WASHINGTON — US Army acquisition chief Heidi Shyu, who has played a major role in the service's weapon systems procurement branch for five years during a tumultuous time, announced she will retire her retirement at the end of January in an email to her staff sent earlier this week.

"This was not an easy decision for me to reach," she wrote in the email obtained by Defense News. "It has been a great privilege to lead the outstanding men and women of the Army's acquisition workforce." Her last day is Jan. 31.

Shyu's retirement, first reported by Inside Defense on Wednesday, falls on the heels of several other major Army leaders' departures. Army Chief of Staff Gen. Ray Odierno retired at the end of the summer and Army Secretary John McHugh announced his departure not long after, delivering his swan song at the Association of the US Army's annual meeting in Washington last month.

Shyu, a former Raytheon executive, has earned a reputation as a defender of the service's science and technology seed corn at a time when the defense budget has repeatedly been subjected to drastic cuts that threaten modernization and technology development accounts.

Over her years as chief, she has grown more and more vocal with her warnings against senseless, across-the-board cuts due to sequestration and regularly called for more budget certainty.

Defense acquisition analyst Jim McAleese, of McAleese and Associates, in a memo on the "near-simultaneous retirements" of both Shyu and the Air Force's acquisition chief, William LaPlante, outlined the roles she played in major acquisition efforts.

The Air Force announced LaPlante's retirement Wednesday. He plans to rejoin MITRE, a not-for-profit organization that operates federally funded research and development centers.

McAleese notes that Shyu oversaw Lockheed's $9 billion Sikorsky acquisition that just closed and observes she was a "staunch protector of [research and development investments into [Future Vertical Lift, the Improved Turbine Engine Program, and] Armor and Lethality upgrades."

Shyu will be staying on long enough, McAleese points out, to oversee both the Joint Light Tactical Vehicle and Common Infrared Countermeasure protest decisions, both due in December.

Shyu also played a vocal role in driving the 2016 National Defense Authorization Act acquisition reform debates. She told Defense News prior to AUSA last month that she was "most concerned" about an acquisition reform policy that would force a program manager to stay in the position through a milestone, which she said would "torpedo his career."

Shyu is also famous for her analogy to describe the difficulties within the defense acquisition process, equating it to a bus where every passenger everyone on the bus(each stakeholder) has a brake and a steering wheel. Her point was to stress that each stakeholder has an agenda and these agendas might not align with program goals.

With everyone on the bus trying to drive, inevitably the bus "flips over" and the program is derailed, she said at a Senate Armed Services Committee hearing earlier this year.

Once Shyu leaves her post, the pool of possible replacements is murky. Shyu's principal deputy Gabe Camarillo would have been a logical choice, at least to take over as the acting acquisition chief, but he's nominated to be the next assistant secretary of the Air Force's Manpower & Reserve Affairs. He's been waiting for Senate action to approve his new job since the spring.

And finding a permanent replacement for Shyu in 2016 could be difficult. Center for Strategic and International Studies analyst Ray DuBois told Defense News that "as we go into the final year of a President's Administration, usually few [Presidential Appointee with Senate confirmation]s get confirmed."

Twitter: @JenJudson