Thursday, April 23, 2015


The questions always come up on the Internet about the strange back-flow of the Saturn V first stage's exhaust plume. What made it happen? Were the engineers surprised to see it? Was it a concern? Did it happen on every Saturn V or just a few? How high did it go? Was it a danger to the vehicle? And often the answers given on the assorted forums and threads are mostly right or over simplified or even completely wrong.

In fact, the flow separation and resulting back-flow WAS expected. It was seen on the early Saturn I and IB launches. If you get the chance to watch the extended on-board staging films from SA-6 as presented by SpacecraftFilms "The Mighty Saturns- Saturn I and IB" DVD set, you can actually see the sooting of the LH2 vent lines on outside of the S-I inter-stage as the back-flow creeps up. It was no hazard to any of the vehicles and yes, it did happen on EVERY Saturn launch vehicle to one extent or another. The Saturn V was the most visually noticeable.

What made this happen? Well, seen below is the reason as stated by the Saturn V engineers. Now you may see folks argue that their classes in rocket engineering says this or that, but to me, the best answer is here, as stated by the folks who designed, built, flew and studied ALL of the data from the actual vehicle. Thus, it is, in my opinion, the best explanation. Highlighting of items are by me in order to bring to greater light some points of the report. The report itself can be fount on the NASA Technical Reports Server.

"Flow separation (from NASA MSFC MPR-SAT-FE-68-3, June 25, 1968; Apollo 6)

Flow separation was observed on the Saturn I and IB flights and was anticipated on the Saturn V.

Flow separation results from the expansion of the F1 engine plume at the higher altitude. The plumes create what could be considered a solid wall to the oncoming free stream. At lower altitudes, the free stream flow can be deflected around the exhaust plume by the exterral plume shock; but as the plumes increased in size, the free stream flow can no longer turn near the plume service. Consequently, the flow on the side of the vehicle separates or begins to turn before it reaches the plumes. Figure 17 – 18 illustrates the flow field which is obtained after separation occurred. 

Hot gas is fed into the separation region from the base region and plume interface. The base region hot gas results from the engine exhaust flows impinging upon one another and forcing some of the hot gases toward the base heat shield. Separation, once induced, will continue until outboard engine shutdown. As the plume diameter expands with altitude, the point of flow separation moves forward along the vehicle.

Flow separation on the AS-501 and AS-502 flights was first observed between 105 and 110 seconds. Measurements have been made of the point of flow separation for various flight time and are shown in figure 17-18. It is noted in this figure that the separation region extended beyond the top of the S-IC forward skirt just prior to stage separation. The observed blackness on the stage may be a carbon deposit rather than paint being burned."

Note that the "Forward Location" in drawing is incorrect and is corrected in the text. Photos of the Saturn V also shows the back-flow extending beyond the FORWARD S-IC skirt.

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