Mercury Rocket Flame

[PhantomWolf]

I noticed while watching a couple of launches that there is a flame out the side of the main jet which I couldn't determine what it is. It might have something to do with the ice falling off, but I can't determine why that would be causing it. Does anyone happen to know?

[Jason Thompson]

Are you referring to the Atlas launches and the billowing flame that doesn't look confined like a rocket exhaust? If so I believe I may know what that is.

The rocket engines use a gas generator to drive the turbopumps that feed the propellant and oxidiser to the combustion chamber. The gas is generated by burning a fuel rich mix of propellant and oxidiser to generate a lower temperature gas in large volumes that can be used to drive moving parts in the engines. The gas drives the turbopumps and is then dumped overboard through a nozzle. In the case of the Atlas, this nozzle was alonside the engines, so you see the three focused plumes of the main engines and a big, darker, billowing exhaust from the gas generator alongside them.

Note that the exhaust was darker than the rocket plume, since it was fuel rich and was therefore incompletely combusted (so it had loads of soot in it), and it was cooler than the rocket plume. Mahy engine designs put that to use by venting this cooler exhaust into the engine nozzle, where it created a barrier between the metal and the intense heat of the rocket enhaust. The effect of this was to produce a rocket plume with a darker region just below the end of the engine bell that extended for a few feet before the brighter rocket exhaust broke through. This is most obvious on the Saturn V launches because of the scale, but you can see it on any other engine cooled by the same mechanism. This includes the four outer engines of the Saturn IB and the sustainer engine of the Atlas. (In the case of the Saturn IB the four inner engines were not cooled in this manner, and the gas generator exhaust was dumped from an outlet in the middle of the cluster, hence you never really see it as prominently as you do on the Atlas launches.)

I hope that answers your question and that someone with a better knowledge of rocket engine design will either tell me I'm right or make me look daft in the not-too-distant future....

[Bob B.]

Jan 19, 2009 9:17:53 GMT -4 Jason Thompson said:

I hope that answers your question and that someone with a better knowledge of rocket engine design will either tell me I'm right or make me look daft in the not-too-distant future....

You are exactly correct. The off-axis plume coming from the Atlas is the turbine exhaust. The following contains a photo of the Atlas engine (through this is an uprated version from that used in the early launches):

www.astronautix.com/engines/ma5.htm

I believe the pipe running down the side of the center engine is the turbine exhaust duct, though I'm not certain. I have a diagram at home somewhere that shows it better.

[PhantomWolf]

Ahhh, thanks very much. It looked strangem probably because I haven't watched a lot of non-Saturn, Shuttle, Delta launches and so too see a lazy looking flame out the side of the main jet just looked all wrong.

[Bob B.]

If you’re interested in rocket engines, also note something else about this photo…

Although all three engines are ignited at liftoff, the two outboard engines are the boosters used during just the first couple minutes of flight. After a couple minutes the outboard engines are jettisoned and the rocket continues to orbit on just the center engine, called the sustainer.

Although both the sustainer and booster engines have about the same diameter at the nozzle exit, note how much wider the booster engines are at the throat. The booster engines have a lower expansion ratio (also called section ratio), which is the area of the nozzle exit divided by the area of the nozzle throat. Low expansion ratios are typical of engines designed to operate in the low atmosphere where there is external air pressure. High expansion ratios, as seen on the sustainer engine, are typical of engines designed to work in a vacuum or near-vacuum.

Since both the boosters and sustainer operate at about the same combustion chamber pressure, the larger throat of the booster tells us that a much larger volume (and hence, mass) of gas will pass through the throat and be expelled by the engine. The boosters burn propellant at a much higher rate and produce much higher thrust than the sustainer.

An engine operates most efficiently when the pressure of the gas exiting the nozzle is exactly the same as the pressure of the surrounding air. As the engine exhaust gas expands in the divergent part of the nozzle, its pressure decreases. An engine operating at sea level, therefore, must expel its gas sooner than one operating at high altitude so that the exhaust pressure doesn’t drop below the ambient air pressure. This is why we see smaller expansion ratios on boosters than on, say, a second stage engine.

This doesn’t mean an engine designed for use at sea level won’t work in a vacuum or vice versa. Engines just don’t work very efficiently in the environment for which they have not been optimized. In the Atlas, the sustainer engine ignites at liftoff and supplements the thrust of the big boosters, but the sustainer doesn’t work very efficiently until the rocket reaches high altitude and the boosters are jettisoned, which is, of course, when you want it to work best.

[Bob B.]

Jan 19, 2009 15:53:53 GMT -4 PhantomWolf said:

Ahhh, thanks very much. It looked strangem probably because I haven't watched a lot of non-Saturn, Shuttle, Delta launches and so too see a lazy looking flame out the side of the main jet just looked all wrong.

Well, it is certainly far more conspicuous in the Atlas than other rockets. As Jason said, many engines inject the turbine exhaust into the nozzle for film cooling, so you don’t get a separate plume of exhaust. Also, engines that use a staged combustion cycle, such as the SSME, inject the turbine exhaust into the combustion chamber and burn the non-combusted fuel to provide extra thrust.

The Titan II also dumped its turbine exhaust, but it is not very noticeable because (1) is it pretty clean due to the type of propellant used, and (2) it’s directed straight aft between the two main engines. Some rockets, such as the V-2, Redstone, and R-7, drive their turbines by decomposing hydrogen peroxide. The resulting exhaust is simply oxygen and water vapor, so that’s not very noticeable either.

[JayUtah]

Open-cycle engines just dump the turbine exhaust overboard, either as coolant or as waste. Closed-cycle engines feed the turbine exhaust back into the propulsive process. The SSME is an example of the latter.

[Bob B.]

Here's a drawing of the Atlas engine clearly showing the turbine exhaust duct:



As you can see, there are separate exhausts from the fuel and oxidizer turbopumps that combine into a single discharge. Just downstream of where the two ducts combine is a heat exchanger. The heat exchanger warms and gasifies LOX that then returns to and pressurizes the LOX tank. I don't recall exactly how the fuel tank is pressurized, but I know of other rockets that use high-pressure helium chilled by immersion in the LOX. The helium is passed through the heat exchange to be warmed and expanded and then routed to the fuel tank. Of course, keeping the balloon tanks of the Atlas pressurized was critical for structural rigidity.

[Jason Thompson]

Jan 19, 2009 10:24:41 GMT -4 Bob B. said:

You are exactly correct.

Yay! Thanks Bob. That's a relief....

[Bob B.]

Jan 19, 2009 9:17:53 GMT -4 Jason Thompson said:

Many engine designs put that to use by venting this cooler exhaust into the engine nozzle, where it created a barrier between the metal and the intense heat of the rocket exhaust. ... This includes the ... sustainer engine of the Atlas.

I didn't know that. Taking a second look at this photo, I'm inclined to say the pipe running down the side of the center sustainer engine is likely the one ducting a portion of the turbine exhaust to the sustainer nozzle extension for film cooling. (It didn't look quite right when I misidentified it earlier, which is why I made the disclaimer that I wasn't sure.) My guess is that the main exhaust discharge is out of view behind the engines.