if hydrogen is used in rockets and produces smoke then why it is called a clean fuel?
Gaseous or liquefied hydrogen is used in rockets.
It is the least polluting fuel that can be used in an internal combustion engine. It is potentially available wherever there is water and a clean source of power. The principle combustion product of hydrogen is water. Hydrogen combustion vehicles do not produce any carbon monoxide, hydrocarbons, particulates, sulphur oxides, ozone, lead, smoke, benzene or carbon dioxide or other ‘greenhouse gases’. The only pollutant of concern is NO which is formed, as in all internal combustion engines, from nitrogen taken from the air during combustion. It is thought that an ultra-lean engine would produce very little NO.