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Originally posted by Henry8minus1
After looking at Dr. Derek Brays power point slides that Bob pointed us to I wanted to bounce my understanding of what happens during the thrust phase. According to pages 34-35 of this presentation about base bleed I conclude that during thrust the base drag is reduced because of the pressure of the thrust. Do I understand this correctly?
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Much of Dr. Bray's power point presention on drag has to do with drag reduction on projectiles, not rockets. Once a projectile leaves the gun barrel, it can only slow down.
To increase the range you have two choices: increase the muzzle velocity or reduce the drag. You can increase the muzzle velocity by increasing the barrel length which is easy and is done on most modern artillery pieces, and/or by increasing the powder charge which increase the pressure in the barrel which dramatically reduces the barrel life-time so this is not done.
Since drag increases as velocity squared, unless the drag coefficient is reduced substantially, you get little addded range by increasing the muzzle velocity, so drag reduction is critical.
Adding a 0.5 calibre boattail typically reduces the drag coefficient by ~50%, but you still have a base vacuum that is generating a lot of drag. Base bleed is the burning of a solid propellant to generate gas to fill in the vacuum and thereby reduce the base vacuum and therefore the base drag. The gas evolution rate is relatively low and it adds no thrust to the projectile. Modern artillery shells with a boattail and base bleed have a drag coefficient that is 80% lower than the old standard flat bottomed projectile.
These drag reduction techniques apply equally well to rockets, however the overall cd for the rocket includes the drag of the fins which can be substantial, so a simple cd based on the rocket diameter will not decrease the overall drag as much as you might expect.
Also there are big differences between subsonic and supersonic fins. Low drag fins designed for use below M=0.7 are relatively thick with chords up to 0.2 or so. Low drag fins for M>1 are thin with chords < 5%.
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Taking this into account then a boat tail rocket with a larger diameter would be less advantageous during thrust than a minimum diameter rocket because both would reduce drag from the thrust, but the boat tail would have a larger cross sectional area. So the boat tail rocket would have to make up for this during the coast phase if it were to perform better. Is this a correct deduction, or am I understanding this wrong?
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No necessarily. If the Cd bt * Area for the boattailed rocket is lower than the Cd md * Area of the minimum diameter rocket, then the boattail wins.
I think you will find that after doing the math, the boat tail wins every time, both on boost and coast.
also to JRThro
You are correct. Recessing the rear centering ring on a rocket increases the base drag, and additionally creates a recirculation zone for the hot exhaust plume to cook the base of the rocket. I don't understand why some designs have this terrible feature. It's better aerodynamically to have the motor tube extend from a flush rear ring since it acts as a psuedo boattail. It does however move the CG rearward, so you don't want to get carried away with the projection.
Bob Krech