The Lunar Reconnaissance Orbiter (LRO) required an innovative and modular approach to the design and development of the electronics needed to control the propulsion and deployment components, as well as the electronics necessary to support safety inhibits for personnel and range requirements. Since these electronics would be designed in parallel with the systems they would interface with, they would need to be flexible enough to quickly accommodate ongoing design changes.
The solution to the problem was the development of the LRO propulsion and deployment electronics (PDE) system that consists of four identical modules and one inhibit unit (IU). The modules each have their own unregulated bus and 1553 interfaces. Each module has 15 commandable switches, which are used to operate the LRO actuators. The IU uses relays to interrupt power to the PDE modules and the RF transmitters until the spacecraft separates from the launch vehicle. The 1-shot PWB (printed wiring board) inside the IU operates the relays. The circuits are activated in flight by separation breakwires (SBW).
The PDE modules control various functions in both the propulsion and deployment subsystems. For the propulsion subsystem, the modules are responsible for actuating the four insertion thrusters (NT), eight ACS thrusters (AT), one high-pressure latch valve (HPLV), two tank latch valves (TLV), four manifold latch valves, and four NASA standard initiators (NSI) in the two pyro valves (PV). For the deployment subsystem, the modules are responsible for actuating the four nonexplosive actuators (NEA) in the two high-gain antenna release mechanisms (HGAR) and eight NEAs in the four solar array release (SAR) mechanisms.
The first step was to architect an electronics box that incorporated the control of the propulsion and deployment components as well as the safety inhibits, so the two sets of functions could stay closely integrated throughout the entire project lifecycle. The first set of functions was designed into a single electronics board that could perform a set number of different types of tasks that could then be copied if a greater number of tasks was needed in the future as the propulsion and deployment subsystems changed. Specifically, each PDE module was designed with its own communications, power, high-power propulsion components control, low-power propulsion components control, and deployment components control.
Each PDE module has the ability to control two high-power propulsion components, eight low-power propulsion components, and five deployment components. Additionally, each PDE module was placed inside its own chassis. Each chassis was designed to allow another chassis to be mechanically attached to it to add capability instead of having to redesign and reanalyze the mechanical structure each time the box needed to grow.
The second set of functions was designed into the IU to address the need for two additional independent inhibits of the propulsion, deployment, and RF transmitter systems on the LRO spacecraft. The IU design uses a mechanical relay on the hot side and another on the return side to interrupt power to the propulsion, deployment, and RF transmitter systems. The control for each of these relays is through a one-shot circuit board design that is activated in flight by detecting separation of the launch vehicle separation breakwires (SBWs). The same mechanical features from the PDE module chassis were designed into the IU chassis so it could also be mechanically attached to the PDE module chassis.
With this unique and innovative design, the project was able to simply add or subtract the quantity of PDE module electronics boards to accommodate the changing propulsion and deployment systems without any concern of redesign or reanalysis to the PDE.
This work was done by Jason Badgley, Brian Batovsky, Nathaniel Gill, Noble Jones, and Kenneth McCaughey of Goddard Space Flight Center. GSC-17037-1