Zero-propellant maneuver

Zero-propellant maneuver (ZPM) is a spacecraft attitude-control technique rather than a commercial brand or corporate product. It uses momentum-storage actuators, such as reaction wheels or control moment gyroscopes, together with the spacecraft's natural environmental dynamics to accomplish rotational operations without expelling propellant. The approach is intended for spacecraft that need to change orientation, damp unwanted rates, or unload accumulated momentum while preserving the finite propellant supply carried on board. Spacecraft commonly use reaction wheels or control moment gyroscopes to rotate and point themselves. These devices are powered by electrical energy and do not consume propellant during ordinary operation. They are nevertheless subject to capacity limits. External disturbance torques, including gravity-gradient effects, solar radiation pressure, atmospheric drag, and other environmental influences, gradually add angular momentum to the storage devices. When the devices become saturated, they can no longer provide the required attitude-control torque. Conventional spacecraft then fire thrusters to unload, or desaturate, the stored momentum. Thruster firings consume a limited resource that is also needed for orbit maintenance and reboost operations. A ZPM addresses this constraint by selecting a non-eigenaxis attitude path that exploits the spacecraft's natural dynamics. Unlike a conventional eigenaxis maneuver, which follows the shortest angular route between two orientations while generally maintaining a fixed rotation axis and maneuver rate, a ZPM allows both the rotation axis and rate to vary over time. The resulting trajectory may cover a larger angular distance and take longer to execute, but it can avoid saturating the momentum-storage actuators and eliminate the need for propellant during the rotational maneuver. The same general optimization can also reduce, though not necessarily eliminate, propellant consumption on spacecraft that rely on thrusters; that related application is described as a reduced-propellant maneuver. The trajectories are generated by solving a nonlinear two-point-boundary-value optimal-control problem for a specified maneuver duration. The optimization incorporates the spacecraft's attitude dynamics, actuator limits, environmental disturbances, and the desired initial and final orientations. A useful analogy is a sailboat that changes course by working with the wind rather than using an engine: the route is not the shortest possible path, but it avoids expenditure of stored fuel. In a spacecraft, the environmental forces play the role of the wind, while the momentum-storage actuators provide the controllable mechanism for shaping the attitude path. The technique was demonstrated on the International Space Station in the mid-2000s. A 90-degree maneuver was conducted on November 5, 2006, in approximately two hours. A 180-degree maneuver followed on March 3, 2007, and took approximately two hours and 47 minutes. The associated optimal-control problems were solved using DIDO software. The ISS demonstrations showed that large-angle attitude changes could be performed without the conventional propellant expenditure associated with thruster-based momentum unloading. The underlying formulation was developed in academic work by Jesse Pietz, while Sagar Bhatt developed the 90-degree ISS demonstration as part of master's-thesis research.

The engineering rationale for zero-propellant maneuvers arises from the limited propellant budgets of spacecraft. Propellant is needed both for maintaining an orbit and for attitude-related tasks such as momentum unloading. Because it must be launched from Earth and cannot normally be replenished, propellant consumption directly constrains the number of reboosts, desaturation events, and other operations that a spacecraft can perform. Extending the useful life of a spacecraft therefore depends partly on reducing unnecessary propellant expenditure. Spacecraft rotational control is often provided by reaction wheels or control moment gyroscopes. These electrically powered momentum-storage actuators can point a spacecraft without firing thrusters, but they accumulate angular momentum when they counter persistent disturbance torques. Gravity-gradient forces, solar radiation pressure, atmospheric drag, and other environmental effects can gradually drive the actuators toward saturation. Once their storage capacity is exhausted, the spacecraft loses the ability to generate the required attitude-control torque through those devices alone. Traditional operations respond by firing thrusters to unload the stored momentum. The ZPM concept changes the attitude trajectory rather than attempting to force the spacecraft along a conventional shortest-angle path. Standard eigenaxis rotation generally uses a fixed axis and a fixed maneuver rate. That simple path can require the actuators to oppose environmental torques continuously, causing saturation even for relatively modest rotations. A zero-propellant trajectory instead varies the rotation axis and maneuver rate and deliberately uses the spacecraft's environmental dynamics. It is therefore usually a longer and slower path, but it can avoid the actuator saturation that would otherwise require a propellant-consuming unload. In mathematical terms, a ZPM is obtained by solving a nonlinear optimal-control problem with specified boundary conditions and a fixed maneuver end time. The solution provides a time-varying attitude and rate command that takes the spacecraft from its initial orientation to its final orientation while respecting the relevant dynamics and actuator constraints. The method is analogous to sailing against a wind by changing course: the vehicle does not follow the geometrically shortest route, but it uses available environmental forces instead of relying on an engine. The technique was developed and applied in research associated with spacecraft attitude control. Jesse Pietz formulated and solved the optimal-control problem used for the International Space Station demonstrations in a master's thesis. Sagar Bhatt developed the 90-degree ISS zero-propellant maneuver in another master's-thesis project. DIDO software was used to solve the optimal-control problems for the ISS operations. The first reported ISS demonstration took place on November 5, 2006, when the station completed a 90-degree rotation in approximately two hours. On March 3, 2007, the station performed a 180-degree maneuver in approximately two hours and 47 minutes. These demonstrations provided operational evidence that large attitude changes could be planned without using thrusters for the rotational maneuver. The broader principle can also be adapted to thruster-equipped spacecraft to minimize, rather than fully eliminate, propellant expenditure; this related approach is known as a reduced-propellant maneuver.

• 2007

  ISS performs a larger-angle demonstration

  [1]Zero-propellant maneuveren.wikipedia.org

  On March 3, 2007, the International Space Station completed a 180-degree zero-propellant maneuver in approximately two hours and 47 minutes.

• 2006

  First reported ISS zero-propellant maneuver

  [1]Zero-propellant maneuveren.wikipedia.org

  On November 5, 2006, the International Space Station performed a 90-degree zero-propellant maneuver in approximately two hours.

A fuel-conserving spacecraft attitude-control method that trades the shortest or fastest rotational path for reduced propellant use and reduced momentum-actuator saturation.

• Spacecraft attitude-control method

  Zero-propellant maneuver trajectory

  An optimized, non-eigenaxis attitude trajectory for rotating or stabilizing a spacecraft without firing thrusters. It uses momentum-storage actuators and the spacecraft's environmental dynamics, such as gravity-gradient, solar-pressure, or aerodynamic effects, to prevent actuator saturation and preserve propellant. The path generally varies its rotation axis and maneuver rate and takes longer than a conventional shortest-angle eigenaxis maneuver.

• Spacecraft maneuver optimization

  Reduced-propellant maneuver

  A related application of the same trajectory-optimization idea for spacecraft that still use thrusters. Rather than eliminating propellant use entirely, a reduced-propellant maneuver seeks to minimize the amount required for an attitude operation by selecting a path that works with spacecraft dynamics and actuator capabilities.

1. 1Zero-propellant maneuveren.wikipedia.org