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Rocket engine test stands mount horizontally. That's surprising to me given that rocket engines are meant to operate vertically. Doesn't gravity effect any part of the system? Valves, gas mixing, etc.

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    $\begingroup$ Rockets might be launched vertically but I wouldn't say they are meant to be operated vertically. I would think they're designed so that gravity doesn't affect the system. That would be bad. Besides, when your rear engines are accelerating you at multiple Gs, that's the force you're going to feel regardless of the direction you're pointed; not the force of gravity. $\endgroup$
    – DKNguyen
    Feb 16, 2022 at 3:23
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    $\begingroup$ maybe the nozzle exhaust is easier to manage $\endgroup$
    – jsotola
    Feb 16, 2022 at 3:55
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    $\begingroup$ when stuff fails, less risk of launching something into the next town. if you make a thick roof, you need thick walls anyway. easier to just have that wall. $\endgroup$
    – Abel
    Feb 16, 2022 at 5:07
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    $\begingroup$ Are tanks mounted horizontally for liquid engines while testing? For example, the recent SLS engine tests were vertical, right? So the question might be valid on some rocket engines, not all rocket engines. $\endgroup$
    – AJN
    Feb 16, 2022 at 5:17
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    $\begingroup$ You might want to check for similar questions on Space Exploration. This is quite likely to have been asked amd answered there already. A sample $\endgroup$
    – AJN
    Feb 16, 2022 at 5:21

2 Answers 2

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Most of the time during a rocket's flight, it and it's exhaust are far away from the ground. So the engine is optimized for this mode, when there is no exhaust reflection off the ground.

If the engine is tested with exhaust directed downwards, then a very big hole needs to be dug for the exhaust. That's expensive.

If the engine is tested with the exhaust directed upwards, some form of protection would be needed to prevent stuff, like rain or debris, from falling into the engine. This would require a tower for instruments to measure the exhaust; it is harder to distribute the load on the large surface on the ground. Small engines can be tested this way, when they can be installed quickly and load doesn't matter and height isn't that much to need a tower for instruments or there are no need for such instruments.

Gravity mostly affects the fuel tank and fuel pump, not so much the burn chamber and the bell.

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Not all rockets test as such. The Space Shuttles RS-25 engines are tested vertically, bracketed to the launch device; however a ramp diverts the exhaust plume at a 45 degree angle. When Testing Rocket engines alone, often tested vertically to assess pressure/thrust exertion for meaningful data. enter image description here

Horizontally tested engines are often the full rocket (Case in point, SHuttle SRB rockets), these tests are because the fuel is solid thus cannot be fed via a hose or pipe and often test the whole assembly to judge structural integrity.

enter image description here

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