Divergences

(i) In "Common Time," Garrard makes an interstellar round trip with the Haertel overdrive and confers with Haertel on his return.

(ii) In "Nor Iron Bars," the Arpe drive replaces the Haertel drive.

(iii) In "This Earth Of Hours," in the further future, Terrestrials representing a Matriarchy and using the Standing Wave, which has replaced the two earlier drives, encounter the civilization at the galactic center, which is called the Central Empire.

(iv) In The Star Dwellers, in the near future, the Haertel drive is called the Standing Wave.

(v) In Mission To The Heart Stars, Terrestrials, representing a peaceful UN world government and using the Haertel drive/Standing Wave, encounter the civilization in the galactic center, which is called the Hegemony of Malis, and discuss Garrard's journey.

(vi) In "A Dusk Of Idols," in the further future, the events of The Star Dwellers are referred back to.

(vii) In A Case Of Conscience, in the near future, Terrestrials, representing a nuclear shelter-based UN world government, use the Haertel drive and discuss Garrard's journey.

Clearly, these works have both continuities and divergences:

(i)-(iii) are the Galactic Cluster trilogy;
(iv)-(vi) are the "Heart Stars" trilogy;
(vii) is Volume III of the After Such Knowledge Trilogy, which starts not with Adolph Haertel in the future but with Roger Bacon in the past.

Mission To The Heart Stars mentions every aspect of Garrard's experience:

his subjective time variations;
the "psuedo-death";
the mysterious aliens encountered.

Thus, appropriately, "Common Time" fits both as the opening story of Galactic Cluster and as a Prelude to a Heart Stars collection. "Common Time," the pivotal events of the two novels and "A Dusk Of Idols" set in a further future would make an impressive "future history" volume.