Shuttle Main Engines: What's Happening?

[Clausius2]

I am going to inaugurate the "What is happening here?" post series. I am interested on knowing what is happening physically in some aspects of flow around aircrafts. And surely, due to the high level of the co-forum-ers people :thumbs_up , you could provide me an answer, if you want of course, and discuss the reasons.

First of all, please open the attached file *.jpg. It shows the exhaust zone of the Shuttle at launch.

Why does it appear that blue colour?. All of us know that it is inside the combustion chamber where the flame and chemical reaction takes place. So that, what do you think is that?. I think it cannot be a detached flame. It is too outside to be a combustor's flame. It is a singular effect that is puzzling me. Surely it has to do with shocks waves interactions at the outlet.

We have seen it a lot of times in TV, but in particular I'm not able to explain it.

 

[sal]

Why does it appear that blue colour?. All of us know that it is inside the combustion chamber where the flame and chemical reaction takes place. So that, what do you think is that?. I think it cannot be a detached flame.

Just to add my ignorance to the pool...

The main engines burn hydrogen/oxygen, and hydrogen burns with a colorless flame. So, the blue color can't be from the flame of the burning fuel.

 

[hitssquad]

Maybe it is atmospheric nitrogen ignited by the heat and pressure of the engine exhaust.

 

[cronxeh]

I think its the air itself that is emitting the blue'ish color. Since hot objects emit color when they cool off - white being the hottest, and red being somewhat cooler, you can compare the color and get an approximate temperature of the region.

Some sample colors given in Kelvin: (0K = -273.15 degrees Celcius, so 273.15 K = 0 degrees C)

1500 k Candlelight
2680 k 40 W incandescent lamp
3000 k 200 W incandescent lamp
3200 k Sunrise/sunset
3400 k Tungsten lamp
3400 k 1 hour from dusk/dawn
5000-4500 k Xenon lamp/light arc
5500 k Sunny daylight around noon
5500-5600 k Electronic photo flash
6500-7500 k Overcast sky
9000-12000 k Blue sky

So the temperature in the blue cone area on the picture should be somewhere around 20,000 Kelvin

 

[enigma]

Hrmm. I'm not sure.

I do know that it isn't a 20,000K combustion. The temperature in the combustion chamber is less than 4,000K, and it can't get hotter than that without a secondary combustion. Even still, it wouldn't be all the way to 20,000 K. That's just too hot. My guess would be some sort of oxidation reaction instead of blackbody radiation (The burners on your gas stove aren't at 20,000 K, but they're blue).

I do know why the rings are so much smaller than the exhaust area. The SSME is designed for optimal performace at a fairly high altitude. That means that the flow is expanded much more than it needs to be for optimal performance at sea level. The pressure of the exhaust gases is much less than sea level at the nozzle exit. This causes the flow to be diverted inward.

Another thing to consider is the fuel to oxidizer ratio. See here. It lists the masswise O/F ratio of the SSME as 6.0, which means that stochiometrically there is more hydrogen than oxygen. This hydrogen at the high temperatures in the chamber will likely be dissociated into a monatomic state. It's possible that it's mixing with atmospheric CO₂ to form methane or some other combustable gas which burns with the atmosphere's oxygen.

 

[sal]

I do know that it isn't a 20,000K combustion. The temperature in the combustion chamber is less than 4,000K, and it can't get hotter than that without a secondary combustion. Even still, it wouldn't be all the way to 20,000 K.

Yes, among other things, water decomposes by around 4000 K, so the reaction's going the wrong way at higher temperatures.

That's just too hot. My guess would be some sort of oxidation reaction instead of blackbody radiation (The burners on your gas stove aren't at 20,000 K, but they're blue).

Right, methane burns blue -- the photons the reaction spits out are the right energy to look blue. The relationship with the flame temperature is weak at best. In general, flame color is not closely related to blackbody temperature.

It's possible that it's mixing with atmospheric CO₂ to form methane or some other combustable gas which burns with the atmosphere's oxygen.

That might make sense... certainly nothing coming out of the shuttle should be burning with a blue flame. OTOH there's not much CO₂ in the atmosphere -- gas #3 is argon, which is (supposedly) inert. CO₂ is #4, and I don't remember what #5 is (helium? nah...)

Is it possible the nitrogen is burning, as hitssquad suggested? I don't know what kind of temperature you need to ignite it, and I have no idea what color it would burn. For some reason I think of a red flame but that's probably not right.

 

[enigma]

I just thought of a possibility:

It may be some ablative material which is burning. There are layers of an ablative insulation which lines the inside of the nozzle. This insulation melts away in high temperatures, protecting the structure beneath it. Carbon is a typical component of it. Methane may be associating in the exhaust flow (which is of a fairly low temperature by the point it gets to the nozzle exit... exit area to throat area is really big in the SSME) from that carbon and unburnt hydrogen fuel.

It's possible that there is a normal shock right where the fire starts. The shock would slow the flow down to subsonic speeds, increasing the local temperature (tremendously... almost to chamber temperature) in the process. That would account for the absence of fire anywhere higher than the exhaust plume, regardless of what is burning.

I don't know about the color of the nitrogen->oxides of nitrogen reaction.

 

[enigma]

Yet another possibility:

I don't know when in the liftoff this picture was taken (other than early). It's possible that we're looking at a unburnt remains of the hypergolic fuels used to start the engine (MMH/N204, IIRC).

 

[LURCH]

I believe that what we are seeing are the tips of the "shock diamonds". These are apparent in many jet engines when exhaust velocity is supersonic.

 

[Clausius2]

I do know that it isn't a 20,000K combustion. The temperature in the combustion chamber is less than 4,000K, and it can't get hotter than that without a secondary combustion. Even still, it wouldn't be all the way to 20,000 K.

I absolutely agree with you.

I do know why the rings are so much smaller than the exhaust area. The SSME is designed for optimal performace at a fairly high altitude. That means that the flow is expanded much more than it needs to be for optimal performance at sea level. The pressure of the exhaust gases is much less than sea level at the nozzle exit. This causes the flow to be diverted inward.

Yes!. I've never heard about that, but I was thinking of something similar since I learned about supersonic nozzles!. You mean, the nozzle is calculated to exhaust with a diamond-shaped jet and oblique shock waves. In the narrower parts of this jet, there is some shockwaves interactions. The jet boundary is a shock wave itself, therefore as the exhausted flow collides with this shock it is formed another shock wave inside because of a compression. Once the flow pass through the narrower jet section, it is developed a Prandtl-Mayer expansion (if anyone does not know about it, please put it across). The Shuttle main engines, also other rocket engines are designed to operate without shock waves at high altitudes. There, the pressure is small enough to produce an entire Prandtl-Mayer expansion at the outlet. But another question is: why? Is this diamond-shaped jet less energetically efficient?. Or does it provide better thrust?

Another thing to consider is the fuel to oxidizer ratio. It lists the masswise O/F ratio of the SSME as 6.0, which means that stochiometrically there is more hydrogen than oxygen. This hydrogen at the high temperatures in the chamber will likely be dissociated into a monatomic state. It's possible that it's mixing with atmospheric CO₂ to form methane or some other combustable gas which burns with the atmosphere's oxygen.

I don't understand it well. First of all, what is this ratio?. It is a ratio relative to the stochiometric one? It could be interpreted as 6 mol of 02 will react with 1 mol of H2. If this is true, in the stochiometric reaction 0.5 mol of 02 reacts with 1 mol of H2, so that the reaction in the Main Engines would not leave any hidrogen to be post-burned, because the mixture will have an excess of 02. I mean, the combustion reaction takes place in a poor mixture. Please, clear it up.

I don't know when in the liftoff this picture was taken

The shuttle was in flight yet. But taking into account it, I read sometime that the Main Engines are started at the liftoff, but they are not powered to the maximum until certain altitude. So that, maybe the engine displayed is not functioning powerfully.

What you've said about the temperature increasing inside the jet is very logical due to shock waves. The zone displayed is one in which shock interactions give off a lot of heat. In my opinion, the hypothesis of some re-burning process seems to be the most accurate.

Thanks to all for replying.

 

[enigma]

Yes!. I've never heard about that, but I was thinking of something similar since I learned about supersonic nozzles!. You mean, the nozzle is calculated to exhaust with a diamond-shaped jet and oblique shock waves.

Sort of. You get highest thrust when your nozzle expands the flow so that the exit pressure is equal to the ambient pressure. Since the Shuttle has the SRB's to provide extra thrust near sea level, the SSME was designed to have optimal performance at a higher altitude. That means that the flow is overexpanded near sea level.

I don't understand it well. First of all, what is this ratio?. It is a ratio relative to the stochiometric one? It could be interpreted as 6 mol of 02 will react with 1 mol of H2. If this is true, in the stochiometric reaction 0.5 mol of 02 reacts with 1 mol of H2, so that the reaction in the Main Engines would not leave any hidrogen to be post-burned, because the mixture will have an excess of 02. I mean, the combustion reaction takes place in a poor mixture. Please, clear it up.

It is the mass flow rate of the oxidizer divided by the mass flow rate of the fuel. For non-toxic fuels (like the Shuttle uses) it is beneficial to run the engine fuel-rich. That makes the reactant molar mass lower, which improves performance.

I believe that what we are seeing are the tips of the "shock diamonds". These are apparent in many jet engines when exhaust velocity is supersonic.

Yes, but jets use (essentially) kerosene for fuel. You can see the byproducts of the combustion. You cannot see a H2/O2 combustion, so we're wondering where the blue is coming from.

 

[Clausius2]

It is the mass flow rate of the oxidizer divided by the mass flow rate of the fuel. For non-toxic fuels (like the Shuttle uses) it is beneficial to run the engine fuel-rich. That makes the reactant molar mass lower, which improves performance.

Now you have puzzled me again. If $$\psi=\frac{m'_{02}}{m'_{H2}}=6$$ is the "oxidizer to fuel ratio", then in 1 second of time, it will be provided 6 units of 02 mass more than H2 mass. The stochiometric reaction:

$$H_{2}+0.5O_{2}--->H_{2}0$$

states that the stochiometric oxidizer to fuel ratio is
$$\psi=\frac{0.5mol*32g0_{2}/mol}{1mol*2gH_{2}/mol}=8$$

Hey! you are all right! The Shuttle mixture is fuel-rich!

Do not be surprised if I start my posts saying " it's impossible!", but after writing some figures I say: " after all you were correct".

It helps me to understand what on Earth are you saying.

 

[Clausius2]

I've got another useful image over here. Take a look if you want. Here this zone is viewed better.

 

[LURCH]

Don't know if this is usefull, but I've noticed that the cone-shaped blue areas in the first pic originally form as full, tear-drop shapes that go from the engine nozzle openeing all the way back to the point. It is only after a second or two that the area of discoloration progresses backward to the "tips" of these cones and dissappears altogether. Could this mean that we are seeing the incineration of the air behind the engine, or perhaps dust particles and impurities in the air? Seems unlikely, now that I say it, since such gasseus and particulate matter would blow out of the way much more quickly than the "blue zone" moves.

 

[enigma]

In that case, LURCH:

My official guess is that the blue cone is formed by the hypergolics being burned to start the H/O reaction.

 

[Clausius2]

But this hypergolics do not last forever. The time during the combustion of this hypergolics must be negligible compared with the total ignition time. Is a casuality that all images I've found have this blue cone?

I will try to read about that, but in my opinion the suggestion of H2 re-burning is the most plausible cause.


Lurch, was this to what you were referring to in the last post? (see the bottom image)

 

[GENIERE]

http://www.engineeringatboeing.com/articles/plume.html

"The exhaust species from a highly efficient oxygen/hydrogen propellant engine, such as the SSME, are primarily water vapor (H2O) and a small amount of residual hydrogen (H) and hydroxide (OH). The flame seen is the byproduct of both thermal and chemiluminescent excitation processes occurring in the main combustion chamber and continuing out the nozzle exit.

The radiated energy from the hot plume is predominantly the blackbody radiation from water vapor emission bands throughout the infrared wave-lengths, while the OH molecules radiate strongly in the near ultraviolet. "

 

[Clausius2]

Thank you. Your link is very useful. Now, you have given off all my doubts.

 

[Spectre32]

Isn't there a "blue flame" similar to that when aircraft use afterburners?

 

[Clausius2]

Isn't there a "blue flame" similar to that when aircraft use afterburners?

I've seen something similar in aircrafts. But I don't know what you mean. It's impossible it's a flame. If you read the link of Geniere, you will see it is a radiation of the structural products of the engine.

 

[Janitor]

Interesting stuff!

Incidentally, years ago I read a book called Ignition!, written by John Clark, a buddy of Isaac Asimov. Some of you might get a kick out of it, if you can get your hands on it.

 

[Chronos]

I haven't read all the posts, but, I do know why the blue plumes appear. At liftoff, the solid rocket engines are the main source of power for a shuttle launch. Once the solid rocket propellent is spent, the primary liquid hydrogent/oxygen propellent rocket takes over. As it exits the troposphere, that plume appears blue in color.

 

[Clausius2]

I haven't read all the posts, but, I do know why the blue plumes appear. At liftoff, the solid rocket engines are the main source of power for a shuttle launch. Once the solid rocket propellent is spent, the primary liquid hydrogent/oxygen propellent rocket takes over. As it exits the troposphere, that plume appears blue in color.

If you do know which is the reason, explain it to us.

The plume appears not only in the troposphere. And the main engines are started just at the liftoff (with less power). The blue color in the plume is a complex effect or reburning and radiation of some internal materials of the pumps and combustion chamber.

Take a look at the Geniere's link.