What is the impact of exhaust vanes, such as those in the A4 and Redstone rockets, on exhaust velocity and overall ISP? As well as this, what is the relationship between ISP losses and the width of the exhaust vane? Does it change with the angle of attack of the exhaust vane?
Sutton edition 7 mentions them without too much detail
Jet vanes are pairs of heat-resistant, aerodynamic wing-shaped surfaces submerged in the exhaust jet of a fixed rocket nozzle. They were first used about 55 years ago. They cause extra drag (2 to 5% less Is; drag increases with larger vane deflections) and erosion of the vane material. Graphite jet vanes were used in the German V-2 missile in World War II and in the Scud missiles fired by Iraq in 1991. The advantage of having roll control with a single nozzle often outweighs the performance penalties.
Thrust loss of 0.5 to 3%