In the above analogy, I think the appropriate question is if the rocket motor is analogous to a bullet and the rocket body is analogous to the barrel of a gun, does the cool air in the barrel of the gun prior to the gun being fired exit the barrel prior to the bullet?
I don't know jack about guns and I don't think that this is a perfect analogy because the bullet fits the entire barrel of the gun and would consequently push the air in front of it. However, I don't believe that the bullet can be pushed by the propellant any faster than the rate of the expanding gasses behind the bullet.
Again, I don't know jack about guns.
I don't know jack about guns either. But I do know jack about physics
I don't think a bullet is an appropriate analogy either, I was simply responding to bradycros' comment from the previous post:
Next you'll be telling us a bullets velocity increases after it leaves the muzzle of a gun in the same magical manner.
It's important to now the difference between acceleration and velocity.
Of course a bullet doesn't travel faster than any gas behind it INITIALLY. They exit the barrel at the same time and have roughly the same velocity. The difference is that the gas doesn't have as much mass as the bullet, therefore doesn't have as much momentum, therefore it decelerates more quickly (more technically, it has a higher negative acceleration). So if you examine three points in time/space:
The barrel exit: The bullet and gas leave the barrel at roughly the same time, at the same velocity. (The gas a bit faster, actually)
6 feet from the barrel: The bullet passes this point before the gas. The bullet is traveling much faster than the gas because the gas has already started to slow down much more quickly than the bullet. They both have to push against the air but the bullet, having so much more momentum, MAINTAINS more of its initial velocity. (This 6 foot mark is not scientific since I don't know jack about guns, but you get the idea. It is at some point beyond the barrel of the gun.)
The target. Finally, the bullet hits the target, but the gas never gets there. Why? Because it has slowed down to a stop before it got there. The bullet is traveling much slower than it did when it left the barrel, but still much faster than the gas at that point.
It is of course NOT a good analogy to the ejection system of a rocket, but you could make that analogy to top end of the rocket motor itself- not the body tube. The gas pushes the clay (and other particles) out much like the propellant in a bullet pushes the round out of the shell. The difference is the particles "cut through" the existing air while the gas has to slow down. This difference in velocity is probably negligible right at the end of the motor tube, but is more pronounced the further you get from the end of the motor. That is one reason why it makes a difference how far the baffle is from the end of the motor.
There is the effect of the "air piston" as bradycros originally mentioned. But the hot and burning particles are important to consider, even though he doesn't think they are. If it was only a system of gas, and no particles, you wouldn't need any protection at all! The "piston of cool air" would simply pop off the nose cone and push the parachute out (with a long enough body tube anyway). But it doesn't work like that, because AS THE COOL AIR SLOWS THE PROGRESS OF THE HOT GAS, IT DOESN'T IMPEDE THE PROGRESS OF THE HOT PARTICLES AS MUCH. So the hot particles have a chance to destroy the parachute before the pressure wave or actual hot gas has a chance to burn the chute or pop the nose cone.
However, in the case of our ejection baffle, hopefully I don't have to worry too much about the hot particles as I hope that they are being trapped by the baffle.
Yes, it looks like they should be. It is a fine design. My intent was not to influence your design, you've obviously already made it! I just had to chime in when an important piece of the puzzle was missing from an explanation of "how baffles work." As long as we're posting information for people to learn from, I thought they should have the whole story.
Now, do I believe that the expanding hot gasses push the cool air in the body tube in front? Well, if they push a bullet out of a gun, I am inclined to believe that they will push the cool air and nose cone out before melting too much of my recovery system, hopefully.
I guess I should just get it built and launch the darn thing and report my results.
Sounds good!
PS: Watch this video:
https://www.youtube.com/watch?v=J9o3N2Of_64&NR=1
Attached is a frame from that video showing the projectiles ahead of the gas.