Drawbacks of Cryogenic Propellants
Boil off Rate
Since these propellants are in extremely low temperature conditions they are very hard to handle. They must be protected from heat so as to prevent boiling of gases. When liquid propellants are stored at temperatures above their boiling point they vaporize. If these vapors are contained in a tank, then the pressure increases with temperature.
Since the tank weight increases with design pressure, a pressure relief valve is generally provided to prevent the tank from over pressurizing and exploding. When the relief valve releases the pressure, some of the propellant escapes from the tank. This lost propellant is referred to as boil-off loss. and proves of no use for propulsion.
For liquid hydrogen the boil off rate per day is 0.127% while that of liquid oxygen is 0.016%. Thus the boil of rates of liquid hydrogen per month is 3.81% while that of liquid oxygen is 0.49%. Boil off rate is governed by heat leakage and by the amount of propellant in the tanks. With partly filled tanks, the percentage loss is higher. Heat leakage depends on surface area, while the original mass of propellant in the tanks depends on volume. So by square/cube law smaller the tank, the faster the liquids will boil off.
To lower the boil off rates, we must protect the propellant tank from the heat of the sun. Tanks at more distance from the sun have lower boil off rates. Lower boil off rates can also be ensured if tanks are maintained at an angle by which their cross sectional area exposed to sun is minimum or tanks are protected by a sunshade.
Highly reactive gases
Cryogens are highly concentrated gases and have a very high reactivity. Liquid oxygen, which is used as an oxidizer, combines with most of the organic materials to form explosive compounds. So lots of care must be taken to ensure safety.
One of the most major concerns is leakage. At cryogenic temperatures, which are roughly below 150 degrees Kelvin or equivalently (-190oF), the seals of the container used for storing the propellants lose the ability to maintain a seal properly. Hydrogen, being the smallest element, has a tendency to leak past seals or materials.
Hydrogen can burst into flames whenever its concentration is approximately 4% to 96%. It is hence necessary to ensure that hydrogen leak rate is minimal and does not present a hazard. Also there must be some way of determining the rates of leakage and checking whether a fire hazard exists or not. The compartments where hydrogen gas may exist in case of a leak must be made safe, so that the hydrogen buildup does not cause a hazardous condition.
Due to cryogenic propellants, various significant thermal stresses are introduced into the launch vehicle. These stresses can damage the structural integrity of the vehicle. Hydrogen reacts with certain materials to alter its grain structure causing it to become brittle, in a process known as hydrogen embrittlement
Zero Gravity conditions
It is difficult to store cryogenic propellants in zero gravity conditions. Generally, the propellants are cooled continuously, allowing excess heat to be carried away as the gasses boil off. The vapors of the gas are vented away. But in zero gravity conditions, the liquid may flash into vapor, and go away from the nozzle that dumps the gas overboard. This pushes the propellant in liquid form, out of the nozzle resulting in wastage of large amounts of fuel into space.
The cryogenic propellants certainly have their own disadvantages. But their advantages outweigh the disadvantages by far. Thus they are preferred for use in rockets.
The next liquid propellant is hypergolic propellant.