What is the plume effect?

Question

I got introduced to the term "plume effect" in an answer I got here.

A simple google search did not explain it, but from the link provided in the answer -- Space Shuttle Technical Conference Volume 1 -- from PDF pages 178 and 195:

Early in the Shuttle program it was anticipated that the exhaust plumes from the SSME/SRB engines would affect the aerodynamic characteristics of the vehicle. This was based on the history of rocket-powered launch vehicles and the resulting plume effect phenomena that was developed over the years. This phenomena is philosophically demonstrated in Figure 17.

For a given engine/rocket motor operating at a fixed altitude and Mach number the exhaust plume phenomena vary with increasing rocket engine chamber pressure. The plume diameter is initially too small to significantly alter the forebody pressure. Thus, the primary effect is the entrainment of the base flow by the high velocity gases in the boundary of the plume and the subsequent reduction of power-off base pressure. As the plume grows in size, it begins to block the base and increase the base pressure. Ultimately, the boundary layer will separate, and a recirculating pattern will develop. For multiple engines, the plumes will impinge upon each other and deflect exhaust flow into the base. Three or more engines can reverse enough mass into the base to choke the volume enclosed by the engines. The effect of the plumes can actually increase base pressure above the power-off level.

Trying to apply the text/illustration (which are confusing me) to the aforementioned answer, I get lost.

• Why is there a 0 high on the y-axis?
• Why does the curve go both below and above the power off horizontal line, isn't there propulsive power regardless of the plume effect?
• Does every rocket go through the stages shown, or is it dependent only on the chamber pressure? For example for the SSME it's given as 20.64 MPa.

(Those aren't multiple questions, nor need be answered in that order. They're just to highlight what has me confused.)

In short, how does a curious best understand the plume effect?

Answer 1

Here's my take at an explanation of the base force - in words and pictures!

Here we see Block I Shuttle^TM sitting on the pad, engines off. The sea level atmosphere surrounds it everywhere and all pressure forces sum to zero.

Immediately after liftoff there is a large "bubble" of reduced pressure under the vehicle because of complicated plume effects, recirculation, etc. Since the pressure on the top of the vehicle is higher, this results in a pressure force pushing down on the vehicle.

At high altitude, the "bubble" has higher pressure than the thin surrounding atmosphere, and the pressure force reverses direction.

Note that this base force is completely different from thrust forces from the engines, aerodynamic drag, or any other force acting on the vehicle!

Now this graph from the other answer may make more sense. (On this graph, a positive force is downwards on the vehicle)

It is extremely hard to predict/calculate this base pressure which is why the shuttle program got it wrong before the first flight, as discussed in the answer linked in the question.