Gyrojet Book Sample Chapter 6

Chapter 6. Introduction to Gyrojets 77

MBA co-founder Arthur Biehl coined the term Gyrojetin 1961, and GYRO-JET became an MBA registeredtrademark, number 799,701, on December 7, 1965.MBA described its Gyrojets as, unguided, miniature,spin-stabilized rockets. Biehl and Mainhardt bothliked the way the word sounded and read even thoughthe Gyrojet was in fact a rocket with self-containedfuel and oxidizer, not a jet, which requires atmosphericoxygen to burn its fuel to produce thrust.

When I asked him about this apparent error, Mainhardtrationalized the discrepancy by explaining that therocket exhaust jetted out of the nozzle ports. Right orwrong, the name stuck and Gyrojet ammunition andfirearms are now the weapons most collectors associatewith MBA. They are certainly the most thoroughlydeveloped, with many more variations than any otherMBA ordnance product. They were also produced inthe largest numbers, with millions of 13mm Gyro-Signal survival flares and launchers being made forU.S. and foreign military organizations.

Gyrojets range in size from the first one made, anexperimental 2.8mm version, to one of the last, a 40mmvariation. By far, the most common Gyrojets, all ofwhich are scarce, are variations of the 13mm.

As noted in chapter 2, MBA was inconsistent when itassigned caliber designations to its Gyrojets. In somecases the company used the metric system withmillimeters, and in other cases it used the Englishsystem with inches. In a few instances it used bothsystems for the same cartridge, e.g., the .30 caliber/7.62mm. As is almost always the case with small armsammunition, the given caliber should not be takenliterally as the exact outside diameter of the rocket. Ihave measured hundreds of 12mm and 13mm Gyrojetsand discovered that the actual outside diameter of most12mm rounds is 12.54mm, and 12.94mm for most13mm versions. A Gyrojet collector without a

precision digital caliper will have a tough time tellingthe difference between apparently identical 12mm and13mm rounds.

Regardless of the caliber or specific model, allGyrojets share a common characteristic: they arestabilized in flight by high-temperature, high-velocitygas jetting through angled ports (holes) in theirnozzles (bases).

The thrust produced by the burning propellant has twocomponents, forward and angled. The forward thrustpropels the Gyrojet ahead and the angled thrust causesit to spin. Some Gyrojets spin clockwise (CW) andothers spin counterclockwise (CCW). Whendescribing Gyrojets in this book as spinning CW orCCW, Ill view the rockets from behind. The greaterthe port angle, the faster the Gyrojet will spin and themore stable it will be. However, high spin rates resultin high hoop stress caused by centrifugal force. In somemodels, such as the so-called .50 BMG Gyrojet, thisstress was high enough to cause the rocket to comeapart in flight, which is why that model failed.

Obviously, if more of the total available thrust wasused to spin the rocket, less was available to accelerateit forward to the target. The challenge for MBA wasto find exactly the right angle for the ports so thatenough spin was created to stabilize the Gyrojet, butnot more. Most Gyrojets have ports angled at about15 degrees, which results in 85 percent of the totalthrust produced being used for forward motion and15 percent used to create spin.

Why Gyrojets?

Why didnt MBA just stick with the Finjets andLancejets it had spent so much time, effort, and moneyto develop? From the beginning, MBA experienced astrong reluctance by its primary target customer, the

Chapter 6. Introduction to Gyrojets

Gyrojet is not a caseless cartridge development. The Gyrojet propellant is a solid fuel and isignited by a percussion primer. When fired, the complete rocket, including the primer, thrusts forwardand accelerates rapidly and accurately toward the target. Nothing is left in the chamber to be ejectedas in conventional guns.

MBAs first news release, July 1, 1965

78 MBA Gyrojets and Other Ordnance

U.S. military, to adopt a totally new small armsweapons technology unlike anything it had seen beforeand that was unproven, especially as the conflict inVietnam was ramping up. Mainhardt and Biehl spentevery available dollar on marketing the Finjets andLancejets they had developed to the U.S. military,including making many trips from California toWashington, D.C., for personal contacts anddemonstrations. At one point, the entire MBA workforce was laid off because of the lack of revenue topay salaries. If a new product had not been developed,MBA would have failed in 1961. Fortunately, thePrince Trusts financial support and a few governmentresearch and development contracts allowed thecompany to survive, if just barely.

Gyrojets, unlike Finjets and Lancejets, could bedesigned to look somewhat like standard military pistolammunition. The similarity in appearance of a .45-caliber Gyrojet, designed to be fired in a modifiedM1911A1 pistol, and a .45 ACP ball cartridge is easyto see, and this was no coincidence. Mainhardt quotedBiehl as suggesting, Why dont we make it look like a.45 ACP? referring to both the Gyrojet ammunitionand the pistol that fired it.

Gyrojets could be fired from handguns and longarmsthat were very similar in appearance to existingmilitary weapons, and in fact, MBA designed itsfirearms specifically to look like then-current militaryrifles and pistols.

Gyrojet ammunition could easily be loaded into clipsor magazines like conventional ammunition, and itwould function well in semiautomatic or full-automatic modes. In short, the technology distancefrom normal military firearms to Gyrojets was a lotshorter than it was to Finjets or Lancejets. By reducingthe apparent differences between the .45 ACP Model1911A1 and the Gyrojet pistol, and between the M16rifle and the Gyrojet carbine, MBA could concentrateits marketing efforts on selling the U.S. military onthe advantages of the Gyrojet over conventional

weapons. MBA listed several advantages anddisadvantages of Gyrojets compared to Finjets in MB-82 [a major 508-page MBA publication from 1962].

Advantages:

Potentially, Gyrojets could be more accurate.

Gyrojets had a greater packing density in agiven volume. They had no fins to take up space.

Gyrojets needed nothing to protect fins.

Gyrojets were easier to fire, and shorterlaunchers could be used.

Gyrojets had less drag than Finjets.

Gyrojets were easier to store.

Gyrojet cases were easier to manufacture.

Gyrojets were the only practical model forspace or vacuum applications, and NASA wasone of MBAs target customers.

Disadvantages:

Gyrojets had smaller Length/Diameter ratios(they were shorter, compared to their diameter)so less propellant could be loaded for a givencaliber.

Gyrojets could not be fired forward fromaircraft unless their spin was extremely highinitially, and an extremely high spin caused themto self-destruct.

Gyrojets had some accuracy-reducing forcesthat Finjets did not have.

Different spin rates were required for differentangles of fire.

Gyrojet manufacturing tolerances were morecritical than for Finjets.

Gyrojet stability and other factors were moredifficult to analyze theoretically.

Fig. 61. .45-caliber Gyrojet (left) and .45 ACP ball round.


MBA touted the following advantages of Gyrojets overconventional weapons:

Almost no recoil. Unlike a regular cartridge,almost all of the pressure involved in firing a Gyrojetwas contained in its case, with practically none actingagainst the firearm to push it back in recoil. This lowrecoil allowed for accurate second and subsequentshots. It could also help reduce training time for newrecruits since they would not have to get used to therecoil of a conventional pistol or rifle.

Very quiet. A Gyrojet firearm has no muzzle blast,and subsonic rockets produce no shock wave. Thosethat did produce a supersonic crack, did so out infront of the shooter where almost all of the Gyrojetsacceleration took place.

Lightweight. A Gyrojets vented barrel was notpressurized during firing, so the gun could be made oflightweight alloys of aluminum, magnesium, and evenplastics.

Less complicated design. All of the Gyrojetrocket (including the spent primer) left the barrel,with nothing left behind that had to be extracted andejected from of the weapon.

Less expensive. Gyrojets could have cost muchless than conventional firearms, at least if they hadbeen made in the same quantities, because of theirlightweight materials, very simple designs, generoustolerances, and few parts, which were easily made bydie casting.

High firing rates. Most of the Gyrojetspropellant burned outside the vented barrel, ahead ofthe shooter. Therefore, very little heat and friction wasgenerated in the gun. This cool operation, combinedwith no extraction or ejection, allowed for very highfiring rates in full-automatic Gyrojets.

Greater effectiveness downrange. Since theGyrojet continued to accelerate after it left the barrel,it had a higher velocity downrange at the target whereit counted most, while the conventional bullet, withits highest velocity at its muzzle, had slowed and lostsome of its terminal performance. The total burningtime for a typical 13mm Gyrojet was 0.12 second, and

burnout velocity was 1,250 fps. The Gyrojets kineticenergy downrange was almost twice that of a .45 ACPbullet.

These advantages sound good, and MBA emphasizedthem. But to be fair, I should also point out threesignificant disadvantages of Gyrojets when comparedto conventional firearms:

Low muzzle velocity. As described and illustratedon page xii of the books introduction, when a Gyrojetis fired, it accelerates slowly, moving forward to rotatethe pistols hammer back and down out of its way andcocking it in the process. It then leaves the barrel at afraction of its final velocity, which is achieved about45 feet downrange in the case of a standard 13mmGyrojet. There are a couple of documented cases ofpersons being shot multiple times with a Gyrojet atpoint-blank range (a foot or less) with little apparenteffect. In one case, after shooting his intended robberyvictim six times (the pistols capacity), a criminal threwdown his stolen Gyrojet in disgust and ran away,leaving a very surprised, but grateful, uninjuredstorekeeper behind. A very low muzzle velocity in adefensive handgun where combat ranges can be quiteshort was a serious disadvantage that MBA neverovercame.

High dispersion, low accuracy. Gyrojet rocketsexperienced a large number of unfavorable ballisticinfluences as they left the pistols barrel at a relativelylow velocity. One of these is called tipping error, wherethe nose of the Gyrojet, no longer being supported bythe barrel, was very slightly tipped down by gravitywhile the base of the rocket was still being supportedin the barrel. This caused the rockets thrust to bemisaligned as it emerged from the barrel. Tipping erroris greater at low muzzle velocities.

In addition, the Gyrojets stability was dependent onits nozzle ports being very precisely aligned with eachanother. In fact, nozzle port misalignment was theprimary reason for the Gyrojets inaccuracy.Unfortunately, the large production quantities MBAhoped for that would have supported the purchase ofexpensive production machinery, which could haveconsistently produced Gyrojet ammunition to verytight tolerances, never materialized. Every Gyrojet,Finjet, and Lancejet made was assembled by hand.


An inexpensive, lightweight military firearm can offerimportant advantages to an Army which has to buy itand to soldiers who have to carry it, but if it cannotconsistently hit its target, it has no real advantage.

In the words of one government critic reviewing areport of a feasibility study of small arms rocketammunition concepts submitted to the AdvancedResearch Projects Agency (ARPA) by the AAICorporation in 1971, [Its] Gyrojet all over again. Ifthe target is close enough to hit, you cant kill it. Ifyou can kill it, you cant hit it.

High Cost. Because they never reached highproduction rates (except for the 13mm flares), Gyrojetweapons and rockets were quite expensive. During atime in the mid-1960s when surplus Model 1903Springfields, M1 Carbines, and 9mm Lugers could behad for $40 each, with their ammunition costing aboutsix cents a round, standard MBA 13mm Gyrojethandguns cost $165 at retail, and 13mm ammunitionwas $31.20 for a 24-round box. If you bought a six-round box, the cost per round was $1.50.

Gyrojet Rocket Components

Regardless of caliber, all production Gyrojet rocketshad the same basic components:

Igniter; a small piece of treated paper foldedto fit inside the grains perforation, or a piece oftreated cotton cord.

Case; screw-machined or deep-drawn carbonsteel, generally copper plated against corrosion.Cases normally had either a typical roundnoseogive or a pointed conical nose.

Nozzle; machined carbon steel, almost alwayswith either two (late) or four (early) drilled,tapered ports. Some low-quantity nozzlesdesigned to save money were stamped.

Primer; standard off-the-shelf small pistolprimer with brass, copper, or nickel cups.

Propellant grain; double-base nitrocellulose,extruded with center perforation and turned toshape on a doweling machine.

Igniter

During early tests, MBA discovered that a separateigniter which provided second fire was required toreliably and uniformly ignite Gyrojet propellant grains.In most early experimental Gyrojets, the BoronPotassium Nitrate (BKNO3) coating on copper wirefuses that were inserted through the grains centralperforation served to uniformly ignite them.

However, when MBA made the shift from pyrotechnicfuses to standard small pistol primers, another igniterhad to be developed because the primer alone was notsufficient to ensure the grains uniform ignition. TheBKNO3 compound had worked well in earlier fuseapplications, and MBA selected it for its first newigniter. A small quantity of it was formed into a pelletand placed in a cutout in the nose of the grain, as shownbelow in Figure 62(A). This ignited the propellantreliably, but the compound was dirty and burned piecesof it tended to fly back in the shooters face. Inaddition, having all of the igniter concentrated at thefront of the propellant grain instead of being moreevenly distributed in the grains perforation made itharder to achieve uniform ignition.

Further experimentation resulted in MBA adoptingtreated paper strips similar to magicians flash paper,shown in Figure 62(B), or nitrated guncotton cord,shown in Figure 62(C). These two methods seem tohave been used interchangeably in late production12mm and 13mm Gyrojets.

Fig. 62. Gyrojet igniters. (A.) BKNO3 pellet in a propellant grainsnose cutout. Early .49 caliber. (B.) Treated paper strip, 13mm wad-cutter. Note the broached ring formed behind the grain to hold itin place away from the nozzle port inlets. (C.) Nitrated guncottoncord, 13mm with powder-metal nozzle. Note the retaining ring in-serted behind the grain to hold it in place away from the nozzleport inlets. Actual size.

A. B.

C.


Case

The typical 12mm or 13mm Gyrojet case was madeof carbon steel. Olin (Winchester), Inc. of East Alton,Illinois, made deep-drawn, copper-plated, steel casesfor MBA, and local machine shops with screwmachines were used for quantity production ofmachined steel cases. Although MBA had thecapability of producing Gyrojet cases in smallprototype quantities in-house, most of its productionconsisted of the assembly of components made byoutside contractors.

Stainless steel was also used for cases, but mainly inthe larger calibers. Cases for dummy presentationGyrojets were often nickel-plated, and a few dummieshave been seen with zinc or cadmium plating.

MBA also sometimes converted unrelated items intoGyrojet rocket cases, including the 18.7mm-diameter,nickel-plated Crosman Powerlet compressed-gasbottles used in air guns, shown next in Figure 64.Mainhardt told me that any strong metal cylinder withone end closed was a potential Gyrojet case, including.45-caliber, 500-grain rifle bullets, which weresometimes nickel-plated.

Some cases were left plain to save money, especiallythose that were expected to be fired in testing beforethey could corrode. Aluminum, sometimes anodized,was tried experimentally in .25 caliber, .50 caliber,and others. Turned brass cases were also tested inseveral calibers, mainly in the .45-caliber range.

MBA preferred to use existing off-the-shelfcomponents whenever possible, so many of the .30-and .50-caliber Gyrojets used conventional bulletjackets, draw pieces, or even complete bullets withtheir cores removed, as cases. Fiberglass was used asthe case material in MBAs 40mm cloud-seedingGyrojet. Typical Gyrojet cases are shown below inFigure 63. Other cases are described and shown laterin the chapters covering their specific calibers andtypes.

A. B.

C. D. E.

F. G.

Fig. 63. Gyrojet Cases. (A.) .25 Caliber, anodized aluminum. (B.) .30 Caliber, Gilding Metal Clad Steel (GMCS).(C.) 13mm, plain steel. (D.) 13mm, copper-plated steel. (E.) 13mm wadcutter, plain steel. (F.) .50 CaliberK.E. (Kinetic Energy), plain steel. (G.) 12mm Long, copper-plated steel. Actual size.

Fig. 64. 18.7mm Gyrojet case, converted, and its nozzle. Thesignificance of the 8 on the nozzle is unknown. Actual size.


Nozzle

Four basic spin-nozzle designs were tested during earlyGyrojet development. One of these was an unusualtype of nozzle that was tried as a means of simplifyingmanufacture and reducing costs. In this design, thebase of the Gyrojet case was simply crimped into acrude nozzle, as shown below in Figure 65. Thisdesign is sometimes referred to by collectors as apinched-base Gyrojet, because of its appearance. Bypinching the rear of the case with a slight twist, angledports were created.

Figure 65 below shows a pinched-base rocket with athree-port configuration. Two, four, and more portswere also tried. These are very scarce, and the onlyones known are in the Woodin Laboratory collection.

The pinched-base Gyrojets propellant was ignited bya fuse inserted through one of the ports. Both staticand flight tests of the design were conducted and thetests indicated that it was at least feasible. The work-hardened brass case used with one version did not openup at internal pressures of about 1,500 psi, and flighttests proved that the rocket was moderately stable.However, the dispersion of this relatively crude designwas very high and it was not adopted. Other pinchedbase specimens are shown in later chapters with theirspecific calibers.

Another nozzle type tried was a broached spiral designin a plastic nozzle. This was done with a heated drillbit which was force-twisted through a nylon nozzleport that had been predrilled to 0.016 inch. The nylonwas melted to conform to the shape of the bit. Whenfired in limited tests, the nozzle quickly eroded,removing all spiral channeling and resulting in asmooth nozzle port with no spin. No actual specimenof this nozzle is known.

The third type of nozzle used a fluted vane in theexhaust stream. This produced only 3 percent of therequired spin, and it was quickly discarded. I have nodrawing or other information about this Gyrojet.However, I think it is likely that the .30-caliberspecimen shown below in Figure 66 is an exampleof this very unusual design.

The fourth type of nozzle was the design that MBAadopted for production. It was a separate nozzle withtwo or more ports drilled at an angle to cause spin.These ports, which were tapered, were individuallydrilled in a fixture called Bertha by MBA machinists,and those who had mastered the complex operationwere highly valued because they were so few innumber. Drilling nozzle ports was very problematic.Expensive special-order, custom, tapered drill bitswere used, and these frequently broke, sometimeswhile drilling the fourth port of a four-port nozzle,which meant that the nozzle and the bit became scrap.

Fig. 66. .30-caliber (7.62 x 38.4mm) Gyrojet with fluted-vanenozzle, actual size . Nozzle shown 2x actual size. Key-chain dummysouvenir. Woodin Laboratory.

Fig. 65. Pinched-base .50-caliber (12.87 x 58.08mm) Gyrojet andits nozzle. This Gyrojet has a small remnant of BKNO3-coatedcopper wire fuse in the center of its nozzle. Actual size.

Fig. 67. Bertha port-drilling fixture. MBA photo.


The basic problem was that drilling ports at an anglecaused the stress on the bit to be not only unequal, butalso constantly changing as the bit rotated, and thiscaused breakage. When one port was successfullydrilled, the fixture holding the nozzle had to be rotatedby hand to position it for the next operation, and soon. Any play in the holding fixture or drill press wouldcause port misalignment.

Mainhardt told me the story of two young engineerswho approached MBA with a design for a machinethat could drill four nozzle ports simultaneously andaccurately. Mainhardt gave them $10,000 and toldthem he would buy the machine if it could be made.Several months later the pair returned to MBA withthe news that they had failed to develop a four-portmachine, but that they had succeeded in making onethat could automatically drill two nozzle portssimultaneously. At the time, MBA was beginning largescale production of the 13mm Gyro-Signal distressflare, which didnt need to be very accurate. Mainhardtsaid that two ports were good enough for the flares,and he bought the machine.

Fig. 68. Gyrojet nozzles, all shown actual size. (A.) .25 caliber, phenolic resin, 2 ports. (B.) .25 caliber, aluminum, 2 ports. (C.) .30caliber, aluminum, 2 ports. (D.) .30 caliber delay fuse, punched steel. (E.) 13mm, steel blank, not drilled. (F.) 12mm, copper-plated steel,2 ports. (G.) 12mm, plain steel, 2 ports. (H.) 13mm, plain steel, 8 ports, outlet (top) and inlet views. (I.) 13mm, copper-plated steel, 4ports. The standard 13mm production nozzle, outlet (top) and inlet views. (J.) 12mm, punched steel, 3 tangs, outlet (top) and inletviews. (K.) 12mm, punched steel, 4 tangs, outlet (top) and inlet views. (L.) 13mm, plain steel, 4 ports. (M.) 12mm powder metal, 4 straightslots. (N.) 13mm, plain steel, 4 large ports. (O.) 20mm, plated steel, 4 ports. (P.) 25mm, plated steel, 4 ports. (Q.) 30mm, plain steel, 4ports. (R.) 40mm, aluminum, steel, and fiberglass, 4 sealed ports.

A. B. C. D.E. F. G.

H. I. J. K.

L. M. N.

O. P. Q. R.

Fig. 69. MBA tapered drill bits. Actual size.


Testing of the two-port nozzles in distress flare rocketsshowed that they worked extremely well. As a result,they were tried in other Gyrojets, where they alsoperformed much better than expected. Because of this,late production 13mm Gyrojets are sometimes seenwith the same two-port nozzles used in 13mm flares.In addition, some 12mm Gyrojets, which were the lastones made, used the two-port nozzle.

Nozzle ports were drilled at angles from 7 degrees upto 35 degrees, with 15 degrees being typical. Earlyexperimental nozzles were made of nylon, aluminum,phenolic resin, and melamine. Each nozzle, whateverthe type or caliber, had a minimum of two ports andsome had up to ten ports.

Most nozzles included a primer pocket and flash hole.Production nozzles were machined, and many werethen copper-plated to prevent corrosion; however,some were left plain. Others were stamped from sheetsteel with tangs formed to deflect exhaust gasses andcreate spin.

Another nozzle was made by compressing powderedmetal in a die under very high pressure and thensintering (heating to just below the melting point torelieve stress) it. These powder-metal nozzles havethree or four slots instead of round ports, and the slotsare either curved or straight. According to Mainhardt,the powder-metal nozzles were the best MBA made,and they are often seen in late 12mm Gyrojets. Theywere inexpensive, accurate, and were made with justtwo operations, compressing into shape and sintering.

Primer

MBA used normal, off-the-shelf, 0.175-inch, smallpistol primers as first fire in its production Gyrojets.When research for this book began, I thought thatnickel-plated primer cups indicated dummy Gyrojetsbecause so many dummies have them. I later acquireda few fired Gyrojets with nickel primers, and I alsofound some unfired live rounds with them. So we knowthat at least some live Gyrojets were loaded with nickelprimers. But not many, except for the .49 caliber.

Mainhardt confirmed that MBA bought primers fromlocal sources like the San Francisco Gun Exchange,and generally didnt care whether they were nickel,

brass, or copper. MBA liked the primers that were usedin hand grenades because they were surefire.Unfortunately, they were not always available whenMBA needed them.

Because the primers stayed with the Gyrojet duringfiring, and were not supported by a breech face atignition, most were heavily crimped in place. As aresult, blown primers were not a problem for MBA.Most production Gyrojets used ring-crimping for theirprimers, but stab crimps and staking were also used.

When MBA assembled Gyrojets, treated cotton cordor paper igniters (second fire) were inserted in thepropellant grains, which were then placed in the cases.Segmented broached rings were then machined, orseparate metal rings were inserted as spacers to keepthe grains away from the nozzles as they burned. Smallpistol primers were seated in drilled nozzles using a

Fig. 610. Standard reloading press used by MBA for loadingprimers in Gyrojet nozzles. MBA photo.


the nozzle port inlets, which it would instantly haveclogged on firing. The cylindrical grains had alengthwise hole, or perforation, and were designed toburn only from the inside out. They were held in placeforward, away from the nozzle port inlets, by broachedrings of metal, as shown in Figure 62(B), or byseparate spacer rings inserted in the case in front ofthe nozzle, as shown in Figure 62(C). Some dummyrounds have solid pieces of rubber rod cut to length tosimulate the live propellant grain, perhaps to duplicatethe weight of a live round.

MBA acquired much of its propellant from theHercules Powder Company. The propellant was thesame used in some military rockets such as theBazooka, and was formed into long strands. Whenpreparing it for loading, MBA first ran the strandsthrough a doweling fixture to size the outside diameter,as shown below in Figure 6-13.

hand reloading press, shown in Figure 610. Theprimers were then crimped in the nozzles in a separateoperation, shown below in Figure 611, before thenozzles were loaded in the cases. Nozzles were securedby cannelures rolled in the cases to match the groovesin the sides of the nozzles, or by the ends of the casesbeing rolled over.

Propellant

MBA used only single pieces of nitroglycerin andnitrocellulose double-base propellant called grains, notto be confused with the grain as a unit of weight. Thispropellant was cheap, was easy to extrude andmachine, would not detonate, and it burned cleanly,producing nontoxic gasses.

MBA never used flaked or granulated gunpowder sinceit could not have been retained in the case away from

Fig. 611. Gyrojet primer-crimping fixture. MBA photo.

Fig. 612. Nickel-plated 13mm Gyrojet dummy section with rub-ber grain. Note the empty nickel-plated primer cup. Actual size.

Fig. 613. Propellant grain doweling fixture. MBA photo.


Sealer

MBA knew that Gyrojets would be exposed to heatand humidity, especially in the jungles of Vietnam,and used two waterproofing seals in its productionGyrojets. One was a clear lacquer-type, food-gradesolution named Humiseal, which was painted aroundthe primer and the case/nozzle joint. The liquidnormally had a small amount of red food dye mixedin so its presence could be verified during visualquality inspections.

The second seal was a thin piece of adhesive-treatedtinfoil applied to the inside of the nozzle prior toloading. The foil was punctured over the primer flashhole but was left intact over the port inlets so moisturecould not seep into the case through them. On firing,the pressure easily blew out the foil over the ports.These seals worked so well the Gyrojets they protectedcould be fired underwater.

The strands were then cut into individual grains ofthe required length, and these were individually turnedon a lathe to form the tapered nose and beveled rearend, as shown below in Figure 614.

Finally, an inhibitor was applied to the outside surfaceof the grains, as shown next in Figure 615. Thisensured that the grain burned only from the inside out.It also insulated the steel case against the extreme heatof firing. After trying several potential inhibitors, MBAfound that titanium oxide worked best. There was onegrade of white outside house paint available in theSan Francisco Bay area with a high concentration oftitanium oxide, Moores Number Eight, which MBAapplied to their propellant grains in a spray booth.When other rocket company personnel visited MBAand asked how the company made such a goodinhibitor, Mainhardt told them to buy a gallon at theirlocal paint store and give it a try.

Fig. 614. Turning propellant grain on a lathe. MBA photo.

Fig. 615. Applying titanium oxide in spray booth. MBA photo.

End of Chapter

This sample chapter is one of 26 chapters in the book, An Introduction to MBAGyrojets and Other Ordnance by Mel Carpenter. See the Book Contents page for acomplete listing of all 26 chapters, plus the Glossary, Patents and Trademarks,Handgun Instructions and Components, Bibliography, and Index.

Gyrojet Book Sample Chapter 6

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