"Cordite" by N J M Campbell -
WARSHIP Annual, Volume 2 (pp 138-140)
CORDITE
This term has been widely used in Britain for any type of solid propellant excluding gunpowder, but where accompanied by additional classification as in 'Cordite MD' or 'Cordite SC' it has a precise meaning and it is now customary to limit the use of the word in this way and to replace it otherwise by 'propellant'. Manufacture of Cordite Mark 1 began in 1889, three years after Vielle in France and Duttenhoffer in Germany had produced the first usable propellant from gelatinised nitrocellulose. For 40 years previously attempts had been made to employ nitrocellulose as a propellant, but it was not until the nitrocellulose had been dissolved in a solvent, later usually acetone, which was then removed, that success was achieved. In 1888 Nobel introduced 'Ballistite' with 40-50 percent Nitroglycerine and the balance Nitrocellulose. This was unusual for the period as the Nitroglycerine served as the solvent for the Nitrocellulose, which was of low Nitrogen content. It had greatly increased unit energy per unit weight, and was less liable to moisture absorption than the straight Nitrocellulose propellant. France, Russia and USA favoured the latter, but Britain, Germany and most other countries preferred a propellant with Nitroglycerine, though acetone was used as the solvent for the high nitrogen nitrocellulose.
Cordite Mark I had the composition 58 per cent Nitroglycerine, 37 per cent Nitrocellulose (13.1 Per cent Nitrogen) 5 per cent Petroleum Jelly (Vaseline). The latter, originally present as a lubricant, served as a moderately efficient stabilizer, as the unsaturated hydrocarbons which it contained would react with the decomposition contents, to which all except perhaps the purest and most carefully made nitrocellulose was liable. Cordite Mark I produced exceedingly hot and erosive gases, and it was soon clear that the search for high energy per unit weight had gone too far. It was mad in thin cylindrical or cord form, the different sizes being known by a number indicating the diameter of the extrusion die in .01 of an inch - thus Cordite 50 for 12-inch Mark VIII and IX guns, Cordite 30 for 6-inch QF, Cordite 5 for the Hotchkiss 3pdr. It is now clear that a mistake was made in choosing solid cylindrical grain form, and it would have been better to adopt a tubular form with roughly constant surface area while burning, as was done in Germany.
For all this, Cordite Mark I was a great advance over the brown powder which it replaced. This material known as 'Prismatic Brown' (Pr Br) or with different processing 'Slow Burning Cocoa' (SBC) , was developed by Rottweil of Dunaberg and instead of the usual 75 per cent Potassium Nitrate, 15 per cent Alder or Willow charcoal, 10 per cent Sulphur in black powder, it contained 79 per cent Potassium Nitrate, 18 per cent Rye straw charred by superheated steam and only 3 per cent Sulphur. It was slower burning and less damaging to guns than black powder, but its rate of burning was extremely sensitive to moisture content. The charges were also enormous, 630lbs for the standard 13.5inch Marks I to IV, and 960lbs for the 15.25inch in eight 120lb bags - the largest charge ever used in any ship. - while the Cordite Mark I charge for the 13.5inch was only 187.5lbs. For 6inch guns, EXE in composition essentially 2/3 brown and 1/3 black powder had been in favour, but the heavy smoke produced by gunpowder charges was impossible for the larger QF guns, and as Cordite Mark 1 was relatively 'Smokeless', a term often applied to such propellants at one time, the first issues to the Royal Navy were for 6inch, 4.7inch, 6pdr and 3pdr QF guns, while the first size 50 for 12inch guns was supplied in 1895.
The excessive heat of the gases from Cordite Mark I were reduced in Cordite MD, the first few batches of which were delivered in 1901. This contained 30 percent Nitroglycerine, 65 per cent Nitrocellulose (average 13.1 per cent N) 5 per cent Petroleum Jelly and the calories per gram on burning (water as liquid) were reduced from 1270 to 1020, a very high figure by present standards but indicating much less erosive gas than with Cordite Mark I. The grain form was unchanged, , though tube was used in the 4inch QF Mark III, and usual sizes were MD45 for heavy guns, and 37, 26, 19, 11, 8, 4.25 for 9.2inch to 3pdr. Cordite MD was a stiff and hard colloid and though it was hoped to use MD55 in the 12in Mark XI, it was found impossible to remove all the volatile solvent in production. The shrinkage in drying was also great, as MD45 could finish at 0.34inch diameter. If properly and carefully made Cordite MD was a reasonably satisfactory propellant, and with its variant MC (qv below) remained standard in the Navy until 1927. Charges had to be increased in weight as compared to Mark I, figures for the12inch Mark IX being 246 and 211 lbs, but the reduced erosion of the gun was far more important.
Very serious trouble due to instability of the nitrocellulose in Cordite MD occurred in the 1914-18 War. It had been known since before nitrocellulose was used ass a propellant, that unless it was correctly made from pure raw material, devastating explosions could occur spontaneously, and this was emphasized by a long series of incident in the ships of several navies, of which the most notorious prior to 1914 were those in the French battleships Iena and Liberte. Testing, safe life and storage procedures had of course been laid down, but in November 1914 the Bulwark and in December 1915 the Natal ere destroyed by internal explosions. The former accident may perhaps have been due to gross carelessness with fuzed Lyddite shell, but there is little doubt that spontaneous explosion of the nitrocellulose in Cordite MD was the cause of the latter. Necessary precautions tend to be reduced or over looked in the interests of wartime production, and by the beginning of 1917 there were indications that an increase in the safety margin against spontaneous ignition in Cordite MD was highly desirable. Thus from April 1917 clean carded cotton sliver was to be used as the source material for the nitrocellulose instead of cotton waste, the nitrating time was to be 2.5 hours and not shorter, to lessen the chances of unstable compounds being formed and resident inspectors were to watch all stages of manufacture. It should be explained that low Nitrogen Nitrocelluloses are made by adjusting the initial composition of nitrating acid, and that the use of wood cellulose as a raw material was of much later date. Other improvements were the addition of a little chalk as a stabilizer, and the use of cracked petroleum jelly to increase the quantity of unsaturated hydrocarbons (hence Cordite MC) both of which had been shown to be desirable in pre-war experiments.
6000 tons of propellants were to be withdrawn and replaced by materials made by the new method, but this was too late to prevent HMS Vanguard blowing up on 9 July 1917 in the worst of all such disaster to British ships. After this catastrophe the supply of new propellant was hastened and by March 1918 all the Grand Fleet, and six months later all ships, had their old outfits replaced. The magazine explosion and fire which led to the scuttle of the Glatton in September 1918 was primarily due to the presence of hot boiler ash and clinker against the magazine bulkhead!
Later work in the 1920sshowed that iron pyrites, a common constituent of some coals and of such things as cinder paths, could have a devastating effect on the stability of nitrocellulose if small particles were present as an impurity, a by no means impossible occurrence. The Sulphur in the pyrites would slowly oxidize to sulphuric acid which corroded the Nitro-cellulose with considerable localized evolution of heat sufficient to cause spontaneous ignition, as the poor thermal conductivity of the propellant assisted the development of high local temperatures.
<<< End of Part One 1 >>>
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WARSHIP Annual, Volume 2 (pp 138-140)
CORDITE
This term has been widely used in Britain for any type of solid propellant excluding gunpowder, but where accompanied by additional classification as in 'Cordite MD' or 'Cordite SC' it has a precise meaning and it is now customary to limit the use of the word in this way and to replace it otherwise by 'propellant'. Manufacture of Cordite Mark 1 began in 1889, three years after Vielle in France and Duttenhoffer in Germany had produced the first usable propellant from gelatinised nitrocellulose. For 40 years previously attempts had been made to employ nitrocellulose as a propellant, but it was not until the nitrocellulose had been dissolved in a solvent, later usually acetone, which was then removed, that success was achieved. In 1888 Nobel introduced 'Ballistite' with 40-50 percent Nitroglycerine and the balance Nitrocellulose. This was unusual for the period as the Nitroglycerine served as the solvent for the Nitrocellulose, which was of low Nitrogen content. It had greatly increased unit energy per unit weight, and was less liable to moisture absorption than the straight Nitrocellulose propellant. France, Russia and USA favoured the latter, but Britain, Germany and most other countries preferred a propellant with Nitroglycerine, though acetone was used as the solvent for the high nitrogen nitrocellulose.
Cordite Mark I had the composition 58 per cent Nitroglycerine, 37 per cent Nitrocellulose (13.1 Per cent Nitrogen) 5 per cent Petroleum Jelly (Vaseline). The latter, originally present as a lubricant, served as a moderately efficient stabilizer, as the unsaturated hydrocarbons which it contained would react with the decomposition contents, to which all except perhaps the purest and most carefully made nitrocellulose was liable. Cordite Mark I produced exceedingly hot and erosive gases, and it was soon clear that the search for high energy per unit weight had gone too far. It was mad in thin cylindrical or cord form, the different sizes being known by a number indicating the diameter of the extrusion die in .01 of an inch - thus Cordite 50 for 12-inch Mark VIII and IX guns, Cordite 30 for 6-inch QF, Cordite 5 for the Hotchkiss 3pdr. It is now clear that a mistake was made in choosing solid cylindrical grain form, and it would have been better to adopt a tubular form with roughly constant surface area while burning, as was done in Germany.
For all this, Cordite Mark I was a great advance over the brown powder which it replaced. This material known as 'Prismatic Brown' (Pr Br) or with different processing 'Slow Burning Cocoa' (SBC) , was developed by Rottweil of Dunaberg and instead of the usual 75 per cent Potassium Nitrate, 15 per cent Alder or Willow charcoal, 10 per cent Sulphur in black powder, it contained 79 per cent Potassium Nitrate, 18 per cent Rye straw charred by superheated steam and only 3 per cent Sulphur. It was slower burning and less damaging to guns than black powder, but its rate of burning was extremely sensitive to moisture content. The charges were also enormous, 630lbs for the standard 13.5inch Marks I to IV, and 960lbs for the 15.25inch in eight 120lb bags - the largest charge ever used in any ship. - while the Cordite Mark I charge for the 13.5inch was only 187.5lbs. For 6inch guns, EXE in composition essentially 2/3 brown and 1/3 black powder had been in favour, but the heavy smoke produced by gunpowder charges was impossible for the larger QF guns, and as Cordite Mark 1 was relatively 'Smokeless', a term often applied to such propellants at one time, the first issues to the Royal Navy were for 6inch, 4.7inch, 6pdr and 3pdr QF guns, while the first size 50 for 12inch guns was supplied in 1895.
The excessive heat of the gases from Cordite Mark I were reduced in Cordite MD, the first few batches of which were delivered in 1901. This contained 30 percent Nitroglycerine, 65 per cent Nitrocellulose (average 13.1 per cent N) 5 per cent Petroleum Jelly and the calories per gram on burning (water as liquid) were reduced from 1270 to 1020, a very high figure by present standards but indicating much less erosive gas than with Cordite Mark I. The grain form was unchanged, , though tube was used in the 4inch QF Mark III, and usual sizes were MD45 for heavy guns, and 37, 26, 19, 11, 8, 4.25 for 9.2inch to 3pdr. Cordite MD was a stiff and hard colloid and though it was hoped to use MD55 in the 12in Mark XI, it was found impossible to remove all the volatile solvent in production. The shrinkage in drying was also great, as MD45 could finish at 0.34inch diameter. If properly and carefully made Cordite MD was a reasonably satisfactory propellant, and with its variant MC (qv below) remained standard in the Navy until 1927. Charges had to be increased in weight as compared to Mark I, figures for the12inch Mark IX being 246 and 211 lbs, but the reduced erosion of the gun was far more important.
Very serious trouble due to instability of the nitrocellulose in Cordite MD occurred in the 1914-18 War. It had been known since before nitrocellulose was used ass a propellant, that unless it was correctly made from pure raw material, devastating explosions could occur spontaneously, and this was emphasized by a long series of incident in the ships of several navies, of which the most notorious prior to 1914 were those in the French battleships Iena and Liberte. Testing, safe life and storage procedures had of course been laid down, but in November 1914 the Bulwark and in December 1915 the Natal ere destroyed by internal explosions. The former accident may perhaps have been due to gross carelessness with fuzed Lyddite shell, but there is little doubt that spontaneous explosion of the nitrocellulose in Cordite MD was the cause of the latter. Necessary precautions tend to be reduced or over looked in the interests of wartime production, and by the beginning of 1917 there were indications that an increase in the safety margin against spontaneous ignition in Cordite MD was highly desirable. Thus from April 1917 clean carded cotton sliver was to be used as the source material for the nitrocellulose instead of cotton waste, the nitrating time was to be 2.5 hours and not shorter, to lessen the chances of unstable compounds being formed and resident inspectors were to watch all stages of manufacture. It should be explained that low Nitrogen Nitrocelluloses are made by adjusting the initial composition of nitrating acid, and that the use of wood cellulose as a raw material was of much later date. Other improvements were the addition of a little chalk as a stabilizer, and the use of cracked petroleum jelly to increase the quantity of unsaturated hydrocarbons (hence Cordite MC) both of which had been shown to be desirable in pre-war experiments.
6000 tons of propellants were to be withdrawn and replaced by materials made by the new method, but this was too late to prevent HMS Vanguard blowing up on 9 July 1917 in the worst of all such disaster to British ships. After this catastrophe the supply of new propellant was hastened and by March 1918 all the Grand Fleet, and six months later all ships, had their old outfits replaced. The magazine explosion and fire which led to the scuttle of the Glatton in September 1918 was primarily due to the presence of hot boiler ash and clinker against the magazine bulkhead!
Later work in the 1920sshowed that iron pyrites, a common constituent of some coals and of such things as cinder paths, could have a devastating effect on the stability of nitrocellulose if small particles were present as an impurity, a by no means impossible occurrence. The Sulphur in the pyrites would slowly oxidize to sulphuric acid which corroded the Nitro-cellulose with considerable localized evolution of heat sufficient to cause spontaneous ignition, as the poor thermal conductivity of the propellant assisted the development of high local temperatures.
<<< End of Part One 1 >>>
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