We do have timed rockets. WGR210 rockets (found on the 109G14, 190A, and Bf.110 G2) explode like flack after approximately 1000 yards.
On the 262, you may be thinking of the R4M rockets on the Me. 262 A-2 (we have the A-1). There were 24, which were fired all at once into a bomber formation with a shotgun effect. 1 rocket was enough to bring down a bomber.