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Obviously you test fire a new engines as many times as you see fit on earth, but how do you know the effect the vacuum of space (or really the lack of gravity/air) will have on a new engine?

Is it just math and the engines that came before at that point? Would there ever be a benefit to somehow test firing in space?

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    $\begingroup$ There are vacuum test chambers for rocket engines. A very powerful pump is neccessary to sustain the vacuum. Of course the maximum mass flow of the pumps should be larger than the mass flow of the engine. $\endgroup$ – Uwe Mar 11 at 15:20
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    $\begingroup$ Derp, that makes a lot of sense, they actually fire them inside those chambers? @Uwe $\endgroup$ – Stickyz Mar 11 at 15:20
  • $\begingroup$ @Stickyz yeah my first thought was the rocket exhaust woud pressurise the chamber quickly. But a re-read says that the evacuation pumps are just that fast. $\endgroup$ – Criggie Mar 12 at 5:37
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There are a number of vacuum test facilities available for testing engines in space and near-space conditions. The NASA White Sands Test Facility has several test stands for space and near-space testing, but they don't have really memorable names:

enter image description here

Test Stand 302 is an insulated 32 ft diameter by 38 ft high (10 m diameter by 11. 6 m high) carbon steel altitude chamber with three interior levels for test article access with a dual-position, vertical or horizontal firing capability, and an altitude capability of up to 100 K ft (30.5 km) for engine firings using the steam ejector system and up to 250 K ft (76 km) non-firing capability with vacuum pumps.

Test Stand 303 is an insulated 11 ft diameter by 39 ft long (3.35 m diameter by 11.9 m long) horizontal carbon steel altitude chamber capable of holding propulsion systems up to approximately 7 ft (2.13 m) in diameter and 25 ft (7.6 m) in length. It is capable of testing single engines or test articles with multiple engines up to 1000 lbf (4.5 kN) total thrust. It has a single-position, horizontal firing capability and an altitude capability of up to 100 K ft (30.4 km) for engines firing using the steam ejector system and up to 250 K ft (76 km) non-firing capability with vacuum pumps.

enter image description here

Test Stand 401 is a 32 ft diameter by 33 ft high (9.75 m diameter by 10 m high) carbon steel altitude chamber capable of accommodating a vehicle with a thrust vector-controlled 25 K lbf (110-kN) thrust engine firing vertically downward. The stand is capable of testing maximum test articles of 15 ft by 15 ft by 45 ft (4.6 m x 4.6 m x 13.7 m). It has three interior levels that can be reconfigured to meet test requirements. It has a dual-position, vertical and horizontal firing capability and an altitude capability up to 100 K (30.5 km) for engine firings using the steam ejector system and up to 250 K (76 km) non-firing capability with vacuum pumps.

There are several more. The altitude of White Sands (about 5k feet) means that even the ambient stands are working at a somewhat-reduced pressure.

NASA AFRL has multiple smaller cells, one of which can go to higher altitudes:

The Space Environment Simulation Facility (1-42C) can perform static tests simulating space altitudes of up to 650K ft. The chamber is a 30 ft. diameter sphere capable of achieving 1 X 10-6 Torr at temperatures of -300F or +400F.

Blue Origin is apparently upgrading and using that facility now.

Apparently, the Glenn Center even had a facility for vacuum testing of nuclear rockets....

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    $\begingroup$ Great answer! Do we know if any of these were used for Merlin or Raptor? I find it crazy they can turn on an entire rocket in a vacuum like setup, but it makes total sense $\endgroup$ – Stickyz Mar 11 at 17:49
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    $\begingroup$ @Sean If an engine is firing with reduced acceleration it’s not working right. Main engines generally work at above 1g, not below. $\endgroup$ – Bob Jacobsen Mar 11 at 23:37
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    $\begingroup$ @BobJacobsen: a) [citation needed] (for instance, the space shuttle's OMS engines produced barely 0.1 G of acceleration, and the J-2 on the Saturn IB/V's S-IVB maxed out at about half a G; b) any upper-stage engine, even one that does produce 1 G or more when firing, is still going to be starting up in a zero-G environment. $\endgroup$ – Sean Mar 11 at 23:41
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    $\begingroup$ @Sean gravity mainly affects fuel feed, and rocket engines for that reason don't use gravity fed fuel systems but active pumping (think turbopumps). $\endgroup$ – jwenting Mar 12 at 4:51
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    $\begingroup$ @jwenting And ullage motors. $\endgroup$ – zovits Mar 12 at 14:29

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