Development of a Cold Gas Propulsion System for the ... - SSL - MIT
Development of a Cold Gas Propulsion System for the ... - SSL - MIT Development of a Cold Gas Propulsion System for the ... - SSL - MIT
would not have been sufficient on its own; there were far too many flow control components available to purchase one of each and try them out to see which one worked best, and the MATLAB model played an essential role in narrowing the field. The CGSE development process illustrates that modeling and testing both have their uses, and it is important to think about when each can be employed most effectively and when it might be necessary to neglect one in favor of the other if resources are scarce. Unexpected things can happen at interfaces. In the CGSE, this is perhaps best illustrated by the issue of the solenoid valve response times. The SV128 was sufficiently fast in the single-stream tests, when the CGSE parts were being tested all on their own. However, when the CGSE was integrated onto the TALARIS vehicle and had to interface with the avionics subsystem, the need to protect the RIO led to the development of control circuitry which slowed the solenoid valve response times to unacceptable levels. Then, additional time had to be spent to investigate the problem and then to design and construct new control circuits. This was not something that had been anticipated in the original design of the CGSE; the solenoid valves seemed quite simple, opening with power on and closing with power off, and they did not in themselves require any special control circuitry. It was only in considering the interface with the RIO that new concerns arose to be addressed. The takeaway lesson is that integration of subsystems should not be taken lightly, and it should instead be recognized as a prime source of emergent behavior. Incremental development takes time, but it does produce solid results. The majority of the CGSE development process was a gradual progression, and the success of this approach was particularly evident in early verification testing. The results of the MATLAB model were used to select CGSE components which were tested individually before being assembled into a full system. Had the components been found to be unacceptable, this single-stream testing step would have saved money, since a full set of components for the entire system had not yet been purchased. And even when the components performed suitably in the single-stream test setup, the knowledge gained helped to direct full flight system testing. When flight system performance was found to be suboptimal in terms of valve response time as mentioned above, it was possible to identify the problem as a difference from the single-stream setup rather than an inherent characteristic of the valve and to trace the source to the interface with the flight computer so that the problem could be resolved. This situation can be contrasted with the validation flight testing, in which schedule pressure led to the abandonment of incremental progression in favor of attempting to advance with larger leaps. If each leap had been successful, it would indeed have resulted in faster progress. However, when difficulties 120
were encountered, it was harder to determine exactly where the problem had been introduced. For instance, when ascent in the 6-DOF setup was found to be difficult, the vertical 1-DOF CGSE altitude tests might have provided a baseline comparison to aid troubleshooting if they had been completed. But because the vertical stand tests had been stopped early, it was much harder to ascertain whether the problems originated in the CGSE, the EDFs, the GNC algorithms, or a combination of some or all of the above. If ongoing flight tests cannot resolve the ascent issue, it may be necessary to return to an earlier stage of testing for more in-depth investigation. Trying to take larger steps forward can mean having to take larger steps backward, and this risk must be kept in mind when planning the development of a system. The ability to identify such universal lessons learned indicates that the development of the CGSE was something of a prototypical engineering problem. However, it was a particular problem that had not been previously addressed. Its solution required the synthesis of several different areas of knowledge and the application of a variety of tools. The end result of this process was a cold gas propulsion system developed specifically to meet the needs of the TALARIS project. Testing of the system is ongoing, and several ways in which it might be improved have already been identified, but so far the CGSE has met all the goals that have been set for it. It is a reliable part of the TALARIS testbed, helping to advance the development of hoppers for lunar and planetary exploration. 121
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were encountered, it was harder to determine exactly where <strong>the</strong> problem had been introduced. For<br />
instance, when ascent in <strong>the</strong> 6-DOF setup was found to be difficult, <strong>the</strong> vertical 1-DOF CGSE altitude<br />
tests might have provided a baseline comparison to aid troubleshooting if <strong>the</strong>y had been completed. But<br />
because <strong>the</strong> vertical stand tests had been stopped early, it was much harder to ascertain whe<strong>the</strong>r <strong>the</strong><br />
problems originated in <strong>the</strong> CGSE, <strong>the</strong> EDFs, <strong>the</strong> GNC algorithms, or a combination <strong>of</strong> some or all <strong>of</strong> <strong>the</strong><br />
above. If ongoing flight tests cannot resolve <strong>the</strong> ascent issue, it may be necessary to return to an earlier<br />
stage <strong>of</strong> testing <strong>for</strong> more in-depth investigation. Trying to take larger steps <strong>for</strong>ward can mean having to<br />
take larger steps backward, and this risk must be kept in mind when planning <strong>the</strong> development <strong>of</strong> a<br />
system.<br />
The ability to identify such universal lessons learned indicates that <strong>the</strong> development <strong>of</strong> <strong>the</strong> CGSE was<br />
something <strong>of</strong> a prototypical engineering problem. However, it was a particular problem that had not<br />
been previously addressed. Its solution required <strong>the</strong> syn<strong>the</strong>sis <strong>of</strong> several different areas <strong>of</strong> knowledge<br />
and <strong>the</strong> application <strong>of</strong> a variety <strong>of</strong> tools. The end result <strong>of</strong> this process was a cold gas propulsion system<br />
developed specifically to meet <strong>the</strong> needs <strong>of</strong> <strong>the</strong> TALARIS project. Testing <strong>of</strong> <strong>the</strong> system is ongoing, and<br />
several ways in which it might be improved have already been identified, but so far <strong>the</strong> CGSE has met all<br />
<strong>the</strong> goals that have been set <strong>for</strong> it. It is a reliable part <strong>of</strong> <strong>the</strong> TALARIS testbed, helping to advance <strong>the</strong><br />
development <strong>of</strong> hoppers <strong>for</strong> lunar and planetary exploration.<br />
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