Defying Atmosphere - How Rocket Engines Get Tested In A Vacuum Before Going To Space

foreign Scott Manley here there's a certain type of person on the internet who likes to tell me that rocket engines can't work in a vacuum these people don't understand Newton's third law of physics and I know that all of you out there understand Newton's third law but many of you are asking me how do you actually test a rocket engine in a vacuum because obviously vacuum of space is very different from the environment on Earth and if you're testing an engine that is going to go out into deep space and you're not going to be able to mess around with it you want to know that it's going to work in that environment there's a lot of things that can go wrong you probably know that uh the second stage engines on a lot of Rockets use very large nozzle extensions because by expanding the exhaust over a wider area you get better specific impulse and better performance but these large nozzle extensions are fragile and if you try to fire one of these engines with a large nozzle extension in the atmosphere then they gas coming out the far end is actually below atmospheric pressure and the air would squeeze in between the rocket exhaust and the wall of the nozzle and when it does that it's called floor separation and it's a kind of chaotic environment that causes the flow to bump around inside there and can easily damage these fragile nozzles so you want to test without that environment there now you could just test the rocket engine without the nozzle extension but then you're not testing blood the way you would actually fly there's other issues that can happen for example when trying to light an engine having air around it completely changes the Dynamics when you try to you when you try to light something then you've got this get hot gas around it and in fact hot gas is Flowing away too quickly because there's no air to constrain it the engine may not light I remember a few years ago Cody's lab doing a demonstration of how to light these little uh model rocket Motors inside a vacuum chamber and it kept on failing until he put it inside a little pressurized actual and lit it there then it blew out the bottom of the capsule so this is a legitimate problem similarly if you're lighting a liquid-fueled engine you may be spraying your propellants into the combustion chamber and then they evaporate too quickly to light properly in a vacuum during early parts of the ignition you've got all these sort of transient conditions and you might have combustion instability right at the very start of the engine that is suppressed by the Earth's atmosphere that uh it wouldn't whether it may become critical when you're actually flying in deep space and then of course there's the problem that when you're operating in a vacuum you can't rely on things like conduction of heat to the environment around you you have to radiate that heat away and you're operating in space is generally a completely different condition because you've got a lot of very cold dark sky around you and a very hot sun in one place so there's a real need to test space vehicles in all these environments so yes you can put spacecraft in a vacuum chamber and in fact you can test a rocket engine in a vacuum chamber the problem of course is that once you start firing a rocket engine it is emitting a lot of high pressure hot exhaust gas and if you don't have a very big vacuum chamber it's going to fill that up very quickly and you're not testing in a vacuum anymore you can actually do this for a very low mass flow engines things like electric thrusters can be tested in a vacuum chamber with your mechanical vacuum pumps you you know you just get it down to whatever pressure you're testing at and then the mass flow rate of these ion thrusters is so low that uh you can generally keep this thing at a usable pressure for a very long period of time however if you're having say something like vassimir where you're using magnetic confinement on a hot plasma the plasma that does come out is so hot that you need a water jacket to cool down the vacuum chamber while you're testing but once you get up to larger engines this simply isn't possible if you think about it a rocket engine is something that's designed to take fluids combust them produce a lot of gas very very quickly this is what they're optimized for and there's no way that you can build a vacuum chamber with a bunch of mechanical pumps that can keep up with this perfectly optimized Beast of a rocket engine so you need to use the rocket engine to actually help you out with a pumping mechanism what you'll typically do is after the rocket engines nozzle you'll have a cylindrical section A long tunnel which is referred to as diffuser and this uses a basically fluid mechanics uh Ventura it's something like the jury effect although it's not technically the same thing you set it up in such a way that the shock waves coming off the lip of the nozzle impinge on the wall of the vacuum tube and then sort of Bounce Down the tube it makes it very hard for molecules to flow in One Direction but very easy to flow in the other direction so what this rocket engine is doing is blowing all the gas down here and if there's any gas in the test chamber near the engine then it will get sucked down this diffuser and exhausted out of the system so you can literally just have like a closed chamber with a diffuser the engine inside the chamber and once that engine lights it will suck all the gas out of the chamber and you're then firing in vacuum conditions now if you want to test ignition under vacuum conditions then you need a door at the end which closes up and you know this can just be sort of closed up and held on and then when you evacuate the chamber literally the air pressure will hold it in place until the engine Fires at which points it pops off and you've got a rocket engine firing down this tunnel it's an ambient vacuum conditions at initial uh initial ignition and it will maintain that during the burn and then once you get towards the end of the burden you have to turn the engine off then you can have a bit of trouble because you've then got all this atmosphere that's pushing to get back in and that can actually damage the engine as it flows back it can damage the chamber if it's not designed properly so you will sometimes then find a system with fast closing shutters that as soon as they detect air flow backwards up the tunnel these shutters will actuate shut very very quickly and provide you a closed tunnel another thing about the diffuser this enclosed tunnel section is that it is dealing with a lot of thermal flux from the engines so you want to make sure that you have this kept cool it's quite common to find these with a water jacket around them so that uh the the tunnel doesn't get too hot so this works for a lot of engines but once you get to these very high expansion ratio engines the pressure of the exhaust is low enough that the atmosphere at the far end can't be kept out so you need to actively you need active pumps to keep the downstream flow at lower pressure and while again you may imagine a raise of vacuum pumps powered by you know the massive power grid or something they actually tend to use something called Steam ejectors and a steam ejector does actually use the Venturi effect the idea is that with a Venturi effect you have a jet of gas and it will entrain atmosphere around it so you will reduce the velocity of this and increase the ABS momentum flux and so you can use that to suck gas out so what you will have is Downstream from the diffuser first of all you'll have typically a cooling system where you spray water in cool down the exhaust and just by cooling the exhaust down and condensing some of it you're actually going to reduce the pressure a whole lot Downstream from that you might then have a steam ejector where the floor continues and then is given more energy by a steam injection system you might have multiple steamage actors to keep the mass flow up you know to you know to basically counteract the it rocket engine that is desperately trying to fill every possible space with gas and you're trying to suck that all out so the great thing about this is that you can prep for a test by having huge boilers reservoirs full of water that is heated to like 250 Celsius that's kept to pressure and as soon as the test starts you just open those Valves and all the energy in this gas in this air steam this water is being used to help clear out the exhaust pathway and you will see you know two three stages sometimes on these exhaust systems that also helps say for example if you've got an engine which has uh like a solid motor which will be outputting acids and uh dust and things like that this the water will actually help capture that and then you can capture the water and use it for you know clean it up before you release it into the environment rather than say spraying a whole bunch of chemicals into the world around you so yeah these facilities can be exceptionally large in fact during the Apollo program they built a system that didn't just test the J2 engine they actually tested the entire third stage of the Saturn V any vacuum chamber with an engine extraction system that allowed them to test the J2 engine and you know these large test Chambers they don't all they don't just replicate a vacuum with the right conditioning you can also add say cooling panels on the wall where you cool it with liquid nitrogen to make the whole chamber cold and then perhaps on one side you will have a thermal lamps that are heating just one side of your system to replicate what you might see in space or you could have have actually the heating Source moving around to simulate say a barbecue rule you can also simulate the RF environment so you can do entire stage you know test the entire system this way if you want for example NASA recently released videos showing testing off the Mars Ascent vehicle propulsion system these are solid rocket Motors but they're going to operate from the surface of Mars and so they had to test it in a vacuum chamber with a diffuser to capture the exhaust but they also had to chill it down to the ambient temperatures that would be expected on Mars to make sure that the solid rocket Motors would actually light under those conditions there's another test I don't think they did this in vacuum pressure but they actually spun the motor around because the second stage of the Mars Ascent vehicle is going to be spin stabilized and you don't want to find out that when you spin your rocket motor that suddenly the fuel doesn't burn in exactly the same way there is a multitude of engineering effects which you want to identify on the ground before you go to space and now is why these engine Test Facilities are so large and complicated and are able to extend to cover as many environments as possible I'm Scott Manley fly safe [Music] foreign [Music] [Music]

Channel: Scott Manley

Views: 345,735

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Id: hrLyzpTV7GU

Length: 11min 21sec (681 seconds)

Published: Fri Aug 18 2023