Original
commentary posted in sci.space newsgroups 1995
Revised
2001
In
the 1960s, project HARP (High Altitude Research Project), was run out of McGill
University in Montreal. The principal engineering direction was from the
Canadian ballistics expert Dr. G. Bull, and funding was from the U.S. Army.
Project HARP involved the use of large guns to fire instrumented ballistic
projectiles and rockets to high altitudes. The HARP program was terminated in
approximately the mid 1960s before the group's ultimate goal of launching an
orbital vehicle was achieved. This message gives some otherwise hard to get
information about project HARP from two older papers in the Canadian
Aeronautics and Space Journal. Material in quotation marks is from the cited
papers. A picture of the HARP gun in
its current dilapidated state on Barbados and further information is at http://www.astronautix.com/stages/harpgun.htm
Paper 1: Bull, G.V. (1964) Development of Gun Launched Vertical Probes for Upper Atmosphere Studies. Canadian Aeronautics and Space Journal 10:236-247.
This
paper was written to accompany a speech made by Bull in Toronto in May 1964. In
the Introduction to the paper, Bull writes:
"During the past several years, both theoretical and experimental investigations have been undertaken to determine the applicability of guns to scientific studies of the ionosphere. Such possibilities have intrigued ordnance workers for many years, but involve a complex mixing of advanced gunnery techniques, scientific experiment considerations and economics....In late 1961, with material support from the US Army, McGill University undertook the development of a 16 inch gun system. In early 1962 this program came under full support of the US Army through the Army Research Office and the Ballistic Research Laboratories"
In
a section on sub-caliber ballistic projectiles, Bull says:
"For example, in the case of a 16 inch naval gun which normally fires shells in the 3,000 lb. class at velocities of 2,800 fps, velocities as high as 6,000 fps can be obtained with shot weights of the order of 400 lbs., the sub-caliber vehicle in this case having a ballistic coefficient considerably higher than the normal shell. By re-design of the gun (i.e. extending the chamber and barrel) to optimize at this lighter shot weight, velocities approaching 7,000 fps are possible."
A
series of sub-caliber "Martlet 2" vehicles were built, which were
sub-caliber and rode the barrel in a fall-away sabot. Canted fins on the
projectile maintained aerodynamic stability, and spun the projectile up so that
it was stable once leaving the atmosphere. These were fired at elevations of
from 60 to 90 degrees from a 16 inch naval gun (on loan from the U.S.) which
was located in Barbados. The gun was bored out to 16.5 inches and made into a
smoothbore cannon. Altitudes of approximately 500,000 to 600,000 feet (100
miles, 160 km) were projected for this arrangement, and early trial reported in
the reference cited went as high as 112 km. Martlet vehicles carried
instruments made from discrete solid-state electronics - they were potted in a
mix of epoxy and sand (!) and the designers did not seem to have any real
trouble getting the electronic to survive the launch acceleration which peaked
at approximately 20,000 g. Martlet vehicles also routinely carried a liquid
mixture of trimethyl-aluminum and triethyl-aluminum to be released at high
altitudes for ionosphere studies. Another option was to carry sodium-thermite
mixes which when ignited would release sodium vapor. If projectiles of a
similar weight were fired for range rather than height then ranges of up to 150
to 200 miles were calculated, depending on the ballistic coefficient. Shots
from the gun were routine and relatively inexpensive. Bull states:
"Normally, loading of the gun can be accomplished in under one half hour, allowing a firing rate of one an hour....Standard service propellant available as surplus (WM/.245) has been used, and the gun geometry has not been modified. Firing programs are planned for the summer and fall of this year [1964] when the gun barrel will be extended and lighter sabots used with propellant designed to match the light projectiles, which should extend the Martlet 2A apogee to 200 km....The economics of the gun launched probe has been as predicted, with the Martlet 2A airframes loaded with TMA/TEA and a flare in the nose cone varying in price between $2500 and $3500, with gun launch costs (propellant and gun wear) included."
After
having discussed ballistic projectiles, Bull discusses gun-launched rockets:
"Gun fired artillery rockets have been developed extensively since World War II and normally must withstand barrel acceleration loads of the order of 30,000 g along with the rotational loads superposed by shell spin. The performance of this type of rocket is only of marginal interest in the vertical probe application where non-spinning (from a stress viewpoint) vehicles are flown at acceleration levels of less than 10,000 g and relatively very large rocket motors are desired with high mass fractions....In May of 1963, work was started on what was designated as the Martlet 3A rocket assist vehicle as part of the HARP program. The objective of this activity was the development of a 16 inch gun launched probe which would carry some 40 lbs. of payload to altitudes in the 500 km range."
The
Martlet 3A and later 3B rocket vehicles were sub-caliber and used various solid
propellants in various configurations. The main problem with gun launched
rockets is supporting the solid propellant during the launch acceleration so
that it does not collapse into the internal cavities molded into the propellant
grain, and a lot of development work was performed to investigate the
performance of various solid propellant grains. From their knowledge of the
performance of the 16 inch gun system and general information about the
specific impulse and mass fraction of solid fuel rockets, it was calculated
that it would be fairly easy to put a payload into orbit using the HARP gun and
a multistage solid fuel rocket.
Orbital Launch Vehicle Characteristics from Figure 31 in the Bull
paper:
|
Total
launch weight |
2000
lbs |
|
Stage
1 weight |
1440
lbs |
|
Stage
2 weight |
403
lbs |
|
Stage
3 weight |
117
lbs |
|
Payload |
40
lbs |
|
Muzzle
velocity |
4500
fps |
|
Mass
fraction |
0.8 |
|
Specific
impulse |
300
sec (vacuum) |
The
first and second stages were to be fired at relatively low altitude, but clear
of the atmosphere. The third stage was to circularize the orbit, and would be fired
horizontally at orbital altitude. Such a vehicle was never built before the
program was shut down, although motors of the first stage size were developed.
The HARP group was also involved in exploring the possibilities of launching
liquid fueled rockets from the gun. These could be thin-shelled as long as they
had no gas spaces in them (you can accelerate a balloon full of water at any g
force you like, as long is it is fully supported during the acceleration).
Paper 2: Eyre, F.W. (1966) The Development of Large Bore Gun Launched Rockets. Canadian Aeronautics and Space Journal 12:143-149.
"The concept of a rocket launched from a gun is not new. It
will suffice to affirm in this paper that the gun launched artillery rocket was
in full development during the Second World War and this investigation still
continues. Like so much work in allied fields, a great deal of what has been
done and is being done is classified and cannot here be repeated....The
conventional solid propellant gun, firing meaningful projectiles, currently
appears able to develop a maximum muzzle velocity of some 6000 to 9000 fps.
Allowing an 80% recovery of muzzle kinetic energy as potential energy, this
corresponds to a ceiling for sounding work of some 800,000 to 1,000,000 ft.
(say 160 to 200 statute miles). Significant improvements beyond this level must
come either from use of a different type of gun or from rocket boost during
vehicle flight, which is here considered."
"Figure 3 shows muzzle velocity vs. shot weight for the Barbados
gun. [HARP]"
"Assumed conditions:
Max. pressure 60000 psi, Fixed charge, 1000 lbs M8M propellant Web size
optimized."
[some approximate data points from Figure 3 graph, and from Figure
4 showing acceleration vs. shot weight]
|
Shot weight, lbs |
Muzzle velocity, fps |
Max. acceleration, g |
|
500 |
7700 |
13,000 |
|
1000 |
6400 |
9,000 |
|
1500 |
5700 |
6,500 |
|
2000 |
5200 |
5,000 |
Eyre
then goes into a long technical discussion related to how to support propellants
of various types in a solid fuel rocket during the gun acceleration. Perhaps
the neatest concept is to simply fill all empty spaces in the rocket with a
fluid which then can support the propellant grain hydrostatically during launch
(sort of a rocket water-bed). The rocket is then accelerated using some form of
pusher plate, which seals the liquid in. The plate drops away after launch, and
the fluid is then vented or drained before ignition. With regard to
practicality and performance, Eyre writes:
"It has transpired in design studies that although structural problems do arise due to the acceleration loads, and additional problems are posed by the necessity to use a folding stabilizer assembly, mass fractions almost as high as conventional rockets can be achieved and the design problems are partially alleviated by an all supersonic flight regime.....Given this condition the advantage of the gun can be seen in that a typical vehicle of mass fraction 0.8 would have an apogee of 176 miles used conventionally, 257 miles at 1000 fps launch, 342 miles at 2000 fps, 435 miles at 3000 fps, 529 miles at 4000 fps and so on."
Eyre
then discusses the fabrication of a full-scale, full bore (16 inch) motor with
a weight of 1450 lbs., designated the Martlet 4A and designed for the Barbados
gun. Further information on this rocket
is at http://www.astronautix.com/lvs/martlet4.htm
"Current work is directed towards development and application of a thin plastic wear resistant coating [they were worried about excessive wear on the rocket casing], and launching of 16 inch motors to investigate scale factor effects. At the time of writing [1966] full bore Aerojet General Corp. grains are awaiting launch. ... At the present time a heavy test program is about to commence with many agencies participating and for the most part full scale hardware ready for launch."
In
summary, up until the time of writing of the later of the two quoted papers in
the mid 1960s, HARP under Dr. Bull appeared to have been highly successful
using a surplus 16 inch naval cannon in firing projectiles to high altitudes
and in firing solid fueled rockets. Bull has been called the most brilliant gun
designer of this century. His comment on vehicle design for guns of different
scales is interesting:
"Obviously since launch weight (i.e. payload) is increasing roughly as the cube of the scale, while peak accelerations are decreasing linearly, the larger the gun the simpler the vehicle engineering problem."