If you read the specs (http://www.astronautix.com/m/mr-103.html) of the thruster...

JetSetWilly on Dec 1, 2017 | parent | context | favorite | on: After 37 years, Voyager has fired up its trajector...

If you read the specs (http://www.astronautix.com/m/mr-103.html) of the thrusters in question, you can see that they do "spew flames" and there is a combustion chamber:

> Combustion Chamber Pressure: 4.35-23.9 atm. Combustion Chamber Temperature: 800 Celsius. Combustion Chamber Cooling Method: radiative. Duty Cycle: 0.008 sec minimum to unlimited maximum burn. 750,000 pulses.

cyberferret on Dec 1, 2017 [–]

But from what I remember reading in the past, those flames are essentially invisible in a vacuum, is that right?