Why did we choose water propulsion?
The easiest propulsion systems to flight qualify are those with low thrust, minimal explosion hazard during launch, minimal flammability, and minimal toxicity. When we designed the propulsion system for OCSD, we borrowed the idea behind Hero’s steam thruster from the ancient Greeks. Water has a low molecular weight, a roughly 100-second specific impulse at a toasty 105o F, and a vapor pressure significantly below 1 atmosphere at typical launch range temperatures. This eliminated the need for pressure vessels, further simplifying the launch qualification process.
Why are we using both closed- and open-loop controls?
Our laser communications system requires a 0.175-degree spacecraft-to-ground-station pointing accuracy, which is much tighter than the 1-degree accuracy that we demonstrated previously in CubeSats. To minimize risk, we are using two independent attitude sensors to allow operation in either a closed-loop or an open-loop mode. Either system is capable of achieving mission goals, but this offers us an opportunity to test each system and to determine their applicability to future missions. In the closed-loop control mode, we expect the system to provide better than 0.1-degree pointing during an overhead pass. The open-loop pointing system is our own new design that relies on cameras that take images of star fields at approximately once per second. We expect these images, combined with data from on-board rate gyros, will provide our satellites with continuous attitude information with less than a 0.1-degree error.
How do we track the satellites?
Of course, successful optical communication requires a compatible optical ground station. The Aerospace Corporations operates two ground terminal telescopes (30-cm and 80-cm diameters, respectively) at Mt. Wilson, CA. An infrared uplink laser beacon on the ground station provides the signal for closed-loop tracking of the ground station by the satellite, which we will test prior to the open-loop, star tracking mode. In the long term, the open-loop mode could reduce ground station complexity and cost.