ANSWER: Only 1
Explanation:
- Gaseous propellants have to be compressed to very high pressures or cooled to very low temperatures in order to achieve a high density.
- Propellants that are usually gases at room temperature but become liquids when cooled to very low temperatures to achieve the high density are called cryogenic liquid propellants.
- Cryogenics is the science and technology of temperatures below 120 Kelvin (-153° Celsius), the limit being defined by the boiling point of methane, a principal component of natural gas.
- The most common cryogenic propellants used in rocket engines are liquid hydrogen (LH2), which liquefies at -253° C, as the fuel and liquid oxygen (LOX), which liquefies at -183° C, as the oxidiser.
- Propellants like oxygen and hydrogen in liquid form give very high amounts of energy per unit mass.
- Hydrogen and oxygen are extremely clean fuels, when they combine they give out only water.
