BiellipticTransfer

Bielliptic transfer, or bi-elliptic transfer, is an orbital maneuver used to change the radius of a spacecraft’s orbit around a central body by employing two elliptical transfer orbits and three engine impulses.

In a typical bielliptic transfer between two coplanar circular orbits, the spacecraft begins in a circular orbit of radius r1. The first burn places it on an elliptic transfer orbit with periapsis at r1 and apoapsis at a much larger radius rB. After coasting to rB, a second burn transitions the spacecraft onto a second elliptical orbit with periapsis at rB and apoapsis at the final radius r2. A final burn at r2 circularizes the orbit. Thus, three burns replace the two burns of a Hohmann transfer.

The total delta-v required by a bielliptic transfer depends on the chosen intermediate radius rB and the radii r1 and r2. For certain large radius ratios r2/r1, the bielliptic transfer can use less total delta-v than a Hohmann transfer. The break-even ratio is commonly cited around 11.9 to 12, meaning that when the final orbit is more than about twelve times the initial orbit, the bielliptic path may be more efficient. However, the choice of rB, thrust limitations, and perturbations can affect practicality.

Bielliptic transfers are mostly discussed in the context of low-thrust or long-duration missions within a single planet’s gravity well, where precise timing and propulsion capabilities permit the additional coast phase. They are less common for modest radius changes but remain a useful option in orbital mechanics for carefully planned, high-ratio transfers.