color of NTR exhaust plume?

[toric5]

I'm looking for the probable color of an NTRs exhaust plume for a sprite i am making for a project. google has been no help, nor has atomic rockets website. any pointer or awnsers. (assume upper limit solid state NTR, but a answer based off of a nuclear lightbulb could work to.)

[Bill Phil]

It's hot hydrogen plasma... I think it would the color of hydrogen gas at that temperature.

[fredinno]

8 hours ago, toric5 said:

I'm looking for the probable color of an NTRs exhaust plume for a sprite i am making for a project. google has been no help, nor has atomic rockets website. any pointer or awnsers. (assume upper limit solid state NTR, but a answer based off of a nuclear lightbulb could work to.)

Seems to be translucent red, at least on Earth. In space, the flame would diffuse, and be barely visible.

[Streetwind]

@toric5 - Hydrogen plasma has a primarily red hue, with some slight pink/purple-ish tint, but solid core NTRs don't get hot enough to shift hydrogen into the plasma phase. As you can see, you need a bare minimum of 10^4 K (= 10,000 K) to get hydrogen there. A solid core NTR will top out somewhere around 3,000 K exhaust temperature. There's no way to transfer more heat to the hydrogen without raising the core temperature further, and if you did that, the core itself would melt and destroy the engine. So for solid NTRs, you're getting all the excitement of... a colorless exhaust gas.

I mean, for a KSP mod, most authors give their NTRs pretty tailflames anyway. Rule Of Cool supercedes realism in that case.

A gas core reactor, like the nuclear lightbulb... that might be able to do it. I say "might", because no such thing has ever been built. It's all just paper stats for now.

 

@fredinno - the flame in that video appears orange because it is actually burning. The NERVA test stand had an ignition source right above the nozzle, to make sure that most of the exhausted hydrogen was combusted safely with atmospheric oxygen the instant it left the engine - as opposed to forming a giant, invisible, uncombusted oxygen-hydrogen cloud enveloping the general area of the test stand that could go off at random.   I'm not sure I remember things correctly, but I think they had at least one major testing accident involving uncombusted hydrogen.

 

Edited April 2, 2016 by Streetwind

[Bill Phil]

Gas cores could get up to 25,000 C

[Streetwind]

Yeah, but that's the fuel temperature. Not the heating imparted into the hydrogen.

[Bill Phil]

6 hours ago, Streetwind said:

Yeah, but that's the fuel temperature. Not the heating imparted into the hydrogen.

The amount going to the propellant depends on exposure time and heat effiency of the system. Most gas core designs use transparent materials, ones that are transparent to the radiation given off by the reactor. Heat effiency is usually pretty good in these designs, and you can run the hydrogen around the whole surface area of the transparent materials.

Gas core rockets use radiative heat transfer over conductive.

Edited April 3, 2016 by Bill Phil

[PB666]

20 hours ago, fredinno said:

Seems to be translucent red, at least on Earth. In space, the flame would diffuse, and be barely visible.

That is because of the background brightness. Its temperature is 2370K which means it emits here:

On the yellow side of orange. On the edges at ground level the hydrogen plasma will react with oxygen in the air, this will cause a color shift on the outside of the plume.

Modern day designs, in theory, seek to achieve temperatures around 4000 which means they would emit closer to green part of yellow.

Besides spectrum you have to consider intensity. In a conventional rocket engine the gases are still reacting in the plume, adding color to the plume, in an NTR its about electron exchange amoung equals, its less energetic.So no additional energy is being add except on the ground, but energy is being lost, cooling the gas. (in space the pressure impulse hits space and expands, PV=NRT which means as the gas expands it cools). 

In space the faint yellow orange should be visible at the center of the plume, and it will fade quickly with distance from the rocket.

 

[magnemoe]

7 hours ago, Streetwind said:

 

@fredinno - the flame in that video appears orange because it is actually burning. The NERVA test stand had an ignition source right above the nozzle, to make sure that most of the exhausted hydrogen was combusted safely with atmospheric oxygen the instant it left the engine - as opposed to forming a giant, invisible, uncombusted oxygen-hydrogen cloud enveloping the general area of the test stand that could go off at random.   I'm not sure I remember things correctly, but I think they had at least one major testing accident involving uncombusted hydrogen.

 

Should think the exhaust temperature of 3000 degree would be high enough to ignite it anyway, 
On the other hand you have start up and shut down so it makes lots of sense to use an igniter. For one you do not want oxygen in the reactor then you start to heat it it up and add hydrogen

[Jesse Samuel]

Would an NTR launch NOT be visible from the ground? As in would it avoid the kind of viewing pleasure the Atlas-V launch gave people recently? 

 

[Nibb31]

You wouldn't use an NTR for launch anyway. It would be an upper stage engine.

[Shpaget]

Make it light blue.

[Jesse Samuel]

1 hour ago, Shpaget said:

Make it light blue.

So there would still be a plume. It would just be light blue. 

 

1 hour ago, Nibb31 said:

You wouldn't use an NTR for launch anyway. It would be an upper stage engine.

I am asking what happens, if you did. 

[Nibb31]

9 hours ago, Jesse Samuel said:

I am asking what happens, if you did.

You would be stuck on the ground. Upper stage engines are designed for low thrust/high Isp.

[Shpaget]

12 hours ago, Jesse Samuel said:

So there would still be a plume. It would just be light blue. 

I'm not saying that.

I'm saying that if the OP is making some sort of graphics, then it should have some visible plume, no matter if it is realistic or not.

And since he's making some, is should not be red/yellow, because that's "reserved" for chemical rockets, nuclear green is not going to happen, so blue it is. Blue also goes well with the high temperature of the exhaust (compared to chemical) and high temperature goes along with blue color. Again, yes I realize that ~2700 K corresponds to yellow light, but realism be damned.

Edited August 7, 2016 by Shpaget

[Jesse Samuel]

5 minutes ago, Shpaget said:

I'm not saying that.

I'm saying that if the OP is making some sort of graphics, then it should have some visible plume, no matter if it is realistic or not.

And since he's making some, is should not be red/yellow, because that's "reserved" for chemical rockets, nuclear green is not going to happen, so blue it is. Blue also goes well with the high temperature of the exhaust (compared to chemical) and high temperature goes along with blue color. Again, yes I realize that ~2700 K corresponds to yellow light, but realism be damned.

Okay, but does anyone here know, if an NTR launch at night would have the same kind of plume the Atlas V launch displayed? 

[pincushionman]

3 hours ago, Jesse Samuel said:

Okay, but does anyone here know, if an NTR launch at night would have the same kind of plume the Atlas V launch displayed? 

If they used an Atlas to launch it, then yes.

[Jesse Samuel]

Can anyone describe what an all NTR launch at night would look like? If it would not look like an Atlas V launch what would it look like? 

[Kryten]

Atlas gives that plume because of water in the exhaust; Soyuz second/third stages and Falcon 9 give similar ones. A pure hydrogen plume might still be visible, depending on how diffuse it is (amateurs get photos of hydrogen plumes from centaur stage fuel dumps all the time), but you won't get a massively visible 'jellyfish' plume.

[Brotoro]

4 hours ago, Shpaget said:

...Blue also goes well with the high temperature of the exhaust (compared to chemical) and high temperature goes along with blue color...

People keep spreading this misinformation...so I keep pointing out that NTRs do NOT run hotter than chemical engines. The Space Shuttel Main Engine ran hotter than the NERVA.

[Shpaget]

4 hours ago, Brotoro said:

People keep spreading this misinformation...so I keep pointing out that NTRs do NOT run hotter than chemical engines. The Space Shuttel Main Engine ran hotter than the NERVA.

You're right.

Here's a useful chart somebody made dealing with various aspects of various fuels and oxidizers.

http://www.thespacerace.com/forum/index.php?PHPSESSID=b8a2b8bf9d202f91a58dc71360581a60&topic=2583.msg17481#msg17481

 

And all that being said, I still stand by my choice of color, as for the OPs purposes, as he presented it, it's art that has different requirements than realistic depiction.

[cryogen]

On 4/1/2016 at 3:52 PM, toric5 said:

I'm looking for the probable color of an NTRs exhaust plume for a sprite i am making for a project. google has been no help, nor has atomic rockets website. any pointer or awnsers. (assume upper limit solid state NTR, but a answer based off of a nuclear lightbulb could work to.)

It's probably invisible, actually. Hydrogen gas has very little emissivity; it's transparent and doesn't glow when heated. Check out photos of the RS-25 engine (shuttle launches, or SLS engine tests); they're almost totally invisible, except for a faint blue glow. Even that glow is probably caused by steam (this person suggests chemiluminescence from radical recombination, I assume OH radicals), and wouldn't show up in a pure-hydrogen exhaust stream.

RS-68 engines, by contrast, have an orange-pink glow, despite using the identical LH2/LOX fuel as RS-25. The difference is that RS-68 is an ablatively-cooled engine, which means the engine nozzle liner is slowly vaporized to provide evaporative cooling. It's this ablated organic material in the exhaust stream that makes it colorful.

I suspect the pink color visible in NTR ground tests is also caused by something ablating; I don't know. If a flight NTR engine involved something ablating, then that could introduce some color; maybe it'd look like RS-68.

 

On 4/2/2016 at 2:16 PM, Streetwind said:

@fredinno - the flame in that video appears orange because it is actually burning. The NERVA test stand had an ignition source right above the nozzle, to make sure that most of the exhausted hydrogen was combusted safely with atmospheric oxygen the instant it left the engine - as opposed to forming a giant, invisible, uncombusted oxygen-hydrogen cloud enveloping the general area of the test stand that could go off at random.  

I think it could be something ablating into the exhaust stream, maybe the protective of the nuclear fuel. That eroded pretty fat in the NERVA tests, but I can't find confirmation they're responsible for the color.

On 4/2/2016 at 2:16 PM, Streetwind said:

@toric5I'm not sure I remember things correctly, but I think they had at least one major testing accident involving uncombusted hydrogen.

Yes, the KIWI B1A test was aborted early because of a large hydrogen fire. According to p. 29-30 of

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920005899.pdf

On 4/2/2016 at 8:48 PM, PB666 said:

That is because of the background brightness. Its temperature is 2370K which means it emits here:

On the yellow side of orange. On the edges at ground level the hydrogen plasma will react with oxygen in the air, this will cause a color shift on the outside of the plume.

That's the rule for emissive blackbodies -- it doesn't work here, since a hydrogen gas is very far from a blackbody (in fact hardly emits anything). You can see in photos that RS-25 engine exhaust barely emits any light. You get closer to a blackbody with massive, solid particles, like the alumina dust in SRB exhaust that makes it glow bright white.

On 8/7/2016 at 1:19 AM, Nibb31 said:

You wouldn't use an NTR for launch anyway. It would be an upper stage engine.

Exactly, an NTR SSTO would be very difficult (see Kirk Sorensen's blogpost), and you don't want an NTR first stage that returns to Earth. 

TIMBERWIND was a NTR 2nd stage with impressive payload improvements -- but the economics would probably be horrible. 

https://en.wikipedia.org/wiki/Project_Timberwind

 

22 hours ago, Brotoro said:

People keep spreading this misinformation...so I keep pointing out that NTRs do NOT run hotter than chemical engines. The Space Shuttel Main Engine ran hotter than the NERVA.

Exactly -- if anything, they're limited to being somewhat cooler. Their enormous Isp advantages doesn't come from higher exhaust temperature, but from the exhaust having much lower molecular mass. 

Edited August 8, 2016 by cryogen
misc edits

[K^2]

8 hours ago, cryogen said:

That's the rule for emissive blackbodies -- it doesn't work here, since a hydrogen gas is very far from a blackbody (in fact hardly emits anything).

With few exceptions, where spectral lines make a large difference, this fact doesn't actually change the chroma. Yes, H2 plume of the NTR is going to be very, very faint for the reasons you describe, but the color of what little light is going to be emitted is going to be red to deep orange as governed by the black body radiation.

[Kryten]

10 hours ago, cryogen said:

It's probably invisible, actually. Hydrogen gas has very little emissivity; it's transparent and doesn't glow when heated. Check out photos of the RS-25 engine (shuttle launches, or SLS engine tests); they're almost totally invisible, except for a faint blue glow. Even that glow is probably caused by steam (this person suggests chemiluminescence from radical recombination, I assume OH radicals), and wouldn't show up in a pure-hydrogen exhaust stream.

RS-68 engines, by contrast, have an orange-pink glow, despite using the identical LH2/LOX fuel as RS-25. The difference is that RS-68 is an ablatively-cooled engine, which means the engine nozzle liner is slowly vaporized to provide evaporative cooling. It's this ablated organic material in the exhaust stream that makes it colorful.

I suspect the pink color visible in NTR ground tests is also caused by something ablating; I don't know. If a flight NTR engine involved something ablating, then that could introduce some color; maybe it'd look like RS-68.

What causes the yellow glow of hydrolox engines at low thrust settings? This can be clearly seen in images of DC-X or New Shepard landings. Given in both cases easy reuse of the engines has been demonstrated, it's very unlikely to be anything ablating.

[DDE]

OP, the convention in Nertea's mods has been to depict it purple, as seen with the plasma above.

If you could somehow contact the guy behind Children of a Dead Earth, he's got a model for calculating exhaust appearance in vacuum:

https://childrenofadeadearth.wordpress.com/2016/04/11/why-does-it-look-like-that-part-1/