Question

"Hydrogen is used as rocket fuel " why???

Answer 1

ʟɪϙᴜɪᴅ ʜʏᴅʀᴏɢᴇɴ ʜᴀs ᴀ ʜɪɢʜ ᴄᴀʟᴏʀɪғɪᴄ ᴠᴀʟᴜᴇ & ʜᴇᴀᴛ ᴏᴜᴛᴘᴜᴛ ɪs ᴠᴇʀʏ ʜᴜɢᴇ ᴛʜʀᴏᴜɢʜ ʙᴜʀɴɪɴɢ.
ɪᴛs ʜɪɢʜ ʜᴇᴀᴛɪɴɢ ᴠᴀʟᴜᴇ ɢɪᴠᴇs ʙᴇᴛᴛᴇʀ ɪɢɴɪᴛɪᴏɴ ᴛᴇᴍᴘᴇʀᴀᴛᴜʀᴇ, sᴜғғɪᴄɪᴇɴᴛ ғʟᴀᴍᴇ sᴜsᴛᴀɪɴɪɴɢ ᴛɪᴍᴇ.
sᴏ, ʟɪϙᴜɪᴅ ʜʏᴅʀᴏɢᴇɴ ɪs ᴘʀᴇғᴇʀʀᴇᴅ.....

ʜᴏᴘᴇ ɪᴛ ʜᴇʟᴘs ʏᴏᴜ
...................................

Answer 2

Propellant is the chemical mixture burned to produce thrust in rockets and consists of a fuel and an oxidizer. A fuel is a substance that burns when combined with oxygen producing gas for propulsion. An oxidizer is an agent that releases oxygen for combination with a fuel. The ratio of oxidizer to fuel is called the mixture ratio. Propellants are classified according to their state - liquid, solid, or hybrid.
The gauge for rating the efficiency of rocket propellants is specific impulse, stated in seconds. 

Actually,initially liquid hydrogen and liquid oxygen is used. The liquid hydrogen reacts, very violently, with oxygen and the reaction becomes exothermic which results in lots of thrust and release of water which further acts as cooling agent!
But, due to less availability of liquid hydrogen now hydrogen gas is used. Its exhaust velocity is maximum in all the chemicals and so it has been chosen and used in preference to other fuels.

Hope this will help u.. :-)