CisLunar Explorers: System Design
The centerpiece of the Cislunar Explorers program is the use of water as a green, dense, and effective propellant.
Water doesn’t burn, of course. But its constituents do. Hydrogen and oxygen have been used in rockets from the early days of space exploration, including the upper stages of the Saturn V used in the Apollo program. Historically, the two are stored separately on the launch vehicle or spacecraft, as cryogenic liquids. To carry enough hydrogen, in particular, necessitates the use of extremely high pressures and extremely low temperatures. It takes a complex apparatus to achieve this: pumps, pressure vessels, and heavy insulation to keep the storage units cold.
There is another way.
Zapping H2O with electricity can overcome the bond between hydrogen and oxygen, decomposing the liquid into a gaseous mixture that readily combusts.
Instant rocket, just add water!
In a space environment, the water will float around the propellant tank and mix with the gas and spoil any attempts to . To fix this problem, we spin the spacecraft like a top around its thruster, flinging the water out like a centrifuge and helping to separate inert from electrolyzed propellant.
3D Printed Titanium Nozzle, Electrolysis Propulsion Prototype
Unlike many other spacecraft, the Cislunar Explorers do not require extremely accurate knowledge of their positions relative to the Earth in order to achieve total mission success. While Apollo and nearly all other lunar missions required an extremely precise orbit, the Cislunar Explorers must “simply” get any lunar orbit and maintain it for as long as possible. These unique mission requirements allow for a very unique method for navigation.
Rather than using Earth-based ranging facilities for navigation (as nearly all other deep-space missions require), the Cislunar Explorers will navigate completely autonomously. After being deployed from the launch vehicle, the Cislunar Explorers will each turn on their commercial-off-the-shelf cameras that enable them to view their surroundings. The view will be dominated by three things: the Earth, the Moon, and the distant Sun. By studying the sizes of each of these objects and their locations relative to one another, the Cislunar Explorers will deduce their locations for themselves. They will not be able to do so with the same level of accuracy that would be expected from Earth-based ranging facilities, but they will do so well enough to navigate themselves to lunar orbit. This technology could enable more deep-space and low-cost cubesat missions.
6U Structure Separation
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 mutations will tend to decay and reduce to a spin about the thruster axis.