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Analytical simulation as a support tool in the decision process “wheels or tracks”
Jourrtal off Terramechanics, Vol. 28, No. 2/3, pp. 201-210, 1991. Printed in Great Britain.
0022-489g/9153.(10+0.00 Pergamon Press plc. © 19c)1 ISTVS
A N A L Y T I C A L S I M U L A T I O N A S A S U P P O R T T O O L IN T H E DECISION PROCESS "WHEELS OR TRACKS"* K.-J. MELZERt and W. KOPPEL'~"
Summary--In the development of a weapon system, the maximization of demands concerning features like fire power, armor and mobility often leads to incompatibilities within the overall systems, to an imbalanced design and excessively high cost. The consequence is a decrease in mission availability. Therefore, system analysis utilizing simulation techniques is being used more and more in the early stages of the development process. This paper describes the application of such simulation techniques to practical cases. First, the development of a system is described by which to select wheeled or tracked ruiming gears for combat vehicles considering the compatibility of fire power, armor, mobility and cost. Secondly, the possible upper weight ranges for wheeled combat vehicles arc discussed, using analytical modeling of vehicle mobility as a basis. The methods used in these cases can also be applied to non-military problems. INTRODUCTION
"DumNc~ the design, development and procurement process, the maximization of requirements concerning system components such as fire power, armor and mobility result in i n c o m p a t i b i l i t y of the s u b s y s t e m s within the overall s},stem, i m b a l a n c e d design a n d excessively high cost". This s u m m a r i z i n g s t a t e m e n t which a r e p r e s e n t a t i v e of the G e r m a n F e d e r a l Ministry of D e f e n s e , who is r e s p o n s i b l e for the p r o c u r e m e n t of a r m o r e d c o m b a t vehicles, m a d e d u r i n g a s y m p o s i u m on "'The A r m o r e d Vehicles of the N i n e t i e s " more than ten years ago led to the following r e q u i r e m e n t : for the future design, d e v e l o p m e n t a n d p r o c u r e m e n t process for c o m b a t vehicles, d e c i s i o n - m a k i n g aids were to be p r e p a r e d that allow the c o m p a t i b i l i t y of the system c o m p o n e n t s fire power, a r m o r , mobility etc. at s u b s y s t e m a n d system levels to be j u d g e d already at the design stage, taking a c c o u n t of cost constraints. T h e d e m a n d for system analysis m e t h o d s was reinforced by the fact that c o m m e r c i a l l y available c o m p o n e n t s have to be t a k e n into c o n s i d e r a t i o n even in d e v e l o p i n g special vehicles: the same considerations of course are necessary if d u r i n g the p r o c u r e m e n t of future logistic vehicles, c o m m e r c i a l c o m p o n e n t s / v e h i c l e s are to be m a d e c o m p a t i b l e with a n d o p t h n i z e d for the c o r r e s p o n d i n g vehicle missions. This p a p e r is a i m e d at giving a very b r o a d overview of the analytical s i m u l a t i o n m e t h o d s available today a n d e x a m p l e s of their a p p l i c a t i o n , which show how the s i m u l a t i o n of the t e r r a i n - v e h i c l e - d r i v e r system can be used to analyse the subsystem "'mobility'" that is to serve as o n e d e c i s i o n - m a k i n g aid in the design, d e v e l o p m e n t and p r o c u r e m e n t process.
*Presented at the 10th International Conference of the International Society for Terrain-Vehicle Systems, Kobe, Japan (August 1990). iBattellc Motor- und Fahrzeugtechnik GmbH, Am Roemerhof 35, D-6000 Frankfurt am Main 01L Gcrmany. 201
202
K.-J. M E L Z E R and W. K O P P E L H I E R A R C H Y OF M O D E L S
Figure 1 gives an overview of the three major levels of methods and models required to fulfill the tasks formulated above, that are basically available today (see also Ref. [11). (1) Level 1 This level includes models and methods that normally are stand-alone modules supporting level 2 but mainly level 3 methods; generally this level serves the description of the overall terrain [2]. Basically, the terrain is described by engineering parameters that interrelate with the vehicle parameters influencing mobility performance. These include slopes, surface composition, soil strength, weather conditions, obstacle geometry, vegetation, etc. The terrain is divided into the following three subsystems: • roads and trails • linear features (creeks, rivers, etc.) • areal (off-road) terrain. For large area investigations, comprehensive data banks are available for the northern, central and southern regions of Germany. Statistical methods allow the comparison with similar geographic regions of Germany, thus extending the potential of applications.
J 11 SUPPORTING MODELS AND METHODS: TERRAIN DESCRIPTION ETC.
TERRAIN-OPERATORVEHICLE SYSTEM
1
zl MODELS AND METHODSDESCRIBING INTERACTIONSON SUB-SYSTEM LEVEL: SOIL-RUNNINGGEAR, OBSTACLE-RUNNING GEARETC.
Fie;. 1. Hierarchy of models.
MDDELS AND METHODS DESCRIBING THE OVERALL SYSTEM
ANALYTICAL SIMULATION IN THE DECISION PROCESS
203
(2) Level 2 This level represents models by means of which the interactions of subsystem level are described (Fig. 1); it can be divided into two subgroups: terrain and scenario. " T e r r a i n " stands for all separate interactions between the vehicle and its components, the operator and the individual terrain characteristics that influence vehicle performance. With this, the analysis of the performance of individual components and its influence on overall vehicle mobility becomes possible (see also Ref. [1]). This includes methods to determine the traction of wheeled and tracked vehicles on different soil types and related strength conditions. Furthermore, two- and three-dimensional dynamic models have to be mentioned for investigating the ride performance of vehicles crossing rough terrain and obstacles; the types of models range from tracked vehicles modelled by classical approaches [3] to very flexible multi-body system models [4, 5]. The second group, "'scenario", then represents methods for taking into account the influence of scenario variables like seasonal conditions, driver's motivation and mission constraints. Undoubtedly, the group of level 2 methods and models is the most important. It is here where basic relations to predict the performance of system components are developed; these provide the basis for establishing overall system models (level 3). (3) Level 3 This level represents models and methods that allow the analysis of the vehicle mobility performance in operational areas on the basis of the overall system " v e h i c l e - t e r r a i n - o p e r a t o r " . This level includes the N A T O Reference Mobility Model (NRMM) which, in its extended version, is being used as Battelle Mobility Model (BMM) in the following examples; the scheme of the model is shown in Fig. 2. Its structure and its application have been frequently described, e.g. Refs [6-11].
EXAMPLES OF APPLICATION (1) Selecting wheels or tracks Figure 3 shows the scheme of a methodology that has been designed to be used for the preselection of wheeled or tracked running gears for armored combat support vehicles of the 20-25 t class [12]. For given requirements for a vehicle such as weapon, space, power train, built-ins, armor, etc., certain basic dimensions and weights are determined; the vehicle is then equipped alternatively with a wheeled and with a tracked running gear. (The selection system contains 1982-state-of-technology components.) The two vehicles are then checked and compared for weapon compatibility, mobility and cost. Within this methodology the issue "mobility" was to be covered by analytical simulation methods of level 3. Part of the input to the overall methodology consisted of the following quantification of mobility. Figure 4 shows four vehicles covering the possible weight range considered; W1 and T1 are comparable to each other and represent the lower end of a defined mobility performance spectrum, whereas the comparable vehicles W2 and T2 are covering the upper limit of the performance spectrum. The mobility performance of all four vehicles was determined using the
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BMM simulation techniques (level 3). The results are shown as mobility profiles (Fig. 5) quantifying the speeds the vehicles can sustain while traveling in a given operational area during a defined season (influence of weather), utilizing the road and trail network at varying percentages. (The crossing of linear features, e.g. creeks, was not considered in this evaluation.) From this result (Fig. 5) it can be seen that the mission of the vehicles (percentage travel on and off the road) will clearly influence the running gear selection. In general, it might be concluded that the tracked vehicles have a better mobility performance as soon as the percentage of off-road travel exceeds about 60%. However, it should be mentioned that the vehicles considered represent first concepts only, far from being optimized. (2) Adding new technology As mentioned before, the four vehicles referred to above represent the state of technology of 1982, including the use of commercially available components. In 1984 an additional early concept was investigated, which was designed on the basis of the
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206
K.-J. M E L Z E R and W. K O P P E L
latest developments of highly advanced wheeled vehicle technology [13]. This concept (8 x 8, 26 t, tires: 17.5-R25), although only on paper, was evaluated for its mobility performance in the same operational area as the vehicles described above. The results (W3) are also shown in Fig. 5. The increase in mobility performance is obvious: even in the most difficult parts of the area, the vehicle W3 approaches the performance of the tracked vehicles T1 and T2. In this context it should also be mentioned that the crossing of linear features was not considered in this evaluation. (3) Where is the [itnit? Hardly any author dealing with the topic of ground mobility can resist expressing his--at least personal--opinion with regard to the question: where is the limit, e.g. in terms of weight, between wheeled and tracked vehicles. Therefore, it may be allowed for these authors to do so yet another time [14]: • The decision as to whether a vehicle is to be equipped with a wheeled or tracked running gear unit should be based solely on the question whether the vehicle can fulfill its defined mission within tactical, environmental and financial constraints. • Therefore no general solution can be given to this problem: the decision must be made on a case-by-case basis for each new vehicle concept. There are, however, some indications as to where we stand today and this may reveal some trends for the future. Generally, it may be stated that tactical vehicles (TV) like 50 t battle tanks will run on tracks. On the other hand, logistic off-road vehicles (LV, trucks) will be mostly equipped with wheeled running gears. In the intermediate zone of off-road tactical support vehicles (TSV) like anti-tank weapons or personnel carriers, the gross vehicle weight (GVW) may range from 7 to about 30 t (in some instances even more). It is within this "'gray zone" that the question "wheels or tracks" becomes the hot issue: Technical'? Tactical'? Philosophical? Emotional? Understandable, if one considers that it has been stated persistently in the past by some schools of thought that for instance the G V W to be put on wheeled running gears for off-road TSV could not exceed much more than about 20 t. Again, it is the way of analytically determining the off-road vehicle performancc, to shed some light into the "'gray zone". Figure 6 is a plot of vehicle cone index (VCI) versus GVW. VCI defines the minimum soil strength that is required to allow one pass of a defined vehicle. VCI incorporates load conditions, ground contact area of running gear, power train characteristics, etc., and is a well suited parameter to 60
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ANALYTICAL SIMULATION IN THE DECISION PROCESS
207
represent traction characteristics for our practical purpose [6, 8]. The figure contains data of vehicles corresponding to the three groups: LV, TSV and TV. Figure 6 allows the derivation of the following trends. In the LV area, VC1 can be reduced further by using more favorable tire geometries and/or central tire inflation pressure systems (CTIPS); in some instances, values as low as those for T V may be reached. With TSV, it appears that G V W larger than 20 t, up to a limit of about 30 t, can be achieved for wheeled vehicles because of advanced running gear technology (including the tire): VCI values reach those of the TV. For the latter, no dramatic changes are to be expected in the near future. Thus, traction together with power requirements may basically become the dominant speed limiting factor for wheeled vehicles which, depending on their specific missions, may have to enter remote areas where they encounter yielding soils, slopes, e m b a n k m e n t s of small creeks, etc.. The second important speed limiting factor is ride quality. If one considers the last 10 to i5 years, a rapid development has occurred with respect to improving the ride quality of wheeled off-road vehicles, e.g. [15]. Ride qualities of wheeled vehicles operating on rough terrain have been achieved (and are still being improved) that are comparable with the ride quality of tracked vehicles. To demonstrate this, results from analytical 2-D dynamic simulations were gathered for some of the vehicles of Fig. 6. The results in terms of ride comfort relations for wheeled and tracked vehicles are shown in Fig. 7. The fact that the two areas for wheeled and tracked vehicles overlap indicates the advancements in the quality of suspension systems of wheeled vehicles: an e x c e p t i o n - - a t least to a certain extent--is the capability to negotiate large obstacles (e.g. e m b a n k m e n t s of creeks). As the reader may have noticed, analytical models according to level 2 (Fig. 1) have b e e e n used so far. And the indication is that it may be possible to push the G V W for armored tactical support vehicles beyond 30 t. However, the final answer depends on the mobility performance of the overall system, which includes analytical simulation with level 3 models (Figs 1 and 2). An excellent example for this procedure is given in [15] where an 8 × 8 experimental armored tactical support vehicle was used to demonstrate advanced wheeled vehicle technology and its influence on mobility in the area of 30 t G V W . Besides intensive hardware testing, mobility system simulations were conducted with the 80 km/h 60
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Battelle Mobility, Model (BMM). The off-road mobility of the 8 × 8 vehicle was investigated in an operational area of a 1:5(),()i)(1 scale topographic map (about 20 x 2(1 km) under extremely wet weather condition using the BMM: gross vehicle weights (GVW) were 28, 32 and 361. Although BMM--like all anah'tical models--cannot simulate the reality,' with 100% agreement, the relative comparison represents a ~aluable additional decision-making aid in the further development phases. Figure S shows the results of the simulation in terms of mobility profiles in comparison with a modern main battle tank (MBT). As can be seen, the mobility of thc wheeled vehicles in the 30-I-GVW range is comparable with that of the tracked vehicle for the given operational condition (percentage of areas inaccessible for all vehicles are indicated). Even at a G V W of 36t, the S × 8 vehicle stays in a comparable range, depending on the vehicle's mission. In order to demonstrate this, the mobility of the S x S at 32 t GVW, compared with the MBT, is shown in terms of isochrones for the same operational area referred to above (Fig. 9). From a defined starting point, the travcl times are given at time intervals of five minutes: the shaded areas represent the 8 × S vehicle and the solid lines lhc MBT (white spots represent immobilization). As can be seen, the S × N vehicle eains time over the MBT after abotlt 25 rain, which is due to accumulating time£peed advantages on firm grounds (see also Fig. ,<,).
(()N(L1 !I)IN(; P,EMA RKS Once again, the possibilities of analytical simulations of mobility and quantifying mobility performance have been demonstrated. The examples included cases where the simulations were incorporated into higher order evalution methods, as well as the classical application cases where mobility performance in operational areas is evaluated analytically to s u p p o r t - - l o g c t h e r with intensive but selected hardware testing --further development and procurement decisions. There seems to be no doubt thai the evaluation methods applied above can also be used in solving similar non-military problems.
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FI~<,";. Oualilati',c mobility profiles of Ihrec wheeled and one tracked ',chicle for cxtrcmdy wet wc;.tthci condition.
A N A L Y T I C A L S I M U L A T I O N IN T H E D E C I S I O N P R O C E S S
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REFERENCES [1] K. -J. MELZER, Analytical m e t h o d s and modelling; state-of-the art report. J. Terramechanics 19, 31 (1982). [2] W. KoeeEL and W. GRABAU, Terrain description for mobility prediction. Proc. 9th lilt. ConJ] Terrain-Vehicle Systems, Barcelona, Vol. 1, p. 160 (1987). [3] P. PARmN~ER. Rechenmodell Laufwerk - R~umliches Rechenmodell. IABG-Bericht B-TF 610, O t t o b r u n n (1976). [4] C. STRAUSS, D Y N A M O D , ein dynamisches 3-D-Simulationsmodell ftir Fahrzeugkonzepte und andere mechanische Systeme. Battelle-Kurzbericht, Frankfurt am Main (1984).
210
K.-J, M E L Z E R and W. KOPPEL
[5] H. HAHN and R. WEHA(ik, Dynamic simulation of terrain vehicles. Proc. 9th hit. Conf. Terrain-Vehicle Systems, Barcelona, Vol. II, p. 491 (19871. [6] M. P. JURKAT, C. J. NUTI.~XLL and P. HA[,EY, The US Army mobility model. Proc. 5th lnt. Con¢i Terrain-Vehicle Systems, Detroit. Vol. IV, p. 1-1 (19751. [7] W. KOPPEL. Rechnerische Simulation als Entscheidungshilfe for die Beschaffung von Fahrzeugen, Wehrtechnik 10, 48 ( 198 I). [8] K. -J. MELZLR, Possibilities of evaluating the traction of tires for off-road transportation vehicles. Proc. 2nd European Conf. Terrain-Vehicle Systems, Ferrara, p. 63 (19831. [91 Z. J. JANOS[. The role of analytical simulation in the US army military vehicle acquisition process. Proc. X X I I FISITA Conf., Dearborn, Michigan, Vol. II, p. 2.157 (1988). [10] K. -J. MLZLR, Mission oriented application of the NRMM support model "Steering for Tracked Vehicles", Proc. 4th European Conf. Terrain-Vehicle Systems, Wageningen, Vol. 1, p. 138 (19891. [11] K. -J. Mt-LZLR, Mission Oriented Component Optimization for Off-Road Vehicles. Truck Technology International. Sterling Publications, London, (199t/). 112] F, PORSCHL AG, Grundsatzstudie zur Entscheidungshilfe ftir die Auswabl von Kampffahrzeugen aut Rad- oder Kettenfahrwerk, Stuttgart, (1984). [13] R. HH~B~.R, Radfahrgestelle fiJr gepanzerte Gefechtsfahrzeuge. Paper presented at the information meeting of "Heeresamt" and Industry, K61n (1984). [14] K, -J. MLLZER, Developments of ground-mobility technology and their application to military vehicles. Proc. XX1 t:TS1TA Cong. Belgrade, Vol. 2, p. 2,335 (19861. [15] A, MJSC'HKL and V. Rt'm. Technologien fiir eine dritte gepanzerte Radfahrzeuggeneration. Jahrbuch der Wehrtechnik 18, 80 (1988).
0022-489g/9153.(10+0.00 Pergamon Press plc. © 19c)1 ISTVS
A N A L Y T I C A L S I M U L A T I O N A S A S U P P O R T T O O L IN T H E DECISION PROCESS "WHEELS OR TRACKS"* K.-J. MELZERt and W. KOPPEL'~"
Summary--In the development of a weapon system, the maximization of demands concerning features like fire power, armor and mobility often leads to incompatibilities within the overall systems, to an imbalanced design and excessively high cost. The consequence is a decrease in mission availability. Therefore, system analysis utilizing simulation techniques is being used more and more in the early stages of the development process. This paper describes the application of such simulation techniques to practical cases. First, the development of a system is described by which to select wheeled or tracked ruiming gears for combat vehicles considering the compatibility of fire power, armor, mobility and cost. Secondly, the possible upper weight ranges for wheeled combat vehicles arc discussed, using analytical modeling of vehicle mobility as a basis. The methods used in these cases can also be applied to non-military problems. INTRODUCTION
"DumNc~ the design, development and procurement process, the maximization of requirements concerning system components such as fire power, armor and mobility result in i n c o m p a t i b i l i t y of the s u b s y s t e m s within the overall s},stem, i m b a l a n c e d design a n d excessively high cost". This s u m m a r i z i n g s t a t e m e n t which a r e p r e s e n t a t i v e of the G e r m a n F e d e r a l Ministry of D e f e n s e , who is r e s p o n s i b l e for the p r o c u r e m e n t of a r m o r e d c o m b a t vehicles, m a d e d u r i n g a s y m p o s i u m on "'The A r m o r e d Vehicles of the N i n e t i e s " more than ten years ago led to the following r e q u i r e m e n t : for the future design, d e v e l o p m e n t a n d p r o c u r e m e n t process for c o m b a t vehicles, d e c i s i o n - m a k i n g aids were to be p r e p a r e d that allow the c o m p a t i b i l i t y of the system c o m p o n e n t s fire power, a r m o r , mobility etc. at s u b s y s t e m a n d system levels to be j u d g e d already at the design stage, taking a c c o u n t of cost constraints. T h e d e m a n d for system analysis m e t h o d s was reinforced by the fact that c o m m e r c i a l l y available c o m p o n e n t s have to be t a k e n into c o n s i d e r a t i o n even in d e v e l o p i n g special vehicles: the same considerations of course are necessary if d u r i n g the p r o c u r e m e n t of future logistic vehicles, c o m m e r c i a l c o m p o n e n t s / v e h i c l e s are to be m a d e c o m p a t i b l e with a n d o p t h n i z e d for the c o r r e s p o n d i n g vehicle missions. This p a p e r is a i m e d at giving a very b r o a d overview of the analytical s i m u l a t i o n m e t h o d s available today a n d e x a m p l e s of their a p p l i c a t i o n , which show how the s i m u l a t i o n of the t e r r a i n - v e h i c l e - d r i v e r system can be used to analyse the subsystem "'mobility'" that is to serve as o n e d e c i s i o n - m a k i n g aid in the design, d e v e l o p m e n t and p r o c u r e m e n t process.
*Presented at the 10th International Conference of the International Society for Terrain-Vehicle Systems, Kobe, Japan (August 1990). iBattellc Motor- und Fahrzeugtechnik GmbH, Am Roemerhof 35, D-6000 Frankfurt am Main 01L Gcrmany. 201
202
K.-J. M E L Z E R and W. K O P P E L H I E R A R C H Y OF M O D E L S
Figure 1 gives an overview of the three major levels of methods and models required to fulfill the tasks formulated above, that are basically available today (see also Ref. [11). (1) Level 1 This level includes models and methods that normally are stand-alone modules supporting level 2 but mainly level 3 methods; generally this level serves the description of the overall terrain [2]. Basically, the terrain is described by engineering parameters that interrelate with the vehicle parameters influencing mobility performance. These include slopes, surface composition, soil strength, weather conditions, obstacle geometry, vegetation, etc. The terrain is divided into the following three subsystems: • roads and trails • linear features (creeks, rivers, etc.) • areal (off-road) terrain. For large area investigations, comprehensive data banks are available for the northern, central and southern regions of Germany. Statistical methods allow the comparison with similar geographic regions of Germany, thus extending the potential of applications.
J 11 SUPPORTING MODELS AND METHODS: TERRAIN DESCRIPTION ETC.
TERRAIN-OPERATORVEHICLE SYSTEM
1
zl MODELS AND METHODSDESCRIBING INTERACTIONSON SUB-SYSTEM LEVEL: SOIL-RUNNINGGEAR, OBSTACLE-RUNNING GEARETC.
Fie;. 1. Hierarchy of models.
MDDELS AND METHODS DESCRIBING THE OVERALL SYSTEM
ANALYTICAL SIMULATION IN THE DECISION PROCESS
203
(2) Level 2 This level represents models by means of which the interactions of subsystem level are described (Fig. 1); it can be divided into two subgroups: terrain and scenario. " T e r r a i n " stands for all separate interactions between the vehicle and its components, the operator and the individual terrain characteristics that influence vehicle performance. With this, the analysis of the performance of individual components and its influence on overall vehicle mobility becomes possible (see also Ref. [1]). This includes methods to determine the traction of wheeled and tracked vehicles on different soil types and related strength conditions. Furthermore, two- and three-dimensional dynamic models have to be mentioned for investigating the ride performance of vehicles crossing rough terrain and obstacles; the types of models range from tracked vehicles modelled by classical approaches [3] to very flexible multi-body system models [4, 5]. The second group, "'scenario", then represents methods for taking into account the influence of scenario variables like seasonal conditions, driver's motivation and mission constraints. Undoubtedly, the group of level 2 methods and models is the most important. It is here where basic relations to predict the performance of system components are developed; these provide the basis for establishing overall system models (level 3). (3) Level 3 This level represents models and methods that allow the analysis of the vehicle mobility performance in operational areas on the basis of the overall system " v e h i c l e - t e r r a i n - o p e r a t o r " . This level includes the N A T O Reference Mobility Model (NRMM) which, in its extended version, is being used as Battelle Mobility Model (BMM) in the following examples; the scheme of the model is shown in Fig. 2. Its structure and its application have been frequently described, e.g. Refs [6-11].
EXAMPLES OF APPLICATION (1) Selecting wheels or tracks Figure 3 shows the scheme of a methodology that has been designed to be used for the preselection of wheeled or tracked running gears for armored combat support vehicles of the 20-25 t class [12]. For given requirements for a vehicle such as weapon, space, power train, built-ins, armor, etc., certain basic dimensions and weights are determined; the vehicle is then equipped alternatively with a wheeled and with a tracked running gear. (The selection system contains 1982-state-of-technology components.) The two vehicles are then checked and compared for weapon compatibility, mobility and cost. Within this methodology the issue "mobility" was to be covered by analytical simulation methods of level 3. Part of the input to the overall methodology consisted of the following quantification of mobility. Figure 4 shows four vehicles covering the possible weight range considered; W1 and T1 are comparable to each other and represent the lower end of a defined mobility performance spectrum, whereas the comparable vehicles W2 and T2 are covering the upper limit of the performance spectrum. The mobility performance of all four vehicles was determined using the
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BMM simulation techniques (level 3). The results are shown as mobility profiles (Fig. 5) quantifying the speeds the vehicles can sustain while traveling in a given operational area during a defined season (influence of weather), utilizing the road and trail network at varying percentages. (The crossing of linear features, e.g. creeks, was not considered in this evaluation.) From this result (Fig. 5) it can be seen that the mission of the vehicles (percentage travel on and off the road) will clearly influence the running gear selection. In general, it might be concluded that the tracked vehicles have a better mobility performance as soon as the percentage of off-road travel exceeds about 60%. However, it should be mentioned that the vehicles considered represent first concepts only, far from being optimized. (2) Adding new technology As mentioned before, the four vehicles referred to above represent the state of technology of 1982, including the use of commercially available components. In 1984 an additional early concept was investigated, which was designed on the basis of the
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206
K.-J. M E L Z E R and W. K O P P E L
latest developments of highly advanced wheeled vehicle technology [13]. This concept (8 x 8, 26 t, tires: 17.5-R25), although only on paper, was evaluated for its mobility performance in the same operational area as the vehicles described above. The results (W3) are also shown in Fig. 5. The increase in mobility performance is obvious: even in the most difficult parts of the area, the vehicle W3 approaches the performance of the tracked vehicles T1 and T2. In this context it should also be mentioned that the crossing of linear features was not considered in this evaluation. (3) Where is the [itnit? Hardly any author dealing with the topic of ground mobility can resist expressing his--at least personal--opinion with regard to the question: where is the limit, e.g. in terms of weight, between wheeled and tracked vehicles. Therefore, it may be allowed for these authors to do so yet another time [14]: • The decision as to whether a vehicle is to be equipped with a wheeled or tracked running gear unit should be based solely on the question whether the vehicle can fulfill its defined mission within tactical, environmental and financial constraints. • Therefore no general solution can be given to this problem: the decision must be made on a case-by-case basis for each new vehicle concept. There are, however, some indications as to where we stand today and this may reveal some trends for the future. Generally, it may be stated that tactical vehicles (TV) like 50 t battle tanks will run on tracks. On the other hand, logistic off-road vehicles (LV, trucks) will be mostly equipped with wheeled running gears. In the intermediate zone of off-road tactical support vehicles (TSV) like anti-tank weapons or personnel carriers, the gross vehicle weight (GVW) may range from 7 to about 30 t (in some instances even more). It is within this "'gray zone" that the question "wheels or tracks" becomes the hot issue: Technical'? Tactical'? Philosophical? Emotional? Understandable, if one considers that it has been stated persistently in the past by some schools of thought that for instance the G V W to be put on wheeled running gears for off-road TSV could not exceed much more than about 20 t. Again, it is the way of analytically determining the off-road vehicle performancc, to shed some light into the "'gray zone". Figure 6 is a plot of vehicle cone index (VCI) versus GVW. VCI defines the minimum soil strength that is required to allow one pass of a defined vehicle. VCI incorporates load conditions, ground contact area of running gear, power train characteristics, etc., and is a well suited parameter to 60
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ANALYTICAL SIMULATION IN THE DECISION PROCESS
207
represent traction characteristics for our practical purpose [6, 8]. The figure contains data of vehicles corresponding to the three groups: LV, TSV and TV. Figure 6 allows the derivation of the following trends. In the LV area, VC1 can be reduced further by using more favorable tire geometries and/or central tire inflation pressure systems (CTIPS); in some instances, values as low as those for T V may be reached. With TSV, it appears that G V W larger than 20 t, up to a limit of about 30 t, can be achieved for wheeled vehicles because of advanced running gear technology (including the tire): VCI values reach those of the TV. For the latter, no dramatic changes are to be expected in the near future. Thus, traction together with power requirements may basically become the dominant speed limiting factor for wheeled vehicles which, depending on their specific missions, may have to enter remote areas where they encounter yielding soils, slopes, e m b a n k m e n t s of small creeks, etc.. The second important speed limiting factor is ride quality. If one considers the last 10 to i5 years, a rapid development has occurred with respect to improving the ride quality of wheeled off-road vehicles, e.g. [15]. Ride qualities of wheeled vehicles operating on rough terrain have been achieved (and are still being improved) that are comparable with the ride quality of tracked vehicles. To demonstrate this, results from analytical 2-D dynamic simulations were gathered for some of the vehicles of Fig. 6. The results in terms of ride comfort relations for wheeled and tracked vehicles are shown in Fig. 7. The fact that the two areas for wheeled and tracked vehicles overlap indicates the advancements in the quality of suspension systems of wheeled vehicles: an e x c e p t i o n - - a t least to a certain extent--is the capability to negotiate large obstacles (e.g. e m b a n k m e n t s of creeks). As the reader may have noticed, analytical models according to level 2 (Fig. 1) have b e e e n used so far. And the indication is that it may be possible to push the G V W for armored tactical support vehicles beyond 30 t. However, the final answer depends on the mobility performance of the overall system, which includes analytical simulation with level 3 models (Figs 1 and 2). An excellent example for this procedure is given in [15] where an 8 × 8 experimental armored tactical support vehicle was used to demonstrate advanced wheeled vehicle technology and its influence on mobility in the area of 30 t G V W . Besides intensive hardware testing, mobility system simulations were conducted with the 80 km/h 60
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Battelle Mobility, Model (BMM). The off-road mobility of the 8 × 8 vehicle was investigated in an operational area of a 1:5(),()i)(1 scale topographic map (about 20 x 2(1 km) under extremely wet weather condition using the BMM: gross vehicle weights (GVW) were 28, 32 and 361. Although BMM--like all anah'tical models--cannot simulate the reality,' with 100% agreement, the relative comparison represents a ~aluable additional decision-making aid in the further development phases. Figure S shows the results of the simulation in terms of mobility profiles in comparison with a modern main battle tank (MBT). As can be seen, the mobility of thc wheeled vehicles in the 30-I-GVW range is comparable with that of the tracked vehicle for the given operational condition (percentage of areas inaccessible for all vehicles are indicated). Even at a G V W of 36t, the S × 8 vehicle stays in a comparable range, depending on the vehicle's mission. In order to demonstrate this, the mobility of the S x S at 32 t GVW, compared with the MBT, is shown in terms of isochrones for the same operational area referred to above (Fig. 9). From a defined starting point, the travcl times are given at time intervals of five minutes: the shaded areas represent the 8 × S vehicle and the solid lines lhc MBT (white spots represent immobilization). As can be seen, the S × N vehicle eains time over the MBT after abotlt 25 rain, which is due to accumulating time£peed advantages on firm grounds (see also Fig. ,<,).
(()N(L1 !I)IN(; P,EMA RKS Once again, the possibilities of analytical simulations of mobility and quantifying mobility performance have been demonstrated. The examples included cases where the simulations were incorporated into higher order evalution methods, as well as the classical application cases where mobility performance in operational areas is evaluated analytically to s u p p o r t - - l o g c t h e r with intensive but selected hardware testing --further development and procurement decisions. There seems to be no doubt thai the evaluation methods applied above can also be used in solving similar non-military problems.
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REFERENCES [1] K. -J. MELZER, Analytical m e t h o d s and modelling; state-of-the art report. J. Terramechanics 19, 31 (1982). [2] W. KoeeEL and W. GRABAU, Terrain description for mobility prediction. Proc. 9th lilt. ConJ] Terrain-Vehicle Systems, Barcelona, Vol. 1, p. 160 (1987). [3] P. PARmN~ER. Rechenmodell Laufwerk - R~umliches Rechenmodell. IABG-Bericht B-TF 610, O t t o b r u n n (1976). [4] C. STRAUSS, D Y N A M O D , ein dynamisches 3-D-Simulationsmodell ftir Fahrzeugkonzepte und andere mechanische Systeme. Battelle-Kurzbericht, Frankfurt am Main (1984).
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K.-J, M E L Z E R and W. KOPPEL
[5] H. HAHN and R. WEHA(ik, Dynamic simulation of terrain vehicles. Proc. 9th hit. Conf. Terrain-Vehicle Systems, Barcelona, Vol. II, p. 491 (19871. [6] M. P. JURKAT, C. J. NUTI.~XLL and P. HA[,EY, The US Army mobility model. Proc. 5th lnt. Con¢i Terrain-Vehicle Systems, Detroit. Vol. IV, p. 1-1 (19751. [7] W. KOPPEL. Rechnerische Simulation als Entscheidungshilfe for die Beschaffung von Fahrzeugen, Wehrtechnik 10, 48 ( 198 I). [8] K. -J. MELZLR, Possibilities of evaluating the traction of tires for off-road transportation vehicles. Proc. 2nd European Conf. Terrain-Vehicle Systems, Ferrara, p. 63 (19831. [91 Z. J. JANOS[. The role of analytical simulation in the US army military vehicle acquisition process. Proc. X X I I FISITA Conf., Dearborn, Michigan, Vol. II, p. 2.157 (1988). [10] K. -J. MLZLR, Mission oriented application of the NRMM support model "Steering for Tracked Vehicles", Proc. 4th European Conf. Terrain-Vehicle Systems, Wageningen, Vol. 1, p. 138 (19891. [11] K. -J. Mt-LZLR, Mission Oriented Component Optimization for Off-Road Vehicles. Truck Technology International. Sterling Publications, London, (199t/). 112] F, PORSCHL AG, Grundsatzstudie zur Entscheidungshilfe ftir die Auswabl von Kampffahrzeugen aut Rad- oder Kettenfahrwerk, Stuttgart, (1984). [13] R. HH~B~.R, Radfahrgestelle fiJr gepanzerte Gefechtsfahrzeuge. Paper presented at the information meeting of "Heeresamt" and Industry, K61n (1984). [14] K, -J. MLLZER, Developments of ground-mobility technology and their application to military vehicles. Proc. XX1 t:TS1TA Cong. Belgrade, Vol. 2, p. 2,335 (19861. [15] A, MJSC'HKL and V. Rt'm. Technologien fiir eine dritte gepanzerte Radfahrzeuggeneration. Jahrbuch der Wehrtechnik 18, 80 (1988).