In the 1950’s and 60’s, the world had rocket fever. Humankind was taking its first steps into space and had sights on the moon. Kids could build rockets at the kitchen table and launch them in the schoolyard. On the darker side, the arms race was well underway with the US and USSR trying to close the fictional missile gap.
All around the world, engineers were trying to do new things with rockets. Among these were Robert Mainhardt and Arthur T. Biehl, who thought rockets could be useful as small arms. Together they formed MBA (short for Mainhardt and Biehl Associates), with an eye toward future weapons – – specifically rocket bullets.
The first project they worked on was the Finjet. Tiny (1 to 3mm diameter) needle-like rockets with steel tips. The rockets were made of injection molded plastic. There were plans to make them of soluble or flammable materials such that fired rockets would eventually burn up or dissolve in a victim’s body fluids. A magic bullet that did damage and disappeared, leaving no trace behind. If this sounds like something 007 would use, you’re not far off. MBA weapons were featured prominently in the James Bond movie “You Only Live Twice” It wasn’t the Finjet which made the screen though, it was their later design, the finless Gyrojet.
The Gyrojet is a cartridge which at first blush looks looked much like a .45 caliber pistol round. A Gyrojet is one piece though – unlike the bullet/shell combination of a standard round. Looks are where the similarities end. Conventional bullets are propelled down the barrel of a gun by an explosion. Gyrojets are powered by onboard rocket fuel. This means the gun itself can be much simpler, as the barrel and chamber don’t have to contain an explosion. Anyone who has shot off a bottle rocket knows that a simple rocket isn’t very accurate. To handle this, Gyrojets are spin stabilized — angled rocket nozzles would spin the bullet up to rotational speeds higher than most rifle rounds. While they may sound a bit like child’s toy rockets, these were definitely deadly weapons. At maximum speed, a Gyrojet round had twice the kinetic energy of a .45 ACP round it resembled.
Gyrojets were different enough that they required their own firearms to launch. MBA produced several models, including a pistol, carbine, and rifle. These weapons were much lighter than their conventional counterparts and utilized smooth barrels. The chamber and barrels weren’t sealed — in fact they had holes, which allowed the rocket exhaust to eject out the side of the weapon.
An especially interesting design feature common to all the Gyrojet launchers is the hammer itself. When you pull the trigger on a conventional weapon, a firing pin is thrust forward into the rear of the cartridge, which is held in place in the breach of the gun. The Gyrojet does just the opposite: the hammer hits the front of the Gyrojet, forcing the entire projectile back into a fixed firing pin. The pin would hit the primer and ignite the solid rocket fuel. The hammer now does its second job of holding the round in place while thrust builds up. It only takes a few fractions of a second. Eventually, the Gyrojet has enough thrust to push the hammer out of the way, cocking the weapon for the next shot. The projectile then continues on its way, spinning out of the barrel and off into the distance. This, of course, meant the Gyrojets were semi-automatic and could be fired as quickly as the shooter could pull the trigger.
Machining Solid Rocket Fuel
Gyrojets used solid rocket fuel, much like Estes rockets, and the Space Shuttle solid rocket boosters. The fuel itself was a double-base nitrocellulose/nitroglycerin mixture. The same fuel used in bazookas and other military rockets. The solid fuel arrived at MBA as a hollow stick. The sticks were cut, then machined down to fit the cone of the Gyrojet noses on a doweling machine and a lathe. Yes, you read that right. MBA was machining rocket fuel on a lathe. With careful control of feeds and speeds, it can be done safely, though it really isn’t something you would want to try at home. I just hope they cleaned out the lathe’s chip tray before anyone cut metal on it.
Getting the fuel to ignite and burn reliably was always a problem. Gyrojets used a normal primer, much like conventional bullets. Behind that bullet though, was a piece of flash paper or cotton which ran the length of the hollow fuel grain. The primer would ignite the cotton which would ignite the entire inner surface of the fuel grain. The fuel would then burn from the inside out.
With rocket fuel sitting inside a thin metal casing, one would want an insulator to ensure the outside surface of the fuel grain didn’t ignite and burn through the casing. MBA tried many chemical compositions for this, but found titanium oxide to work best. In a clever hack, they simply went down to the local paint store and bought the white paint which had the most titanium oxide. It happened to be Moore’s Number Eight. The paint was sprayed on the fuel grain, which was then inserted into the casing. The titanium oxide worked so well that other rocket companies began asking MBA how they were doing it. The answer was simple – head down the local paint store!
Iterating on Nozzle Design
The key to the Gyrojet’s spin stabilization was its nozzles. They were also the toughest part to manufacture. MBA tried many designs, from crimped casings to fluted vanes. The design that won out was a series of precise holes in the rear of the casing. The holes were tapered, acting as rudimentary de Laval nozzles.
Tapered drill bits aren’t very common, so MBA had to have them custom made. The holes also had to be drilled precisely across from each other, at a very accurate angle. MBA created a fixture called Bertha. It consisted of a rotary table mounted on a drill press at an angle. This allowed the angle to be precisely controlled and two, four, or more holes to be drilled accurately. Even with this fixture, any misalignment of the nozzle holes would cause the bullet to fly poorly.
Bertha was eventually replaced with a custom automatic drilling machine, but the drilled holes weren’t the last word on the Gyrojet. MBA began experimenting with sintered powdered metal. Mainhardt believed these were the most accurate of all the Gyrojet nozzles made. One can only assume that sintered nozzles would have become the standard if MBA had gone into full production.
The Fizzle of the Gyrojet
While they were innovative, Gyrojet never caught on in a major way. The the two primary downsides were muzzle speed and accuracy. A Gyrojet projectile would be deadly at 40 feet. But out of the barrel of a pistol, it might not even have enough momentum to pierce the skin. That’s not to say that getting hit point blank by a Gyrojet would be pleasant — bruises or broken bones were quite possible. But it chances are it would go much better for the target than being hit point blank by a conventional bullet.
The other problem was accuracy. Since the Gyrojet left the barrel at such a slow speed, effects like crosswind were much more pronounced than they were with a conventional bullet. Even tipping error came into effect. Nozzle alignment played a huge part in the accuracy of the Gyrojet. If one of the nozzles was miss-drilled, the Gyrojet would corkscrew through the air rather than fly straight. This was all summed up quite succinctly by a DARPA staffer:
“[It’s] Gyrojet all over again. If the target is close enough to hit, you can’t kill it. If you can kill it, you can’t hit it.”
While Gyrojet didn’t keep MBA afloat as a bullet, it did quite well as a flare. The company sold many spin stabilized flares and pen launchers to the US and other militaries. Spin stabilized rocket flares can still be bought today.
Shift to Less Lethal
On May 4, 1970, Kent State students were shot by national guardsmen. This incident shifted Mainhardt’s focus to nonlethal (less-lethal) munitions. Robert C. Mawhinney, a manager at MBA, pioneered the bean bag shot, which is still used as a less-lethal weapon today, although following the lineage back to this time in the company’s history is a bit tricky. In 1980, MBA was bought by Tracor. Tracor itself has since changed hands many times and is now part of BAE. After leaving Tracor, Mainhardt formed Trebor, Inc to focus on less-lethal weapons, eventually going bankrupt. The last of the Gyrojet inventory was sold under yet another company of Mainhardt’s, named PSI, in the early 1990’s.
Today Gyrojets are something of an oddity. No one is making guns, bullets, or parts anymore, so they’re commanding incredibly high prices at gun shows. Recently TAOFLEDERMAUS had the opportunity to try out a Gyrojet pistol and carbine. Thanks to his expertise with high-speed photography we have some amazing footage of Gyrojets in operation. With nearly 50-year-old munitions, it’s not surprising that there were a few surprises while filming. One of the cartridges blew out its primer. This created a huge hole in the rear of the shell, causing it to kick much harder than a normal Gyrojet firing operation. While exciting, no damage was done to the pistol or shooter.
While Hackaday doesn’t normally dive into things like bullets and munitions, the Gyrojet story was too good to pass up. The many engineering challenges it took to make a viable rocket bullet were amazing — especially considering this was all done without the aid of CAD or CNC machines.
If you want to read more about Gyrojets, your first stop is Mel Carpenter’s Gyrojet.net. Without a doubt, Mel is the global expert on Gyrojets. He’s written an incredible 400+ page book about Gyrojets and other MBA munitions. The book is based on exhaustive research including interviews with the MBA staff. It was used as a primary source for this article. You can pick it up here.