Transfer Orbit

Author: CislunarSpace
Website: https://cislunarspace.cn

Definition

A transfer orbit is an intermediate trajectory used to move a spacecraft from an initial orbit to a target orbit. In cislunar missions, transfer orbit design is one of the core problems in orbital mechanics, requiring trade-offs between transfer time and fuel consumption. The shape of a transfer orbit depends on the geometric relationship between the departure and target orbits, available thrust, and mission time constraints.

Key Elements

Basic Types of Transfer Orbits

Transfer orbits can be classified by thrust mode:

Impulsive Transfer: Assumes thrust is applied instantaneously, achieving orbital changes through a finite number of velocity increments $\Delta V$ . The classical Hohmann transfer is a two-impulse transfer representative, suitable for transfers between near-circular orbits.
Low-Thrust Transfer: Thrust is continuous but small in magnitude, resulting in a gradually evolving spiral trajectory. Electric propulsion systems commonly use this mode, offering higher fuel efficiency at the cost of longer transfer times.
Gravity-Assisted Transfer: Uses celestial gravity to change orbital energy and direction without additional fuel consumption. Lunar gravity assist is a key technique in cislunar transfers.

Energy-Time Trade-off

The core challenge in transfer orbit design is the Pareto optimality between energy and time. Transfer time $t_f$ and total velocity increment $\Delta V_{\text{total}}$ are inversely related:

\Delta V_{\text{total}} = \sum_{i=1}^{n} |\Delta \mathbf{v}_i|

where $n$ is the number of impulses. Generally, shorter transfers require larger $\Delta V$ ; conversely, slower transfers save fuel but extend mission duration.

Specifics of Cislunar Transfer

Key differences between cislunar and interplanetary transfers include:

Three-body gravity field: Both Earth and Moon gravity act simultaneously, requiring transfer orbit design within the restricted three-body problem framework
Lunar gravity assist windows: Precise control of perilune altitude enables significant orbital energy changes via lunar gravity
Multiple target orbits: Targets include DRO, NRHO, Halo orbits, each imposing different design constraints on the transfer orbit

Application Value

Transfer orbit design has critical applications in cislunar missions:

Crewed lunar missions: Transferring from Earth parking orbit to lunar orbit or the lunar surface
Cargo resupply missions: Delivering supplies to mission orbits like DRO or NRHO while optimizing fuel consumption to maximize payload mass
Constellation deployment: Using multi-launch combined with elegant transfer orbit designs to place multiple satellites into different target orbits
Emergency return: Free-return transfer orbits provide crewed missions the ability to safely return to Earth without additional propulsion

References

Hohmann W. Die Erreichbarkeit der Himmelskörper[M]. 1925.
Wei Z et al. Research on lunar gravity-assist injection into cislunar distant retrograde orbit families[J]. 2026.
Broucke R. Periodic orbits in the restricted three-body problem with Earth-Moon masses[R]. 1968.