While some of Lieutenant Colonel Koler’s men were employed in evacuating the flood-stricken Vietnamese, others were modifying three of the squadron’s helicopters to carry a new weapons system which had been developed specifically for use on the UH-34D. The TK-1, an externally mounted combination of M-60 machine guns and 2.75-inch rocket launchers, was first used on 19 November in support of a Tiger Flight mission conducted just south of the Song Thu Bon about 17 miles from Da Nang. Two armed UH-34Ds expended 90 rockets and 500 rounds of 7.62mm ammunition on enemy positions during prelanding strikes. The effectiveness of the new system could not be determined after this particular strike, but an estimated 10–15 Viet Cong were killed in a similar action by the armed UH-34Ds the next day. The transport aircraft armed with the TK-1 would continue to escort troop carrying helicopters regularly throughout the remainder of the year. At best, however, the TK-1 was of only marginal value. The inherent limitations of the UH-34D, which possessed neither the maneuverability nor the speed to conduct truly effective attacks, reduced the overall value of the system. Because of these limitations the Marines seldom relied solely on the UH-34D for fire suppression during assault missions. The system would eventually be phased out in 1965 with the arrival of Marine jet attack squadrons in Vietnam. [Illustration: MAJOR MARINE HELICOPTER OPERATIONS SECOND HALF 1964 ] Two improvements, one in the physical facilities available to the task element and the other in the size and composition of its security detachment, were made shortly after the Marines began using the UH-34Ds in the gunship role. On 25 November, HMM-365 moved its aircraft and maintenance equipment across the airfield into a newly constructed hangar just west of the strip. The second change took place the next day when the security force from the 2d Battalion, 9th Marines was replaced by Company L, 3d Battalion, 9th Marines, reinforced with engineers, 81mm mortar teams, and counter-mortar radar personnel. This adjustment came in response to the reports of the growing Viet Cong threat to Da Nang. Designated the Security Detachment, Marine Unit Vietnam, the 255-man organization was under the command of Major William F. Alsop, the battalion’s executive officer. Captain John Sheridan, the company commander, retained tactical control of the infantry unit.