Exhaust Gas effect on drag

[DaveL]

Re; datadoc's post, I think the USU's thrust estimate is a little optimistic.

To achieve 6000 lbs of thrust with a propellant flow of 1000 lb/min requires an exhaust exit velocity of 11600 ft/sec (7900 mph). That would require convergent/divergent de Laval nozzles at the exhaust tips and compression waves (shock diamonds) would be visible. Any further chemical reaction after the exhaust tips has no bearing on thrust produced. Looking at the photo in the first post I see sub-sonic deflagration in the exhaust, thrust would be nowhere near 6000 lbs.

[Stan Back]

I think I may have to go back to school . . .

[datadoc]

 Davel.,I too questioned the 6k force, but I had no frame of reference. What I did know is the funny car that took flight weighted over 2,300 lbs. and the headers were not pointed straight down after the ends burned off, so there was a vector involved. Using the 4 tires spring rates and the change in ride height data that would also suggest the the thrust wasn't in the 6k range but maybe 1/2 that amount. But again the header angle during the test still had a vector component, the pipes where not pointing straight up.  Another effect of header thrust with these cars is if one cylinder goes out it is very difficult for the driver to keep the car going straight. Bottom line is there is a great deal of thrust, after laying the headers back from the original position the cars picked up a significant performance gain.