A selection of U.S. Self-propelled Guns
The self-propelled guns created in the United States at the end of WW1 and the early 20s have been poorly documented and understood. The objective of this article1 is to provide the context of the creation of these vehicles and provide a brief overview of the progress made in their development and the termination of the development of SPGs in 1923.
The U.S. Army at the outbreak of WW1 was a very small and limited force. Whereas the U.S. Navy had heeded the writings of Admiral Mahan about global force projection through the creation of a powerful blue water navy, the U.S. Army was sufficient to maintain superiority over other armed forces within the American continents. To effectively participate in WW1 the U.S. Army had to undergo a huge expansion in size and major upgrades in equipment.
The artillery arm of the U.S. Army in 1914 was composed of two branches: the field artillery tasked with support of the infantry and cavalry and the coast artillery tasked with manning of heavy, and often obsolete guns, in fixed emplacements to protect the U.S. coastline.
The heaviest guns deployed by the field artillery were the 4.7 inch (120mm) M1906 gun, 48 of which were on strength in 1917 and the 6 inch M1908 howitzer, 100 of these had been built. Both of these guns were quite short ranged compared to European designs. The bulk of the U.S. artillery park was composed of 3inch (75mm) horse towed light field guns.
The U.S. Army was in a unique position in the early years of WW1 where the equipment and doctrines of both sides of the conflict could be observed. It was rapidly apparent that the U.S. Army was deficient in mobile heavy artillery and railway mounted artillery. The time scales available to rectify this deficiency meant that new artillery pieces could not be designed, built and tested by local industry. Fortunately, Britain and France were quite willing to allow U.S. industry to build their heavy gun designs under licence since their local industries were very stretched because of the huge losses of manpower and materiel on the Western Front.
The artillery pieces the U.S. Army selected for mobile heavy artillery were:
The mechanisation of the artillery arm had been under investigation since 1915. Numerous trials were conducted with trucks and tracked vehicles to assess their utility as artillery towing vehicles2. The conclusion of these trials were that four-wheel trucks were superior to conventional trucks and that tracked vehicles were superior to both trucks and horses for artillery towing over bad ground. To mobilise the newly adopted heavy field guns the U.S. Army adopted, in part, the French doctrine of using heavy trucks to move artillery pieces by road for "strategic mobility". That is, moving the artillery by road between various parts of the front line away from the combat zone. For "tactical mobility", that is, within the combat zone, Holt tracked artillery tractors were purchased in 2½, 5, 10, 15 and 20-ton capacities. The introduction of mechanised towing did not meet universal acceptance since the traditional towing of guns with horses was entrenched. The next logical step that the French and British3 took of mounting a gun on a tracked carrier capable of firing from the vehicle without the setup delays of towed guns was also accepted as a reasonable solution to the mobility problem by the Ordnance Department, the U.S. Army Department charged with the specification, development and testing of new weapon systems.
The Ordnance Dept appear to have started their investigations of vehicle mounted guns with the 3 inch anti-aircaft gun M1917. There was experience that anti-aircraft guns had to have much higher mobility than field artillery. Both the British and French had mounted adapted field guns on trucks to provide this mobility. The 3 inch AA gun was somewhat heavier than light field artillery pieces and also bulkier so truck mounting may not have been feasible. Although a wheeled trailer mount was developed, similar to the German 88mm AA guns, the limitations of a wheeled towed vehicle appear to have prompted the Ordnance Dept to consider self-propelled options. It is thought that the Ordnance Dept requested design concepts from interested manufacturers. The Holt Tractor Company approached the Ordnance Dept4 with a proposal for a tracked anti-aircraft (AA) vehicle armed with a 3 inch AA gun. This was tested by the Ordnance Dept in 1917 and although it had many deficiencies the concept was judged worthy of further exploration. The original Holt vehicle was refitted with an 8 inch howitzer and tests showed that it was possible to fire the heavier gun from the tracked vehicle. J. Walter Christie's design for the 3inch AA gun was for a narrow 4-wheeled vehicle with large outriggers which could be deployed to stabilise the gun for firing.
In 1918 Pliny E. Holt, the chief designer of the Holt AA vehicle joined the design staff of the Rock Island Arsenal, Illinois5. Design and construction under Holt's supervision commenced to build tracked vehicles carrying the 8 inch howitzer, 155mm GPF and 240mm Schneider howitzer known as Marks I to III respectively. These vehicles were based on components of the Holt artillery tractors and proved to be very heavy and also slow. It should be noted that the American concept was of a gun carried by a minimal tracked vehicle rather than a self-contained weapon system carrying gun crew, ammunition as well as the gun which was the British and French approach to SPGs.
The Rock Island Arsenal produced an SPG based on the French St Chamond SPG6, the Mark IV, with a petrol-electric powered tractor with a tethered electrically powered gun vehicle.
The Ordnance Dept testing showed that although the Marks I to III SPGs had some promise future designs would have to be much lighter and smaller. The War Munitions Board saw the logic of SPGs to improve the battlefield mobility of the artillery and to reduce the costs associated with horse transport. Production contracts for 50 Mark I, 50 Mark II and 250 Mark III and Mark IV/IVA self-propelled guns were let in 1918 with delivery expected in early 1919. The Armistice in Nov 1918 lead to the reduction of the production contracts to a few experimental vehicles.
The Ordnance Dept in 1918 constructed a pair of light SPGs based on the Holt 2½ and 5-ton artillery tractors carrying the 75mm M1916 gun. Although these SPGs were improvised they demonstrated the viability of SPGs carrying light field guns.
At the end of WW1 the U.S. Army convened two boards to examine the experience gained and make recommendations for the future development of the artillery arm. One of the boards, presided over by Maj. Gen. Andrew Hero7 was convened in France taking submissions from serving troops. The report concluded that age of horse transport had ended and mechanised transport for artillery pieces was the only way forward. The board recommended:
"53. The consensus of opinion is that every piece of artillery that can be successfully adapted to motor traction would be motorised. The Board believes that very great expenditures for the purpose of developing tractor-drawn or tractor-carried artillery would be fully justified (p.24)"
The other board was presided over by Maj. Gen. William Westervelt considered artillery calibers, types of ammunition and transport best suited for the U.S. Army. This report8 noted:
"… for we have the broad field presented by the possibilities of the gun mounted directly upon a self-propelled vehicle. Already, the self-propelled caterpillar gun mount is well along in the experimental stage and has passed to fact from fancy, and while the weights are excessive, the gun traverse limited, sight relaying necessary, the results arrived at indicate final success in the near future. This success will be realised in a gun using, possibly, a pedestal mount, possessing perfect stability, all round fire, 90 degrees elevation, mounted upon a caterpillar track tractor. (p.49)"
and recommended:
"These two mounts, the 75 and 155mm guns, should be immediately developed to the utmost, paying particular attention to mobility and lightness consistent with strength and stability. (p.52)"
The effect of these reports on the traditionalists in the artillery arm wedded to horse transport can be easily imagined.
The development of SPGs by the Ordnance Dept continued after the end of the war with an emphasis on much lighter, smaller vehicles. Holt designed a 75mm armed SPG, the Mark VII, using components of the 2½-ton artillery tractor and although modestly successful was considered to be overweight. Holt then produced a 75mm gun/105mm howitzer armed vehicle, the Mark VI, designed from the ground up rather than adapting components from its commercial vehicles. Christie also submitted a similarly armed SPG which could run on wheels or tracks as well as a wheel/track vehicle mounting the 4.7 inch anti-aircraft gun.
The Christie 155mm armed SPG was recommended by the Ordnance Dept for production after extensive testing of the original vehicle and two further modified vehicles.
Holt produced a pair of lighter and smaller SPGs carrying the 8 inch howitzer (Mark IX) and either the 155mm howitzer or 4.7inch M1920 gun (Mark X)9.
At the start of 1923 the Field Artillery Board conducted a "trial" comparing the Holt Mark VI and Christie 75mm SPGs. Both of these vehicles were badly worn by this time but this did not deter the board from concluding that the SPGs were very unreliable and unable to meet the requirements of the artillery. The recommendations of this board were10:
"1st. That further experimentation on 75mm gun and 105m howitzer self-propelled mounts be absolutely
discontinued so far as any use for divisional artillery is concerned.
2nd. That when, and not until, experiments with heavier calibre self-propelled mounts may indicate the
possibility of a very material weight reduction, further work be undertaken on a self-propelled mount for 105mm
howitzer and 75mm gun with a view to developing, if possible, a special vehicle that may have some value as a
mount for a special gun, as an anti-aircraft or anti-tank one.
3rd. That the conditional recommendations for work on a special gun mounting be considered as based largely upon
an unwillingness on the part of the Board to take a definite stand against experiment, within reason with smaller
calibres; since it is the belief of the Board that development along this line, if such development be possible,
may be more difficult if absolutely confined to the larger calibres"
The language of the report is in similar form, that is, difficult to understand, but at least the headline is clear "No SPGs for the US Army on our watch". This combined with financial restrictions on the U.S. Army meant the end of SPG development, aside from a few purely experimental vehicles, until WW2.
Don Boose, David Keough and Jeffrey Nester for their suggestions and help with the creation of this article