# What is so difficult about the ISS zero propellant maneuver?

Nazareth Bedrossian (from Draper Laboratory) et al. describe the ZPM in this paper. What I want to know is:

• Why is planning and executing the maneuver so difficult it had not been done before 2007?
• What are the sources of errors that can occur during the maneuver?
• How does the fact that the ISS is not a rigid body complicate the attitude control problem for large maneuvers?
• 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)?

Notes and fun facts:

• The ISS is equipped with four 4760 Nms Double Gimbal Control Moment Gyroscopes
• Each CMG has a flywheel nominally spinning at 691 rad/s (6600 rpm)
• Each CMG can produce a maximum output torque of 258 Nm
• There are software limitations on gimbal rates of max. $0.014 \frac{rad}{s}$ ($0.8 \frac{deg}{s}$) and gimbal acceleration of max. $0.0007 \frac{rad}{s^2}$ ($0.04 \frac{deg}{s^2}$) to extend CMG lifetime and lower the probability of failure.

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

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

and also directly from NASA at: https://ntrs.nasa.gov/search.jsp?R=20080009592

I believe the ZPM was first performed before 2007. The paper seems to be about using the ZPM to flip the ISS completely around ("reorient the International Space Station 180 degrees"), not just using ZPM for day-to-day attitude control. From the summary on the first page (emphasis mine):

The flight test established the breakthrough capability to simultaneously perform a large angle attitude maneuver and momentum desaturation without the need to use thrusters.

and later, on page, 2,

In the early 00’s, the ZPM concept was used...

i.e. the "breakthrough" in the paper you linked was how much the ZPM moved the station, not just that the ZPM moved the station.

My copy of the Reference Guide to the International Space Station, published in 2006, has a page (p.67) on "Guidance, Navigation, and Control" that states "The preferred method of attitude control is the use of gyrodynes, Control Moment Gyroscopes (CMGs)...."

That page goes on to say that "CMGs are, however, limited in the amount of angular momentum they can provide and the rate at which they can move the Station. When CMGs can no longer provide the requisite energy, rocket engines are called upon," (emphasis mine) which implies to me that CMGs are not sufficient to permanently maintain the orientation of the ISS in the event that thrusters were lost, which maybe sort of answers your last question.

I can't help with your second or third questions.

Why is planning and executing the maneuver so difficult it had not been done before 2007?

I'm just an armchair astronomer but having read the papers it sounds to me like this is an optimization. It's a little like a few other inventions that evoke the "Well of course that's obvious" feeling. But like many great ideas it might not have been so obvious before.

It was stated in the published paper that programming for rotating using propellant was the simplest thing to do. So they did that first. Now they must have realized there was a better way and they did it.