How will GomX-4B manage angular momentum when using butane thrusters to move?

Question

Phys.org's ESA's next satellite propelled by butane describes GomX-4B's butane powered cold gas thrusters, and how it will test them and use them to navigate to different distances from its partner GomX-4A. See Gunter's Space Page. Also see ESA.

The image below is shown in the article. If I understand correctly this represents one sixth of the 6U spacecraft - or 2x0.5 U at one end. It's said that there are four thrusters, and one pair will be used primarily with the other pair as backup.

Where will the thrusters be located? Can they vector, or are they fixed wrt the spacecraft's center of mass? How will the spacecraft ensure controlled, directed thrust without picking up spin around any axis, using only two fixed thrusters? Are there reaction wheels for this, and other types of thrusters for unloading of accumulated angular momentum?

below: "Thruster chip for the GomX-4B CubeSat’s propulsion system, designed by Nanospace in Sweden. Elements such as flow channels and sensors, chamber and nozzle are fitted into a 1x2 cm chip, just 1 mm thick by using microelectromechanical systems technology, otherwise known as MEMS. In terrestrial terms, MEMS is already a very mature technology platform: there are such devices all around us, in our cellphones, watches and cars." From ESA here

Open in new window for full size:

below: Cropped and reduced from here. Open in new window for larger size.

GomX-4B’s cold-gas thruster system takes up two half-CubeSat units at one side of the nanosatellite, with two spherical titanium tanks filled with liquid butane. It has four 1 mN thrusters, typically to be fired in pairs while keeping one set in reserve. Credit: Nanospace

Answer 1

tl:dr: It uses reaction wheels during the burn and magnetotorquers to unload the wheels later.

The GOMX-4 web page talks about the common satellite bus (emphasis added):

Both satellites apply the same architecture based on the GomSpace 6U platform with only different hardware configuration. ... the ADCS subsystem on-board is an upgraded version for large cubesat sizes using powerful reaction wheels, external gyro sensor and adapted to integrate different star tracker technologies, in addition to the magnetorquer, sun sensors and magnetometer successfully used in previous missions. The space-ground communication capacity of both satellites are improved by the use of High Speed Link in S-band which supports the nominal UHF communication link for uplink and downlink.

(The web page also has some system diagrams, but they're of such poor resolution to be not worth including here)

Together, that's a pretty thorough suite of tools for sensing and controlling attitude.

Attitude management has been a bit of an operational issue:

One of the main lessons learned about these (butane test) propulsion maneuvers is the saturation of the reaction wheels at 4000 rpm which has been reached during firing longer than 3.5 minutes. Therefore, all the burning performed in this mission has been split into several consecutive steps of less than that duration until the desired total burning time and targeted orbit is reached.

That seems to indicate that the thrusters aren't perfectly aligned through the center of mass, or perhaps not properly balanced: As the thrusters fire, the reaction wheels have to provide torque to offset them, which builds up the wheel angular momentum. Once the wheels reach their top speed, the burn has to stop until the wheels can be unloaded via the magnetorquer.