There's a massive difference between HDPE (which is a hydrocarbon) and PVC (which contains chlorine and is awful to burn). Might be some plasticizers in the mix as well, but mostly they're just H, C, and O. Cl makes the real nasty stuff.

Rocket fuels can have some pretty nasty stuff in them as well, of course...

Some plastics can burn cleanly.

The whole "don't burn plastic" only applies if you don't know what type the plastic is.

I see you've bought into the whole "plastics = bad" trend, but not all plastics are the same.

Pure HDPE is just H and C, basically like candle wax with a much higher molecular weight. Products of combustion are H2O and CO2.

Chlorinated and fluorinated plastics are the ones that produce lots of toxic emissions when burned.

Why is burning plastic worse than the existing fuels? Pretty much all fuel mixes for rockets are nasty stuff.

Most fuel mixes for rockets are just Kerosene and Liquid Oxygen, at least in the USA. Seems like most of the next generation are aiming for Methane and LOX.

Burning plastic incompletely would most likely produce more noxious stuff than Kerolox or Methalox engines. Maybe on par with existing Solid Rockets or Hypergolics.

Rockets leave the launch site very quickly. And they launch from the shore towards the ocean.

No one is allowed to be near the launch path or within 2-3 miles of the launch.

What's the difference to other fuels? Except for few plastic is actually much better...