Moving U.S. Forces: Options for Strategic Mobility Section 12 of 12
February 1997

Appendix D

Details About CBO's Analysis

To compare the capabilities of various options for strategic mobility, the Congressional Budget Office (CBO) used two tools: a spreadsheet model called the Airlift Cycle Assessment System (ACAS) and a simulation called the Sealift Factors and Closure Approximation Tool (SeaFAXT). Those models allowed CBO to estimate the amount of time required to deliver cargo during the early part of two major regional conflicts.

Airlift Estimates

For the purposes of this study, the Department of Defense (DoD) provided CBO with estimates of airlift requirements for the halting phase of the most difficult scenario of the Mobility Requirements Study Bottom-Up Review Update (MRS BURU)--a major regional conflict on the Korean Peninsula followed shortly by another in the Persian Gulf region. Although two of the options in Chapter 6 preposition some of the equipment that would otherwise deploy by airlift during a conflict's halting phase, CBO did not adjust DoD's data on airlift requirements for any of the alternatives. Using those requirements, CBO estimated how much cargo various airlift fleets could deliver to the conflicts using the Airlift Cycle Assessment System.

ACAS is a deterministic spreadsheet model that the Air Force developed to quickly estimate airlift delivery capability. It uses standard Air Force planning factors that describe the average payload, speed, and maximum flying hours of airlift planes. ACAS uses those and other factors such as the availability of aircrews as inputs to a series of equations that calculate airlift cycles and delivery time.

The ACAS model does not simulate the loading, departure, and landing of individual planes from specific points of embarkation and debarkation as other, larger simulations do. Instead, it calculates how quickly a fleet can deliver a specific amount of bulk, outsize, and oversize cargo by distributing the weight among the airlift fleet based on the average payload of each type of aircraft. Since real-world airlift deliveries are constrained by the shape and volume of individual pieces of equipment as well as by total weight, the ACAS model may understate the amount of time required to deliver cargo. However, CBO calibrated its estimates using simulation results that DoD provided for airlift fleets with 120 or 140 C-17s.

Sealift Estimates

To assess how quickly different fleets of ships could deliver cargo to two major regional conflicts, CBO developed the Sealift Factors and Closure Approximation Tool. SeaFAXT simulates the flow of cargo on board individual ships between various ports of embarkation and debarkation, tracking their delivery time. By simulating individual ships, SeaFAXT allows analysts to look at how congested sealift operations might become if there were too few berths, or if ships were too long or had too deep a draft for a given port, or needed cranes and other equipment to unload their cargo.

Table D-1
Comparison of Assumptions in the MRS BURU and CBO's Analysis
Mobility Requirements Study
Bottom-Up Review Update CBO's Analysis
(2001 time frame) (2007 time frame)
Airlift (In primary  88 C-141s No C-141s
aircraft authorized) a 104 C-5s 104 C-5s
55 C-17s Number of C-17s varies
37 KC-10s 37 KC-10s
26 KC-135s 26 KC-135s
Option III includes 30 C-33s
Civil Reserve Air Fleet Stage II for one MRC Same as MRS BURU
Stage III for two MRCs
Land-Based Prepositioning Equipment at various sites Same MRS BURU, except 
that Option II would add
240,000 square feet to each site
Afloat Prepositioning 8 LMSRs for Army prepositioning Same as MRS BURU, except that
1 auxiliary crane ship  Options I and IV would add or
heavy lift ships remove one LMSR, respectively
miscellaneous containerships
lighter-aboard ships
3 Marine squadrons
Surge Sealift 8 SL-7 fast sealift ships Same as MRS BURU, except that
11 LMSRs Option V would add one LMSR
44 other RRF ships
Sustainment Shipping b 11 U.S.-flag vessels Same as MRS BURU
90 effectively U.S.-controlled ships
43 allied vessels
SOURCES: Congressional Budget Office and Department of Defense, Joint Chiefs of Staff, Mobility Requirements Study Bottom-Up Review  
                  Update (February 13, 1995). 
NOTE: MRC = major regional contingency; LMSR = large, medium-speed roll-on/roll-off ship; RRF = Ready Reserve Force; RORO =  
           roll-on/roll-off ship.
a. Includes planes withheld by the Joint Chiefs of Staff for other missions.
b. Assumes no maritime reform. Number of U.S.- flag vessels excludes ships that would be withheld for commercial purposes during wartime.

Like most simulations, SeaFAXT does not optimize: in other words, it does not pick the best type of ship for a given sealift load or for a given port or berth. Nor does SeaFAXT determine in what order units would be sent to a conflict. CBO relied on military planning guides, unclassified data from the MRS BURU, and historical experience to determine the inputs for SeaFAXT. CBO also calibrated its estimates with actual sealift deployment times observed during Operations Desert Shield and Desert Storm.

Assumptions Behind CBO's Estimates

To estimate delivery time, CBO's analysis followed the assumptions of the MRS BURU as closely as possible. For example, it phased in the number of airlift planes and aircrews available for deployment based on time-lines in the MRS BURU for reserve call-up, activation of the Civil Reserve Air Fleet, and the speed with which maintenance personnel can prepare military airlift planes for operations. When necessary, CBO used other information about the average capability of planes from Air Force Pamphlet 76-2, which lists standard airlift planning factors.

For assumptions about the number of planes that airfields in the Persian Gulf region and on the Korean Peninsula could service (that is, maximum on the ground, or MOG), CBO began with the most restrictive constraints that the Institute for Defense Analyses used in its 1993 study of the C-17.(1) CBO then tightened or loosened MOG values to match DoD's estimates for airlift fleets that included 120 or 140 C-17s.

For sealift forces, CBO followed the MRS BURU's assumptions about when ships from the Ready Reserve Force would become available at ports of embarkation, the number of berths and the amount of cargo processed per day at ports in the United States and at each regional scenario, and the amount of time U.S. forces would need to move equipment from their home bases to embarkation ports. When necessary, CBO used supplemental information from the Military Traffic Management Command's Logistics Handbook for Strategic Mobility Planning (94-700-2) of April 1994.

For the MRS BURU, DoD analysts assumed that the United States would face two major regional contingencies in 2001, and thus the authors projected what number of planes and ships would be available by then. As a result, the study assumed that the Air Force would have 88 C-141s and 55 C-17s (both in primary aircraft authorized) available for airlift missions, among other lift assets (see Table D-1). For the options in this study, however, CBO estimated the delivery times of airlift fleets that included no C-141s, replacing them entirely with some number of C-17s or a combination of C-17s and C-33s.

Similarly, CBO varied the amount of prepositioned equipment and the number of sealift ships that would be available to DoD. To identify specific units the military might preposition, CBO referred to DoD documents on units that would be sent to two major regional conflicts. In order to keep its analysis unclassified, CBO made subjective assessments about which units were likely to deploy during the halting phases of the conflicts. For its calculations, CBO did not include any equipment that the Army designates as inappropriate for prepositioning.

1. W. L. Greer, Cost and Operational Effectiveness Analysis of the C-17 Program, Report R-390 (Alexandria, Va.: Institute for Defense Analyses, December 1993), pp. D-22, D-23.

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