HY steels are preferred for rocket motor casings because any increase in engine structure mass requires more propellant, which increases the overall rocket thrust needed. Lighter engine structures are desirable so the maximum amount of the total vehicle consists of useful payload and propellant rather than deadweight supporting structures. HY steel is preferred for rocket motor casings because solid rocket motors require high internal temperatures and pressures for efficient combustion and thrust, and HY steel can withstand these demanding conditions.

1. Why are HY steels preferred for rocket motor casings?
Solution
In most of thel rocket vehicles, the payload represents a very small fraction of the overall vehicle
mass at launch, most of the mass consisting of propellant and engine structure. Because any
required engine structure must be lifted into orbit along with the intended payload, any increase
in engine structure mass requires an increase in the amount of propellant used to launch the
vehicle. Engine structure is essentially payload that the engines must lift. Because of this, a rapid
increase in the amount of fuel required, and hence the overall rocket thrust required, occurs as
engine mass increases. A rocket with the lightest possible structure is desirable, so that the
maximum fraction of the total vehicle will consist of useful payload and propellant, rather than
supporting structures. These structures, such as engines and mechanical connections, are
essentially deadweight.So this is desirable to minimize the mass of support structure in the rocket
vehicle. Specifically, high internal temperatures and pressures are needed for proper combustion
and efficient thrust in a typical solid rocket motor. So HY steel is desirable.