or accidents. New HH-60W helicopters are under development to replace both the "G" and "U" model Pave Hawks. HC-130N and HC-130P Combat King HC-130J Combat King II HH-60G and HH-60U Pave Hawk K – Tanker The USAF's KC-135 and KC-10 aerial refueling aircraft are based on civilian jets. The USAF aircraft are equipped primarily for providing the fuel via a tail-mounted refueling boom, and can be equipped with "probe and drogue" refueling systems. Air-to-air refueling is extensively used in large-scale operations and also used in normal operations; fighters, bombers, and cargo aircraft rely heavily on the lesser-known "tanker" aircraft. This makes these aircraft an essential part of the Air Force's global mobility and the U.S. force projection. The KC-46A Pegasus began to be delivered to USAF units starting in 2019. KC-10A Extender KC-46A Pegasus KC-135R and KC-135T Stratotanker M – Multi-mission Specialized multi-mission aircraft provide support for global special operations missions. These aircraft conduct infiltration, exfiltration, resupply, and refueling for SOF teams from improvised or otherwise short runways. The MC-130J is currently being fielded to replace "H" and "P" models used by U.S. Special Operations Command. The MC-12W is used in the "intelligence, surveillance, and reconnaissance" (ISR) role. Initial generations of RPAs were primarily surveillance aircraft, but some were fitted with weaponry (such as the MQ-1 Predator, which used AGM-114 Hellfire air-to-ground missiles). An armed RPA is known as an "unmanned combat aerial vehicle" (UCAV). MC-12W Liberty MC-130H Combat Talon II MC-130J Commando II MQ-1B Predator MQ-9B Reaper O – Observation These aircraft are modified to observe (through visual or other means) and report tactical information concerning composition and disposition of forces. The OC-135 is specifically designed to support the Treaty on Open Skies by observing bases and operations of party members under the 2002-signed treaty. OC-135B Open Skies R – Reconnaissance The reconnaissance aircraft of the USAF are used for monitoring enemy activity, originally carrying no armament. Although the U-2 is designated as a "utility" aircraft, it is a reconnaissance platform. The roles of the aircraft vary greatly among the different variants to include general monitoring, ballistic missile monitoring (RC-135S), electronic intelligence gathering (RC-135U), signal intelligence gathering (RC-135V/W), and high altitude surveillance (U-2) Several unmanned remotely controlled reconnaissance aircraft (RPAs), have been developed and deployed. Recently, the RPAs have been seen to offer the possibility of cheaper, more capable fighting machines that can be used without risk to aircrews.