pressure bleed holes?

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Rex R

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idle thought. would there be a noticeable reduction in drag if one were to use bleed holes say just in front of a launch guide or along the base of a fin? connect the bleed holes to the area of reduced pressure at the base of a rocket.
Rex
 

Steve Shannon

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Wouldn't you end up with low pressure inside the parachute compartment then, meaning ambient outside pressure would be holding your rocket together? Your ejection charge would then have to overcome that much more pressure to separate the rocket.
How much drag do vents add, I wonder?
Interesting thought...


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OverTheTop

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Interesting thought. Are you talking about letting air in or out? Assuming out (pressure differential as rocket ascends), surprisingly I think they would reduce drag slightly if they were were behind the guide rail. The flow of air would essentially "fill the gap" of air (lower pressure zone) behind the lug. That would allow the air passing the lug to experience less turbulence and drag as it passes the back of the lug.

NASA and other private companies are currently experimenting with technologies using either blowing air from or applying suction to flying surfaces. This allows the point of transition of flow from laminar to turbulent, and hence the detachment point of the flow across the wing or fin, to happen much further along the chord. This is useful in either increasing the angle of attack limit, or reducing the drag on the aircraft (improving fuel efficiency). The same can be applied to the vertical tail so a small tail can still have the same control authority as a standard larger tail. That allows enough authority to keep the plane straight under asymmetric thrust conditions, but with a smaller tail surface area. The idea is to improve fuel economy by reducing the skin friction drag (less area) and by reducing vehicle mass.

So the short answer is probably, but almost immeasurably I think. If it makes sense aesthetically to remove holes and put them somewhere else then go for it. Don't do it just for less base drag. There would be insufficient mass flow out of the hole (in relation to the freestream velocity) of the rocket to make any significant effect I think.

On reading your post again I suspect you mean taking the air into the rocket. You are in effect trying to reduce the frontal area (sort of). If you could move it through without changing the total pressure (a measure of energy, sort of) it might work. Unfortunately moving air through hoses or compartments needs to be done exceptionally carefully to not lose that energy (in a jet engine for example it is very important for efficiency). So for what we are trying you would need a long skinny tube. If you try blowing down a long skinny tube you know that the back-pressure from the tube is significant. So that, in the rocket, would translate the total energy of the airflow into drag on the rocket before it got out the back end I think.

Total pressure (fluid dynamics concept) is really the pressure which is the sum of the static pressure in the area, plus the dynamic pressure, plus the pressure head due to height difference of the ends.
 

Rex R

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well I was working on my booster which has the inter-stage vented out the back, got me to wondering if one could slurp air around the motor mount(aft of the forward centering ring, which would limit the potential boundry layer control to the aft end). which got me thinking about the potential higher pressure areas in front of fins etc. this would also tend to reduce base drag.
Rex
*edit*
depends on the speed of the truck, below 45 mph you do better with the windows down. :)
 
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Steve Shannon

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Wouldn't you end up with low pressure inside the parachute compartment then, meaning ambient outside pressure would be holding your rocket together? Your ejection charge would then have to overcome that much more pressure to separate the rocket.
How much drag do vents add, I wonder?
Interesting thought...


Steve Shannon
When I thought about this again I realized that I was completely wrong about the ejection charge having to overcome the ambient pressure. When the apogee charge goes off ideally the velocity will be near zero and the placement of vent holes in the shadow of fins, rail guides, or plumbed to the base won't matter because at near zero velocity those locations won't have low pressure.
In fact, maybe venting to those lower pressure areas during the high velocity region of the flight could serve as an alternative to shear pins. With low pressure inside the parachute compartment the higher ambient pressure would be holding the rocket together instead of allowing it to drag separate.
Rex, you may be on to something.


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Rex R

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one question would be, how much vacuum could you pull? for a 3" rocket a difference in pressure of 0.1 psi would generate 1.4 pounds of force holding two sections together.
Rex
 

Steve Shannon

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one question would be, how much vacuum could you pull? for a 3" rocket a difference in pressure of 0.1 psi would generate 1.4 pounds of force holding two sections together.
Rex
You need to build a wind tunnel...


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Buckeye

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idle thought. would there be a noticeable reduction in drag if one were to use bleed holes say just in front of a launch guide or along the base of a fin? connect the bleed holes to the area of reduced pressure at the base of a rocket.
Rex
This is called "base bleeding" in aerodynamic circles.
 

OverTheTop

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The use of diffusers in high-performance cars is a similar theme. They take the high velocity air (producing downforce) under the car and then expand it a decelerate it back to the ambient pressure at the rear, The tricky bit is decelerating the air gently enough to maintain something like laminar flow and reducing the loss of total pressure and thus drag. They do in fact add to the airflow under the car as well, as they are designed to optimise downforce normally.

https://www.formula1-dictionary.net/diffuser.html
https://www.caranddriver.com/features/the-physics-of-diffusers-how-to-make-a-car-really-suck-feature
https://en.wikipedia.org/wiki/Diffuser_(automotive)

In airconditioning a duct diffuser to slow the air (by expanding the cross-section) can taper no more than about 7-8 degrees without losing efficiency and dropping too much pressure. Be gentle with the air :)
 

Buckeye

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I don't think automotive diffuser is the right analogy, here. That's all about Bernoulli pressure recovery. The OP wants to duct air from a stagnation point and dump it in a wake region. Automotive base bleed or wheel house air curtain is more similar to that.
 

OverTheTop

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They are all about letting the air through and reducing drag I think. Sometimes other effects are needed along the way but looking after drag is always part of the equation. Just a matter of how much it matters.
 

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