[natosaichek]

I've worked on satellites, and yeah - everything is super bespoke, very low quantity, very expensive. There is probably a qualification unit, or a flight spare that may be available for many subsystems, but maybe not. Integration is a long and complicated process. Pulling apart this bot, with however many fasteners, joints, etc, and then reassembling it correctly would be a decidedly non-trivial project that could easily take a month or two, given that testing has to be performed at each step along the way to ensure that every sensor and actuator is fully functional throughout the integration process. This style of traditional aerospace assembly / integration is not particularly efficient. The only reason it is done this way is that for these kinds of missions you only get one shot, and total cost is ridiculously high so everything must be done correctly.

[frankus]

Any idea why they would use brushed motors? When every gram counts I would think ditching the mechanical commutator would be a no-brainer, but maybe adding another leg to the H-bridge is a bigger penalty?

[cpgxiii]

In 2003, small brushless DC motors were way less mature and available as they are now, particularly for low-speed/high-torque applications*. Brushless controllers are much more complicated than brushed controllers, particularly on the control software front, so sticking with simpler and more reliable brushed controllers for a space application makes sense (remember, it probably needed to be radiation hardened - doing that for an H-bridge is much easier than a BLDC controller).

*A notable example of this is in the world of RC cars, where rock-crawlers only very recently have started switching to brushless motors using field-oriented control to deliver acceptable very-low-speed behavior. Until FOC controllers became available, brushed motors offered much better low-speed handling.

[wolverine876]

Without disagreeing with your point, would availability be an issue in this case? They need one or two, have an enormous budget, and if the technology exists can make their own.

[cpgxiii]

Availability is often strongly correlated with technical maturity. Small brushless motors with FOC didn't become widely available and mature until really the late 2010s. Arguably the foundation of nearly all of DJI's product lines is due to their early mastery of small brushless motor control (drones, gimbals, lens controls, robots, etc), and that's a company founded in 2006, well after the events of the article.

You can get good control of brushed motors with just a couple of transistors. Good brushless control means FOC, which really requires a fairly capable microcontroller in addition to all the power electronics for variable-frequency drive. While brushed motors certainly have limitations, those were quite well understood by the early 2000s (to the point here that assessing whether or not damage had occurred was "just" a question of "have these few transistors suffered from voltage applied in an unintended manner"). Brushless motors involve way more components with way more integration required to make then small. Far more complexity and potential failure modes need to be understood.

[wolverine876]

> Availability is often strongly correlated with technical maturity.

I see what you mean. Yes, agreed.

[Saris]

Closed loop control of brushless motors is just more complex, in addition to needing 3 phase AC output, you also need either hall sensors or an encoder of some kind to be able to start the motor smoothly, and you need a dedicated IC or MCU for each motor to manage commutation and read the sensors.

I don't think FOC type controllers were anywhere near common back then either, which is needed to run a brushless motor smoothly.

There is just so much more that can go wrong with a brushless setup, vs brushed where you just apply power and that's it.

[tonyarkles]

I'd guess it's mostly because it was 2003 and decent BLDCs were not super common-place yet. There were some older forms (steppers, PMSMs, etc) but they generally didn't have very good torque/weight performance. Brushed motors would probably have been the answer at the time.

[euroderf]

Related: NASA's monster rocket costs several times more than SpaceX's monster rocket.