The system should produce acceptable parts while operating automatically in the harsh environment of Psyche’s surface, and without prematurely deteriorating. It should also adhere to NASA’s guidelines throughout its construction, transport, and operation. The targets required to meet these functions are all defined by benchmarking previous space travel and current additive manufacturing techniques. Several targets were also derived from the customer’s needs. By meeting these quantitative targets, overall success of the system will be achieved.
There are a few targets and metrics that are critical to the success of the mission. They must be achieved while aligning with the requirements set by the sponsor. The critical targets were determined by the engineering design team by looking at which functions would result in complete failure of the system without them. The critical targets and metrics will ensure that the system will be able to take an input material and output a desired output. The absence of the other targets and metrics may not lead to an immediate mission failure like the critical targets and metrics will. The critical targets and metrics of the system are to receive input material, relay instructions to the output system, regulate temperature, and produce desired output. Due to the freezing temperature in the asteroid belt, being able to regulate the temperature of the system is paramount to mission success. A major motivation of the system is to utilize the surface materials of Psyche, therefore inputting materials is critical. If the controls of the system are not working, the system will not be able to produce the desired output, which is another critical target and metric.
Minimizing outside vibrations in the system will be crucial in efficient and stable printing. Any vibrations above 50 Hz have been shown to affect printing quality in additive manufacturing designs. Excessive vibration can lead to issues in layer adhesion, print precision, and cause imperfections in the final surface. A stable surface will be key to reducing vibrations. Loose components could also lead to a rise in vibrations. Damping systems may be necessary to prevent resonance. Additive manufacturing systems show the highest number of faults when the vibrations match the resonant frequency of the system itself. Our system will aim to keep outside vibrations below the resonant frequency. The “shields system from radiation” function is used to limit the degradation of electronics from ionization damage. The metric of radiation shielding is the allowable dose rate based on displacement damage results for NASA spacecraft electronics. The target is to allow less than 0.02 rads per second [1]. The lowest dose rate of all the electronic devices was used to account for any potential parts the design may need. For scale, humans can handle 0.62 rads per year [2]. The system needs to be protected from unintended particulates. The finest quality requirement for space-grade electronics for NASA was ~0.015 μm particles, which should be sufficient protection for electronics such as a laser, focusing lens, or open circuitry. [3] In the same NASA cleanliness report, the highest cleanliness level, level 1, would be 1.08 particles of size 1 μm per 0.1 m2 or 10 particles per liter. Moving components that could be damaged due to particle interaction would benefit from maintaining a cleanliness level of 1, or ISO 1. In the “compensate for internal force” function, the system must be able to withstand and compensate for forces faced in the device to ensure prints are not disrupted during printing and total failure of the device is avoided. Moving components in the device can create moments and forces that can potentially destabilize our device and cause it to move from the base. This is crucial because any unstable movement can cause materials to shift around leading to inaccurate/failed parts. To prevent this from happening the target for mitigating internal forces is obtaining 0N (Newtons). The system must operate in deep space conditions approximately 2.5 – 3.3 astronomical units from the sun (375 – 495 million km). Certain portions of the asteroid will be exposed to the sun for long periods while others will be in darkness. According to a study conducted to model the surface temperatures of 16 Psyche, the surface experiences temperatures from -213 to -23 degrees Celsius (-351 to – 9.7 degrees Fahrenheit) [4]. Metal additive manufacturing processes rely on melting the metal input material to form a solid rigid body. The metals of interest to the system are nickel and iron as they are the metals speculated to be the highest in abundance, their melting points being 1452 and 1538 ℃ respectively. The system must be able to withstand these temperatures to preserve its integrity and ability to function. To this end, the system must be able to regulate its internal temperature to ensure that no systems are compromised by extreme temperatures. Space-grade electronics used in missions with comparable conditions have an operating range of -35 to 65 ℃ with batteries needed for energy storage having a more limited range of -5 to 35 ℃ [5]. The “receive instructions from user” function will be achieved through a radio transmission from NASA's Deep Space Network. An internal solid-state drive with a database of CAD files will be included in the system, so a transmission can be sent to command the system to execute any given file. For execute commands, the target radio frequency waves are in the “S” band (2-4 GHz) due to their minimal attenuation through Earth’s atmosphere when compared to other frequencies. For brand-new CAD files not included in the database, the challenge of sending larger amounts of data arises. Greater atmospheric attenuation, free space loss, and noise levels require the use of higher frequencies. The target radio frequency waves for this application are in the “Ka” band (27-40 GHz), and a high-gain antenna will be used to compensate for the losses at these frequencies. The “relay instructions to output system” function will occur through a microcontroller’s logical inputs. When the system receives input instructions, they will be decoded into commands in the C/C++ language and relayed to the output system through internal logic. There will be a microcontroller to execute the system’s internal logic with a target operating voltage of 3.3 volts. The success of this subsystem is determined by the output subsystem receiving accurate logical commands, which are transmitted by a series of voltage changes at either 0 or 3.3 volts. It can be measured by observing the action of the output system when commands are transmitted. The “relay status to user” function will operate similarly to the “receive instructions from user” function. At three consecutive stages in the manufacturing process, a radio frequency wave will be transmitted for reception by NASA’s Deep Space Network. The target frequency is in the “S” band (2-4 GHz). It will be sent before, during, and after the operation of the output subsystem. The signal will contain information that can be decoded to commands in the C/C++ language. They correspond to any of the following stages: “awaiting command”; “successfully received command”; “received a faulty command”; “previous command canceled”; “printing in progress”; “error during printing”; “successfully completed print.” The “receive input material” function operates by utilizing a specific amount of material which in our case the material will be in powder form, used for additive manufacturing. Our device will be looking to print small to medium parts from a supply of powder. In powder manufacturing devices, they use about 10vol% - 20vol% (Percentage by Volume) of their powder stock in making a specific part (Li, 2024). The remaining volume of powder is then recycled and used again. For our device, the target volume of input material per part will be less than 20vol% of powder. This allows us to print complete parts of small to medium size while being able to recycle and reuse the remaining powder. The goal of the additive manufacturing system is to use materials from Psyche to manufacture a rigid body structure. The output of the system must have some measure of accuracy to what is desired. Industry metal additive manufacturing systems can produce parts up to a tolerance of ± 0.0015 inches (0.38 millimeters) [6]. Due to the extreme conditions and variety of factors that could impact the precision of the system, the system should meet the geometric specifications of the test artifact within a margin of 0.5 millimeters.
Vibrations can be measured with an accelerometer or a vibration meter. These devices can be placed on the print bed and print head gantry. These sensors would be placed onto locations that directly affect print quality. These tests can be repeated in several important impact locations. The results can then be analyzed signaling points of extreme vibration. Vibrations could also be induced in order to ensure stability up until the target [10]. The “shield system from radiation” function will be tested during prototyping. There are dosimeters that can measure the dose rate from ionizing radiation sources. At Goddard Space Flight Center they have a radiation effects facility (REF) that uses gamma sources and particle accelerators as their testing radiation sources to perform damage testing of spacecraft electronics [1]. The REF is available to universities and NASA centers on an as-available basis. NASA may have its own methods of testing for cleanliness but ensuring the ways particulates could get in is blocked should prevent dirtiness as the machine is in use. Because the team will likely be using powder within the manufacturing process, the bare minimum should be protecting against those particles, particles as small as 10 μm. One solution could be precision parts with gaps smaller than the expected particle size. More solutions for prevention will be addressed in the concept selection process. From the point the device is initially manufactured, it should meet the cleaning requirements and stay clean. Simulations could be used to test theoretical allowances if the cleaning requirements are more or less precise than necessary. The “receive instructions from user” function will be tested while prototyping. Since the design of the communication systems between Earth and Psyche are outside of the scope of this project, testing the success of this subsystem will be based on its actions after instructions are inputted. If the subsystem performs according to its command, then it has succeeded. The commands are received as code in the C/C++ language. If the subsystem performs incorrectly, debugging is required until success is achieved. The “relay instructions to output system” function will be tested during prototyping. For success, two things need to happen. The first: signal is sent at 3.3 volts. It can be measured using a voltmeter, multimeter, or oscilloscope. In the case of failure, there is either a power supply issue or a faulty electrical connection. The second: proper commands in the C/C++ language are transmitted. In the case of failure, debugging will be required. Continuous testing and debugging will occur until this subsystem performs correctly. The “relay status to user” function will be tested during prototyping. Similar to the “receive instructions from user” function, no radio wave transmissions will be tested. Instead, this subsystem will output messages through an LED screen. Success of the subsystem will be measured by the appropriate messages being displayed at each stage of the output systems operation. If successful, it is assumed that the messages can easily be imposed on the radio waves that will be sent through an external communication system. Testing the input material amount of the device will require demo parts to be printed by the device during prototyping. After the demo parts are printed, the left-over volume of powder will be measured and compared to the total volume that was initially placed in the device to get the vol% that was used to make the parts. This will then be compared to our target of using less than 20vol%, and if it's over, design optimizations will be made. Testing the system for temperature regulation will require simulating the surface temperature of Psyche. Testing for such low temperatures would require the use of a thermal vacuum chamber which can simulate low temperatures comparable to deep space conditions [11]. The system must be able to perform within desired parameters during a thermal vacuum test. A testing method for precision proposed for additive manufacturing systems by NIST (the National Institute of Standards and Technology) is to have the system produce a test artifact from which geometric characteristics can be observed and compared to a control version of the artifact [12]. The system will be tested against the theoretical measurements of the test artifact using precision calipers for measurements.
More needs need to be met for mission success, other than the listed functions. The first is that the system must have as minimal mass as possible. Another metric will be power. The device will require a power input of less than 3.5kW which will mark the target [7]. This number was benchmarked by Powder Bed Fusion Additive Manufacturing. For reference, the Mars Curiosity Rover generates about 2.5kWh per day [8] The total build volume is necessary for establishing the size for which parts can be printed. Another metric is the total mass of the system. The target mass is less than 1650 kg benchmarked off previous additive manufacturing systems. [9] To provide some scale the mass of the previous Psyche spacecraft was 2747 kg. For the print volume, a minimum of 8 liters (200 mm cube) would be a good goal as it would provide more opportunities for weight distribution given that the printer used for the 1650 kg mass reference has a print volume of 15.625 liters (250 mm cube). [9]
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