What occurs when the injector orifice size is reduced in an ideal hybrid rocket motor?

When the injector orifice size is reduced in an ideal hybrid rocket motor, the effect on the overall performance is primarily tied to the amount of oxidizer being introduced into the combustion chamber. A reduction in orifice size limits the flow rate of the oxidizer, which in turn influences combustion characteristics.

The total impulse of a rocket is a measure of the total change in momentum provided by the motor over the duration of its burn, driven by both the amount of propellant consumed and the specific impulse, which reflects the efficiency of the combustion. Reducing the injector orifice size while keeping the other parameters constant results in a decrease in the flow of oxidizer into the combustion zone. This reduction can lead to a relatively stable burn rate, meaning the total impulse remains constant, but the lower oxidizer flow also leads to a decrease in the average thrust produced during the burn, as the engine is not producing as much thrust at any given moment due to the limited oxidizer availability for combustion.

In summary, with a smaller injector orifice, while the total impulse can stay constant, the average thrust output decreases because of the limited combustion efficiency associated with the reduced oxidizer flow.