Sierra Space Net Launcher
Capstone Project
Orbital space debris poses a significant threat to current and future space missions, from damaging satellites to hindering the ability for future space travel. Sierra Space tasked the team to develop a Net Launcher to identify, capture, damp, and retrieve space debris of varying sizes. This project contains several mechanisms, sensors, and control systems to enable net capture of an unresponsive satellite.
My Components: Damping Subsystem
The damping subsystem assembly includes the Hatch Door Assembly, Linear Assembly, Rotating Assembly, Restraining Hook Assembly, and Burn Wire Assembly. The purpose of the damping subsystem is to dampen the rotation of the RSO around the tether axis (the major axis of the net launcher assembly). The Linear Assembly is designed to translate up and down, and it uses springs to increase or decrease the frictional force between itself and the Rotating Assembly through a frictional ring of HDPE plastic. The Rotating Assembly is able to rotate around and translate with the Linear Assembly. The Hatch Door Assembly rotates and translates with the Rotating Assembly, and it acts both as the lid of the net launcher assembly in its stored configuration and as a contact surface for the RSO to rest against and protect the vehicle during damping. Spring hinges are preloaded and act to force the doors open once an RSO has been identified for capture. Finally, the Restraining Hook Assembly is used to hold the assembly in place, therefore hindering any translational or rotational movement of the overall subassembly until the net launcher is in use, and the Burn Wire Assembly holds the Hatch Door Assembly shut in the stored configuration and allows them to open when desired.
Hatch Door Subassembly
The hatch door assembly serves two important purposes for the functionality of the net launcher design. Firstly, the bottom hatch door has a seal that is used to enclose the top of the net launcher from the surrounding environment and to act as a lid to keep the net inside the net retainer. Secondly, in the open configuration, the hatch doors act to increase the surface area in contact with the RSO when the RSO has been brought back to the net launcher and therefore as a protective barrier between the RSO and Sierra Space’s vehicle.
Linear Subassembly
The linear assembly is the sub-mechanism of the overall damping mechanism designed with only one degree of freedom: to translate in the direction of the net launcher boresight. This allows for the force of friction while damping the RSO’s rotation around the net launcher boresight to be modulated and controlled by the net launcher control system. The top plate is connected to guide rods traveling through Delrin bushings in our original design, and the bushings are press-fit into the bottom plate. This allows for translational motion of the top plate with respect to the bottom plate. The bushings have since been changed to flange-mounted linear ball bearings, which will be further explained below.
Design Iteration:
Original Design: The original design featured guide rods through manufactured Delrin bushings. Tolerances were based on binding analysis and consultation of Machinery’s Handbook.
Final Design: The final design featured off-the-shelf linear ball bearings with ground and polished steel guide rods traveling through them. Mounting and clearance holes were added to the static ring to accommodate this change.
Supporting Analysis: This change was the result of testing of the linear damping mechanism. Significant binding was occurring with the guide rods and bushings, and any load eccentricity resulted in immediate and not easily reversible binding. This issue was attributed to a lack of high positional tolerance from machining. The solution was to test linear ball bearings and offer clearance fits for the shafts of the linear ball bearings and for the flange mounting holes on the static ring. These clearances were RC clearances based on the Machinery’s Handbook.
Original Design: The original design featured 6061 Aluminum guide rods with an RC fit with the bushings as specified in the Machinery’s Handbook.
Final Design: The final design increased the length and material of these guide rods. The length was made longer so that the bottom of the rods do not rest inside the linear ball bearings in the linear damping assembly’s uncompressed state, and the material chosen was 6 mm diameter ground and polished stainless steel. A flat was added at the bottom for easier installation of the guide rods.
Supporting Analysis: Evaluation tools in the updated CAD model were used to find the minimum required length of the steel guide rods for use with the linear ball bearings. 6 mm ground and polished stainless steel was chosen for its diametrical tight tolerance to be used in conjunction with the 6mm inner diameter linear ball bearings.
Rotational Subassembly
The rotational assembly is designed to rotate around the linear damping assembly. During the second and final phase of the damping process, damping the RSO’s rotation around the axis of the net launcher boresight, the RSO will come into contact with the hatch doors (a subassembly of the rotational assembly as depicted below). At this point, the rotational assembly will match the rotation of the RSO and will rotate around the net launcher. It remains in contact with the linear assembly through the guide rods connecting the top and bottom plates and also bears against the frictional surface of the linear assembly. It’s through the interaction of the linear assembly with the rotational assembly that the only remaining axis of rotation of the RSO is dampened.
Design Iteration:
Original Design: The original design featured six 6061 Aluminum guide rods in rotational contact with the rings constituting the linear damping assembly.
Final Design: For the final design, the original 6061 Aluminum guide rods were turned down and Delrin sleeves were added to slide over them. Only three Delring sleeves and guide rod combinations were used as opposed to the original six.
Supporting Analysis: This design alteration was made as a result of testing the rotational damping assembly. The friction present in the system was too high due to tolerance stack-up and aluminum on aluminum grinding, and Delrin was chosen as the contact surface for the inner aluminum rings of the linear damping assembly and for rotating around the aluminum guide rod at its center. An RC fit was chosen for fitting the Delrin sleeves over the Aluminum guide rods, and the clearances were verified during manufacturing.
Burn Wire Subassembly
The Burn Wire Assembly holds the Hatch Door Assembly shut in the stored configuration and allows the hatch doors to open upon actuation by the control system. It utilizes a Nickel Chromium (Nichrome) wire which acts as a resistive heating element to burn through a nylon tiedown. Current is passed through the wire via actuation by the control system causing the nichrome to heat to temperatures exceeding 300 Celsius; the contact between the wire and the nylon tiedown causes the nylon to melt, releasing the spring actuated hatch doors.
Restraining Hook Subassembly
The Restraining Hook Assembly is used to hold the damping assembly in place, therefore hindering any translational or rotational movement until the net launcher is in use. It utilizes an electromagnetic solenoid to lock a spring-loaded hook in place. When the solenoid is activated, the spring rotates the hook backwards, releasing the damping assembly
My Role:
Systems Engineer
The first major takeaway from a systems engineering perspective is to move through design iterations efficiently and with a goal of learning something new or crossing an idea off of a list of possible options. It is also important to first analyze the problem and the merit of potential solutions based on theoretical models and equations where applicable. If an idea can be eliminated as a possible solution with some quick hand calcs as opposed to after purchasing hardware, then that is the first route to consider.
A systems engineer must also ensure that all components and subassemblies mate together as required and are able to be assembled. This starts in CAD where it is important to include fasteners and analyze the assembly order of operations. Including fasteners makes certain that a systems engineer is always considering how components are fastened together and keeps their mind focused on how that fastener will physically be installed. Without including fasteners in a CAD assembly, it can be easy to forget they are needed until it’s time to assemble. Just before assembly, as well, it’s important to implement some sort of quality control procedures to verify that the machined parts are within the specified tolerancing limits. Tolerance errors, and worse tolerance stack-up, can lead to assemblies that don’t mate correctly or don’t mate at all and must be re-machined.
