NASA successfully conducted tanking demonstration tests for its artist 1 mission on September 21, but during the test it once again faced an issue that has been plugging the mission a liquid hydrogen leak. The second launch attempt had to be scrubbed because a liquid hydrogen leak appeared multiple times. If this fuel is so difficult to work with, why has it become the rocket fuel of choice for space expedition? Let's find out. Liquid hydrogen and liquid oxygen are both cryogenic gases, meaning that they can only be liquefied at extremely low temperatures. Due to this, the fuel poses enormous technical challenges. Liquid hydrogen must be stored at about minus 270 degree Celsius and should be handled with extreme care. The tanks of rockets fueled with liquid hydrogen must be insulated from all source of heat, including the exhaust from the rockets engine and air friction during flight. But despite these challenges, hydrogen offers some inimitable advantages. It is the element with the lowest molecular weight among all known substances and burn very intensely at higher than 2600 degree Celsius. When combined with liquid oxygen, liquid hydrogen yields the highest specific impulse with respect to the amount of the propellant consumed. According to NASA, one of the first and most important projects that texted using liquid hydrogen as fuel was conducted by the United States Air Force between 1956 and 1958. Even though very few people were aware of it, at the time and even now, the Air Force spent over $100 million on the project. That is more than a quarter billion in today's dollar. The test project was code named Suntan and it was an effort to develop a hydrogen fuel airplane that was aimed to be a successor to the Lockhead U2, which was the 5 plane of choice for the Air Force at the time. Even though the project was cancelled before completion, it led to the development for the first rocket engine that flew using hydrogen. That's all for now. Keep watching news site for further updates. Thank you.