Final answer:
The duration that a refueling vehicle engine can operate during in-shelter refueling operations with closed aircraft doors depends on specific regulations, which vary. Therefore, without exact guidance, a specific time cannot be determined, and operators should consult relevant regulations or procedures.
Step-by-step explanation:
The question of how long the refueling vehicle engine should be allowed to operate during in-shelter refueling operations with the aircraft entry doors closed is related to safety protocols and procedures in an aviation or aerospace engineering context. While the exact duration can vary depending on the specific regulations and standards set by aviation authorities or military specifications, there is a general concern about the accumulation of potentially hazardous fumes and ensuring adequate ventilation.
The hint provided suggests calculating the operation time based on a 12-hour continuous operation or a reduced power operation over a 24-hour period. However, without exact regulatory guidance or operational standards mentioned in the question, it is not possible to give a specific number of minutes for engine operation. Instead, it is crucial for operators to consult the appropriate regulations or standard operating procedures to determine the safe operational limits in their specific situation.