Thanks to Matt and Gunnar
The reason for the acorns
Tests on the prototypes showed severe tail vibration at speeds over 380mph. This was famously fixed with the ‘acorns’, but once a solution had been found through trial and error there was little further thought given to how it worked.
Aerodynamic theory has moved on since then, as has the ability to model aerodynamic behaviour. so here is the reason behind the problem and the fix:
The tail presents two symmetrical aerofoils at 90 degrees to each other. Where they are both at their thickest the airflow is accelerated by up to 40%, and the pressure drops accordingly. At flying speeds there is subsequent flow separation, rapid flow deceleration, and extreme turbulence. This is the classic ‘interference drag’.
You can see this problem zone as the blue blob on the (unmodified) tail in the pressure plot on the left (blue=low pressure)
The graphic below shows a velocity plot ‘slice’ close to the fin/horizontal tail junction, at 400mph. Red=fast flow and blue= slow. Air passing close to the junction is accelerated up to 480 mph in this low-pressure zone, then immediately slowed to under 15 mph (the dark blue), in the space of a few inches. As Gunnar observed, it’s not surprising pilots reported rudder vibration at speed.
The plots below are Gunnar’s model at 400mph. Adding the acorns reduces the severe pressure drop (the blue area) and the local velocity increase, indicated by the reduction in the orange area (this is the portion of local air accelerated to over 525mph)