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High Mobility Agility (HIMAG) Vehicle

High Mobility Agility (HIMAG) was amedium weight class variable parameter test bed mounting a 75MM medium caliber anti-armor automatic cannon. The HIMAG was developed to provide the Armor & Engineer Board with a system on which they could vary parameters in order to obtain optimum system performance. Component parameters that coul be varied included suspension spring rates, number of roadwheels, fire control performance levels, gun controls sight displays, etc. Data obtained from HIMAG testing was used in developing future system requiremerts.

Western armies could field high mobility-agility (HIMAG) armor vehicles which would so speed up the tempo of mechanized forces that cavalry and renerve forces would no longer have a mobility advantage. The ability to airlift these light mechanized forces could treble the mobility of ground maneuver forces. In the 1970’s the Defense Advance Research Projects Agency (DARPA ) developed a program to demonstrate new technologies for future tanks. One of the major component technology areas in the program is a vehicle characterized by high mobility and agility and hence the program named HIMAG.

The High Mobility/Agility, or HIMAG, vehicle was built as part of the Armored Combat Vehicle Technology Program to resolve a number of basic questions about the degree of mobility and agility attainable and effective in the present state of combat vehicle technology. It was also intended to be used for assessing the effectiveness of a gun with an automatic loading system capable of high rates of fire, and for evaluating several different gun fire control system configurations. In principle, the HIMAG test bed provided a basis for decisions about several important aspects of the design of future combat vehicles.

The hull and suspension were developed first. The vehicle was powered by a Continental AVCR-1360 diesel that could be tuned to up to 1500 gross horsepower coupled with the Allison X-1100-1H transmission and hydro pneumatic suspension that could tilt the hull forward, backward or to the sides. The weapon system followed. The main weapon of choice was the brand new 75mm MC-AAAC hypervelocity smoothbore cannon firing extremely potent APFSDS rounds.

A critical element in the HIMAG program was the demonstration of the feasibility of a medium caliber automatic gun. The weapon pod proposed for evaluation was the ARES 75 mm , solid propellant, rapid fire cannon using APFSDS and HE inert rounds. This 75mm gun was designed to fire in various modes and incorporates the capability for both rapid or single shot fire.

The primary objective for both HIMAG-A and HIMAG-B was to provide mobile test beds for the DARPA 75mm solid and liquid propellant guns and to explore to the fullest extent, the novel vehicular options that such a weapon might now make possible. It is essential that such weapons be tested on a light-weight chassis to determine feasibility and to quantify the effectiveness. If the HIMAG-B is dropped, the option for tank-equivalent firepower on such small combat vehicles will probably not materialize.

The elimination of one of the two brassboard guns called for in HIMAG programs would result from proposed cuts. However, it was extremely doubtful that either program could be completed with only one gun. Each program requires the firing of up to 40-short bursts to determine barrel erosion and to characterize sustained rates of fire and tube life. The HIMAG schedule requires some parallel test firing with both guns.

The Combat Vehicle Technology Program's High Mobility Agility (HIMAG) Vehicle Chassis Tests, conducted in 1978 end 1979, provided an opportunity to explore the prediction of human performance requirements and the implications of high mobility tracked vehicle design for driver performance. Results supported the general hypothesis that cross-country driving on the higher horsepower per ton vehicles was significantly different from the same task on the M60Al or Ml13.

The Advanced Combat Vehicle Technology [ACVT] program was a joint Army and Marine Corps project to develop technology for designing and building armored vehicles in the post-1985 time frame. The program examined potentials for improving weapon systems, armor, and mobility/agility performance particularly as advances in these areas could be combined to produce combat vehicles of greater battlefield lethality and survivalility.

In a seating position test, the program examined a supine or reclining position versus a prone posture for armored vehicle crewmen in an attempt to find a good way to lower vehicle silouette. The prone position was medically and physically unsatisfactory, but the supine or reclining was determined to be effective.

The HIMAG chassis test was conducted principally to gather engineering data on high performance tracked vehicles. The HIMAG chassis was the program's first experience with a highly instrumented test bed, and there were some severe dependability problems, in both the automotive and instrumentation areas, that plagued the test schedule and events. More than three-fourths of the available test days were involved in some form of repair activities. However, all required tests were completed successfully, except soft soil testing, for which the test program lost the weather window at Fort Knox. That hole was later closed with some soft soil tests with surrogates at Waterways Experiment Station (WES).

The principal special test chassis was the HIMAG vehicle whose gross weight, center of gravity, suspension spring and damping rates, and wheel travels could be widely varied. The HIMAG was used for ride and shock tests, dash tests, and traverse tests with various drivers. The second Ljjspecial test chassis was the twin-engine M113 developed for research purposes by the U. S. Army Engineer Waterways Experiment Sta tion (WES) and referred to as the M113 HOTROD. The M113 HOTROD (86 gross horsepower per ton (hp/ton) and the ATR (36 gross hp/ton) were used in special tests to examine the effects of high speeds on the motion resistance offered by soils.

The test work was designed to develop quantitative data relating specific measures of vehicle performance to the engineering characteristics of a vehicle configuration and of the terrain and to driver behavior. Emphasis was placed upon obtaining a wide range of variations in vehicle and terrain so that trends could be seen clearly and analytical models could be checked as broadly as possible.

More than 1900 mobility/agility tests were conducted with 21 high performance and 2 contemporary vehicles. Eighteen distinct configurations of the HIMAG variable high mobility/agility test bed were tested to explore mass and suspension effects on performance. Tests were also conducted with the ATR with and without the 13.5-ton turret, the M113 HOTROD and two contemporary vehicles, the M6OAl MBT, and the M1l3AI APC. The test vehicles provided a range in gross vehicle weight from 9 to 52 tons, in gross hp/ton ratios from a low of 14 for the M6OAI MBT to a high of 86 for the M113 HOTROD, and in sprocket hp/ton ratios from a low of 8.4 for the M6OAl MBT to 28.9 for the M113 HOTROD.

Five principal types of engineering tests were run--acceleration-deceleration (dash), ride dynamics, obstacle-impact response (shock), turning, and controlled-slalom (maneuver). Two types of tests were conducted to test tactical performance--a 20-km traverse test through many quantitatively defined terrain types for vehicle speed -and driver response evaluation, and hit-avoidance tests to determine the survivability attributed to vehicle mobility/agility. The majority of tasts were conducted at Fort Knox, Kentucky, with some special soft-soil tests conducted in a floodplain, near Vicksburg, Mississippi.

Mobility/agility performance depends on design balance, terrain, weather, and a specified mission profile. It cannot be assessed on the basis of a single vehicle parameter.

The results of the ride and shock tests showed that the effects of suspension jounce travel (i.e., the vertical travel of a roadwheel from its static equilibrium position to the bump stop) depended on the degree of suspension damping, suspension spring rate, vehicle weight, and surface roughness. Reduced jounce travel combined with soft springs and low damping caused a progressive increase in suspension "bottoming" (roadwheels striking the bump stops) as the surface roughness or obstacle height increased. This condition became worse for this type of suspension if the vehicle weight was increased. However, the shock effects caused by suspension bottoming could be effectively reduced with increased damping.

Sprocket horsepower per ton is definitely a prominent factor in mobility/agility performance. Yet it is obvious that a vehicle wiqth high sprocket horsepower per ton and poor suspension will be able to use that power only on smooth terrain surfaces where ride and shock are not limiting factors. Likewise, the mobility/agility advantages of high horsepower per ton are quickly diminished in deformable soils if the vehicle's ground pressure does not provide sufficient flotation to prevent excessive sinkage and soil motion resistance; or in curves and sharp turns during evasive maneuvers if the vehicle's center of gravity is too high for stability; or it the vehicle's dimensions prevent effective maneuvering in the dense forests, such as those found in Germany and certain tropical areas of military interest.

A principal concern was to determine if the soil motion resistance increased significantly at high speeds in a manner similar to the exponential increase in resistance offered by water to high-speed boats. If the increase was significant, there would be practical limits on power trains beyond which large increases in motion resistance would largely offset power increases, resulting in only small gains in speed. Until this program, power trains in cross-country vehicles had not permitted speeds where such soil resistance rate effects, if they existed, were encountered. Over the speed range from 10 to 30 or 40 mph there appeared to be no significant increase in motion resistance; i.e., up to at least 40 mph there was no evidence that increased power will not provide proportionally increased speeds in normal weak soil conditions.

A comparison of the performance between the professional test drivers and military drivers was used to determine the degree that trained military drivers would exploit the increased mobility capability of the HIMAG chassis. It would be wasted effort and money to design and build vehicles withl 50 or 60 percent increase in mobility capability if military drivers only use 10 or 15 percent of the increased capability. For the two HIMAG configurations the familiar drivers reached 90 to 95 percent of the speeds achieved by the test drivers over the entire course. The unfamiliar drivers achieved 87 percent for the HIMAG 5, but the somewhat unstable behavior of the lighter, tail-heavy HIMAG 2 had a significant influence on those drivers not familiar with the course and they achieved only 79 percent of the test drivers' speeds. the military drivers actually exploited more of the available mobility capability from the two HIMAG configurations than the two contemporary vehicles.

Tests compared the relative performance of eight selected vehicles for four distinct types of mobility in both dry and wet conditions in West Germany and Middle East terrains, respectively. The vehicles were ranked in each case according to performance. The four mobility types - dash, traverse, maneuver, and cross-country - were representative of those often encountered in tactical situations. The relative performance of the vehicles varies according to the type of mobility, the area of operations, and the terrain conditions. The variations showed no particular pattern with respect to gross vehicle weight or sprocket horsepower per ton.

Generally the HIMAG was a top performer except in t1e German cross-country terrain where its size severely restricted maneuverability through the denser German forests. The Ml MBT encountered the same problem. In most cases all the lighter concept vehicles outperformed the Ml MBT. The Ml MBT demonstrated excellent maneuver performance (in open, level terrain) except in wet German terrain where its performance fell below that of the CFV. The M113Al APC and the M60AI MBT were consistently the worst performers.

Subsequent war gaming indicated these differences between the Ml MBT and the concept vehicles were not tactically significant. The results also reflect that vehicle performance depends upon the combined effects of the vehicle, the mission, and the terrain and does not vary directly with weight or horsepower.

High mobility/agility provides an increased hit-avoidance capability, but the reduced effectiveness to fire-on-the-move while maneuvering violently may result in only a marginal payoff in survivability. The results of the hit-avoidance tests revealed that a vehicle capable of performing fast, quick maneuvers can gain an additional measure of hit avoidance. In the simplest sense, against opposing guns, a maneuvering vehicle moves out of the way of a projectile already in flight causing what is referred to as target-induced error. Likewise, a fast, agile target affects the ability of a gunner to accurately track the target in his sight. The gun turret drive and fire-control-computer system are also affected. This type of error, which occurs before the round is fired, is referred to as system-induced error. Finally, the fast, agile target reduces exposure time to opposing gunners. These three factors--increased target-induced error, increased system-induced error, and decreased exposure time created by a fast, agile maneuvering vehicle decrease the probability of being hit. Further, a maneuver that minimizes exposure time while maximizing accelerations seen by the firer could be considered optimal.

While most of the HIMAG critical incidents were associated with high speed and driver error, there were, apparently, also engineering design features which contributed to loss of control or machiaie failures. The critical incidents in which vehicle component failure was presumed to be the prime cause were not especially associated with high speed. For example, in wet and muddy terrain, the visibility limitations imposed on the HIMAG driver by the inadequate windshield washer system severely hampered his performance.

Systems measures (e.g., mean course speeds, number of crucial incidents) are expected to reflect operator or crew performances, but are known to be influenced heavily by machine variables. Without very special instrumentation for the purpose, it is difficult to obtain clean measures indicative of raw performance distinct from such system measures.

The turret that housed the 75mm gun was manned but the two crewmembers (gunner and commander) were sitting in the basket under it, protected by the hull armor from threats. This upgraded vehicle was tested in Yuma and Fort Knox between March 1980 and March 1981, after which it was dismantled and its components used elsewhere. The tests showed that, simply put that the hydro pneumatic suspension performed well, but the 75mm gun didn’t. Gun accuracy was a constant problem. The first shot accuracy was comparable to the 105mm M68 but anything over 2-round burst caused significant accuracy loss.

The Combat Vehicle Technology Program's High Mobility Agility (HIMAG) Vehicle Chassis Tests, conducted in 1978 and 1979, provided an opportunity to explore the prediction of human performance requirements and the implications of high mobility tracked vehicle design for driver performance. Preliminary analysis and projections, based on the vehicle concept during construction, were ccmpared with data gathered during driver training and 20 kilometer testing.

Results supported the general hypothesis that cross-country driving on the higher horsepower per ton vehicles was significantly different from the same task on the M60AI or M113. course speeds, driver throttle use, driver errors, and critical incidents showed a differential pattern on HIMAG trials. Human factors and human engineering design deficiencies in the driver compartment, some of which were predicted in preliminary analysis and training but were not resolved, probably limited HIIMAG speed and maneuver.