spill holes

[rocketsonly]

Hello. Does anyone know how much spill holes can effect a rockets descent rate? Is there an online calculator? Obviously size and weight of rocket are pretty important.

I'll make some random numbers.. Say my rocket weighed 1lbs with a 24in hexagon parasheet, and a 5 in spill hole.

Thanks.

 

[shockwaveriderz]

Yes the spill hole size can effect the descent rate.....or reefing the shroud lines....or both...

My only suggestion in using a descent rate calculator is to to use it with the parachute size minus the spill hole size..

for example a 24" chute with a 5" spill might be equivalent to perhaps a 20" chute actual?

Anybody have an equation to perhaps determine chute size equivalency with and without spill holes?


There are descent rate calculators on the net:

https://www.rocketreviews.com/tools.shtml

using 15 ft/sec as the recommended minimum descent rate with both a 24 and 20 " chute the descent rate varies by almost 7 feet per sec from 24 fps to 17 fps....

check the drift difference using 24 and 17 fps respectively from say 1000 ft..

 

[gregzo]

I agree - the spill hole can effect the desent rate. However I believe it is much less of an effect than reducing the total size of a 24" chute w/ a 5" spill hole to a 20" equivalent.

The area of a 24" chute minus the area of the 5" center you remove for the spill hole gives an equivalent chute of 23.47" in total surface area.
24" chute = 452.3 sq in
5" hole = 19.6 sq in
sqrt((452.3-19.6)/3.14(pi))=23.47"
Unless the hole in the middle makes it drop a lot faster than a simple calculation of total area remaining in the chute - I believe this is accurate.
Let me know if you or others think differently because this is how I calculate - I want to change that if I am wrong.

 

[shockwaveriderz]

gregzo:

I did some similar calcs and came up with the same results......but I wasn't for sure if my results were correct or not....

I find it interesting that a 24" chute with a 5" spill hole versus a 24" chute with no spill hole is equivalent to about a 23.5 " diameter chute....

If this is true then the descent rate between the 2 chutes should be essentially the same which doesn't seem to jive with reality..... something is missing...

 

[Bowhunter]

So spill holes helps drifing?

 

[AndyC]

Originally posted by shockwaveriderz
gregzo:

I did some similar calcs and came up with the same results......but I wasn't for sure if my results were correct or not....

I find it interesting that a 24" chute with a 5" spill hole versus a 24" chute with no spill hole is equivalent to about a 23.5 " diameter chute....

If this is true then the descent rate between the 2 chutes should be essentially the same which doesn't seem to jive with reality..... something is missing...

The reason these figures dont seem to add up:

The total area of a chute is only part of the story. In flight, the chute adopts a 'domed' shape. The fabric at the edge of the canopy is almost parallel to the airflow, and produces relatively little drag. The fabric at the centre of the chute is perpendicular to the airflow, and produces a lot of drag. The spill hole is cut from this centre section, and so has a disproportionately large effect on the rate of descent. Hope that makes sense .

Andy

 

[gregzo]

I "think" the spill holes are there to help a parachute stay equalized. If there is any unbalance on the shroud line loading, or in the dimensions of the chute, it will "puff" and release causing strange behavior. The hole keeps it all in balance and falling at a constant rate rather than a quick decent as the chute unloads followed by a continual slowing until it unloads again. If the touchdown occurs during an unloading......
Rocket Rage says they dynamically balance all their chute shroud lines to prevent this and not use a spill hole???
I'm just parroting some of the stuff I have read about this - I'm really not sure.