Have you ever tried to balance a top without spinning it first? It tips over immediately under the influence of gravity. A similar thing happens to spacecraft under various influences such as:
- Gravitational gradients.
- Solar radiation pressure.
- Magnetic fields.
- Spacecraft thrusters.
A spinning top is much more resilient to tipping over from torque. This is because of the conservation of angular momentum. It can be considered the rotational analog of conservation of linear momentum: “An object at rest tends to remain at rest, an object in motion tends to remain in motion with the same speed and direction, unless acted on by a force.”
The Cislunar Explorers already need to spin in order to enable the propulsion system to function. So, we designed the spacecraft to take advantage of passive spin-stabilization and create synergy between the propulsion and attitude control systems.
When the Cislunar Explorers fire their attitude thrusters, they create a torque about their center of mass. If they were not spinning, they would enter a continuous tumble. Instead, the thruster torque tilts the spin axis, adjusting spacecraft attitude by a finite amount. The water sloshing about the propellant tank helps damp any nutation about this new spin axis that are caused by the sudden adjustment.
The necessary spin is effected by the twin spacecraft design. As shown in the figure below, the two L-shaped spacecraft launch stowed together as a 6U rectangle. A spring-loaded separation mechanism pushes them apart and spins them up on command.
The spacecraft are major-axis spinners, and with the effect of the sloshing water, all tumbles and nutations will tend to decay and reduce to a spin about the thruster axis.