Q: Why is planning and executing the maneuver so difficult it had not been done before 2007?
A: The 1st ZPM was performed in Nov 2006 when the ISS was rotated 90 degrees. The paper describing this maneuver can be obtained at https://www.academia.edu/4543547/First_Ever_Flight_Demonstration_of_Zero_Propellant_Maneuver_Attitude_Control_Concept
Also downloadable (without requiring a login) at: https://www.researchgate.net/publication/268554560_First_Ever_Flight_Demonstration_of_Zero_Propellant_ManeuverTM_Attitute_Control_Concept
Re difficulty in planning and executing maneuver:
- As ZPM's take longer than an equivalent thruster maneuver (it can take up to 5-10 X longer time), the ISS operational timeline must be altered and that impacts many other subsystems which have to be coordinated. Anything that disrupts an existing operational timeline increases risk and additional work has to be performed which costs more and since ZPM's were not included in ISS budget that means the money has to come from somewhere else.
- Because ZPM's are slower than thruster maneuvers, separate thermal analysis has to be performed to evaluate impact of a "stale sun", ie the sun shinning too long on a particular spot on the ISS. Note that maneuvers that take less than an orbit (about 90min) do not need any additional analysis. Since the ZPM's take longer than 90min additional work has to be performed which costs money.
Finally, about the timing, it is a complex combination of availability of solution and its maturity, opportune timing where management and operations line up with new technology. Just having a good idea and an implementation was not enough as you also need to change perception and status quo that this is actually possible.
Q: What are the sources of errors that can occur during the maneuver?
A: Some of the error sources that directly affect how much momentum is used during the ZPM:
- Uncertainty in knowledge of ISS inertia. Other than knowing the inertia of each ISS component there is also the inertia variation due to rotating solar arrays.
- Uncertainty in initial conditions (attitude & rate).
- Uncertainty in aerodynamic drag that results in a disturbance torque on the ISS
Q: How does the fact that the ISS is not a rigid body complicate the attitude control problem for large maneuvers?
A: The general answer is none. The ZPM in general does not excite the ISS flexible structure unlike thruster firings because it uses smooth CMG actuators. The design of the ZPM is based on rigid body dynamics. There is a feedback CMG control system wrapped around the ZPM commands that includes flex filters that suppress the impact of structural vibrations on attitude and rate measurements.
Q: Assuming loss of propulsive attitude control (i.e. failure of Russian segment's thrusters), can ZPMs result in off-nominal situations (e.g. the Station spinning beyond control authority of the CMGs)?
A: The general answer is no. Remember that there is a stable feedback attitude controller wrapped around the ZPM and the design always includes substantial momentum margin (at least 50%). The feedback controller's job is to get the ISS to track the ZPM trajectory. The momentum margin is designed to deal with mismatches between modeled and actual ISS dynamics that can result in using more momentum than anticipated. So as long as the CMG momentum is not saturated we would have control authority. And how do we know that? Because before flight, there is extensive analysis on the impact of ISS uncertainties on the maximum momentum that would be required during a ZPM. In the end we are taking a calculated risk that an out of control situation will not occur. Theoretically it is possible that momentum could saturate but the likelihood of that happening is very small.
Interestingly the opposite happened. In 2007 the Russian segment computers all failed at the same time resulting in loss of automatic thruster control while the Shuttle was docked to the ISS. If thruster control was not recovered, Shuttle undocking could have put the ISS in such a spin. And a ZPM was one of the two options considered to recover a spinning ISS. Analysis showed that an ISS spinning up to 0.1deg/sec could be recovered and returned to stable attitude using a ZPM. Thankfully the computers were recovered before the Shuttle had to undock. A reference for this is the paper https://www.academia.edu/5497196/ISS_Contingency_Attitude_Control_Recovery_Method_For_Loss_Of_Automatic_Thruster_Control
Also downloadable (without requiring a login) at: https://www.researchgate.net/publication/273441779_ISS_Contingency_Attitude_Control_Recovery_Method_For_Loss_Of_Automatic_Thruster_Control
and also directly from NASA at: https://ntrs.nasa.gov/search.jsp?R=20080009592