Kerosene + LOX rockets have big, bright exhaust plumes, at least in the atmosphere; second stages are are a different matter: Why is the flame of the Falcon 9's 2nd stage (nearly) invisible?
But the kerosene burning Black Arrow looks spooky to me in that it seems to be hovering above the ground. Only after careful inspection can I detect a faint exhaust glow below it in the photo below.
Question: How did the kerosene-burning Black Arrow have transparent exhaust? What is it about the chemistry that makes the kerosene exhaust plume almost invisible?