How are vacuum optimized engines tested without disintegrating them? is an excellent question as it's attracted quite a number of informative and diverse answers.
@Uwe's answer informs us that there exist at least one or two low-pressure test chambers where engines can be fired for tens of seconds and the exhaust cleverly managed/diverted to maintain a pressure below 50 Torr for tens of seconds which is pretty amazing!
From NASA Plum Brook Station In-Space Propulsion Facility Test Stand Characterization Hot Fire Test Brian K. Jones, John C. Zang, Hal F. Weaver, Nicholas A. Connelly, and Gerald M. Hill, NASA Glenn Research Center, Plum Brook Station, Sandusky, OH, 44870, United States
(see also ESA's Maintaining Vacuum)
A test facility modification to enable small scale altitude propulsion testing at the NASA Glenn Research Center’s In-Space Propulsion (ISP) Facility was verified with a hot fire test campaign. As the facility’s primary steam supply system undergoes refurbishment, the alternate facility configuration, known as the “vacuum accumulator” mode, would enable rocket engine testing up to 10,000 lbf thrust. The NASA Johnson Space Center developed the vehicle for the verification test campaign: the Integrated Cryogenic Propulsion Test Article (ICPTA). Constructed primarily from assets of the former Morpheus Project, the ICPTA provided an integrated liquid oxygen (LOX) / liquid methane (LCH4) propulsion system including a 2,800 lbf thrust main engine. The ISP Facility’s vacuum accumulator configuration leveraged the large test volume of the facility and a diffuser insert to maintain altitude conditions. During hot fire, the ICPTA main engine “started” the diffuser insert constructed for the test campaign. As a result, the test chamber upstream of the diffuser insert remained at altitude conditions throughout the hot fire. Upon engine shut down, a backflow deflector mitigated blow back into the test chamber by restricting the mass flow and redirecting it away from the test article. The test campaign successfully characterized the performance of the vacuum accumulator configuration. In addition, it provided an opportunity to collect data for an integrated LOX / LCH4 propulsion system in an altitude and thermal vacuum environment.
I see “vacuum accumulator”, "diffuser" and "backflow deflector" but after a read through I still don't understand how this actually works.
Question: How do you test-fire vacuum engines in a vacuum chamber? Why doesn't it fill up almost instantly from the exhaust and spray (deluge) water?
Figure 1. A cutaway view of the In-Space Propulsion Facility. In the center of the test chamber, the facility’s diffuser provides the interface between the test chamber and the spray chamber. The steam ejectors are located outside of the test building in the yard on the right.
Figure 9. Hot Fire 2.6 test chamber and spray chamber pressures during a 27 second main engine hot fire test in the lowered position with the backflow deflector.