How does propulsion work?

Why are cryogens hard to handle?

Cryogens are substances that would be gasses at normal Earth conditions, but have been chilled and condensed to form liquids. They are hard to handle because they exist only at extremely cold temperatures. This means that special precautions must be carried out to protect the workers from these chemicals and to protect these chemicals from the heat in normal surroundings. Otherwise they will rapidly boil off and become useless for propulsion.

Substance Boiling Point Specific Gravity
Oxygen 93K (-297.3F) 1.1
Nitrogen 77K (-195.8C) 0.967
Hydrogen 20K (-252.8C) 0.0695

Another problem with cryogens is that they are essentially highly concentrated gasses. This means the chemical properties of the gas are magnified. Oxygen as a gas enables things to catch on fire. Liquid oxygen (LOX) combines with most organic material to form explosive compounds. There have been accidents where liquid oxygen was spilled onto asphalt which caused an explosion when a truck was driven over the spill. The small amount of heat and pressure caused by the tire was enough to trigger an explosion in that concentration of oxygen.

Nevertheless cryogens are often used in rocketry because the benefits in increased specific impulse and controllability outweigh the problems.

How are rockets designed?
What is a propellant?

What are the types of rocket propulsion?
What is chemical propulsion?
How do things burn?

What are some rocket propellants?
What is specific impulse?