Technic History Novels

There are 12 and they form a pattern:

3 with van Rijn, 1 without;
3 with Flandry, 1 without;
3 with Flandry, 1 without.

This establishes first that there are 2 major continuing characters and secondly that 3 entire novels feature neither. We may add that the 2nd and 3rd novels with van Rijn also feature his protege, Falkayn, another major continuing character, and that the last novel with Flandry is mainly about his daughter, potentially a new continuing character. Thus, there are at least 2 other important characters. In fact, there are several more but I am just seeing what can be learned by skimming through the novels. To be more precise about the pattern:

3 with van Rijn, 1 between van Rijn and Flandry;
3 with Flandry, 1 contemporary with Flandry;
3 with Flandry, 1 post-Flandry.

This establishes that the novels alone cover 4 periods. The novel contemporary with Flandry introduces the pro-Terran human Desai and the anti-Terran Chereionite Aycharaych. Both later interact with Flandry. Desai expounds to Flandry the theory of Empire which Aycharaych, Flandry's opponent, probably applies to subvert the Empire. Thus, another two important characters.

In fact, the more numerous shorter works in the History add several more periods: 

3 before van Rijn;
3 after Flandry;
a total of 5 between.

(By the 5 between, I mean the Troubles, the early Empire, 2 stages of the colonisation of Avalon and the war between the Empire and Avalon.) It all looks rather symmetrical but perhaps only because readers seek order where none was planned?