Re: 2 tires down, I must be doing something fundamentally wrong here...
When you say "tire" do you mean the rim/spokes? I've never heard of a tire being ruined by a chain coming off.
If you are trashing your rim then you have an alignment/tension issue with the drive chain, drive sprocket, tensioner and rear wheel sprocket. Yes, you can run without a chain tensioner, many have done it successfully IF you have a way of maintaining proper chain tension. That will be the rear wheel mounting, called drop outs, that allow you to move the rear wheel to adjust chain tension. Another deciding factor is if you have clearance between the chain and the bike frame. Use the search feature and read about chain tensioners. There is lots of information there and ways around the tensioner issue.
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