Real Rocket Plumes As Depicted In Space VS Scifi

[Spacescifi]

 

The media I presume does not depict rocket plumes in space correctly. Even Ad Astra showed bulb shaped plumes in deep space, and many video games do too.

From what I have seen, the answer seems to be that it depends.

I have also seen that a funnel cone plume is fairly common in space, but I wonder if it would even be possible to get a bulb shaped plume in space since there is no air pressure to pinch the plume at all.

I know plume color depends on propellant plus the heat imparted to it. I have read that super efficient antimatter thermal rockets would not have much of a visible plume, since so little propellant is required for high thrust.

So what is it? What do you know on this subject?

 

 

[razark]

Note the plume shown here:

and here:

 

Edited October 4, 2019 by razark

[Guest]

Hydrogen plumes are practically invisible in space. High mass flow methane NTRs might have a "heat blur" effect visible behind them. For chemical engines, you get yellow/reddish flame and whispy smoke. All of them are conical. With high energy engines, the plume will look like a beam, only slightly wider at the end than at the engine, because the particles are travelling very fast. The higher the Isp, the smaller the angle of the cone.

[Spacescifi]

2 hours ago, Dragon01 said:

Hydrogen plumes are practically invisible in space. High mass flow methane NTRs might have a "heat blur" effect visible behind them. For chemical engines, you get yellow/reddish flame and whispy smoke. All of them are conical. With high energy engines, the plume will look like a beam, only slightly wider at the end than at the engine, because the particles are travelling very fast. The higher the Isp, the smaller the angle of the cone.

 

Thanks.

So with high energy engines, how wispy the exhaust is depends on how much propellant we are burning. More means higher thrust, more wispy means weaker thrust but more propellant burn time?

Or is it the other way around?

[Guest]

It depends on the exhaust products. Specifically, the refractive index versus vacuum. This causes the "heat blur" effect. Hydrogen has a very low refractive index, so it's barely visible, if at all. Most other propellants will have a visible distortion.

Of course, if your exhaust is hot enough, it will glow on its own, with blackbody radiation depending on temperature. You generally want to avoid that as much as possible (you want it to stop glowing while still inside the nozzle), but for high energy engines, it may be visible. A beam-like plume might glow white or even blue, if it's very hot.

[Spacescifi]

46 minutes ago, Dragon01 said:

It depends on the exhaust products. Specifically, the refractive index versus vacuum. This causes the "heat blur" effect. Hydrogen has a very low refractive index, so it's barely visible, if at all. Most other propellants will have a visible distortion.

Of course, if your exhaust is hot enough, it will glow on its own, with blackbody radiation depending on temperature. You generally want to avoid that as much as possible (you want it to stop glowing while still inside the nozzle), but for high energy engines, it may be visible. A beam-like plume might glow white or even blue, if it's very hot.

 

Hydrogen is tricky though since people say it cannot be stored for long periods without it all boiling/vaping off.

Unless future us figures out a way to prevent that. So spacehip/starship propellant will likely be a variety. One for deep space, and another for launch.

Also, I read somewhere that violet colored light gives off more energy than blue, so would'nt a purplish plume be ar a higher energy than blue?

Although I do know that a white plume beats all (the sun is blazing white in space).

[sh1pman]

34 minutes ago, Spacescifi said:

Also, I read somewhere that violet colored light gives off more energy than blue, so would'nt a purplish plume be ar a higher energy than blue?

You can’t get purple plume (unless you mix in some purple substance) because black body radiation always includes longer wavelengths that make it more white. At very high temperatures it’s bluish-white, and increasing the temperature just makes it brighter.

[Spacescifi]

5 minutes ago, sh1pman said:

You can’t get purple plume (unless you mix in some purple substance) because black body radiation always includes longer wavelengths that make it more white. At very high temperatures it’s bluish-white, and increasing the temperature just makes it brighter.

 

Now U see why all the media scifi spaceships have blue plumes. Guess they got that right at least.

However what about these?

 

Then again... violet lightning often happens in the rain, so there is that.

[kerbiloid]

purple = red + blue

You should have something 2000 K and 5000 K hot at once.
(In fact, of course cooler).
So, only either a mixture, or some specific atom with additional red in spectre.

Edited October 5, 2019 by kerbiloid

[sh1pman]

8 hours ago, Spacescifi said:

Then again... violet lightning often happens in the rain, so there is that.

Only because atomic emission spectra of oxygen and nitrogen has strong lines in the blue and purple portions of the visible spectrum. 

[Guest]

12 hours ago, sh1pman said:

You can’t get purple plume (unless you mix in some purple substance) because black body radiation always includes longer wavelengths that make it more white. At very high temperatures it’s bluish-white, and increasing the temperature just makes it brighter.

Actually, this is because it would require more than an infinite temperature:

If you heat up a gas enough, it will become more blue, but it doesn't go any further. Of course, it will also become brighter overall. 

12 hours ago, Spacescifi said:

Unless future us figures out a way to prevent that. So spacehip/starship propellant will likely be a variety. One for deep space, and another for launch.

We have figured that out already. Zero-boiloff tanks are a thing, though they are heavy. 

Edited October 5, 2019 by Guest

[sh1pman]

2 minutes ago, Dragon01 said:

Actually, this is because it would require an infinite temperature:

Didn’t you read what I wrote? At infinite temperature it’s bluish-white, not purple. Look at the picture you attached, it doesn’t go into purple area. That’s because black body spectrum at infinite temperature includes all visible wavelengths, and purple is combined with higher wavelengths, resulting in light blue. If you filter out higher wavelengths, you’ll see purple.

[Guest]

24 minutes ago, sh1pman said:

Didn’t you read what I wrote? At infinite temperature it’s bluish-white, not purple. Look at the picture you attached, it doesn’t go into purple area. That’s because black body spectrum at infinite temperature includes all visible wavelengths, and purple is combined with higher wavelengths, resulting in light blue. If you filter out higher wavelengths, you’ll see purple.

Fine, it would require more than infinite temperature. Anyway, it's really more blue than white at extreme temperatures. 

[magnemoe]

23 hours ago, razark said:

Note the plume shown here:

and here:

 

And this explains why we have visible rocket plumpes in movies and KSP. 
The engine is not running, complain
But yes having them expand more would make sense 

However how KSP players tend to cluster LV-N engines this could well give weird results then the plumpes overlap 

[IncongruousGoat]

The mistake that really gets me is when movies (haven't seen it in a show yet, but I don't really watch shows so YMMV) get the in-atmo plume wrong. Specifically, use of a dense, cloudy, off-white solid-fuel plume where it just isn't even vaguely appropriate. The Martian did it with the MAV (which is either hydrolox or methalox based on ISRU chemistry), but the worst offender I can think of is Hidden Figures, which mixed CGI footage with real historic launch footage for the launch of Mercury-Redstone 3. In the historic footage you can see the largely transparent ethanol-LOX plume clear as day. In the CGI? Big, cloudy, off-white, <insert profanity of choice here> SRB exhaust.

I personally blame the Shuttle for ruining several generations' view of what a rocket launch looks like, but that may just be my overall dislike of the Shuttle leaking over.

[kerbiloid]

6 hours ago, IncongruousGoat said:

The Martian did it with the MAV (which is either hydrolox or methalox based on ISRU chemistry)

That plume looks very familiar, I have already seen it in some videogame video.
Idk in what game, but a futuristic shuttle started from probably Rio-de-Janeiro (because the Rio statue) in same clouds of white smoke. Funny, but its plane-like first stage unlikely could be solid-fuel, too.

Edited October 6, 2019 by kerbiloid

[Xd the great]

36 minutes ago, kerbiloid said:

That plume looks very familiar, I have already seen it in some videogame video.
Idk in what game, but a futuristic shuttle started from probably Rio-de-Janeiro (because the Rio statue) in same clouds of white smoke. Funny, but its plane-like first stage unlikely could be solid-fuel, too.

Looks like solid fuel to me too.

I also suggest looking at exhaust from different fuel mix.

High carbon rockets have smoky/yellowish exhaust from incomplete combustion, and a bit of free carbon.

Liquid hydrogen engines have practically invisible exhaust (water vapour is essentially colourless)

N2O4/UDMH or N2O4/MMH engines have less visible exhaust due to more complete combustion/lower carbon content. Same for methaneLOX engines.

[Spacescifi]

13 minutes ago, Xd the great said:

Looks like solid fuel to me too.

I also suggest looking at exhaust from different fuel mix.

High carbon rockets have smoky/yellowish exhaust from incomplete combustion, and a bit of free carbon.

Liquid hydrogen engines have practically invisible exhaust (water vapour is essentially colourless)

N2O4/UDMH or N2O4/MMH engines have less visible exhaust due to more complete combustion/lower carbon content. Same for methaneLOX engines.

 

I take it you mean near invisible in space?

Since this methane rocket plume is visible in the air.

https://m.youtube.com/watch

?v=e7kqFt3nID4

Edited October 6, 2019 by Spacescifi

[Xd the great]

25 minutes ago, Spacescifi said:

 

I take it you mean near invisible in space?

Since this methane rocket plume is visible in the air.

https://m.youtube.com/watch

?v=e7kqFt3nID4

Yep, thanks for correction