By Cliff Carlisle

By 1944 the Japanese forces had been exposed to the US 2.36” anti tank rocket launcher commonly called the Bazooka.  Apparently they saw a need for a similar weapon.  In 1944 they adopted a 70mm rocket launcher as the Type 4.  Unlike the US rocket that used fins to stabilize it in flight, the Japanese rocket had angled ventures in the base to spin the rocket for stabilization.  The 20cm (8”) HE rocket used by the Japanese was also spin stabilized so they were familiar with the production of this type of rocket.

The launcher was made in two parts that were joined in the middle similar to the US 3.5” Rocket Launcher.  From the pictures in a Japanese book on artillery, it was made to be fired from the prone position.  The front half of the launcher had a bipod that looks like one from a Type 99 LMG attached to it.  The gunner lay with his body at approximately a 45 degree angle to the bore on the left side while the loader was positioned similarly on the other side.  The firing mechanism appears to be crude but effective.  The pistol grip & trigger mechanism are attached to the rear half of the launcher.  A cable runs from the trigger to the rear of the launcher where the hammer is located.  The hammer with firing pin is mounted on an arm that looks like a mouse trap mechanism.  The arm is above the bore and out of the way of loading the rocket when it is in the cocked position.  Pulling the trigger pulls the pin holding the arm in position & the arm swings around under spring pressure striking the primer & igniting the rocket.

The 70mm rocket, like the 20cm one, used a mortar fuze.  There would be no set back when the rocket was fired to arm an artillery fuze.  The Japanese mortar fuze for the 81mm & 90mm use a simple shear wire to make it bore safe.  The wire goes through the brass body & aluminum firing pin plunger.  Upon impact the plunger is forced back shearing the wire and freeing the plunger to strike the firing pin to detonate the round.  This system would work well with a rocket & was an already available item in the Japanese supply system.

The photo shows the components of the 70mm rocket.  Above is the main body.  It has a central diaphragm to separate the explosive cavity from the propellant cavity.  Below, left to right, are the end cap with mortar fuze, shaped charge cone, perforated plate that went below the propellant charge and the base plate with angled ventures to spin stabilize the rocket in flight.  Missing from the rocket is the primer.  The base plate is threaded for a primer but it is not the size of any of the 3 Army primers in use.  What type of primer it used is unknown at this time.  The body of the rocket is marked with a + (0.5% to 1.5% overweight) and a 20.6 (June 1945) loading date.

The rocket is a shaped charge round.  In a shaped charge round the explosive is placed around the outside of the cone in the lower part of the HE cavity.  The cone is hollow & made from fairly thin metal.  When the charge is set off by the fuze the thin walls of the cone are much easier for the force of the explosion to destroy than the heavy steel walls of the shell.  This allows the major portion of the explosion to be directed down the center line of the cone.  The detonating wave, jet of gases & molten metal from the cone are traveling at a velocity of over 32,000 feet per second.  There will be a point where the force of the explosion is concentrated in one small area.  If this point is correctly figured & the distance from the cone to that point is the distance to the fuze (which is the point of contact), the major part of the explosion will be directed to this small area & will blow a hole that looks like it has been cut with an acetylene torch.  To give an example of the effectiveness of this type of projectile, the book “The Illustrated Encyclopedia of Ammunition” shows a 250mm (9.8”) thick piece of armor plate that had been penetrated by a 75mm shaped charge projectile.