Unless you go through the ROTC program or an Academy, they won't train you to be an engineer. You can get good training to be a mechanic or any of many types of technician by simply enlisting.

Getting into an Academy is really tough, involves being sponsored by your congresscritter, and lots of other stuff. I haven't done it, and don't recommend it to anyone unless they want to be a career officer.

I spent three years in the NROTC program with the original goal of becoming an officer in the Marines. The application process is not unlike the college application process, and if you get in, you'll go to school with your tuition paid, and some other expenses covered. You'll wear the uniform two days a week, and will have some of your time devoted to military instruction while you are in school. You'll also spend two to six weeks each summer in full time military training.

In my case, I got up to the summer between my junior and senior years, when Marine candidates go to Officer Candidate School (Bulldog). Around the second week of OCS, I realized I was in a place I didn't want to be, and ended up leaving the program. Since the government had paid for my first three years in college, I was obligated to serve some active duty. I chose to lateral over to the Navy, and spent two years in the Seabees. I received about four months' training to be a mechanic, and then deployed to various locations fixing trucks and construction equipment.

Enlisted training can also be (depending on which service you join, ASVAB scores, length of comitment) a number of other jobs. You can learn to service aircraft, keep shipboard powerplants working smoothly, repair electronics, become a gunsmith (armorer), maintain computer networks, or just learn to live in the woods with a rifle and kill bad people. That last one tends to be difficult to transfer to the civilian world, but many police departments will look well upon it.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.