It's like looking for a needle as all eyes are on the Hall thrusters.
TL;DR:
Why does the Psyche Spacecraft use cold gas thrusters?
Psyche was always designed for the main propulsion to be electric and was always going to have cold gas thrusters to make up for the shortcomings of electric propulsion (i.e., low thrust, high power)—the focus was on making it an EP (electric propulsion) spacecraft with cold gas thrusters sharing the same xenon, then during its development this changed:
September 15-20, 2019
... while the project’s scientific objectives and partnership structure are well established, the technical design has matured as detailed engineering design proceeds.
Some notable developments include:
• Better understanding of mission requirements has led to a change from a shared xenon cold gas propulsion system to a dedicated nitrogen cold gas propulsion system.
The original concept design
used xenon cold gas thrusters for secondary propulsion, but these were changed to nitrogen cold gas thrusters in order
to provide sufficient thrust during the Science Phase (26-month science phase makes observations and collects data as it orbits the asteroid at different altitudes).
There are two primary drawbacks to the use of Hall thrusters for a reaction control system: the low thrust generated
and high power consumed. An additional, small propulsive attitude control system is needed to accomplish short
duration, power-critical and time-critical maneuvers, such as immediately after launch vehicle separation and
contingencies where the solar arrays have lost lock with the sun.
during detailed design it
was realized that the amount of Joule-Thompson (aka Joule–Kelvin or Kelvin–Joule effect, describing temperature change of gas when it is forced through a valve while kept insulated so no heat is exchanged with its environment) cooling across the xenon regulator at the cold gas thruster (CGT) flow rates is two orders of magnitude above that for the Hall thruster subsystem.
• An SPT-140 at full power (16mg/sec) with the pressure dropping from 2700psi to 37psi requires
1W of heat to mitigate the temperature drop.
• Two xenon cold gas thrusters, regulated at 100psi, with a combined flow rate of 1.3g/sec require
86W of heat to mitigate the temperature drop.
to maintain the xenon CGT flow rates, a new regulator design and a massive increase in heater power would be
required. The risk and cost of a new component design were deemed unacceptable for the mission
Second redesign:
Implementing an unregulated xenon system, which would maintain the operational advantages of shared tankage without the regulator. An unregulated system requires that the CGTs be capable of operating at a range of pressures varying from 2700 psi to 37 psi. Without dramatically increasing the size of the CGT coil, the thruster’s orifice had to remain small
in order for the thruster valve to open at high pressure. Achieving 0.25 N at End Of Life would have required maintaining 350 psi pressure in the tanks, resulting in a hold-up xenon mass of 90kg. This effectively eliminated the advantage of a shared tank EP/CGT spacecraft.
Final design:
final iteration uses a separate regulated nitrogen CGT system based on previously qualified hardware,
with no large qualification or redesign efforts required.
Unless an excessive
amount of contingency propellant is used, the system will meet the required 0.25 N thrust level through end
of mission.
Nominal 3-axis attitude control is accomplished using a pyramid of four Honeywell HR 16-100 reaction wheels
operated near zero momentum. To increase torque authority and disturbance momentum capacity, the nominal
configuration uses four wheels, but 3-wheel operation is fully supported, providing three for four redundancy. Backup
attitude control for use during initial detumble and cold gas system (CGS) safe mode is provided by a regulated nitrogen cold gas system
with six thrusters per branch. The CGS also provides an alternate RWA momentum unloading capability to supplement the use of EP-based momentum control.
Difference to Dawn:
Unlike Deep Space 1 and Dawn, which used the EP gimbals for direct attitude control of two axes while the EP
thruster is operating, the Psyche Guidance, Navigation and Control subsystem uses (Reaction Wheel Assemblies) RWA's for full 3-axis control even while thrusting.
The approach is similar
to that used for Maxar's GEO communications satellites. The RWAs provide 3-axis control of the spacecraft attitude
while a low-bandwidth momentum control algorithm positions the EP gimbal to direct the thrust vector near the
vehicle center of mass. Small offsets to the EP thrust vector produce torques that allow near continuous unloading of
two transverse axes of RWA momentum.
Swirl disturbance created by Hall thrusters:
Hall effect thrusters create a “swirl” disturbance torque that must be account for in operation. Since swirl torque is aligned with the thrust axis, it can not be directly unloaded using a two-axis gimbal and continuous unloading.
During cruise, when the inertial thrust vector changes only slowly, the remaining axis of momentum (along the thrust vector) will be unloaded every few days using short (~15 min) EP-unload burns using a thruster on the opposite side of the spacecraft. Once in orbit around Psyche, where the inertial thrust direction will change more quickly, continuous unloading can be used to accomplish most of the required momentum control so dedicated EP-unload burns are expected to be rare. If required, EP unloading can be supplemented with momentum adjustment burns using the CGS thrusters.
The spacecraft requires no chemical propulsion and... will be the first mission to use Hall Thrusters beyond lunar orbit.
In summary:
This seems like an odd choice for me, considering it is a multi year mission that will presumably change attitude a lot when inspecting the Psyche asteroid or testing its laser communication system.
The CGT is mainly for precise attitude control when required and a minor role as backup to the EP in safe mode. The EP still plays the major role when it comes to altitude changes during the science phase. EP is also the main thruster for momentum changes, with CGT as backup.
Why was this approach chosen over a hypergolic thruster system for attitude control that would probably run out later, giving the spacecraft a longer operational lifespan, especially since it seems that many missions end because attitude control fuel runs out, like Dawn.
Dawn gave them experience that led to a design change away from that spacecrafts method of an all EP attitude control system. So a direct comparison can no longer be made. An all EP system, that included shared CGT attitude control, led to some disadvantages that an established flight-proven commercial CGT did not present.
Nitrogen CGT was probably less toxic to implement compared to hypergolic too. Compared to the EP system, CGT allowed precise momentum management, propulsive maneuvers not feasible with the Hall thrusters, and allowed for multiple contingency scenarios including hardware failures without impacting the mission.
Adding a flight proven, commercially reliable CGT, gave the mission an alternate (and back up, with large enough fuel budget for contingencies) system that did not negatively impact the spacecraft design.
Links:
http://electricrocket.org/2019/192.pdf
This seemed to be the best document I could find that detailed the initial xenon-CGT-shared electric design, the alternatives in light of the drawbacks encountered, and the implementation of separate nitrogen cold gas thrusters, when they would be used, the fuel budget and integration with the spacecraft.
