The most efficient way to transfer between circular orbits of similar sizes is the Hohmann transfer orbit, requiring two burns. For larger orbit changes, the bi-elliptic transfer maneuver may be more efficient, involving two half ellipses outside the target circular orbit. The radius relationship between the inner and outer circles determines the efficiency of the bi-elliptic transfer orbit. The sudden curvature change at the furthest point of the elliptical orbits, shown in a graph, is notable. The Hohmann transfer orbit is a special case of the bi-elliptic transfer orbit. The accuracy of the graph has been compared to other sources, highlighting the maneuver’s practicality over aesthetic representation.
https://www.johndcook.com/blog/2024/12/03/bi-elliptic-transfer/