SetDirectory[NotebookDirectory[]]
 Do[filep = OpenWrite["s_rk.txt", FormatType -> OutputForm],                        Do[Write[filep, "  ",
 SetPrecision[FortranForm[fun1[ t2,x]], 5],
 "  ", SetPrecision[FortranForm[x], 5]], {x, 0.001,
 k51[t2], 0.01}],
 Close[filep];{t2, 0.2, 0.5, 0.1}];