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Gorgeous photo! Here's some #SundayScience about rocket exhaust (thread) /1
2/ I study rocket exhaust and how it interacts with planets' surfaces and atmosphere. There is a lot of interesting physics in this. Note how this rocket plume gets narrower right after it leaves the nozzle. That's because it comes out at lower pressure than the surrounding air. 
3/ The job of the rocket nozzle is to expand the plume. If the nozzle is too small, then the plume is underexpanded, so it gets bigger right after leaving the nozzle. Here's an underexpanded plume from the Apollo Saturn V 2nd stage. See how it flares wider?
4/ That picture is very high in the atmosphere where the air is very thin, so it would be impractical to have a nozzle big enough to perfectly expand that plume. The Rocket Lab plume is slightly over expanded, so the plume gets narrower after leaving the nozzle. That's because...
5/...it is tuned to be perfectly expanded at higher altitude where air is thinner. It is slightly overexpanded at ground level so it displays Mach structure. You can see the Mach diamonds better in this image of the Armadillo Pixel lander. The jet oscillates in width.
6/ The jet goes too wide, then swings back too narrow, then too wide repeatedly. Every time it gets too narrow, the compressed gas gets very hot and radiates extra light. The bright spots at these narrow spots are called Mach diamonds. You can see them in the Rocket Lab plume.
7/ At those points, the gas molecules slam into each other so hard that energy from the collisions gets smashed into the electrons that orbit those molecules. Then, the electrons want to give away that energy, so it goes into photons, which is the light from the Mach diamonds.
8/ From this rocket you don't see multiple Mach diamonds. Instead, because there are so many jets side-by-side, they mix with each other after the first diamond. At the bottom all that is left is turbulent gas, no more jets.
9/ The center of the jets are supersonic and are called the Potential Core. As the jets mix with surrounding atmosphere, the supersonic potential cores taper off, ending at a definite length. We call that the Jet Extinction Length. (I can't tell exactly where it is in the photo.)
10/ The extinction length is vital to predict how much of a crater it will make when landing on a planet. You don't want the supersonic cores to touch the dirt, because dirt cannot resist supersonic flow. Everywhere this jet core touches dirt immediately becomes a hole.
11/ For the Viking landings, the engines originally had one nozzle each. Tests showed it would blast too much soil so they were replaced by a "showerhead" of 18 tiny nozzles. Just like the Rocket Lab plume, this would mix the jets better, shortening the extinction length.
12/ Carl Sagan standing next to a model of a Viking lander. You can see the showerhead nozzle next to him. This was vital to ensuring the spacecraft landed safely and did not overly disturb the soil where measurements were going to be taken.
13/ This artist picture of the Curiosity Skycrane plumes is not realistic. Can you guess why?
14/ Answer: @Doctor_Astro showed that the jet extinction length is vastly longer than shown in this image. In fact, because the Mars atmosphere is so thin, the mixing is very weak, so the jets reached all the way to the soil, despite the height of the SkyCrane.
15/ She predicted that the jets would touch the soil, creating narrow holes. The gas then has to come back out of those holes, so it would shoot regolith back at Curiosity as it was being lowered. She predicted this would happen before the landing occurred. Did it occur?...
16/ Yep! Curiosity had gravel sprayed all over its instrument deck. In fact, one of the wind sensors was broken, we think from gravel hitting the wires at high speed. You can see the gravel on Curiosity in this picture. This is why plume science is important.
17/ You can see two of the craters where Curiosity was set down by the SkyCrane. They are the smoothed depressions in the top center. They don't look like the narrow holes I was describing, which would shoot soil and gravel back toward Curiosity. Can you guess why not?
18/ We think it is because these are not the original craters. These are what we call the Residual Craters. After the jet turns off or goes away, the original crater collapses leaving only tbis. The gas flow had been keeping the deep narrow holes open. We see this in experiments.
19/ Here's a high speed video of a jet cratering sand. The aerodynamic forces of the jet hold up the sides of the hole much steeper than what is normal for sand. The narrow hole (before collapse) is what shoots regolith back at the spacecraft.
20/ So now you have enough information to answer the Apollo Hoax theorists about why there was no crater on the Moon under the Lunar Modules. Think in terms of overexpanded plumes, jets cores, and catering physics...
21/ How big does a nozzle have to be to perfectly expand a plume in a lunar landing? Well, the surrounding atmosphere is zero pressure, so to expand the plume all the way down to nearly zero pressure the nozzle would need to be gargantuan! The nozzle would weigh too much to fly.
22/ So the plume coming out of the Lunar Module is vastly, vastly u nderexpanded, and as it cpmes out of the nozzle it fills the entire hemisphere under the lander, and some of the gas even blows completely around the lander and goes UP! So, there is no "jet".
23/ Because there is no jet, the gas forces are spread around widely on the soil with no sharp boundaries. On Earth or Mars the surrounding atmosphere creates the jet which is like a post-hole digger. On the Moon the plume just scours the surface. No crater.
24/ If the Lunar Module wasn't so close to the Moon, it actually would have a jet, because the Moon actually does have a very thin atmosphere. But the pume coming out of the LM would have to expand to bigger than the Moon before its pressure matched the surrounding atmosphere.
25/ So if you put the LM in space away from the Moon and fired the engine, it would expand to giant size and then oscillate in the thin interplanetary medium. We actually do see jets in the thin medium of space! Here's an example, a jet shooting out from a giant black hole.
26/ That jet is shooting out of the M87 galaxy, from the supermassive black hole at the galactic core. Here it is in three different wavelengths. It even has internal structure, reminiscent of Mach disks in rocket plumes! (Image credit here chandra.harvard.edu/photo/2001/013…)
27/ We shouldnt try to take the similarity too far, but it does have the following in common with rocket plumes. First, it is a jet. A jet is a fluid traveling at higher speed than the surrounding fluid. (In this case, the surrounding fluid is the sparse intergalactic medium.)
28/ Second, it emanates from a sourxe of high energy before expanding into the surrounding medium. In a rocket, the high energy is from fuel burning in the combustion chamber. With galactic jets, it is the energy of matter being extremely squeezed by a black hole.
29/ Third, something is holding the jet together until it reaches the extinction length. In rocket plumes, it is the surrounding atmospheric which has low enough viscosity that it doesn't mix too much. It galactic jets I'm no expert but I think an embedded magnetic field helps.
30/ Fourth there is internal structure that radiates light at certain distances down the jet. In rocket plumes it is the shock waves from the plume over- and under- expanding in the surrounding air. In galactic jets it is also caused by embedded shock waves.
31/ In a galactic jet's shockwaves the electrons are accelerated to spin around the embedded magnetic fields and then the electrons give off photons. That's a little different than the reason electrons radiate in a rocket shock wave, but the similarity makes me happy :)
32/ Finally, we might think that this embedded magnetic field with free electrons swirling around is unique to the physics of galactic cores, too exotic for rocket exhaust. We would be wrong. It turns out, rocket exhaust is actually an electrically charged plasma. How exotic! :)
33/ The rocket exhaust is so hot as it travels down the nozzle, that the atoms can't hold onto all their electrons. As atoms and electrons run into the sides of the nozzle, some absorb into the metal. But electrons are far smaller than atoms, so they go into the metal more often.
34/ As a result, the rocket or lander builds up a negative charge, and the rocket plume is positively charged. It is actually an electrical current shooting into space, or onto the surface of the Moon. We aren't completely sure yet what effects this has on the lunar soil. But...
35/ We think it comtributes to the extended dust levitation we measured after the engines shut off. Here is Apollo 15 after landing and again about 30 seconds later. It took that long for the dust to clear, which is surprising!
36/ One theory is that the rocket plume just got done spraying electrical charge all over the surrounding soil, so all the dust is positively charged and it is all repelling each other, continuously hopping off the surface, until the charged dust hops far away.
37/ This might have something to do with the change in surface texture and therefore reflection of sunlight off the soil all around the landing sites, which @Ryan_N_Watkins has measured using data from the @LRO_NASA spacecraft. Her analysis of Chang'e 3: sciencedirect.com/science/articl…
38/ In the gorgeous Rocket Lab photo, you can't see it but the rocket there is becoming negatively charged, too. When the voltage is big enough, it pulls positive charge from the surrounding air so the rocket doesn't charge up to infinity.
39/39 We haven't figured out the details yet on how a lunar lander neutralizes with its environment. More research is needed!
I hope this rocket plume nerd fest was interesting! 😅
I hope this rocket plume nerd fest was interesting! 😅
*Mach disks
Appendix (40/39): I see that I accidentally wrote "Mach diamonds" at least twice where I should have written "Mach disks". The diamonds are the oscillations in jet diameter. The disks are the bright spots where the diamonds are at their narrowest point. Sorry for the confusion!
