Modelling time travel in fiction

Here I go with every model of time travel I can think of.

Time travel is impossible

Fixed History model

The timeline is preserved through dumb luck

The timeline is actively protected by an intelligence of some kind

The timeline is preserved by quantum

Time travel isn't always possible

Malleable History model

Sensitive History model

Pause for thought: Why single-modifiable-history models don't work

Multiple History model

Time loops

Two histories create each other

More imaginative structures

Helix

Two timelines going past each other in opposite directions

One timeline, which suddenly reverses direction