Final answer:
The query is related to aerospace engineering, focusing on the limitations and management of fuel in an aircraft or spacecraft's main and external tanks. Design choices, such as the sequence of using liquid and solid fuels, affect performance and safety.
Step-by-step explanation:
The subject in question is related to aerospace engineering, specifically dealing with the design constraints of aircraft or spacecraft fuel systems. When considering the limit on fuel in the main (fuselage) tank with fuel in external tanks, there are several factors to take into account. Aircraft and spacecraft might have different fuel systems, with some using a combination of internal and external tanks. The distribution of fuel, and when and how fuel is used from these tanks, are crucial for the performance and safety of the vehicle.
For aircraft, external fuel tanks can extend the range but come with trade-offs like increased drag and weight. They are typically used up first to allow for their drop if necessary. For spacecraft, the sequence and timing of engine firings are critical. In a hypothetical spacecraft, engines firing at 3 seconds (green flame) might use a liquid fuel/oxidant mixture from internal tanks, while another set of more powerful engines that fire at 4 seconds (yellow flame) might use a solid fuel mixture, perhaps from drop-away boosters. These design choices would be factored into the overall fuel limit considerations.