bigone5500
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Is there an online calculator anywhere?
For example, all things being equal you would need a larger hole for a rocket going to 10,000 feet in 7 seconds, than you would for the same rocket going to 5,000 feet in 12 seconds.
Greg
All the above calculations provide 1 sq. inch of venting for 2000 cu. inch of internal volume. If you divide the volume of air trapped inside a rocket compartment by the volume flowrate of air through a hole and do the dimensional analysis, you will find the ratio is in units of seconds. Volume /( volume/second) = seconds.
The conductance air flow through a hole is ~10 liters per second per square centimeter, or ~4,000 cubic inches per second per square inch. Since the VernK formula uses 2,000 cubic in. per sq. inch. and the venting rate is ~4,000 cubic in. per sq. inch per second, the VernK time constant is 2,000/4,000 ~ 0.5 seconds. In 1 tiime constant, the pressure in the confined has reduced to 1/e = 0.367 = 36.7% of the actual outside pressure difference. For example, if the outside pressure difference is dropped -1.7 psi in 1 second (0-Mach 3 in 1 second), the internal pressure is ~0.624 psi higher than the outside. The time constant for the VernK is ~0.5 seconds, so the change per second is 2 time constants = (0.367)^2 = 0.135 = 13.5%. For the same example, the internal pressure would be only 0.23 psi.
Translated to a 4" diameter rocket, if the airframe is not vented, in 1 second the internal separation force would be 1.7 psi x 12 sq. in. = 20 pounds. Using the VernK formula the internal pressurization separation force would be 0.135 x 20 pounds = 2.7 pounds. If the rocket continued upward at the same velocity in 3 second, the internal pressurization load would be approximately 3 x 20 pounds = 60 pounds, about what it takes to break (3) 2-56 shear screws. If vented by the VernK formula, the load would still be only 2.7 pounds.
This should illustrate why the VernK calculation is so useful, and why you need to vent high acceleration, high flying rockety even when you use shear screws.
Bob
calculator did not calculate hole or circumference sizes in inchesI have updated the spreadsheet. Changes to it are consolidation of the scale calculator and scale chart sheets.
I am looking to add more functionality to the spreadsheet. If you have any suggestions please post them here.
View attachment 124862
what about adding Bob's PSI math and making a 'ballpark" starting point for DD charges?
Thread necromancy here.
I have a nose cone alt-bay with a volume of about 135 cubic inches.
Can anyone tell me how big/how many static pressure ports I need? I can't find a calculator that works on volume
I plan to put them equally spaced around the perimeter of the nose cone 1" above the break between nose cone and body tube.
I was going with 1" above the break to account for the swell of the shoulder. The nosecone is a laid-up fiberglass gelcoat one from Polecat Aerospace and I'm trying to get an unrestricted/clean flow into the nosecone.
By my calculations three 3/16" diameter holes should do the trick.
Now the trick will be to get holes lined up so that I can see the RRC2+ status LED flash.
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