Grantville Gazette 46

Naval Armament and Armor, Part Two: Ready, Aim, Fire

In part 1, I provided an overview of how warships were armed in the seventeenth century and later in the old time line, and considered the choices between muzzle and breechloading, and smoothbore and rifling. I also explained how cannon were manufactured. Here, I look at how the guns were mounted, laid, sighted, and fired, and at their internal ballistics. I also review the propellant options.

Gun Mounts

There are two basic gun mounts. First, the cannon could be mounted on a mobile carriage which recoils by rolling or sliding. Secondly, it can be mounted on a fixed pivot on the bulkhead, or a pivotable turntable on the deck; the recoil force must then be absorbed by the ship structure.

Mobile carriage. The wrought iron guns of the Mary Rose (sunk 1545) were mounted on "wooden beds," said to resemble "hollowed tree trunks," and these were equipped with one pair of wheels. In contrast, her bronze guns were mounted on four-wheeled truck carriages. (Konstam, 40). The gun carriages of the Vasa were of the latter type.

When a gun fires, the Law of Conservation of Momentum applies. Momentum is mass times velocity; the backward momentum of the cannon must equal the sum of the forward momenta of the projectile and of the gases that escape out the muzzle. The cannon being a lot heavier than the projectile, the effect upon it is less dramatic, but still quite visible; the cannon recoils backward.

The recoil is arrested eventually as a result of friction (rolling or sliding), gravity (the deck was cambered so the backward movement was slightly uphill), and elastic tension (the carriage was fastened to the hull with ropes, "breeching," that stretched taut when the gun moved backward enough). If the ropes broke, you had the proverbial "loose cannon on deck." The distance of recoil would depend on the weight of the cannon and shot, the powder charge, the elevation of the gun, and the particulars of the restraint. A 24-6.5 fired with a six-pound charge at point blank elevation had a recoil of 9.4 feet. (Beauchant 21). On narrow-beamed ships, port guns could be staggered relative to those on starboard to allow more recoil room. (Ireland 47).

It's worth noting that if the gun is elevated, the force of recoil is partially horizontal and partially vertical. While the gun carriage rolls backward as a result of the former, the deck must absorb the shock of the vertical component. That's one of the reasons that bomb ketches, whose principal armament was a large mortar, had a strongly-reinforced mortar bed to absorb the shock.

Fixed Carriage. Initially, pivoted guns were light weapons. However, some of Chapman's designs had pivoted heavy guns, and the nineteenth-century British and American navies toyed with the concept of providing a ship with fewer but heavier, more versatile artillery pieces. Both long guns and carronades were placed on pivots. (ChapellaHASN 238, 319, 422).

Early pivot designs had to be combined with raised decks or cut-down bulwarks, which exposed the pivot gun crews to small arms fire. This problem was corrected by a mount introduced during the War of 1812. With improvements to sturdiness, it could be used with a "long" 18-pounder. (319).

With a pivot mount, guns could be given a broad field of fire, but this meant that to avoid obstruction, a ship had to carry fewer (but perhaps larger) guns. Larger ships nonetheless retained broadsides; it took time to abandon the notion that the rank and seniority required to command a large warship shouldn't be based on the number of guns, but rather on the weight thrown. Hence, pivot guns tended to be used mainly in smaller vessels until the 1840s. (422). Eventually, design philosophy changed, and the big guns (say 10" up) were mounted on turntables and the smaller guns (9", firing 72 pound shell, or smaller) in broadside. (Canfield).

When pivoted guns became heavy enough to need to be mounted on a turntable, the designer had to decide whether to protect the crews from enemy fire and if so, whether the armor would rotate with the gun (true turret) or be a fixed part of the hull, a semicircular parapet (hooded barbette) that the gun fired over. The "hood" could be a light hood, just to fend off splinters, or a heavy one, to resist shells directly. If there was no protection at all, just a turntable, that was an open barbette.

The problem with the hooded barbette was that it limited the gun's range of elevation, whereas the true turret's disadvantage was weight (you probably want to use an auxiliary engine to turn it).

Another option was the disappearing gun; after the gun fired, its turntable would sink more deeply down inside a barbette for reloading. This design was used on HMS Temeraire (1877). It worked, but it was expensive to build and slow to reload, and was deemed a failure.

A true oddity, the British Wolverine (1798), had eight main deck guns which could be switched from side to side by thwartship tracks or skids, and which also had pivot mounts. (ChapelleHASN 422).

Recoil Reduction. With a muzzle loader, recoil had the advantage that it ran the gun into a reloading position. With a breech loader, recoil is simply annoying.

To reduce the recoil distance, you need to supply some countervailing force. If the gun was on a slide mount, the slide could curl upward on the inboard side, and the carriage's recoil would be slowed by gravity. Friction brakes were sometimes used to slow the recoil of wheeled cartridges. Pneumatic (compressed air) brakes were experimented with, but there were problems with air leakage.

The most successful recoil brake was of a hydraulic nature. The carriage was connected to a piston that fit into a liquid-filled cylinder. As the carriage recoiled, the piston was thrust into the cylinder, encountering fluid resistance. Tapered grooves in the cylinder allowed some liquid to pass from one side to the other, thus altering the dynamics of the system. A typical recoil liquid was a mixture of glycerin and water.

After the recoil was exhausted, the carriage had to be returned to the firing position. In our period, this was done manually. Later, gravity, springs, pneumatics or hydraulics were used to effectuate the return, and an additional brake might be used to soften the end of the "counter-recoil."

Admiral Simpson's ironclads have guns with hydraulic recoil and hydraulic counter-recoil (and, for that matter, hydraulic gunport control and ammunition hoisting). However, it's important to note that the hydraulic systems were salvaged from mining equipment, not made from scratch. Hence, only a few ships can be so equipped.

One method of avoiding recoil is to fix the gun securely and sturdily to the ship structure. This is not a Third Law violation; the force and momentum are transmitted to the entire ship, and that is so massive that the firing of a single gun is not going to have a discernible effect. (A full broadside would probably roll the ship substantially, and could strain the hull, which is why broadsides were actually rippled, not simultaneous.)

The catch is the word "sturdily." The part of the ship structure to which the gun is attached must be sturdy enough so as to withstand the force and transmit it to the rest of the ship. It would not be very good for continued employment as a ship designer if the bulwark broke off.

While this is less likely to be an issue for an ironclad, in which the gun is connected to the armor, it's a concern with wooden ships. Still, wooden bomb ketches were constructed in such a manner as to absorb the shock of firing a heavy mortar. And "non-recoil carronades were first used by the Arrow in 1796. . " (Blake 140).

The recoilless guns of land warfare use a different cheat; they eject a counterblast of equal momentum (mass * velocity) in the opposite direction at the time of firing. This may be propellant gas, or liquid or solid material that is forced out by the gas. The problem, of course, is that it is dangerous to stand behind the breech end of the cannon in the path of the counterblast. (Not that standing behind a recoiling cannon was smart.) The German Bohler 78 mm had the counterblast fired obliquely upward, to reduce the risk to crew (Hogg 135); but this would also require a larger blast to compensate for the angle, and create a downward force on the deck. Also, these recoilless systems are very wasteful (~80 %) of propellant.

An intermediate solution is a muzzle brake. This is a baffle attached to the muzzle end; the gases escaping with the projectile are deflected sideways and upward, so that they don't create a backward reaction force on the gun carriage. (Payne 265).

Gun Laying

A gun is elevated vertically, and traversed horizontally, so that with the chosen projectile and charge, and discharged at the correct moment, it will strike the target. The greater the range, the more important it was that the gun be elevated to compensate for the fall of the projectile, and traversed to lead the target.

Elevation (the angle between gun bore and horizontal, not the height of the gun above sea level) was relatively straightforward. Since about 1450, cannon were cast with trunnions-short lugs extending on either side of the barrel to serve as an axle. This fitted onto the gun carriage, and the barrel pivoted up-and-down around it.

Maximum elevation was dependent on the geometry of the barrel and carriage, but probably was about 15°-the highest value typically given in nineteenth-century gunnery tables. One source says the limit was about 7°, but that ships could hit a more elevated target by firing on the up-roll. (Volo 256). Douglas (252) proposed that nineteenth-century ships be equipped with "dismantling guns" that could achieve at least 30° elevation.

In the 1630s, changing barrel elevation was a little tricky. There was a wedge (quoin) under the breech end. The barrel would be lifted off the quoin with handspikes, and then the quoin would be moved forward or backward to adjust the elevation angle. That was enough for varying the degree of positive (above horizontal) elevation, but depressing the barrel below the horizontal was trickier. A wad had to be rammed down the muzzle so the ball wouldn't roll out, and it might be necessary to insert an additional or thicker quoin under the breech so the barrel would point downward. It should be noted that when the gun recoils, the position of the quoin may be disturbed.

You might logically wonder whether this cumbersome quoin system was adopted because no one had thought of equipping the gun with an elevating screw. The elevating screw per se had already been invented, as is evident from drawings by both Leonardo da Vinci and Albrecht Durer. (Kinard 70). However, it was not used in the field artillery of the Thirty Years' War (Guthrie 15), let alone in the technology-lagging naval artillery. Deane (48) says that a Jesuit, in 1650, was the first to equip a land gun with an elevating screw. As for naval guns, the British introduced elevating screws around 1790, for use on carronades. (Lavery 132).

A screw provides mechanical advantage-it is the equivalent of an inclined plane that has been coiled up. The pitch of the screw determines how much the gun is elevated per turn; the smaller the pitch, the slower the elevation, but the finer the control. Modern tests on nineteenth-century 6-pounders revealed that each turn elevated the piece by 30–60 arc-minutes, and that the obtainable accuracy of elevation was about 2 arc-minutes. (Hughes 19).

Without the elevating screw, it took at least four men to change elevation: at least two with handspikes to lift the breech end, the "first Captain" to sight the gun and judge when it was at the right elevation ("Raise!" "Lower!" "Well!"), and the "second Captain" to adjust the quoins to hold the gun ("Down!) at that elevation. With the screw, one man could sight the gun while turning the screw to suit.

Nonetheless, to make a rapid, albeit crude, change in elevation, quoins were apparently faster, which is why carronades were also given molding under their breeches. (Lavery 132). Quoins were also needed if the elevation change was greater than that permitted by the screw (Douglas 163).

In the case of field guns, "the heavier pieces like the 18-and 24-pounders were still elevated by quoins as late as the early 1800s." (Manuoy 55). Quoins were also still used with siege guns. I suspect that this was because there were technological limitations at the time on the pitch or the compressive strength of the screw, and therefore on how heavy a weight could be lifted. The logical solution was to increase the mechanical advantage by using gears. And from there, the next step was to provide power assistance, e.g., from an auxiliary steam engine or an electric motor, rather than relying on manual operation.

In canon, elevation screws are apparently in use by the Danes in 1634. Offord, "The Bloody Baroness of Bornholm" (Grantville Gazette 18).

Traversal. For a target which is not moving relative to your gun, you traverse the gun so it points horizontally at the target. If the target is moving, you must "lead it" — point to the place it will be when the projectile arrives.

The wheels of the standard naval carriage all rolled forward and backward, and therefore would not have made it any easier to turn the barrel toward the bow or stern. The carriage had to be turned to or fro by brute force.

I own a storage cart with four swivel casters, i.e., wheels with a pivotable connection to the cart. A cannon, of course, is a lot heavier than a storage cart, but internet searching reveals that some caster manufacturers (e.g., Hamilton) claim that their casters can support up to ten tons. Of course, I have no idea whether we have the metallurgical skills to duplicate these casters at a reasonable cost, but it shows that the idea of putting such on a cannon carriage isn't absurd. But perhaps it would make the cannon too easy to move sideways, causing them to shift as the ship pitched.

A pivot mount, of course, would make traversing much easier, and could be equipped with a traversing screw or gear. With a simple slide mount, traversing the gun would be impossible. However, for carronades, the slide bed itself was mounted on a pivot, and on the inboard end there were two small wheels, whose positions established the radius of the traverse. Since the recoil motion was on the slide, and didn't affect these wheels, they could be positioned to roll circumferentially, making the traversal much more efficient. (Blake 140).

There were basically two ways of mounting a turret; it could rotate around a central shaft (Ericsson's USS Monitor) or on a circular track with ball bearings (Eads' USS Winnebago) (cityofart.net).

There were aircraft and tank turrets that were manually rotated, but naval turrets were larger and heavier. While the earliest naval turrets were hand-cranked (Kinard 237), the USS Monitor was equipped with a steam "donkey" engine to turn its turret, and that quickly became the mid-nineteenth-century norm. However, steam engines radiate heat, making conditions in the turret unpleasant, and of course there's the risk of scalding the crew if a leak occurs. There was some experimentation in the late-nineteenth century with compressed air systems, but the necessary high working pressures posed dangers of explosion. By the early-twentieth century, turret power was either hydraulic (British) or electric (American) in character. (Fullam 214ff).

Elevation Measurement

It does you no good to calculate and adjust the elevation of the gun if you can't judge whether you have done so correctly.

Pre-Ring of Fire (RoF), elevation was determined using a gunner's quadrant, first described by Tartaglia (1545). This was an L-shaped instrument with a plumb bob and an arc scale. One arm of the L was placed inside and parallel to the bore; the angle at which the plumb bob intersected the scale was read off.

Elevation may also be read off by a clinometer. A viscous liquid might half-fill a disk, and then the level of the liquid (an artificial horizon) is compared to an angular scale inscribed on the face of a disk. This is analogous to the aircraft inclinometer.

Or an object, a bubble or a bead, moves inside a tube filled with a viscous liquid, as in the spirit level used by carpenters, and the tube is graduated to show the angle of inclination.

Unfortunately, the spirit level doesn't provide much of an angle range. So a military clinometer has the spirit level mounted on a pivotable arm, which points to a scale that specifies the "zero" angle for the spirit level. The arm is attached to a frame whose base is placed on a receiver attached to the gun barrel. Since the outside of the gun barrel is not parallel to the bore, this receiver must be adjusted, just like gun sights, or an offset must be dialed in. To set the gun to a desired elevation, you lay the clinometer on the receiver, adjust the arm to point to the desired value on the main scale, and elevate the gun until the level bubble in the level vial is centered.

Some kind of pocket clinometer, most likely of the kind used by geologists, came through the Ring; see Jones, "Schwarza Falls" (Grantville Gazette 5).

Gun Sights

Open Sights. The simplest method of sighting was to sight along the "line-of-metal," the top of the cannon, directly at the target. However, the cannon was wider at the breech end than the muzzle end, so the line-of-metal was depressed 1–3° (Douglas 293; Beauchant 16) below the line-of-fire, depending on the exact geometry of the cannon. This could be corrected for by adding a "dispart," a vertical sight at the muzzle end, with a height equal to half the difference in diameters. If the bore wasn't quite center, this still wouldn't be quite right, but a gunner could customize the dispart for the peculiarities of a particular gun.

The tangent sight was an adjustable rear sight. The sight was on a bar, graduated either in degrees or ranges, and fitting into a socket at the center or on the side of the breech. The name is derived from a trigonometric relationship, the required height of this rear sight is the product of the distance from the rear sight to the front sight, by the tangent of the required angle of elevation. This may seem a simple concept, but it doesn't appear to have been used on artillery until it was introduced by de Gribeauval in the late-eighteenth century. (Cummins 25). Bear in mind that its use implies setting a specific angle of elevation, rather than just sighting on an aiming point. (Ruffell).

In canon, the guns of Simpson's 1634 navy have ring-and-post sights. (1634: The Baltic War, (TBW) Chap. 38).This combines a front (post) and a rear (ring) sight.It can be advantageous for the "ring" part to have several concentric circles; these can be useful in "stadiametric ranging" (measuring the angular width of a target of known actual width). A V-or U-notch is a possible substitute for the ring.

You have to hold your head just right to keep the ring and post aligned. It's also hard to use if the target is far away; bear in mind you are trying to keep in focus the target, the rear sight and the front sight, all at different distances.

Telescopic Sights. In Cooper, "Seeing the Heavens," Grantville Gazette 16, I described the state of the telescopic art as of the RoF. In 1640-41, William Gascoigne mounted telescopic sights-essentially, a Keplerian telescope with crosshairs in the focal plane-on various scientific instruments, including a micrometer and a sextant. However, the first documented use of a telescopic sight on a firearm was in 1835, and that was for use with a percussion ignition sporting rifle. (Pegler 50). And as far as I know, the first use of a telescopic sight with artillery was in 1857. (Strauss 587).

If this long time lag from the invention of the telescope to its use in gunnery surprises you, consider this: it doesn't matter if you can see the pimple on the enemy helmsman's nose if your powder and shot are so inconsistent in character, and ship motion so erratic, so you can't even hit the enemy ship with more than one shot in ten.

In canon, the best of Krak's Shooters have been given up-time telescopic sights for their flintlock rifles. (Flint, 1633, Chapter 35).

Since telescopes provide a magnified image, they necessarily have a narrowed field of view, and the eye needs to be close to the eyepiece to get that view.If the telescope is attached to a cannon, you must move your head away quickly when you fire, lest recoil result in an unpleasant experience.

Reflector Sights. A half-silvered diagonal or curved mirror can be used to overlay a virtual image of an illuminated crosshair, harmonized with the gun bore, over the field of view. While optical tricks using partially reflective mirrors are much older, the reflector gunsight reportedly was invented in 1900. A reflector sight is easier to align with the target than is an open sight. But please note that open sights were still used four decades later.

In shooting at a distant target, you need to allow for the "drop" (from gravity) and the "lead" (to anticipate relative target motion). So you have to offset the line of aim from the line of bore so that the projectile would hit the target.

Initially, gunners had to offset manually. However, analog computers were developed to calculate this offset and manipulate the optics accordingly. On these "computing gun sights," the gunner had to estimate the size of the enemy and fit the image within a reticle so the range could be calculated stadiametrically. Anti-aircraft guns had fancier "predictors."

Gyro Sights. The gyro sight was developed during WWII for aircraft (and anti-aircraft) use. The reflector was linked to a spinning gyro and this made it possible for the sight to compensate for the aircraft's own motion by adjusting the reflector. (Jarrett 190).

Firing Mechanism

The period gun has a vent (touch hole) that connects the powder chamber to the outside world. In preparing to fire the gun, the touch hole was filled with a "priming" powder, and some powder was deposited on the barrel just behind the touch hole. A linstock (forked staff) was used to bring a lit "slow match" (a slow-burning fuse, made by impregnating a rope with a saltpeter solution) over to the surface powder, igniting it. It, in turn, ignited the powder in the touch hole proper, which ignited the powder in the chamber. (Little 145).

Unfortunately, this process tended to erode the vent. Consequently, come 1697, gunners inserted disposable metal (tin) tubes into the vent. The tubes were filled with a paste of powder, gum and water, and loose powder was sprinkled on top. In 1778 the British Navy replaced the metal tubes with goose quills. (Rufell).

After 1700, it became customary to use the slow-match just to light a "portfire," a paper tube, closed at one end, filled with a mixture of gunpowder, sulfur and saltpeter in a linseed oil base; it burned rather like a motorist's emergency flare. (Peterson 66).

I imagine that we will leapfrog portfires and proceed to mechanical ignition. The first such was the Douglas flint lock (1778), which was actuated by a lanyard that pulled its trigger. In 1842 it was replaced by the Hiddens percussion lock; a hammer struck a percussion cap. (EB11/Ordnance).For the reasons why the 1633 NUS army was armed with flintlocks, not percussion locks, see Grantville Firearm Roundtable, "Flint's Lock" (Grantville Gazette 3).

The "firing interval" is the time elapsed from when the gun captain activated the ignition mechanism to when the primer actually ignited. (There would of course be a further delay until the projectile actually left the gun barrel). With the percussion lock, the firing interval averaged 0.13 seconds. (Meigs 195).

The percussion cap contains a primer, a fairly sensitive explosive mixture that in turn sets off the explosive. The first primer developed was mercury fulminate (1807). According to canon, certain reckless souls are making it. See Offord, "Dr. Phil Zinkens A Bundle" (Grantville Gazette 7); Offord and Boatright, "The Dr. Gribbleflotz Chronicles, Part 2: Dr. Phil's Amazing Essence Of Fire Tablets" (Id.); Mackey, "The Essen Steel Chronicles, Part 2: Louis de Geer" (Grantville Gazette 8); Evans, "Thunder in the Mountains" (12); Zeek, "One Fine Day" (20); Offord, "A Change of Hart" (25); Howard, "The Baptist Basement Bar and Grill" (32).

While these stories emphasize the dangers of manufacturing mercury fulminate, there are other problems. Specifically, it was found (in 1897) that the mercury in the primer became amalgamated with the brass of nineteenth-century cartridge cases, embrittling them. These cases were a large part of the cost of a cartridge and the "brass" (couldn't resist) wanted to be able to reuse them.

Accordingly, mercury fulminate was replaced with potassium chlorate (historically, first synthesized in 1786). That, too, has appeared in canon, in the percussion caps for the French "Cardinal" rifles. (TBW Chap. 27, 45).

It's not a panacea. When a gun using a potassium chlorate primer is fired, the priming reaction generates potassium chloride, which is deposited on the bore. This salt greedily absorbs water, causing rusting. In OTL, it wasn't until 1922 that potassium chlorate was identified as the cause of the rust, but Grantville's gun buffs may already know about the problem.

A non-corrosive primer, based on lead styphnate, was patented in 1928. It's likely that gun owners in Grantville have heard of it. Condensed Chemical Dictionary reveals that this is the legal label name for lead trinitrosorcinate (311), that the latter is made from magnesium styphnate and a lead salt, the former in turn being made from magnesium oxide and styphnic acid (312), and that styphnic acid is made by nitration of resorcinol (830; cp. 759).

An alternative to the percussion cap is the friction tube. This is described in detail in EB11/Ammunition; essentially this is a T-shaped device, the vertical branch communicates with the vent hole, and the horizontal branch contains a copper friction bar surrounded by a "friction composition." The lanyard causes the friction bar to be pulled out, igniting the composition which ignites the powder in the vertical tube.

The great advantage of electric ignition was that it reduced the firing interval. However, the reliability of the power source is the sticking point. While it would be possible to have the ship carry a generator and run lines from it to all the guns, that would mean that a shot that took out the generator rendered them useless. Hence, each gun must have its own battery. And developing a working battery itself took some time. An 1894 article (Morgan) noted that while electrical ignition had until then been in limited use, the Bureau of Ordnance had recently adopted a zinc-carbon dry battery as well as a new electric primer design. By WW I, electric ignition was the norm, with percussion systems as backup.

Internal Ballistics

The term "ballistics" was coined by Marin Mersenne in 1644. Ballistics may be divided into three broad categories: interior (internal) ballistics, explaining what happens inside the gun barrel; exterior ballistics, describing the flight of the projectile through the air; and terminal ballistics, dealing with its penetration of the target.

Gun designers manipulate the internal ballistics of a gun so that it projects the desired projectile at the desired muzzle speed without bursting the gun.

A good propellant is one whose ingredients react very quickly ("deflagrate") to form a gas. At normal temperature and pressure, this gas would occupy a much greater volume than the original ingredients; but initially the volume of the gas is limited by that of the propellant (powder) chamber, and so there is instead an increase in pressure. The deflagration reaction also generates heat, which further increases pressure.

As the reaction continues, the pressure reaches the point that it's sufficient to overcome the friction holding the projectile in place ("shot-start force"), and it starts traveling down-bore.

The propulsive force on the projectile is the pressure times the area of the projectile base; the acceleration it feels is the force divided by the mass of the projectile. For spherical shot, the base area is proportional to the square of the diameter, and the mass to the cube, so acceleration is inversely proportional to the diameter.

You can cheat to some degree by using a sabot as a gas check. The sabot ensures that the pressure is exerted on the largest possible area, but the projectile may be subcaliber (narrower than the bore) and therefore less massive.

For a given caliber and shape, stone projectiles have a lower sectional density (mass/frontal area) than lead or iron and that means that for a given barrel length, they require less force to accelerate them to a given muzzle velocity. Less force means less pressure which means a smaller charge. Stone projectiles can therefore be fired from lighter cannon than metal ones of the same weight. As of the early-seventeenth century, stone throwers (pedreros) were being phased out in Europe, but they remained popular in the Ottoman Empire.

As the projectile moves, the volume available to the gas increases, which tends to reduce the pressure. On the other hand, if the deflagration reaction is still going on, the newly-produced gas and heat will tend to increase the pressure. One can thus draw a pressure-time or pressure-travel curve, and the location of its peak will depend on the specific characteristics of the powder. Likewise one can draw velocity-time or velocity-travel curves for the projectile. Sample curves appear in EB11/Ballistics.

Once the powder is completely consumed, the propulsive force on the projectile can only decrease as it moves down-bore, and once that propulsive force is less than the resisting forces, any further travel will reduce the projectile's speed.

After the projectile exits the muzzle, the pressure on it drops precipitously, although it may experience a brief period of additional acceleration from the escaping gases.

Internal Ballistics: Barrel Stress

A gun barrel, in essence, is a pressure vessel, containing the gases generated by the rapid combustion of the powder. We want to design the barrel so the gun is safe to fire, without inordinately increasing its weight and cost.

One way to do this was to put the metal where it was needed, i.e., where the pressure was greatest. It was certainly known to the down-timers that the thickness had to be greater at the breech than at the muzzle-they figured twice as much (see Manucy 37 for the detailed thickness variation)-but they had no quantitative knowledge of the pressure variation. That was revealed by nineteenth-century experimentation, as discussed below.

It is also important not to impair the barrel's function. Until the mid-nineteenth century, it was customary to bedeck cast cannon with a variety of ornamentation. However, these protuberances acted as "stress raisers," weakening the gun. (Hazlett 147-8, 221).

For a thin-walled (thickness not more than one-tenth diameter) cylindrical pressure vessel, the hoop stress is pressure * radius / thickness, in which case thickness should be proportional to bore diameter if pressure held constant. However, a cannon can't be considered thin-walled; the cannon of the Santissimo Sacramento (sunk 1668) had a maximum barrel thickness about equal to the bore diameter. (Guilmartin).

The thick-walled tube hoop stress formula, if external pressure is ignored, is

(Ri2*p/(Ro2-Ri2)) * (1+ Ro2/r2)

with

Ri inner radius

Ro outer radius

p internal pressure

r radius at which stress is calculated. (Labossier; McEvily 53).

So, at r=Ri, stress is

p*(Ro2+Ri2)/(Ro2-Ri2)

and at r=Ro, it is

2*p*Ri2/(Ro2-Ri2)

It's immediately evident that the stress is greater at the inside radius (Ri) than at the outside radius (Ro); the gun will crack first on the inside and the crack will grow each time the gun is fired.

Wall thickness of course is Ro-Ri and bore diameter is 2*Ri. When increasing the barrel thickness, each additional layer decreases the stress inflicted by a given internal pressure, but with diminishing returns (Table 2–1):

If we express Ro as k*Ri, so thickness is (k-1)*Ri, then the inside stress is proportional to

(k2+ 1)

– -

(k2- 1).

The even more complex "Gunmaker's formula," for built-up guns (see part 1), appears in EB11/Ordinance.

I mentioned pre-stressing in connection with cannon manufacture; this is to "make the outer layers of metal in the barrel bear a greater proportion of the bursting load." (Payne 264).

****

With real guns, the pressure varies according to the position of the projectile. In 1861, the distance-pressure curve for a 42-pounder with the powder of that time might feature a maximum pressure of 45,000 psi, dropping to one-tenth that by the time the shot exited the muzzle. (Bruce 138). Hence thickness (and thus weight) can be reduced if you know how pressure varies.

It is possible to control the curve to some degree by powder design. A progressive powder (burn rate increases with time) reduces the peak pressure, and thus the required barrel strength. This also reduces barrel wear, which tends to be more dependent on peak pressure than average pressure (Rinker 43). And it's likely to provide the highest exit velocity. On the other hand, if you have a short barrel, then use a degressive propellant, so you develop high projectile velocity quickly. (ES310).

Pressure Measurement. In 1842, U.S. Army Chief of Ordnance George Bomford "had holes drilled at regular intervals along a cannon barrel. Pistol barrels were then fastened into the holes, each loaded with a bullet. Opposite each barrel he placed … a ballistic pendulum" (see below). This allowed him to generate a projectile displacement-pressure curve. That, in turn, permitted the design of guns to have metal exactly where it was needed. It's not clear to me how much attention the rather hidebound Navy paid to these newfangled Army notions, but by 1850 John Dahlgren had designed a 135-pounder shell gun with a soda bottle shape. (Park 113).

A somewhat less Rube Goldbergesque sensor was the Rodman indenting gauge (1858). Its tube, like Bomford's pistol barrel, fitted into a drilled hole in the barrel wall, and the expanding gases moved a piston with a gas check, which in turn moved a knife that indented a copper disc. The depth of the indentation was compared to that achieved with a matching disk (from same copper bar) and knife using a standard testing machine. (VNEM). In the Noble crusher gauge (1860), the Rodman disc was replaced with a cylinder of copper, resulting in the pressure being expressed as so many "copper units of pressure" (CUP). For guns developing lesser pressures, lead cylinders were used. (EB11/Ballistics; Barnett 195ff; Buchanan 306).(The Rodman or Noble gauge could be inserted behind the cartridge, but this had its own limitations.)

We've been focusing on pressure, but the deflagration also results in an increase in temperature. The temperature can reach 5,550°F, and barrel steel melts at 2,500°F (Rinker 62). Fortunately, the projectile is only in the barrel for something like ten thousandths of a second. Still, gun barrels can definitely overheat. It's therefore very important that barrels have a high thermal conductivity so heat is dissipated quickly.

Gustavus Adolphus experimented in the 1620s with "leather cannon" for field use. This was actually a thin copper barrel with leather wrapped around it and bound with wire, cord and canvas; we know this because one prototype (test-shot in 1628) survived. (Brzezinski 18). Leather-like ceramics, glass and plastic-is a poor conductor of heat, and the leather cannon had a tendency to overheat and burst; they were superseded by bronze pieces. A Mythbusters version fired a cannonball at 450 mph, but blew out its breech in the process (episode 141).

Guns often were designed with separate powder chambers; these were narrower than the bore (to reduce stress) but communicated with it. They could be cylindrical, spherical or conical in shape. Spherical chambers offered the greatest muzzle velocity, but were difficult to construct, load and clean, and strained the gun most. Conical offered the worst muzzle velocity, so cylindrical became the happy compromise. (Jeffers 98).

The maximum quantity of powder that could be used in the gun was limited by the gun's bursting strength and the size of its powder chamber). One pound of 1820s powder occupied 30 cubic inches. (Beauchant 104).

Barrels can suffer permanent bore expansion as a result of exceeding the "elastic limit," catastrophic rupture, gas leakage, fatigue (micro-cracking), and erosion/wear. Barrels can be inspected for deterioration in a number of ways, including measuring the bore diameter deep inside the barrel with a long-handled inside caliper, and visually inspecting it with a borescope. A rigid borescope would be something like a periscope with a magnifier and a light attachment. A flexible borescope uses optical fibers and thus requires a higher tech level.

Internal Ballistics and Windage

Bore-windage had several effects. First, gas could escape around the ball, reducing the effective pressure driving the projectile. This reduced muzzle velocity and wasted energy, but also eased the stresses on the gun barrel. Secondly, as the ball progressed down-bore, it would glance off the walls of the bore. Each bounce drains some of the kinetic energy of the ball, thus further reducing muzzle velocity. Also, the direction and spin the ball emerged from the muzzle would be dictated by its last bounce. Obviously, this affected range and accuracy. All the bouncing around was also bad for the gun barrels. (Douglas 81).

The direct energy loss from escaped gas is proportional to the ratio of the annular area to the bore's cross-sectional area. Since windage is small, this ratio is roughly inversely proportional to the bore diameter. By my analysis, the indirect loss, from inelastic collision with the bore wall, will be proportional to 1-r? where r is the "coefficient of restitution" (the kinetic energy after collision as fraction of that before collision, for each collision) and n is the average number of bounces, which is proportional to the barrel length divided by the windage (as diameter difference).

Well, that's all theoretical. In practice, Douglas (70) says that one-quarter to one-half of the force of the powder was lost in consequence of the early-nineteenth-century standard windage. Douglas urged that windage should just be a fixed allowance, rather than one proportional to the gun's caliber. Only the degree of expansion due to heat, he reasoned, would be dependent on caliber (amounting to 1/70th caliber at white heat); rusting of the shot and fouling of the bore wouldn't be. He suggested reducing windage to 0.1–0.15 inches. (74ff).

The maximum pressure usually obtained in the late-nineteenth century was 15 tsi in rifled guns and 3 in smoothbores-this shows how much difference windage makes! (Barnett 196).

Muzzle Velocity

Range is definitely a function of muzzle velocity. The following table shows expected ranges for an early-nineteenth-century 24-pounder fired at a 45° elevation:

(Douglas 43).

Suppose that the work done by the powder in moving the projectile down the bore is proportional to the powder charge. If so, then the kinetic energy obtained must be proportional to the charge, and the muzzle velocity is then proportional to the square root of the powder charge relative to the weight of the shot. (Sladen) and this was generally assumed by early-nineteenth-century writers on gunnery (Beauchant 45; Douglas 53, 57).

In 1828, Beauchant proposed the following formula:

MV = 1600 sqrt (2 powder weight / shot weight)

This leads to the following results:

(Beauchant 45, 133)

This rule is probably good enough for our purposes, although I suspect that "1600" is a bit high for 1630s guns. Assuming powder quality is 75 % of early-nineteenth-century levels, we could use "1200" instead.

However, the work done on the projectile per pound of powder is not really constant regardless of the charge. It's dependent on the expansion ratio of the full bore relative to the initial charge volume, and thus depends on the length of the bore and the size of the charge. (Sladen 32).

Grantville has the Encyclopedia Britannica 9th edition, and its "Gunmaking" article provides Noble's table of the theoretical maximum work done by gunpowder per pound of charge ("specific work"), as a function of the expansion ratio. One can therefore calculate the expansion ratio, interpolate the "work/pound" from the table, and plug it into this formula:

Vmuzz= sqrt (2 g k e wp/ ws)

where g is gravitational acceleration (322 fps), k is the specific work per pound, e is the efficiency of the powder relative to the theoretical maximum, wpis the weight of the powder, and wsthe weight of the shot.

The efficiency of a gun is less than 100 % because some energy will be lost by heating the barrel and, for rifled guns, in rotating the projectile. EB9 suggests an efficiency ("factor of effect) ranging from 0.6–0.65 for field guns up to 0.85-0.95 for heavy guns.

In the nineteenth century, the "gold standard" for predicting muzzle velocity was Sarrau's monomial (for quick powders) or binomial (for slow powders) approximation. This had a couple of adjustable parameters to account for the differences between powders and these were determined by measuring the muzzle velocity for the same powder fired in two dissimilar guns. You could then apply the same formula to any other gun using the same powder.

The Sarrau formula was available in a few texts for general readers, including Johnson's Universal Cyclopaedia (1895), and the Encyclopedia Britannica 10th edition (the 1902 supplement to the 9th edition), but these are not, as far as I know, among the books that traveled back with Grantville.

Optimal Bore Length. Bore length is probably 92–94 % of the length of the piece. (Douglas 293). In general, the longer the effective bore (from projectile starting position to muzzle), the greater the muzzle velocity for a given powder charge; the muzzle velocity in turn determines range and penetrating power.

But there are definitely diminishing returns. Experiments have been conducted in which a barrel is successively cut down and the new muzzle velocity determined. In 1862, Benton (130) reported that for a small change in length of a 12-pounder, the velocity was in fact proportional to the fourth root of the length. Another writer says that it's proportional to something between the square and cube root of the length of the bore (Douglas 101). I have seen the opinion expressed that there was no advantage to making a sixteenth-century gun longer than ten feet (Rodger 215). A 24-pounder of Douglas' time would have a bore length of 5.5–8.92 feet (293).

Theory also predicts diminishing returns. If the powder burns at a constant rate, slowly enough so the last of the powder is consumed just as the projectile exits the barrel, and the gas expansion is isothermal, the muzzle velocity will be proportional to the cube root of the barrel length. If we instead assume that the gas expansion is adiabatic (no heat lost), then it will be proportional to the fifth root of the length. (Denny 183-5). Note that this requires that the powder charge be proportional to the length of the barrel, which was not usually the case!

The analysis above is for black powder propellant; with smokeless powder, the pressure-position curve is different, and the optimal barrel length depends on the shape of the curve. (Denny 67ff, 188ff).

The advantage of increasing length is not so much the increased muzzle velocity, but rather that one can then use a slower-burning powder and thus reduce the maximum pressure-permitting reduction of barrel thickness and increasing barrel life. (Sladen 34).

Is there a limit beyond which increasing length has no effect or even reduces muzzle velocity? If the force propelling the projectile merely diminished as it traveled down bore, then there would be no bore length at which muzzle velocity was maximized, merely diminishing returns from lengthening it. But there is such a length, because the projectile's movement faces opposition even as the propulsive forces decline.

Benton (128) suggested three opposing forces: (1) friction, (2) inelastic collision, and (3) the pressure of the air in front of the projectile, and urged that if the length is increased too much, keeping the charge constant, the muzzle velocity will decrease.

Friction comes into play only for rifled barrels, where the projectile engages the rifling. The frictional force is presumably constant throughout the length of the barrel, whereas the propulsive force declines as the projectile moves down-barrel. It's possible to show that if you assume constant burn rate, isothermal expansion, constant frictional force along barrel, and optimal projectile length (powder completely burnt, and frictional force equal propulsive force, just as projectile reaches muzzle), the length at which the frictional force equals the propulsive force must be proportional to the mass of the projectile and the square of the muzzle speed, and inversely proportional to the frictional force in the barrel (this results from combining Denny equations N7.6, N7.9, and N8.2). Since the length of the region of contact is kept small, the frictional force at any given moment should be proportional to the circumference of the bore and thus to the diameter.

For smoothbores, interior collisions slow down the projectile, but as noted in the discussion of "windage," they should be less common as the diameter increases.

Like friction, outside atmospheric force is a constant resistive force, but it's proportional to the area and thus to the square of the diameter of the bore.

Powder charge. The expectation was that up to a point, increasing the powder charge (relative to the shot weight) would increase muzzle velocity. Obviously, once the projectile left the muzzle, any unconsumed powder would fail to provide any further boost to its speed.

There was great controversy, however, as to whether a point could be reached where any further increase in charge would actually reduce the muzzle velocity. Robins was insistent that this could not possibly be the case. However, Benton (130) reported a progressive decrease in muzzle velocity for a 36-pounder firing charges ranging from 36–77 pounds, and Farrow (289) suggests that the charge yielding the maximum velocity is half to two-thirds projectile weight. Still, it's not clear from the underlying physics why this diminution should occur.

Even if there weren't diminishing returns vis-a-vis muzzle velocity, the amount of powder used would be constrained by the size of the powder chamber, fear of bursting the gun, and the recoil.

It has also been reported that the maximum velocity charge increases with the length of the gun. This makes sense as, for the same rate of acceleration, it gives more time for useful consumption of powder.(Simpson 177).

Multi-Chamber Guns

High-Low Pressure Gun. These have a divided propellant chamber, with two compartments separated by a plate with holes. The powder is ignited in the first compartment, generating a high pressure. Because of the constricted communication with the second, the pressure there is lower, resulting in a lower muzzle velocity but also a lower recoil. If you are wondering why not just use a conventional gun with a low powder charge, it's because the high pressure results in a better "burn" curve, and only the first chamber needs a thick wall. The concept was first implemented in the Panzerabwehrwerfer 600 (1945) and copied in the British Limbo depth charge launcher (1955) and later the American M79 grenade launcher.

Lyman-Haskell Multicharge Gun. The American government tested a 6-inch multicharge gun in 1883. This had five powder chambers, one at the breech, and the remaining four distributed along the length of the bore. The charge at the breech was smaller than the others, and the nearer the powder chamber to the muzzle, the faster burning the powder used. The theory was that the pressure created by their deflagration would also be distributed, allowing one to achieve a much higher muzzle velocity without overstressing the barrel. The breech charge was ignited in the usual way and the other charges by the passage of the combustion gases propelling the projectile.

The multicharge gun, with 119 pounds of powder, accelerated a 111-pound projectile to a muzzle velocity of 2004 fps, but with a barrel pressure of only 31,550 psi. In contrast, the Krupp 5.9 inch gun used a single charge to propel a 112.2 pound shell, achieving a muzzle velocity of 1676 fps with a pressure of 40,320 psi. So what's the catch, other than the profligate use of powder?

Well, the multicharge gun was 25 feet long (50 calibers). It therefore was quite heavy (25 tons), whereas the Krupp gun weighed only 3.8. The Ordinance Board was of the opinion that the multicharge gun's performance should be compared, not to guns of equal caliber, but to those of equal weight. While the 10-inch gun, weighing 18 tons, had a lower muzzle velocity (1400 psi), its 400 pound projectile carried greater kinetic energy and, in the Board's opinion, was likely to have more penetrating power. (Walker; Haskell).

Propellants (Gunpowder, etc.)

Gunpowder is a mixture of saltpeter, charcoal and sulfur. The saltpeter (potassium nitrate) is an oxidant, and it burns the charcoal (carbon), forming carbon dioxide gas. The sulfur combines with the potassium ion of the saltpeter, forming potassium sulfide, and in the process generates a lot of heat. Since the heated gas is confined by the gun, that results in an increase in pressure. And that's what pushes the projectile out. It also stresses the barrel, so you can't use too much powder and how much can be used depends on its burn rate.

Among the down-timers, there's no consensus as to the proper formula for gunpowder (black powder). Just one master gunner, Peter Whitehorne (1560), presented 20 different recipes, with saltpeter content of 16–84 %, charcoal of 8-64 %, and sulfur of 8-28 %. (Walton 123). EB11/Gunpowder says that the following formula was used in Britain in 1647: 66.6 % saltpeter, 16.6 % charcoal, 16.6 % sulfur. By 1781, the proportions were 75-15-10, the ones given in H. Beam Piper's Lord Kalvan of Otherwhen. Other formulae of possible interest included 52.2-26.1-21.7 (Germany 1596), 68.3-23.2–8.5 (Denmark 1608). 75.6-13.6-10.8 (France 1650), and 73-17-10 (Sweden 1697). Even in the nineteenth century, different countries had different preferences, with saltpeter 70–80 %, charcoal 11–18.5 %, and sulfur 9.5-13 %. (Beauchant 149). The proportions given in the modern EB (2002CD) is 75-14-11. The Medieval Gunpowder Research Group, using a replica of the Loshult Gun, found that muzzle velocity peaked at a saltpeter content of about 72 %. (MGRG2).

The burn rate is proportional to the burning surface. It thus is dependent in part on initial particle ("grain") size; the smaller the particles, the greater the total surface area for a given weight of powder, and the faster the "deflagration" reaction. However, the reaction shouldn't be too fast; you want it to continue until the projectile reaches the muzzle. Thus, the grain size must be matched to the barrel length; muskets used finer powder than did "great guns." The term "powder" became a bit misleading; the "grains" can be several inches in diameter-please look at Fig. 1 in EB11/Gunpowder.

The quality of gunpowder has improved over time. In 1587, gunners used "serpentine," which was floury. Because of the small particle size, it was necessary to leave part of the powder chamber empty, to provide oxygen. The powder also absorbed moisture readily. The charge for a culverin was equal to the shot weight, and for a cannon, half that weight.

By 1625, "corned" powder, which was granulated, was common. (Lavery 135). The size of the grains could be controlled by sieving. In 1673, a culverin used a two-thirds shot weight charge, and a cannon, one-half.

Improvements were also made in the preparation of the components of gunpowder. By 1740, the charges ranged from 40 % for a 42-pounder to 66 % for a 9-pounder. (Id.) In 1783, "cylinder powder" was introduced, although it didn't come into common use until 1803 (Rodger 421). It incorporated a better grade of charcoal. The wood was placed in cast iron cylinders, and heated over a stove, rather than charred in a kiln. (Id.; Douglas 201). This permitted reducing the standard charge to one-third the weight of the ball for ordinary guns, and a mere 8 % for carronades (Lavery). The method is described by EB11/Gunpowder but without discussion of its advantages over former practice.

You could use less if you were trying to conserve powder, or were hoping to produce more splinters if the shot didn't hole the target. A one-sixth charge is sufficient to "drive a ball from any large gun through the side of a ship at 1100 yards" but for a 24-pounder would require twice the elevation as a one-third charge, thus reducing accuracy. (Douglas 54).

In the mid-nineteenth century, the increase in gun size led to incompatibility with the ordinary black powder; it burned too quickly, creating conditions that strained the gun. A slower-burning "brown powder," described in EB11/Gunpowder, was introduced.

The shape of the grains is also significant. Normally, as deflagration continues, the particles are consumed inward, reducing the total burning surface and thus reducing the burn (regressive burn). This is experienced with all solid grains, whether they be spheres, cylinders or plates.

In 1860, what EB11 calls "shaped powders" were introduced. The grains had one or more perforations so they were consumed both inward and outward, resulting in a constant (neutral burn) or even increasing (progressive burn) burn rate. EB11 describes how they were made.

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In the late-nineteenth century, gunpowder was largely replaced by nitrocellulose-based propellants (the so-called "smokeless powders"). These produced less smoke and flash, burned progressively, and caused less erosion to the barrels. They are classified as being single-base (nitrocellulose) or double-base (nitrocellulose combined with nitroglycerin or some other liquid organic nitrate).

Ballistite (1887) was 40 % nitrocellulose and 60 % nitroglycerin (EB2002CD/explosive). Cordite was similar; 37 % nitrocellulose, 58 % nitroglycerin, 5 % Vaseline (Rinker 34) or later 65-30-5 (EB11/Cordite). EB11 doesn't say anything about stabilizers, but EB2002CD suggests diphenylamine.

A member of a Civil War reenactment group would probably be familiar with Pyrodex, which was developed in the 1970s. It's essentially black powder with various additives so it burns more cleanly-less fouling of the bore, less smoke. However, the formula of Pyrodex is proprietary, and the person who developed it (Powlak) lost his life in the process.

The decomposition products of black powder are 43 % gaseous and 57 % solid, the latter being responsible for the smoke of the proverbial "smoking gun." In contrast, modern smokeless powder is more than 99 % gaseous. Gases can be accelerated to higher velocities than solids, for a given internal pressure. Consequently, black powder has a low "specific impulse" (pounds thrust produced per pound propellant burned per second)-~50–70 seconds-whereas double base powders provide ~180–210 seconds. (Guilmartin 300).

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Average muzzle velocities increased over the nineteenth century, from 1575 fps for ordinary black powder, to 2133 fps for prismatic powder and 2225 fps for early (1885) smokeless powder. (Breyer 38).

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With black powder, the principal manufacturing considerations were "strength", freedom from fouling, and proneness to deterioration. These were affected by composition, density, moisture content, and grain size, shape, hardness and "glazing."

Until 1868, powder density was measured by "cubing"-weighing it in a box of standardized volume. This was improved upon by the mercury densimeter. (Farrow 313).

There was no quantitative test for hardness; the grain would be broken between finger and thumb. (Smith).

Powder strength will vary from manufacturer to manufacturer, from lot to lot, and even from barrel to barrel. (Dahlgren 180). Even at the end of the black powder era, powder manufacture was an art, not a science. In 1881, 150,000 pounds of Westphalian Company prismatic powder was rejected because it didn't meet the standards; the representative blamed it on manufacturing "during very cold weather." (Buchanan 325).

Powder strength was originally tested by setting a small amount afire in the open air, and observing the results. Eprouvettes ("provers"), which ignited the powder in a confined space simulating a cannon barrel, provided more useful data (von Malitz 163ff). An early eprouvette was described by William Bourne (1578). This was essentially a box with a hinged and ratcheted lid and a small fuse hole. A set quantity of powder was placed inside, and set off. The force of the combustion gases would drive the lid upward, and the lid would be kept from dropping back into place by the ratchet. The angle reached by the lid was a measure of the strength of the powder.

In Bourne's eprouvette, the propulsive force was resisted by the weight of the lid, but some later devices used a spring mechanism, and Du Me's eprouvette (1702) employed water resistance. Also, some were engineered so the propelled object moved linearly rather than angularly. And, instead of measuring that movement, one could measure the eprouvette's recoil.

Most of the eprouvettes just worked on an indicator object like Bourne's lid, but the mortar eprouvette actually fired a projectile at a fixed angle, usually 45°, so the power was inferred from the range achieved.

In storage, gunpowder could become damp, and once its moisture content rises above 1 %, it begins losing explosive power. (Kelly 59). Keeping it away from seawater isn't enough because it can actually absorb water vapor. (Douglas 199). It follows that what's needed is airtight storage or, if that isn't possible, storage together with some desiccant.

Powder was examined for dampness and if damp, it was dried. This was a ticklish operation as the drying could melt the sulfur or even explode the grains. (200). If the powder were past redemption, one could at least attempt to recover the saltpeter, which was a rare and valuable commodity in Europe (201).

Alternative Propellants

Steam. Jacob Perkins received a British patent in 1824 for a steam gun. This was no "paper patent"; in an 1825 demonstration an 800 (or 900) psi boiler projected one ounce musket balls out a barrel, achieving penetration of quarter-inch iron plate and eleven inches of pine at a range of 35 yards. Moreover, he developed a rapid-fire gravity feed enhancement. (Smith). The rapid fire version was later a major attraction at the National Gallery of Popular Science (1832). Some outrageous claims were made for how fast it was, but I am inclined to believe the ten balls/minute that Perkins' son asserted in 1861. (Bruce 138).

In 1828, Perkins designed for the French a 1500 psi steam gun, with a barrel six feet long and three inches caliber, firing four pound balls. It worked, but its range was only half that of a conventional cannon of the same caliber. Not only was the barrel pressure much lower than in a "powder" gun, it may have suffered much more acutely from bore-windage because of that difference. Another problem was weight; the 1825 model had a five-ton boiler. (BPHS).

EB11/Explosives alludes to the Winans (Dickinson) steam gun, built for the Confederacy. It was never put to use; Mythbusters Episode 93 suggests that it would have gotten off five rounds a second and had a maximum range of 700 yards, but expressed doubt that the impact velocity beyond point blank range was high enough to be lethal.

The concept of using steam to throw a projectile wasn't new; Leonardo da Vinci had speculated that Archimedes had used a steam cannon at the siege of Syracuse, and drew one. In 2006, an MIT team figured out how to implement Leonardo's concept. They were deliberately coy about the particulars of steam generation, but they built a steam cannon designed for 3,500-4,000 psi, and fired a one pound projectile (i.e., equivalent to that of a robinet) with a muzzle velocity over 300 m/s. The bore was 2 feet long and 1.5 inches diameter. They were able to fire one round every two minutes. (MIT).

While I am sure the projectiles made a satisfying whizz, the fact remains that "steamer" muzzle velocity is low compared to that achieved with powder guns. A bit of a back-of-the-envelope calculation puts this into perspective. Let's assume that we have a large enough reservoir of steam so that we can maintain constant pressure. Let's also assume that the barrel is horizontal (so we can ignore gravity), frictionless, and without windage. If so, the projectile accelerates at a constant rate and the muzzle velocity will therefore be

sqrt (2*L*P*A/m),

where L is bore length, P pressure, A cross-sectional area of the projectile and m mass. For a standard projectile (one pound, one inch diameter) this reduces to

24.56 sqrt(LP) (L inches, P psi).

Perkins' 1828 gun thus has a theoretical ideal muzzle velocity of 1345 fps, and the MIT gun, 951. But note that the actual muzzle velocity for the MIT gun was a bit less than a third of the theoretical value.

Compressed Air. The blowgun is the earliest compressed air weapon, limited in propulsive force by the ratio of the volume of air one can huff (about 60 cubic inches) to the bore volume of the blowgun (14 cubic inches for a six footer with a half inch caliber). (Gurstelle 142). A "pneumatic rifle was built at the beginning of the seventeenth century," and some Austrian jaegers carried the model 1780 rifle (300 m/s muzzle velocity), which was a great weapon for covert operations against French occupation forces. (Rossi 232).

The USS Vesuvius (1888) carried three 15-inch pneumatic "dynamite" guns. Compressed air from a 1000 psi reservoir was fed into the barrels, which were only 55 feet long (!), partially below deck, and mounted at a fixed elevation of 16 degrees. Range was changed by adjusting the pressure. The guns couldn't be traversed; you aimed the ship to aim the gun. The guns fired finned projectiles filled with up to 600 pounds of dynamite; this high explosive was too sensitive to be used in an ordinary gun and indeed even the muzzle velocity of the pneumatic gun had to be limited. The maximum range was 5,000 yards, with a subcaliber (6") shell. (NAVWEAPS; NAVSOURCE; Hamilton; Clark).

Because of the low pressure, a 20-inch gun could have a steel or aluminum bronze barrel that was one half an inch thick. In trials, the gun had good accuracy, and could fire about one round a minute. (Zalinski). The Zalinski gun is discussed approvingly in EB11/Pneumatic Gun.

The secret to understanding the dynamite gun is to think of it, not as a gun, but as a torpedo launch system. Ship armor had reached the point at which ordinary shells weren't reliably penetrating it. The projectiles fired by the dynamite gun were conceptualized as "aerial torpedoes," traveling faster and farther than any underwater torpedo and exploding underwater against the unarmored bilge of the enemy craft (Parkerson 83).

At a U.S. Naval Institute proceeding, the commentators conceded that the gun would be useful for countermining, that is, using explosives to set off enemy mines-stationary targets. They were less sanguine that it could be used effectively against a rapidly closing foe, as the elevation limited the zone of danger for the target and ranges are difficult to estimate. If the pressure were reduced because the enemy was close, the projectile would have a lower velocity and be more vulnerable to deflection by the wind. (Zalinski). The naysayers doubted that the countermining advantage was sufficient to justify building a ship with the dynamite gun as the main armament

In practice, USS Vesuvius proved reasonably useful for shore bombardment-the quietness of the pneumatic action meant that the enemy didn't hear the guns fire-but the system was quite obviously impractical for use against another warship. USS Vesuvius would have been outranged by conventional guns, and the inability to traverse the gun other than by turning the ship meant that a fast attacker could evade its fire. (McSherry).

It doesn't appear that scaling down the gun to a size suitable for turret mount would have been productive. The US Army tried out the Sims-Dudley dynamite gun, which fired a 2.5 inch caliber shell carrying five pounds of nitro-gelatin. Since the army couldn't carry compressors around, the gun used black powder to compress air, and then the compressed air to project the shell. The length of the gun-and-carriage was 14 feet and the muzzle velocity was just 600 fps, yielding a range of just 900 yards. (McSherry).

Compressed air projection reappeared in an anti-aircraft gun format in World War II. The Mark I Holman projector had a 4.5 foot smoothbore barrel and used compressed air bottles to fire fragmentation grenades up to 30 rounds/minute to perhaps 600 feet. Its advantages were that the low barrel pressures meant that it didn't need high-strength steel, its recoil was small, and of course it didn't need any cordite. Its disadvantage was that it was quite inaccurate. (Wikipedia/Holman Projector).

Liquid Propellants. These became popular in rocketry, but for artillery, despite a half-century of effort, their time hasn't yet come. (McCoy).

This article continues in Part 3, "Hitting the Target."