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Solid fuel ramjets (SFRJ) are a hybrid between solid rocket engines and liquid fuel ramjets (LFRJ). They are simpler and more contained than LFRJs (just like solid rocket engines). They are also more fuel efficient than solid rocket engines (just like LFRJs). Since they are a type of jet engine, they can only operate within Earth's atmosphere. Therefore, they can be good to use on any current system that has rocket engines, but never leaves the Earth's atmosphere. An example could be missiles on military aircraft.