Recommendations for a New and Improved ORCA 

Modeling System Blast Module 

by Owen P. Litt 

ARL-TR-3320 September 2004 

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Army Research Laboratory 

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ARL-TR-3320 September 2004 

Recommendations for a New and Improved ORCA 

Modeling System Blast Module 

Owen P. Litt 

Survivability/Lethality Analysis Directorate, ARL 

Approved for public release; distribution is unlimited. 
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Recommendations for a New and Improved ORCA Modeling System Blast Module 

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Owen P. Litt 

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14. ABSTRACT 

The blast module of the Operational Requirement-based Casualty Assessment (ORCA) modeling system computer software 

package was reviewed and compared to the WRAIR blast injury model (INJURY). As a result, recommendations are presented 

in this report to improve ORCA by expanding its capabilities consistent with INJURY results and to expand the ORCA inputoutput capabilities. The recommended improvements include incorporating a newly developed ORCA mathematical 

lethality/injury blast effects model and its computer implementation, ORCA model changes, and expanded user interaction 

capabilities. 

15. SUBJECT TERMS

ORCA model, casualty assessment, blast overpressure, lung injury, lethality, mathematical computer module, injury severity, 

human estimates 

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Owen P. Litt 

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iii

Contents 

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. New Injury and Lethality Algorithms 3

3. Data File Headers 8

4. Display of New and Modified User Interface Features 10

5. Need for Orca Model Case Studies Computer Library for Analysts 13

6. Need to Obtain Computer Hard Copy Outputs 14

7. Conclusion 15

List of Abbreviations, Acronyms, and Symbols 16

Distribution List 17 
iv

List of Figures 

Figure 1. Lethality/lung injury levels vs. n-work. ..........................................................................5 

Figure 2. Lethality/lung injury vs. n-work......................................................................................8 

List of Tables 

Table 1. Lung injury/lethality and n-work......................................................................................5 

Table 2. INJURY: n-work vs. lung injury. ....................................................................................6

Table 3. Lethality/lung injury vs. n-work. ......................................................................................9 

Table 4. Lethality/lung damage vs. selected n-work. ...................................................................13 

Table 5. Lethality/lung damage levels vs. selected n-work...........................................................13 
v

Acknowledgments 

The author gratefully acknowledges Dr. Edward Davis (U.S. Army Research Laboratory, 

Aberdeen Proving Ground, MD) for the many valuable discussions and his technical 

contributions to this report.
vi

INTENTIONALLY LEFT BLANK.
1

1. Introduction 

The Operational Requirement-based Casualty Assessment (ORCA)1, 2 modeling system is a 

computer software package that provides a comprehensive standardized casualty assessment 

methodology associated with weapon-induced injuries. With it, anatomical damage and the 

effect on individual performance can be determined for exposure to fragments, chemical agents, 

and blast shock waves. Each of these sources of injury is modeled and analyzed in an 

independent computer code module. Each module characterizes bioresponse to these sources of 

injury and estimates soldier performance degradation.

The ORCA blast module has been reviewed in order to determine which changes and additions 

should be made to improve and expand its capabilities. The current blast module was compared 

to the Walter Reed Army Institute of Research (WRAIR) blast injury model (INJURY). Five 

recommendations resulted from this review, which include a new ORCA mathematical model 

(which has been completed) and its computer implementation, ORCA model changes, and 

expanded user interaction capabilities. These recommendations follow. 

1. Incorporate the newly developed mathematical/computer model, which calculates “lung 

injury” and the likelihood of lethality for all levels of blast overpressure (BOP) in the 

ORCA input time history data files. The values obtained by the model are equivalent to 

those in INJURY. 

2. Expand the data file headers with which the user is able to select input options with respect 

to the experimental setup. These options include the following: 

 • BOP inputs in different units of time and overpressure, 

 • target and gauge orientation with respect to the blast shock wave, and 

 • the target mass and ambient air pressure as a user input. 

3. Improve and expand the display of results in order to include all the added and modified 

features. 

4. Include an option to generate a permanent folder of computer files consisting of selected 

elements, including the input parameters, of a given case study. 

5. Develop a simple procedure by which to make hard copies of selected parts of the ORCA 

inputs and results. 

 1 Kaltenbach, E.; Kilion, E.; Partch, J.; Streit, B. Operational Requirements-Based Casualty Assessment (ORCA) User’s 

Manual; ARA Project No. 0573; Applied Research Associates, Inc.: Albuquerque, NM, 2001. 

2

 Applied Research Associates, Inc. ORCA Computer Software Computer Code, version 2.43a; Albuquerque, NM, 2000. 
2

The term “lung injury” refers to the fractional, or percentage of, loss of respiratory function. 

INJURY was developed by WRAIR and Jaycor.3 It is a computer software package developed 

from a mathematical model and its associated computer code. Its purpose is to predict auditory 

injury and lung injury for blast shock-wave anterior impacts. A description of the model and its 

computer code are not presently available. The only document at our disposal is a user’s manual 

published by Jaycor in May 1995.4

 It is the baseline model against which ORCA results are 

compared for correctness. The inputs to INJURY are BOP time history data files. 

The analysis of the BOP data yields “normalized work” (n-work)—a result from which lung 

damage and likelihood of lethality can be determined. The n-work is calculated from the peak 

overpressure and specific impulse (impulse per unit area) as a measure of lung damage resulting 

from incident blast shock waves. 

ORCA follows the same initial process of analysis as INJURY—the determination of n-work 

from a BOP time history data file. Beyond the determination of n-work, the two models are 

different. For each value of n-work, INJURY calculates probability distributions for five levels 

of lung injury and the likelihood of lethality. There are no corresponding values of likelihood of 

lethality in ORCA. ORCA presently generates a table of five levels of lung injury, which 

correspond to five values of n-work. These five levels are not all consistent with the lung 

damage levels in INJURY. In ORCA, the five lung injury levels are presented for the purpose of 

display. The injury levels and corresponding incapacitation are determined by n-work at each of 

the five injury thresholds. The most obvious inconsistency in the ORCA lung injury levels is the 

highest injury threshold, the lethal lung damage threshold, at which ORCA assigns 100% lung 

injury at n-work = 2. In INJURY, the likelihood of lethality, at n-work = 2, is 47%. This 

obviously does not correspond to 100% lung injury. As a result, it is necessary that changes will 

be made to the five n-work thresholds, which correspond to five levels of incapacitation. For 

consistency, these thresholds should also correspond to the five levels of lung injury in ORCA. 

At present, the data file header consists of the number of lines of BOP time history data to be 

processed. In addition, time history data files must be read in units of milliseconds and pounds 

per square inch (psi). The BOP data is assumed to be in one of the four possible load (reflective) 

modes with no user option for the other three or for the two possible incident (side-on) modes. 

To increase the generality of the user input, the data file should begin with an options header. 

This header should include time units, BOP units, the target mass, and the ambient air pressure. 

In addition, it should be able to include target orientation and pressure gauge orientation (side-on 

or incident mode). However, the model is presently based on the configuration of an 

experimental thorax model with pressure gauges only on the anterior side. It is not clear that 

 3 Walter Reed Army Institute of Research (WRAIR). Applied Science and Engineering Technology; INJURY Software 

Computer Code, version 4.00 Beta; WRAIR: Washington, DC, 1995. 

4

 Jaycor. Applied Science and Engineering Technology; INJURY User’s Manual, version 4.00 Beta; Jaycor: San Diego, CA, 

1995. 
3

including other target orientations is presently feasible. In INJURY, side-on pressure gauge 

orientations are included, but the inputs are converted to the BOP as measured with experimental 

thorax-model pressure gauges. This means that it is assumed that the BOP always impacts a 

target that faces the blast source. 

It is easy to display the result, but it is difficult to make hard copies. Also, there is not presently 

a procedure for storing the inputs and results of a specific case study. If this capability is added 

to ORCA, a library for case studies could be developed in which several inputs or results could 

be directly observed. 

The five sections of this report address the five recommendations listed in section 1. The first 

consists of a discussion of the data file header in its present and expanded format. 

Section 2 begins with a discussion of the present analysis methodology. The strengths and 

weaknesses are described, and a new analysis model which eliminates the present limitations in 

the blast module of ORCA is introduced. 

Section 3 consists of recommended displays of new and modified features. This includes the 

specifics of the expanded data file header format. Also, it includes displays of the suggested 

target configuration options. Additionally, it presents suggested tabular and graphic displays of 

the improved analysis model, which is described in section 2. 

Sections 4 and 5 consist of a brief description of improvements, which would result in enhanced 

user conveniences. In section 4, development of an ORCA case-studies computer library is 

suggested. This would make it possible for the user to look up past studies instead of having to 

rerun them. In section 5, a procedure with which to easily generate sets of hard copies is 

recommended. 

2. New Injury and Lethality Algorithms 

New algorithms of lung damage and the likelihood of lethality have been developed for 

application in ORCA. This section briefly describes a comparison between INJURY and ORCA 

and the inconsistencies between the two models. It is assumed that where INJURY and ORCA 

disagree, that INJURY is correct because ORCA was developed with the methodology used in 

INJURY. The new formulations provide corrections to these inconsistencies. 

INJURY predicts the likelihood of lethality and the probability of five levels of lung injury for 

all levels of n-work. The levels of lung injury are none, trace, slight, moderate, and severe. The 

total probability of lung injury at any of these levels is the sum of the value at that level plus the 

value(s) of all levels beneath. The total probabilities are as follows: severe lung injury is the 

sum of the values of moderate, slight, trace, and none; slight lung injury is the sum of the values 

of slight, trace, and none; trace lung injury is the sum of the values of trace and none; and no 
4

lung injury is the value of no lung injury. The extent of lung injury, or lung damage, or loss of 

respiratory function is not addressed. Therefore, the lung injury labels are a qualitative 

designation of increasing levels of lung injury, or lung damage, or loss of respiratory function. 

Lung injury levels in ORCA are qualitative, as in INJURY, and not directly related to resulting 

incapacitation. Incapacitation is determined by five threshold values of n-work, which 

correspond to four of the five levels of lung injury that are found in INJURY, and a fifth level 

(lethal injury), which does not correspond to an injury level in INJURY. The five levels of lung 

injury and their corresponding values of n-work are trace-0.09, slight-0.25, moderate-0.60, 

severe-1.58, and lethal-2.08. These five levels of lung injury do not appear to be consistent with 

corresponding values of lung injury in INJURY. 

In developing more representative injury levels corresponding to n-work and the corresponding 

values of likelihood of lethality, it is suggested that these values be changed. The values of 

n-work correspond to the maximum values, in the INJURY lung injury probability distributions 

(found in figure 1), of trace, slight, moderate, and severe lung injury, and the n-work value 

corresponding to a 99% likelihood of lethality, in INJURY, for lethal lung injury in ORCA. The 

new values were derived from a consultation with Dr. Edward Davis of the U.S. Army Research 

Laboratory. 5 They are more representative of the loss of respiratory function corresponding to 

the likelihood of lethality at a given n-work level. The lung injury labels (trace, slight, moderate, 

severe, and lethal), the n-work levels, the corresponding levels of likelihood of lethality (lth), and 

the new suggested lung injury levels (L) are shown in table 1. 

These values of L are obtained by applying the probabilities (which are normalized to 1) of each 

level of INJURY, which corresponds to a specific value of n-work, to calculate an injury level in 

ORCA. In INJURY, each level of lung injury has a probability value ( Pi ) as follows: 

• P1 = probability of severe injury; 

• P2 = probability of moderate injury; 

• P3 = probability of slight injury; and 

• P4 = probability of both no injury and trace injury. 

For each value of n-work, the total probability of all levels of injury is 1: 

 1 P1 + P2 + P3 + P4 = . (1)

The equation for lung damage is as follows: 

L = 0.56 + 0.4* tanh(0.32* n − work) . (2) 

 5

 Davis, E. U.S. Army Research Laboratory, Aberdeen Proving Ground, MD. Technical consultation, 2002.
5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

N-WORK

TRACE / SLIGHT / MODERATE / SEVERE 

/ LETHALITY

 SEVERE P1

 MODERATE P2

 SLIGHT P3 

 TRACE P4

 LETHALITY : LETH-1+LETH-2

Figure 1. Lethality/lung injury levels vs. n-work. 

Table 1. Lung injury/lethality and n-work. 

Lung Injury N-Work Lth L 

Trace 0.10 0.0000 0.57 

Slight 0.40 0.0014 0.61 

Moderate 1.00 0.1400 0.68 

Severe 2.40 0.6100 0.82 

Lethal 6.60 0.9900 0.95 

The likelihood of lethality is also calculated in INJURY. The following equations, in which H

is the likelihood of lethality, have been developed in order to yield similar results in ORCA: 

For , n − work ≤ 0.5

H = 0. (3) 

For n − work >0.5,

tanh(0.42 * 0.3) H1 = n − work − , (4) 
6

Table 2 consists of selected levels of likelihood of lethality and the probability distribution of the 

four levels of lung injury (loss of respiratory function), in INJURY, as functions of n-work. 

Table 2. INJURY: n-work vs. lung injury. 

N-Work Lethality Severe 

 

Moderate Slight Trace + No Injury 

Total Injury 

Distribution 

(N) (Lth – 1 + Lth – 2) (P1) (P2) (P3) (P4) (P1 + P2 + P3 + P4)

0.01 0 0 0 0 1 1 

0.05 0 0 0 0.003 0.997 1 

0.1 0 0 0.0006 0.0464 0.953 1 

0.2 0 0 0.022 0.32 0.658 1 

0.4 0.001407725 0.006 0.218 0.579 0.197 1 

0.6 0.024834704 0.039 0.453 0.452 0.056 1 

0.8 0.072865096 0.108 0.585 0.29 0.017 1 

1 0.136675136 0.203 0.615 0.176 0.006 1 

1.2 0.208662512 0.31 0.583 0.105 0.002 1 

1.4 0.283260378 0.414 0.522 0.064 0 1 

1.6 0.356831498 0.511 0.451 0.038 0 1 

1.8 0.427189219 0.595 0.381 0.024 0 1 

2 0.493104182 0.667 0.318 0.015 0 1 

2.2 0.55394055 0.728 0.2626 0.0094 0 1 

2.4 0.609430137 0.778 0.216 0.006 0 1 

2.6 0.659542755 0.819 0.177 0.004 0 1 

2.8 0.70441027 0.852 0.1453 0.0027 0 1 

3 0.744276973 0.8791 0.1191 0.0018 0 1 

3.2 0.77946274 0.9012 0.0976 0.0012 0 1 

3.4 0.810333659 0.919 0.081 0 0 1 

3.6 0.837278422 0.934 0.066 0 0 1 

3.8 0.86068987 0.9453 0.0547 0 0 1 

4 0.880951268 0.955 0.045 0 0 1 

4.2 0.898426694 0.963 0.037 0 0 1 

4.4 0.913454894 0.969 0.031 0 0 1 

4.6 0.926345874 0.9744 0.0256 0 0 1 

4.8 0.937379569 0.9787 0.0213 0 0 1 

5 0.946806013 0.9822 0.0178 0 0 1 

5.2 0.954846537 0.985 0.015 0 0 1 

5.4 0.961695611 0.9875 0.0125 0 0 1 

5.6 0.967523047 0.9895 0.0105 0 0 1 

5.8 0.972476366 0.9912 0.0088 0 0 1 

6 0.976683167 0.9925 0.0075 0 0 1 

6.2 0.980253424 0.9936 0.0064 0 0 1 

6.4 0.98328163 0.9945 0.0055 0 0 1 

6.6 0.985848768 0.9953 0.0047 0 0 1 

6.8 0.988024097 0.996 0.004 0 0 1 

7 0.989866741 0.9966 0.0034 0 0 1 

7.2 0.991427094 0.9971 0.0029 0 0 1 

7.4 0.992748054 0.9975 0.0025 0 0 1 

7.5 0.993330385 0.9977 0.0023 0 0 1 
7

 ) 0.9417 *exp( ( 1)

2 H2 = − n − work + , (5) 

and 

H = H1 + H2 . (6) 

The first column consists of a range of values of n-work corresponding to trace or no injury and 

0 likelihood of lethality to nearly 100% likelihood of both severe lung injury and lethality. The 

second column is the predicted likelihood of lethality as a function of n-work. The third, fourth, 

fifth, and sixth columns are the probabilities of severe, moderate, slight, and trace and no lung 

injury. The last column is the sum of the four levels of lung injury; the total is 1. This is 

expected since the four levels represent all possible injury levels—none and trace, slight, 

moderate, and severe. 

Figure 1 is a graph generated from table 2. The probability distributions of the injury levels and 

the likelihood of lethality are graphed as functions of n-work. These are consistent with the 

values of lung injury probabilities and lethality likelihoods found in INJURY. The lung injury 

and lethality levels were obtained from extensive BOP studies applied to INJURY. In these 

studies, both ambient air pressure and target mass were varied for selected external BOP time 

history data files. The results provided a sufficient number of injury and lethality results to 

generate a smooth curve in the range of n-work corresponding to likelihoods of lethality between 

0 and almost 1. 

As part of this effort, an algorithm was developed, as previously described, in order to generate 

the INJURY values of likelihood of lethality as a function of n-work for application in ORCA. 

The lung injury levels, which correspond to the likelihood of lethality values, were applied to the 

development of an algorithm with which to calculate lung injury as a function of n-work. 

Table 3 consists of many of these results. Figure 2 is a graph generated from table 3. Lung 

injury and lethality are graphed as functions of n-work. 

The lung injury algorithm is the result of improved estimates. It is therefore assumed that they 

are well within error bar limits. This method of determining lung injury makes it possible to 

select a good estimate of lung injury for determining levels of incapacitation, which result from 

blast shock-wave impacts. 

Table 3 consists of lethality and lung damage likelihoods as a function of n-work. Figure 2 

graphs levels of lethality likelihood and lung injury as functions of n-work. The lethality 

likelihood levels are the same as those in INJURY for corresponding values of n-work. The 

levels of lung injury were developed as a result of consultations with Dr. Davis.5

 Note that the 

likelihood of lethality is nearly 0 at a lung injury level of 0.6, and that the likelihood of lethality 

is nearly 1 at a lung injury level of 0.95. 
8

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

N-WORK

LETHALITY / LUNG INJURY 

 LETHALITY

 LUNG INJURY

Figure 2. Lethality/lung injury vs. n-work. 

3. Data File Headers 

In order to implement the first objective, a list of user data file header input options would appear 

as a user menu. For each header input, a list of available units would appear to be selected by the 

user. After completing the selection of input options, the list would appear with all of the user 

selections. The user would have the choice of entering the selected options as the data file 

header or of changing any of the header-input options. The options would be entered by adding 

them as a header to the data file or perhaps as a separate header file. 

The list of header inputs would include the following user options: 

• OVERPRESSURE: (available units). 

• TARGET MASS: (number, available units). 

• AMBIENT PRESSURE: (number, available units). 

• TARGET ORIENTATION: (one of six possibilities). 
9

Table 3. Lethality/lung injury vs. n-work. 

 Lth-1 Lth-2 Lethality Lung Injury N-WORK

 Tanh(0.42*N – 0.3) 0.9417*Exp(– (N+1)^2) (Lth 1 + Lth-2) 0.56 + 0.4*Tanh(0.32*N) N 

0 0 0 0.561279996 0.01 

0 0 0 0.566399454 0.05 

0 0 0 0.572795633 0.1 

0 0 0 0.585565105 0.2 

–0.13123865 0.132646375 0.001407725 0.6109222 0.4 

–0.04796317 0.072797874 0.024834704 0.635869993 0.6 

0.035984456 0.03688064 0.072865096 0.660220163 0.8 

0.119427299 0.017247837 0.136675136 0.683802768 1 

0.201216439 0.007446073 0.208662512 0.706470476 1.2 

0.280292976 0.002967402 0.283260378 0.728101447 1.4 

0.355739852 0.001091646 0.356831498 0.748600815 1.6 

0.4268185 0.000370718 0.427189219 0.767900828 1.8 

0.492987967 0.000116215 0.493104182 0.785959821 2 

0.553906919 3.36308E-05 0.55394055 0.80276024 2.2 

0.609421153 8.98397E-06 0.609430137 0.818305992 2.4 

0.65954054 2.21542E-06 0.659542755 0.83261938 2.6 

0.704409766 5.04313E-07 0.70441027 0.845737846 2.8 

0.744276867 1.05974E-07 0.744276973 0.857710747 3 

0.779462719 2.05569E-08 0.77946274 0.868596299 3.2 

0.810333655 3.68105E-09 0.810333659 0.878458804 3.4 

0.837278421 6.08473E-10 0.837278422 0.887366221 3.6 

0.86068987 9.2847E-11 0.86068987 0.895388113 3.8 

0.880951268 1.30783E-11 0.880951268 0.902593966 4 

0.898426694 1.70055E-12 0.898426694 0.909051862 4.2 

0.913454894 2.0412E-13 0.913454894 0.914827468 4.4 

0.926345874 2.26171E-14 0.926345874 0.919983315 4.6 

0.937379569 2.31338E-15 0.937379569 0.924578306 4.8 

0.946806013 2.18429E-16 0.946806013 0.928667422 5 

0.954846537 1.90385E-17 0.954846537 0.932301587 5.2 

0.961695611 1.53183E-18 0.961695611 0.935527656 5.4 

0.967523047 1.13775E-19 0.967523047 0.938388496 5.6 

0.972476366 7.80074E-21 0.972476366 0.940923128 5.8 

0.976683167 4.93723E-22 0.976683167 0.943166922 6 

0.980253424 2.88461E-23 0.980253424 0.945151823 6.2 

0.98328163 1.55577E-24 0.98328163 0.946906584 6.4 

0.985848768 7.74575E-26 0.985848768 0.948457015 6.6 

0.988024097 3.55989E-27 0.988024097 0.949826229 6.8 

0.989866741 1.51031E-28 0.989866741 0.951034875 7 

0.991427094 5.91496E-30 0.991427094 0.95210137 7.2 

0.992748054 2.13842E-31 0.992748054 0.95304211 7.4 

0.993330385 3.94581E-32 0.993330385 0.953469943 7.5 

ORCA would have header file input codes with which to convert the selected units to the units of 

the existing ORCA code. Other inputs, such as target mass, ambient pressure, and target 
10

orientation, are presently fixed in ORCA. These sections of the code would need to be rewritten 

in order to accommodate the new user options. 

4. Display of New and Modified User Interface Features 

In order to introduce new display options, existing menus need to be expanded to include them. 

The additions and modifications include the data file header inputs and the contents of table 3 

and figure 2 (n-work as a function of lung damage levels—fractional loss of respiratory 

function—and the likelihood of lethality). They might also include the contents of table 2 and 

figure 1 (n-work as a function of the probability distribution of lung damage levels and the 

likelihood of lethality). This last option demonstrates clearly that there is a range of possible 

lung damage levels from a blast shock-wave impact on a soldier. The possible difficulty with it 

is that it was generated in INJURY for the values of n-work listed in table 1. If in ORCA, the 

lung injury probability distribution is to be displayed for a value of n-work, which is not included 

in table 1, there is no present algorithm to generate the results. If this is considered important for 

clarification, or some other purpose, one of two methods can be applied. The first is an 

embedded look-up table of n-work, as a function of the probability distribution of lung damage 

levels and the likelihood of lethality, which can be generated. It would be applied by displaying 

the closest value of n-work (in the look-up table) to the value obtained in ORCA. The second is 

an algorithm, which could be developed and added to ORCA. 

The data file header menu display would consist of two parts. The first is the user-options menu. 

The second is a display, as it will be applied to the BOP time history data file. The options menu 

might look something like the following: 

OVERPRESSURE (UNITS)

• kPa

• Pa

• 2 kg / m

• 2g / cm

• db

TARGET MASS (AMOUNT, UNITS)

• lb

• kg

• g
11

AMBIENT PRESSURE (AMOUNT)

• psi

• kPa

• Pa

• 2 kg / m

• 2

g / cm

• mm

TARGET ORIENTATION (OPTIONS)

• ANTERIOR (FRONT) 

• POSTERIOR (BACK) 

• LATERIAL-LEFT (LEFT SIDE) 

• LATERIAL-RIGHT (RIGHT SIDE) 

• SUPERIOR (TOP) 

• INFERIOR (BOTTOM) 

The suggested target orientation menu consists of six target-impact orientations with respect to 

the direction from the location of the blast shock-wave source. For a given BOP time history, 

left and right, front and back, and top and bottom cannot be assumed to yield the same levels of 

lung damage because of the nonsymmetrical structure of the lungs and the anatomical elements 

that surround them. 

This target orientation menu cannot presently be implemented because the present lung damage 

experimental target, from which the n-work algorithm was developed, has pressure gauges only 

on the front surface. Its implementation would require the design and fabrication of a new 

experimental target, an experimental program, and the development of five new n-work 

algorithms. 

Expanding the target orientation choices from one to six would be a significant improvement to 

ORCA. However, the most likely scenario is an oblique blast shock-wave impact. Therefore, 

BOP data from the pressure gauges on the surfaces of as many as three of the six principle 

directions would contribute to the determination of n-work. It is not clear if these BOP time 

histories would contribute independently or synergistically to the determination of n-work. This 

can be determined by an expanded experimental program, which would include oblique 

directions from the blast shock-wave source and the target. If this were implemented, the target 

orientation options menu would be replaced with a new user input, which includes the position 

and orientation of the target with respect to the blast shock-wave source. 
12

TARGET CONFIGURATION (OPTIONS)

• STANDING 

• LYING DOWN 

• SITTING 

• DESCRIPTIONS AND LOCATIONS OF NEARBY 

SURFACES 

At present, n-work is determined from the unlikely scenario of a perpendicular blast shock-wave 

impact on the front surface of a target. It does not include secondary reflected shock waves. At 

best, this can only result in a crude approximation of a blast shock-wave impact on a target. 

The target-configurations menu is important because of the secondary blast effects that can result 

from nearby surfaces and objects. Nearby surfaces and objects are sources of reflected shock 

waves. These reflected shock waves could be a significant added source of BOP shock-wave 

impact on the target. These added shock waves would impact the target from directions, and 

with BOP time histories, according to the locations, distances, orientations, structural 

configurations, masses, and sizes of the reflecting surfaces. 

The configuration of the target is also important. Standing, lying down, or sitting can alter the 

BOP time history characteristics of an impacting blast shock wave. 

The second display of the data file header options would include the same header titles and the 

options selected by the user. 

DATA FILE HEADER (OPTIONS)

• OVERPRESSURE (UNITS) 

• TARGET MASS (AMOUNT, UNITS)

• AMBIENT PRESSURE (AMOUNT) 

• TARGET ORIENTATION (OPTIONS) 

• TARGET CONFIGURATION (OPTIONS) 

Upon review, the user would have the option of applying the selected data file header options to 

the BOP time history data file or returning to the user-options menu. 

The n-work as a function of lung damage and lethality, could also be displayed in two parts—the 

graph in figure 2 and the specific values which correspond to the value of n-work calculated by 

ORCA for a given BOP data file and the user-selected header options. The display could be in a 

form like table 4. 
13

Table 4. Lethality/lung damage vs. selected n-work. 

Lethality Lung Damage N-Work 

(Lth) (L) (N) 

0.00141 0.611 0.40 

For n-work as a function of the lung damage probability distribution, the simplest way to add it 

in ORCA, as previously stated, is to use INJURY to obtain more n-work results. The results 

could be added to table 2. For any n-work result in ORCA, the values of the lung damage 

probability distribution, corresponding to the closest value of n-work in the augmented version of 

Table 2, would be displayed. The display could be in a form like table 5. 

Table 5. Lethality/lung damage levels vs. selected n-work. 

N-Work Lethality Severe Moderate Slight Trace + No Injury 

(N) (Lth) (P1) (P2) (P3) (P4) 

0.40 0.0014 0.006 0.218 0.579 0.197 

5. Need for Orca Model Case Studies Computer Library for Analysts 

In order to develop a library of ORCA studies, a system of computer files and folders can be 

developed, in which the results of these studies are stored and retrieved. A method with which to 

easily transfer ORCA inputs and results is an important element. Perhaps one or more menus 

with selection and transfer options would be very helpful to the analyst. As displayed inputs and 

outputs appeared on the monitor, the menu would be used to transfer selected items to existing 

folders and files or to create new folders and/or files. An option to transfer selected items to 

more than one folder and/or file should also be included. 

For each selected category, there should be one or more layers of more specific sublevels. An 

initial suggestion is to select the highest level categories as folder labels. The inputs to the 

development of a list of categories could begin with the following list: 

• blast shock-wave description, 

• target description, 

• blast/target interface description, and 

• blast/target environment description. 

The following list consists of some of the suggested files within each folder: 

• BOP time histories, 

• BOP header files, 
14

• configuration diagrams, and 

• descriptive text files. 

The folders and files would be generated and developed from three sources—ORCA library 

folder and file menus, external files, and library folder and file procedures for internal editing. In 

addition, an index with pointers would provide a relatively simple method with which to search 

for and access information addressing specific blast/target configurations. 

6. Need to Obtain Computer Hard Copy Outputs 

It is presently difficult to generate hard copies of selected displayed results of ORCA studies. 

Therefore, it is recommended that the method of selection and printing of results be simplified. 

There are so many displayed results connected to any ORCA study that a procedure of at least 

two steps may be necessary in order to select a desired combination of hard copies. These steps 

include generating temporary files by selecting from the displays in an ORCA study. These 

temporary files could be reviewed and changed until the desired combination of displayed results 

is achieved. A user-options menu could then be used by the user to print the selected results 

and/or to save them in a permanent file. A similar procedure could be applied to the case studies 

library previously described. 

A hard copy options menu could also be developed with which to generate view graphs, 

document sections, or selected hard copy pages from ORCA displays or from the case studies 

library. This menu would be included as one of the hard copy options menus. 

Sections 1 and 2 describe the most important of the suggested changes to the blast module of 

ORCA. They are also the easiest of the suggested changes to implement. The values of n-work 

in ORCA have been confirmed to be the same as those of INJURY for the same BOP time 

history data files. The improvements will result in a new calculation and display in ORCA—the 

likelihood of lethality, which results from the loss of respiratory function (lung injury), for each 

value of n-work. It will also result in the improved prediction of levels of loss of respiratory 

function, which is consistent with the corresponding levels in INJURY. 

The expanded header will add capabilities to ORCA in processing BOP time history files. It will 

also make it possible for the user to specify the ambient pressure and the mass of the target. 

The suggestions for an expanded analysis, including target orientations options and nearby 

reflective-surfaces options, are relatively ambitious. Their implementation would require the 

development of experimental targets, a matrix of experiments, and the development of 

mathematical models for application in ORCA. 
15

7. Conclusion 

The ORCA Modeling System is presently in its final stages of its validation, verification, and 

accreditation process. At the completion of this process, ORCA will be operational and usable 

for many types of casualty assessment studies. This report describes suggested improvements 

for the ORCA Modeling System blast module. Implementation of any or all of these 

improvements will simplify the procedures and expand the capability of the module. 

Some of the recommended changes are not presently applicable. For example, the target 

orientation options assume instrumentation and analysis applications, which don’t presently 

exist. 

If none of the additions and/or changes described in this report are implemented, ORCA will still 

be an important and useful tool. After a review of the recommendations in this report, the 

decision to implement none, or some, or all of them will be based on the merit, usefulness, cost, 

necessary recourses, required time, and future objectives to which ORCA could be applied. 
16

List of Abbreviations, Acronyms, and Symbols 

WRAIR Walter Reed Army Institute of Research 

ORCA Operational Requirement-based Casualty Assessment 

INJURY Blast Effects Injury Computer Software Package 

BOP Blast Over Pressure 

n-work Normalized work 

Lth Lethality 

L Lung injury 

P1 Probability of severe injury 

P2 Probability of moderate injury 

P3 Probability of slight injury 

P4 Probability of trace injury 

H Probability of lethality 

H1 First of two elements of probability of lethality 

H2 Second of two elements of probability of lethality 

tanh Inverse tangent function 

exp Exponential function 

kPa Kilopascal 

Pa Pascal 

kg/m2 Kilograms per meter squared 

g/cm2 Grams per centimeter squared 

db Decibels 

lb Pounds 

kg Kilograms 

g Grams 

Psi Pounds per square inch 

mm millimeters 
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