This paper proposes a highly robust and low power flip-flop cell with complete double-node-upset (DNU) tolerance for aerospace applications. The proposed cell is constructed from a master latch and a slave latch. The master latch comprises two C-elements as well as one clock-controlled C-element; the slave latch is similar to the master but has an extra keeper to avoid high-impedance state of the output-level C-element. Simulation results demonstrate that the proposed cell can provide complete DNU-tolerance and also demonstrate that the proposed cell can reduce power dissipation by roughly 65% on average when compared with existing radiation-hardened flip-flop cells.