Cost-Effectiveness of Conventionally and Nuclear-Powered Aircraft Carriers
GAO/NSIAD-98-1 -- August 1998

UNDERWAY REPLENISHMENT EXTENDS THE ENDURANCE OF CARRIERS ========================================================= Appendix III The Navy operates a Combat Logistics Force fleet of about 40 ships that resupply combatant ships at sea with several commodities. The ships carry significant amounts of these commodities, for example, ship and aviation fuel (DFM and JP-5, respectively), ordnance, and other supplies such as ship and aircraft fuel, ordnance, and food (see table III.1), which enables combatant ships to operate at sea almost indefinitely, if required, without ever needing to go into ports to replenish their stocks. The force represents additional days of sustainability for the naval force by serving as an extension of the combatant ships' bunkers, magazine and store rooms. Table III.1 Capacities of Selected Combat Logistics Force Ships Other Class Speed Fuel\ a Ordnance supplies ---------------------- ---------- ---------- ---------- ---------- (knots) (barrels) (tons) (tons) (T-) AE-26 20 \b 6,000 \b (T-) AFS-1 20 18,000 \c 7,000 AO-177 20 150,000 625 420 (T-) AO-187 20 180,000 \c \d AOE-1 30 177,000 2,500 750 AOE-6 30 156,000 1,800 650 ---------------------------------------------------------------------- Note: T-class Combat Logistics Force ships are operated by the Navy's Military Sealift Command. These ships use civilian, instead of military, crews but may have a small military detachment aboard. A majority of the non-AOE class ships are now operated by the Military Sealift Command. \a Reflects a combined total for DFM and JP-5. \b Primary mission is ordnance replenishment. Limited quantities of fuel and other supplies are also available. \c No ordnance carried. \d Primary mission is fuel replenishment. Limited capacity to carry other supplies. Source: Navy. A comparison of these capacities with average daily ship and aviation fuel consumption and ordnance expenditures reflected in table III.2 shows that daily fuel consumption represents only a small percentage of the fuel capacity carried by Combat Logistics Force ships. Table III.2 Average Daily Fuel and Ordnance Consumption Rates for Selected Ship Classes DFM JP-5 Ordnance Ship class (barrels) (barrels) (tons) ---------------------------- ------------ ------------ ------------ Carrier (CV) 2,700 6,500 70-150 Carrier (CVN) \a 6,500 70-150 CG-47 725 \a \a DD-963 710 \a \a DDG-51 710 \a \a ---------------------------------------------------------------------- \a No quantities shown. Source: Center for Naval Analyses report.\1 The conventionally powered cruisers and destroyers that are a part of carrier battle groups are dependent on underway replenishment support by Combat Logistics Force. Compared to a conventional carrier, they have smaller fuel storage capacities and relatively high fuel consumption rates at higher speeds. Table III.3 compares the approximate range and endurance of these ships as well as of a conventional carrier. Table III.3 Battle Group Ship Range and Endurance at Various Speeds Speed (knots) 18 22 26 30 -------------------------------------- ------ ------ ------ ------ CV Range (nm) 8,600 7,800 6,300 5,100 Daily fuel consumption (percentage of 5 7 10 14 total load) Days endurance 20 15 10 7 CG-47 Range (nm) 6,200 5,600 4,600 3,300 Daily fuel consumption (percentage of 7 9 14 22 total load) Days endurance 14 11 7 5 DD-963 Range (nm) 5,100 4,800 4,200 3,300 Daily fuel consumption (percentage of 8 11 15 22 total load) Days endurance 12 9 7 5 DDG-51 Range (nm) 4,300 4,000 3,500 2,800 Daily fuel consumption (percentage of 10 13 18 26 total load) Days endurance 10 8 6 4 ---------------------------------------------------------------------- Note: Ranges rounded to nearest 100 nautical miles. Fuel consumption and days endurance rounded to nearest whole number. Source: Our analysis of Navy data. As shown in table III.4, the other ships in the battle group require a higher proportion of the fuel during a transit than a conventional carrier requires. Thus, from a practical standpoint, the time penalty for refueling is more associated with the rest of the battle group than with the conventional carrier. Table III.4 Battle Group Propulsion Fuel Underway Replenishment Requirements During Transits Underway replenishment requirement -------------------------- Percent of total ------------------ Transi Total Remainder t (barre of battle Transit distance (nm) speed ls) CV group\a ---------------------------------- ------ ------ ------ ---------- 4,800--Norfolk, Va. to the Eastern Mediterranean Sea ---------------------------------------------------------------------- 20 52,370 0 100 24 91,953 43 57 28 106,28 37 63 6 8,600--Norfolk, Va. to the Persian Gulf via the Suez Canal ---------------------------------------------------------------------- 20 158,65 25 75 6 24 158,65 25 75 6 28 236,27 34 66 6 12,000--San Diego, Calif. to the Persian Gulf ---------------------------------------------------------------------- 20 229,56 34 66 4 24 250,60 32 68 9 28 342,56 35 65 2 ---------------------------------------------------------------------- \a For this analysis, we used a battle group configuration that included one conventional carrier, two CG-47 class cruisers, two DD-963 class destroyers, and two DDG-51 class guided missile destroyers. This configuration is consistent with the Navy's standard carrier battle group. We also assumed that underway replenishments would occur when the ships' fuel levels reached 30 percent of capacity and that the ships were then refueled to full capacity. Source: Our analysis of Navy data. The presence of a station ship\2 in the battle group extends the group's range considerably compared to those shown in table III.3. Table III.5 reflects an AOE's impact on the ability of notional conventional (CVBG) and nuclear (CVNBG) carrier battle groups to reach their destinations. As in the previous analyses, these battle groups consist of a carrier (CV or CVN), two CG-47s, two DD-963s, two DDG-51s, and one AOE-1. As the table shows, the capabilities of the two groups are about equal. Table III.5 Battle Group Comparative Transit Capabilities With AOE Support (illustrative transit destinations and distances) Transi t Transit distance (nm) speed CV\a CVN\a ---------------------------------------------- ------ ------ ------ 4,800--Norfolk, Va., to the Eastern Mediterranean Sea ---------------------------------------------------------------------- 20 Yes Yes 24 Yes Yes 28 Yes Yes 8,600--Norfolk, Va., to the Persian Gulf via the Suez Cana ---------------------------------------------------------------------- 20 Yes Yes 24 No Yes 28 No No 12,000--San Diego, Calif., to the Persian Gulf ---------------------------------------------------------------------- 20 No No 24 No No 28 No No ---------------------------------------------------------------------- \a A "Yes" indicates that the battle group completes the transit with at least 30 percent propulsion fuel remaining, collectively. Source: Our analysis of Navy data. To further illustrate the information presented in table III.5, we compared the estimated remaining fuel levels of these battle groups when they reached their destinations, assuming that the battle groups sailed at a constant speed of 20 knots. We assumed that the ships would be fully refueled whenever they reached 30 percent of their fuel capacities. We also assumed that the diesel fuel marine (DFM) carried by AOEs represented 60 percent of their total fuel capacity. For example, the distance from Norfolk to the Eastern Mediterranean Sea is approximately 4,800 nautical miles and could be covered in about 10 days. The conventional carrier would have arrived with over 40 percent of fuel remaining and would not have needed refueling during the transit. The carrier could steam another 2 days at a constant 20 knots before reaching 30 percent of capacity (consuming about 6 percent of capacity per day). Once refueled the carrier could operate about another 12 days at a constant 20 knots before again reaching 30 percent. The destroyer and cruiser escorts of both battle groups would arrive with between 53 and 90 percent of their fuel remaining, depending on the type of ship and interval since their last at-sea refueling. The AOE supporting each battle group would have about 54,000 barrels of ship fuel remaining when arriving on station, if no other Combat Logistics Force support was provided. In another instance, the distance from Norfolk to the Persian Gulf is approximately 8,600 nautical miles and could be covered in about 18 days. The conventional carrier would arrive in the Persian Gulf with about 65 percent of its fuel remaining, having been refueled once during the voyage. We estimated that the carrier could operate another 6 days at 20 knots before reaching 30 percent fuel remaining. The AOE would have enough capacity to refuel the DDG-51s twice but could only refuel the CG-47s and DD-963s once, if not refueled itself during the voyage. The DDG-51s would arrive in the Persian Gulf with about 80 percent fuel remaining, while the CG-47s would have about 30 percent fuel remaining. The DD-963s would not be able to reach the Persian Gulf. In this case, either the AOE would need to be refueled or another oiler, such as a T-AO-187, would need to accompany the battle group. In the latter case, all combatant ships would reach the Persian Gulf with over 60 percent fuel on board, and the oilers would have over 55,000 barrels remaining. On this voyage, the carrier would require about 25 percent of the replenishment fuel, while the escorts would require the remainder. The CG-47s and the DDG-51s in a nuclear carrier battle group would arrive at nearly full fuel capacity, having been replenished two and three times, respectively, while the DD-963s would have about 65 percent fuel remaining. The AOE, however, would essentially be empty. We believe that on a voyage of this distance, either the AOE would be replenished itself at some point or another oiler would accompany the battle group. Additionally, the distance from San Diego to the Persian Gulf is about 12,000 nautical miles and could be covered in about 25 days at a sustained speed of 20 knots. With the refueling support of one AOE and no additional Combat Logistics Force ships, only the carrier in the conventional battle group would reach the Persian Gulf. It would have about 25 percent fuel remaining. None of the conventional battle group's escorts would reach the Gulf. In the case of the nuclear carrier battle group, the CG-47s would arrive with about 40 percent fuel remaining, and the DDG-51s would have about 15 percent fuel remaining. The DD-963s would run out of fuel before reaching the Gulf. A voyage of this distance would most likely require additional Combat Logistics Force support. If another oiler, such as a T-AO-187, accompanies each battle group, all the ships of both battle groups reach the Persian Gulf with no additional support provided. The conventional carrier would arrive with about 70 percent fuel remaining, while the escorts would have from about 40 to 85 percent fuel remaining. The conventional battle group's two oilers, however, would essentially be out of fuel, unless they were resupplied during the voyage. In this example, the conventional carrier required about 35 percent of the battle group's overall underway refueling requirement. In the nuclear carrier battle group, the escorts would also have between 40 and 85 percent of their fuel remaining, and the two oilers would have about 65,000 barrels remaining. -------------------- \1 Center for Naval Analyses Report 205, Sizing the Combat Logistic Force, June 1993. The Center for Naval Analyses used 1990 and 1991 fleet data contained in the Navy Energy Usage Reporting System for fuel consumption, and aircraft fuel and ordnance consumption based on the final days of Operation Desert Storm. \2 Station ships travel with carrier battle groups. They carry petroleum products, ordnance, and other supplies and are generally replenished by shuttle ships operating from land-based facilities worldwide.

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