>>but I'm curious about the phrase...'let rip'<<

I'm with you, etaoin. The phrase is familiar, but the use is not. It may be a question of agency, 'to let rip,' 'let her rip,' 'to rip a f*rt,' all involve the action of an agent on something else. In this case, that something else is, probably unintentionally, given as the agent itself 'a flare let [itself] rip,' rather than, '[the sun] let rip [a flare]'. Or?

But the image is approriate. Here are the results of a quick Sherlock search:

\Rip cord\ (A["e]ronautics) A cord by which the gas bag of a balloon may be ripped open for a limited distance to release the gas quickly and so cause immediate descent.

to rap out, to utter hastily and violently; as, to rip out an oath. [Colloq.] See To rap out, under Rap,

2. To hasten. [Obs.] --Piers Plowman.