* One of the simplest ideas for putting payloads into space is
also the oldest. The idea of blasting an object into orbit goes back
to the 17th century and Isaac Newton's classic treatise on math and
physics, PRINCIPIA MATHEMATICA.
Newton was not serious about space flight. His famous
illustration of how a cannon mounted on top of a mountain could, if
given a big enough charge, fire a cannonball that went clear around
the Earth was simply an illustration of elementary orbital
mechanics. However, in the 19th century, French romantic novelist
Jules Verne imagined sending humans to the Moon using a giant
cannon.
Verne's giant cannon was impractical, and illustrated some of the
problems with the idea of simply shooting an object into space with
a gun. Unlike a rocket, an artillery shell fired upward loses energy
continuously after launch, which means that it must have a
tremendous muzzle velocity. Since the length of a "space gun" is
necessarily limited, this implies thousands of gees of acceleration
-- and the large muzzle velocity also means that the projectile will
have to endure severe friction and heating effects while trying to
fly up through the dense lower atmosphere.
In any case, 19th-century artillery was too primitive to make the
prospect of putting a payload into orbit a serious proposition.
However, development or large and powerful artillery pieces
progressed rapidly after the turn of the century.
By 1918 Germany had developed an artillery piece of unprecedented
range. This weapon was known as the Wilhelmgeshuetze, or Paris Gun.
It had a bore of 357 millimeters and a barrel length of 30 meters.
The Paris Gun fired a 106 kilogram shell, driven by an explosive
charge of 200 kilograms that produced an acceleration of 7,500 gees
and a muzzle velocity of almost 6,000 kilometers per hour. The gun's
maximum range was 126 kilometers, with the shell reaching a peak
altitude of almost 42 kilometers during its three minutes of flight.
While similar large long-range artillery pieces were used as late
as World War II, the development of aircraft and rockets provided a
much more effective way to deliver munitions over long distances,
and the development of bigger and more powerful artillery pieces
ended.
Use of such large guns for space launch remained a possibility,
however. The maximum possible muzzle velocity of an artillery piece
charged with nitrocellulose explosives is sufficient to launch small
probes to high altitude for atmospheric sounding applications, and
in the mid-1960s experiments along this line were performed using
lengthened US Navy surplus 406 millimeter (16 inch) guns.
The effort was designation HARP, for High Altitude Research
Project, and was the brainchild of Gerald Bull and a group at McGill
University in Canada -- with support by Charles Murphy of the US
Army Research Office and Aberdeen Proving Ground.
Bull's group devised a fin-stabilized projectile named Martlet
for cannon launch. As the Martlet had a much smaller diameter than
the cannon bore, it was fired using a snug-fitting "sabot", or shoe,
that was discarded after the Martlet left the muzzle.
About 200 Martlet 2s were launched with the 406 millimeter guns,
with most of the launches from the island of Barbados in the
Carribbean but a few from Yuma Proving Ground in Arizona. The
Martlet 2s carried various payloads, including chemical releases and
ruggedized instruments. They were fired to altitudes of up to 180
kilometers.
Smaller projectiles were launched from 127 millimeter and 178
millimeter (5 and 7 inch) guns to altitudes of about 75 kilometers
from Yuma and the US National Aeronautics & Space Administration's
(NASA's) launch facility at Wallops Island, Virginia. A total of
about 570 ballistic projectiles were launched in the course of HARP.
While HARP blasted projectiles into space, the McGill group was
driving the development of cannon-launched rockets to put payloads
into orbit. Their Martlet 3 design was a discarding-sabot
solid-propellant rocket with a diameter of 190 millimeters (7.5
inches), and was to be launched from a 406 millimeter gun.
The Martlet 3 was to lead to the Martlet 4, which was to be a
multistage cannon-launched rocket with a launch mass of 1.2 tonnes
and a payload capacity of 90 kilograms to low Earth orbit (LEO); it
would be given a muzzle velocity of 5,400 KPH. The McGill group also
considered a three-stage rocket design that could put 295 kilograms
into a 185 kilometer orbit using all solid fuel, or 590 kilograms
into a 1,100 kilometer orbit using all liquid fuel. This vehicle
would be launched from a 813 millimeter (32 inch) gun.
Development of these cannon-launched projectiles proceeded to the
point where subsystems were test-launched, demonstrating survival
under accelerations of up to 10,000 gees. Subsystems included
solid-rocket motors, an IR horizon sensor, a spin-rate sensor, Sun
sensors, NiCad batteries, a solenoid-operated cold gas thruster, and
various support electronics modules.
The McGill group eventually concentrated on a rocket-propelled
variant of the Martlet 2, named the Martlet 2G-1, as a minimum
alternative to the ambitious Martlet 4. The Martlet 2G-1 would have
been able to put a two kilogram payload into LEO, making it an
excellent demonstrator for a cannon-based "nanosatellite" launch
system.
Unfortunately, funding for HARP eventually dried up and
disappeared, even though the Martlet 2G-1 and various Martlet 3
rockets had been designed and were under construction.
Although HARP was discontinued, it was the most impressive effort
ever made to blast payloads into space using a cannon -- and in fact
appears to be the only project that ever succeeded in doing so. It
was also groundbreaking in developing rocket technology for launch
by artillery, and in developing instrument and guidance systems that
could withstand the stresses of being fired out of a gun.
Eventually, guided munitions that could be fired out of cannon,
such as the American Copperhead laser-seeking 155 millimeter round,
were developed and deployed, but Bull's dream of using a cannon to
put a payload into orbit remains unrealized.
* The story of Gerald Bull didn't end with HARP, however, and
took a turn straight out of James Bond (and in fact was dramatized
in a movie made for US TV). Bull was embittered by the termination
of HARP, and in 1980 served a short term in a US prison as part of a
plea bargain for charges of smuggling arms to South Africa.
After he was released, Bull was unable
to interest anyone in the US in his superguns, and so moved to
Brussels and peddled his designs to anyone who would pay -- first to
the Chinese, then to the Iraqis. This was a fatal mistake in
judgement; Bull was gunned down in front of his home in early 1990,
apparently by agents of the Israeli Mossad intelligence agency.
Three weeks after Bull's death, British customs seized components
of an extremely large-caliber gun that were being readied for
shipment to Iraq, disguised as pipe sections. After the Iraqi defeat
in the Gulf War in 1991, UN inspectors operating in Iraq discovered
an incomplete 350 millimeter supergun with a fixed elevation, and
parts for an even bigger 1,000 millimeter supergun.
* The muzzle velocities that can be obtained with a cannon driven
by nitrocellulose explosives are limited, and so research has been
conducted on alternatives.
One such alternative is the light gas gun, which was invented in
the postwar period as a means of performing hypersonic experiments
with missile warhead reentry vehicle designs, and studying the risks
of space debris to spacecraft.
Obtaining high velocities in a cannon requires a gas with a high
speed of sound, exerting high pressures on the base of a projectile
through a long barrel. The speed of sound squared varies inversely
with the molecular weight of the gas and directly with the gas
temperature, meaning that a hot gas of low molecular weight makes an
excellent propellant for a space gun.
A light gas gun uses a piston to rapidly compress a reservoir of
helium gas. This reservoir is sealed off from the gun barrel by a
diaphragm; when the diaphragm breaks, the hot gas expands rapidly
and blasts a projectile down the barrel. Light gas guns using
hydrogen instead of helium are expected to have even better
performance.
There have been several research programs conducted on light gas
guns. One of the most significant was led by John Hunter of the US
Lawrence Livermore National Laboratory. Hunter is now promoting a
commercial scheme for a light gas gun, appropriately named the Jules
Verne Launcher, for delivering small payloads to orbit.
* Electromagnetic guns have been one of the most prominent
alternative technologies for space cannon. Research has been
conducted on two different approaches: railguns and coilguns.
A railgun consists of a pair of copper rails, mounted in an
insulating barrel, with the rails connected to a rapidly switched
high current source. An armature on the projectile to be fired
completes the circuit, resulting in a magnetic force that drives the
projectile down the barrel. This armature is usually actually a
plasma arc ignited at the base of the projectile.
Switching such high currents has proven tricky in practice.
Railguns also suffer from erosion of the rails after a few launches,
and the designs based on plasma arcs have difficulties with
uncontrolled arcing around the projectile or to the muzzle. Railgun
enthusiasts have proposed designs that they claim will be able to
boost a ten kilogram projectile to 36,000 KPH, but so far railguns
have been restricted to lab-scale systems with muzzle velocities no
greater than 21,600 KPH.
Coilguns are a little more intuitive in design. They consist of a
series of pulsed electromagnetic coils that accelerate a projectile
to high velocity. They are more mechanically complicated than
railguns, but since there is no direct contact between the
projectile and the coils they avoid the erosion and arc-over
problems of railguns.
"Mass drivers" based on coilguns were considered for launching
payloads from the Moon at least as far back as the 1960s, and
small-scale models have been built for decades. NASA has designed a
coilgun that can accelerate 10 kilograms to 39,600 KPH; an enhanced
version of this device has been proposed to boost a 300 kilogram
rocket to 36,000 KPH, allowing it to put a 150 kilogram payload into
LEO.
However, so far coilguns have lagged railguns in performance. A
major drawback to both railguns and coilguns is that any facility
using them would be big and very expensive.
* A new alternative for a space gun, the ram accelerator, has
been promoted by Abraham Hertzberg and colleagues at the University
of Washington since 1988.
The ram accelerator consists of a long, sealed tube filled with a
mixture of fuel and oxidizer, such as hydrogen and oxygen. A
projectile resembling the centerbody of a ramjet is shot into the
tube at a velocity of about 3,600 KPH, igniting the mixture and
blasting the projectile down the tube, which acts like the outer
cowling of a ramjet.
It is possible to accelerate the projectile in several distinct
modes by varying the fuel-oxidizer mix in different sections of the
launch tube, with the sections isolated by thin diaphragms that are
ruptured by the projectile as it speeds up the tube.
While there have been proposals to build ram accelerators to
launch one-tonne projectiles for delivering supplies to LEO, so far
these devices have remained lab experiments. The University of
Washington group is currently operating a three stage, 120
millimeter ram accelerator that launches 4.3 kilogram projectiles
with a muzzle velocity of 4,320 KPH.
* All the options for space cannon face the same constraints that
Jules Verne's Moon gun would have had to contend with.
The fact that a projectile leaving the muzzle of the space cannon
loses energy from that instant on means that it has its highest
velocity during the part of its flight path that moves through the
densest parts of the atmosphere.
As a result, the projectile must be able to withstand frictional
heating and must also be given additional muzzle velocity to
overcome the losses it will suffer. A simple calculation based on a
1-kilogram cubic projectile launched at a muzzle velocity of 39,600
KPH at sea level shows that it will lose 20% of its velocity and a
good part of its ablative thermal protection in the first 16
meters of flight.
One way of minimizing these losses is to launch the projectiles
from the top of a mountain. Calculations show that launch energy
requirements are cut by almost a third if the cannon's muzzle is
placed on a mountaintop at an altitude of 4.6 kilometers (15,000
feet).
The energy requirements for launching large payloads with a space
gun are still extreme, however, and so the approach appears best
suited to launch of large numbers of small "hardened" payloads.
Constellations of "nanosatellites" for communications or similar
applications could be placed in orbit at relatively low cost, using
the space gun as a "first stage" for launch of a rocket-boosted
projectile. Such a projectile would weigh about a tonne and carry
about a 60 kilogram payload; the space gun would have to accelerate
it to a muzzle velocity of 9,000 to 14,400 KPH.
None of the space gun technologies investigated to date have been
scaled up to this size, and doing so would require a major capital
investment that would demand high launch rates for a long period of
time to break even. However, building such a gun does not require
the development of any major new technologies and remains an
interesting possibility for the future.
