Saturn V exhaust question

[Jason Thompson]

Just out of curiosity, what causes the exhaust during the S-IC burn phase of the launch to end up engulfing the whole rear end of the vehicle to what appears to be at least halfway up the first stage? I can understand it spreading out due to the decreasing pressure, but what makes it travel forward of the nozzle exit? Was the effect anticipated, or was it a total surprise when it happened the first time? Did it cause any concern about tank integrity?

[gwiz]

As you say, the exhaust expands in the low pressure of the upper atmosphere. This presents the flow past the vehicle with a large bluff obstruction at the rear end. In the normal manner of fluid flow, it slows down as it approaches the obstruction and the pressure rises. Because of the skin friction along the sides of the vehicle, the air closest to it, the boundary layer, loses energy as it goes aft, and in an adverse pressure gradient it can reach the point where it has zero velocity. This is known as flow separation. From the separation point aft, there are two regions of flow, an outer one with velocity in the original direction and an inner one with circulating flow, moving in the opposite direction along the skin. This is a common situation for flow about bluff objects, but is something you try to avoid in aircraft design as it can lose lift and increase drag, hence the use of streamlined shapes without separation.

In the case of the Saturn V, you have a region of re-circulating flow at the aft end which is essentially like a smoke ring or a doughnut rotating through its hole. This vortical flow mixes with the exhaust at its aft end and brings some of the hot gas forward along the side if the vehicle up to the separation point, then out and back again.

I expect it was anticipated, they certainly did wind-tunnel tests. You can see the same effect on high-altitude photos of the Shuttle and other rockets.

[Jason Thompson]

Thank you. Most informative.

[Count Zero]

So much for my theory. I though that, as the plume spread, the radiant heat from it was burning the paint off.

[JayUtah]

No, although the F-1 engines themselves had to be insulated against the heat from the portions of the plume that were drawn into the separation.