passengers to would be
about 7g though in the space shuttle it
was around 3g and in Soyuz it's 4.5g,
so to be on the safe side you're
probably looking at less than 5g for
longer periods of time. The length of the
barrel
if you allow the acceleration to
increase safely from zero to 7.8km/s
would need
to be greater than 50 kilometers long,
depending upon the level of G you were
going to inflict on your passengers. This
pretty much rules out any conventional
gun powered by an explosive charge but
an electromagnetic railgun could start
at a much lower speed and reach top
speeds high enough to reach orbital
velocities. This idea has actually been
proposed to launch payloads into orbit
which are not affected by the high
g-forces such as food, water and fuel but
even limiting the acceleration to a
1000g it would still require a
track over a kilometer long. With current
technologies this would be very
challenging but not impossible the
biggest issues being the arcing of the
electrically conducting track in the
Sabot and the atmospheric heating of the
projectile at hypersonic speeds. Coil
guns which use high-power electromagnets
to propel the projectile with no
physical contact would eliminate the
wear on tracks but aren't currently
capable of reaching 7km/s.
The best launch sites would be
near the equator to take advantage of
the Earth's rotational speed and 2 to 3
kilometers up the side of a mountain in
less dense air and where noise would not
be an issue. This would still rule out
transporting delicate satellites and
humans but could dramatically reduce the
cost of getting raw materials into orbit.
So it looks like Rockets will be with us
for the foreseeable future if only to
get delicate payloads into orbit unless
of course we can build a space elevator
but that's a whole other story
altogether
and in which case don't forget to check
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