F-35 Joint Strike Fighter pilots will have to wait until 2022 to fire the U.S. military's top close-air-support bomb after the Small Diameter Bomb II enters service in 2017, JSF officials explained.

The Small Diameter Bomb II (SDB II) is an upgrade from previous precision-guided air-dropped weapons because of its ability to track and hit moving targets from up to 40 miles. However, the F-35 will not have the software package required to operate the bomb loaded onto the fifth generation fighter until 2022, officials said.

The delay in getting the SDBII onto the F-35 will reduce the aircraft's ability to provide close-air support to ground troops. It plays a role in the debate over the aircraft's ability to adequately fulfill the mission of the A-10 Warthog if Air Force officials are allowed by Congress to retire the close-air-support aircraft.

Air Force leaders renewed their intent to retire the A-10 by 2019 in February with its budget proposal. Officials said the Air Force needs to transfer resources being used to support the A-10 over to the development of the Joint Strike Fighter. Air Force leaders have said the F-35 will be one of many aircraft that will backfill the A-10.

The JSF office has already discovered that the SDB II does not fit onto the F-35B -- the Marine Corps variant -- without modifications to the aircraft's weapons bay. The Pentagon is not in a rush to make those changes before the F-35B reaches initial operating capability this year because the weapon won't work until the right software package is installed.

"When we get to the Block 4's of the F-35s those are going to be great CAS (close air support) platforms -- when we get there. So we've got to continue to move down that with respect to the systems," Air Force Gen. Herbert "Hawk" Carlisle, Commander of Air Combat Command, told reporters March 6.

GPS and laser-guided weapons such as Joint Direct Attack Munitions have been around for decades, however, they have primarily been designed for use against fixed or stationary targets.

A key part of the SDB II is a technology called a "tri-mode" seeker — a guidance system which can direct the weapon using millimeter wave radar, uncooled imaging infrared guidance and semi-active laser technology.

"Really, in the close-in CAS fight, and the most challenging being danger close where you have adversaries and friendlies in very close proximity -- we have to be able to support the ground component at that point. We need the ability to deliver weapons rapidly. We need the high magazine, we need precision and we need to be able to control the yield," Carlisle said.

The SDB II recently completed successful live-fire testing and is slated to enter full-rate production later this year. Ultimately, the Air Force plans to acquire 12,000 SDB II weapons — which will enter service by 2017, service officials said.

Most of the testing of the SBD II thus far has been on an Air Force F-15 Eagle fighter jet, however the weapon has been fitted and tested on the F-35 Joint Strike Fighter. Engineers are also working on plans to integrate the bomb onto the F/A-18E/F Super Hornet and F-16 as well, Raytheon officials said.

The Air Force has done some electronics testing with the SDB II and the F-35A and done a successful "fit" test to ensure the weapon can be carried in the internal weapons bay by the aircraft, JSF officials said. However, the weapon will need the JSF program's 4a software drop before it can be operational on the F-35A – and that is not slated to happen until 2022.

The JSF program developmental strategy is, in part, grounded upon a series of incremental software drops," each one adding new capability to the platform. In total there are more than 10 billion individual lines of code for the system, broken down into increments and "Blocks," F-35 program office officials explained.

The Marine Corps short-take-off –and-landing variant of the JSF, the F-35B, is slated to reach operational status later this year with software block 2B. Block 2B provides basic close air support such as the ability to fire an AMRAAM (Advanced Medium Range Air to Air Missile), JDAM (Joint Direct Attack Munition) or GBU 12 (laser-guided aerial bomb), JSF officials said.

The Air Force plans to reach operational status with its F-35A in 2016 using the next iteration of the software, called 3i. Described as a technical refresh of Block 2B, 3i will also enable the aircraft to drop JDAMs, GBU 12s and AMRAAMs.

JSF officials point out that the F-35A will have substantial close-air support capabilities when it reaches full operational capability in 2018. This includes the ability to fire an internal gun and drop a range of munitions including AIM-9X weapons, AMRAAMs, GBU 12s, GBU 31s and the Small Diameter Bomb I.

The SDB II will be integrated with what's called JSF software Block 4a – a next-generation iteration of the software for the aircraft which service engineers are already working on.

Block 4 will be broken down into two separate increments; Block 4a is slated to be ready by 2021 or 2022 and Block 4B is planned for 2023. The first portion of Block 4 software funding, roughly $12 million, arrived in the 2014 budget, Air Force officials said.

Block 4 will also increase the weapons envelope for the U.S. variant of the fighter jet. A big part of the developmental calculus for Block 4 is to work on the kinds of enemy air defense systems and weaponry the aircraft may face from the 2020's through the 2040's and beyond, service officials explained.

-- Kris Osborn can be reached at kris.osborn@military.com