The military drives use a special military fuel instead of just hydrogen fuel, so you probably just don't have any of the correct type of fuel. (MD's also produce radioactives, which are illegal some places, like at sol, so consider that when you buy one)
Don't even TRY combat until you can successfully manually navigate a complete jump, from take off, thru hyperspace, to docking, without using the autopilot even once. And the docking should be at a rotating space station. That is the MINIMUM competancy necessary for combat.
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