X-2
preliminary name for Spartan
X-3
preliminary name for high-performance third stage (Advanced Spartan)
XDC
control center for Nike-X system
xmit
transmit
xmsn
transmission
xmtr
transmitter
XO
executive officer; expenditure order
XRL
Extended Range LANCE
xstr
transistor
xtal
crystal
XYrDev
Ten Year Device