English: Delta-v required for Hohmann (thick black curve) and bi-elliptic transfers (colored curves) between two circular orbits as a function of the ratio of their radii. The delta-v is normalized to the orbital speed in the initial orbit, and this is plotted against the ratio of radii of the final and initial orbits. For the bi-elliptic transfers the value of the apoapsis radius, normalized to the radius of the initial orbit, is shown next to the curve.