[Shaheen Mongam (OP)]

United Launch Alliance (ULA) has launched the world's heaviest rocket of all the time last week. I observed that its having an almost smokeless exhaust compared to other rockets. Why is it so

[chiralSPO]

I believe (and wikipedia confirms: https://en.wikipedia.org/wiki/Delta_IV#Delta_IV_first_stage) that the first stage of a Delta IV rocket uses liquid hydrogen and liquid oxygen as propellants. This combination will only result in gaseous water (no smoke), which is also very hot, so it may have time to dissipate before condensing into vapors, which might be visible.