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Descent rate: OR vs Alt3

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dhkaiser

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Descent rate: OR vs Alt3

Flew a Wildman Drago on my L1 flight with a Top Flight Recovery 36" parachute. This rocket weights in at 33.9oz no motor(BRB900 in the nosecone, a Chute Release and Alt3. OpenRocket says a ground hit rate of 18.5 ft/s. The Alt3 says it was 13 ft/s. Looking at the TFR web page I NOW see they recommend a 30" for a 2 lb rocket (1.75-3lb).

So why is OR so far off?

I also have a Madcow DX3 SD waiting for it's first flight. 44.6oz with the BRB900, CR and Alt3. OR says 17 ft/s with a 48" Rocketman chute, or 22 ft/s on a 36" Rocketman. Now I'm not sure which to use.
 
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rharshberger

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Accuracy of the sim is only as accurate as the information you put in. The Alt3 is recording actual data not trying to predict it. If the Cd's are not accurate then the descent rates OR predicts will be off, dont assume the Cd OR has for a given chute is accurate unless you have verified it with the manufacturer.
 

crossfire

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For that rocket I would have advised no larger than a 30" chute. If I flew it I would have used a Par24. The WM Drago is a very tough rocket. We all need to think about landing surface, wind speed, and rocket design, launch site, and weight when choosing a chute. I have questioned the Sim files for years. They always seem to recommend a larger chute than needed. Even for the DX-3 where I flew I would use a Par30. A Par36 would even be on the big size. By the way WM will get a good laugh out of calling the DX3 a WM kit.
 
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dhkaiser

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So where does one get accurate numbers for the Cd? Using the Alt3 data from my Drago flight and working backwords I get a Cd = 1.4. Is that reasonable?
 

rharshberger

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So where does one get accurate numbers for the Cd? Using the Alt3 data from my Drago flight and working backwords I get a Cd = 1.4. Is that reasonable?
Not all the manufacturers have actual Cd's posted where they are easily found. Some you have to dig around a bit, and others just require concting them.
 

dhbarr

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Should be able to backulate TFR numbers from their descent rate, since they give weight and velocity targets. Wouldn't be surprised to see a 1.4ish pop out.
 

MikeyDSlagle

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From another thread:

The XT CD.98
The standard chutes CD 1.34
Crossfire chutes 1.55

So your extrapolation is pretty close. If I had to guess, I'd say a Rocketman chute would be close to a Skyangel, which would be a CD of 1.34 for 36" and 1.87 for a 44". But I'm sure rocketman woukd have an answer.

Mikey D
 

dhkaiser

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Got this from TFR today...

The CD Par9-Par24 1.09
Par-30-58" 1.34
Par70-120" 1.40
The XT chutes are .98

Have not heard back from Rocketman.
 

Kruegon

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Just a theory for me at the moment, perhaps someone with more knowledge could expand.

It would seem to reason humidity, elevation, and temperature would affect descent rates as well. We all know that hot, humid days produce lower flight altitudes. High humidity was once described to me as "trying to launch through water". The closer to MSL you go, the denser the air. And we all know hot air rises.

So couldn't any of these factors account for a slower descent rate, Hot air rising against a full chute. Denser air mass from a lower elevation. Thicker air due to humidity?
 

dhbarr

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Isn't water vapor less dense than nitrogen at the same temperature and pressure provided both are gaseous?
 

Banzai88

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I always believe the data my RRC3 or JL Altimeter tells me over what a sim predicts. I go back and adjust the sims, as necessary, to get something more 'real world' from them.

Remember, the more hemispherical under load a 'chute is, the more lift it will achieve with cross winds and that radically alters descent rate.
 

Kruegon

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Isn't water vapor less dense than nitrogen at the same temperature and pressure provided both are gaseous?
I'm not sure myself, but wouldn't concentration per cubic foot cause it to be more effective than its nitrogen counterpart in such dense quantities?
 

dhbarr

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Air is about 80/20 N2/O2 plus change. Water is H2O.

H=1, N=7, O=8 by number, double that for the Atomic Weight.

So, air is mostly big ugly 28-weight N vapor and a fifth slightly larger 32-weight O vapor. Water vapor is a relatively svelte 18.

So to break records you want a calm-before-the-storm in Hartsel ( but watch out, since your descent rate will also increase).

No?
 

dhkaiser

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Still waiting for rocketman to give the Cd's for 3 and 4 ft chutes. Anyone have those numbers?
 

John Beans

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Nice thing about Chute Release is that you can size (oversize) your chute for a soft landing, not minimize it to limit drift. After you are confident of your inflation times pick a larger chute, open it low, land it slow.
 

dhbarr

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Nice thing about Chute Release is that you can size (oversize) your chute for a soft landing, not minimize it to limit drift. After you are confident of your inflation times pick a larger chute, open it low, land it slow.
Does anyone have a study for inflation-vs-weight for various Cd designs?

Anybody need a technical paper idea and have access to the roof of a tallish building? :)
 

John Beans

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Does anyone have a study for inflation-vs-weight for various Cd designs?

Anybody need a technical paper idea and have access to the roof of a tallish building? :)
By Cd, do you mean the rocket's flight Cd, or the parachute's? Not sure either really has anything to do with inflation rate, though parachute Cd would influence the deceleration *after* inflation.

There has been speculation about what makes a rocket tumble flat vs. tumbling more vertically (maybe the individual balance of the two rocket halves, etc.).
One philosophy is "I don't know whether my rocket will tumble flat, so I'll add a drogue because the descent rate under drogue can be simulated reliably" based on simple weight and drogue diameter/Cd. And SCRocketFan has shown via recorded AltimeterThree data that a drogue smooths out the descent and that the pressure seen by deployment sensors is less noisy.

I think inflation rate is most influenced by folding and wrapping technique. There have been stories of folks sticking to their old ways of folding, then adding Chute Release as an afterthought and having that open slowly. For instance, wrapping your shrouds around the bundle, then adding a blanket wrapped around it, then adding Chute Release will probably inflate slowly because there are layers that need to come off and then unroll and then inflate. Whereas a fresh approach which which whips away the blanket as soon as ejection happens and then lets the chute fall open (rather than unroll) as soon as the Chute Release triggers probably inflates significantly faster. Hopefully that makes sense the way I've described it.
 

dhbarr

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Absolutely. Hopefully I'm not just agreeing because it matches my design intent ( hi co2 pop, lo JLCR flop ).

I literally meant the opening speed of a reefed chute which suddenly becomes unreefed assuming the while train is descending in a mostly vertical line.

Expressed differently: how low can I reliably do my final open :).

I suppose I'd need to make a bunch of chutes with different designs that all have the same surface area....
 

dhkaiser

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Rocketman never replied to my inquiry concerning the Cd of his 36 and 48" parachutes. I used Mikey D's figures from post #10 in this thread, ie 1.34 for the Rocketman 36" chute. On my Madcow DX3 SD (44.6oz with no motor) Open Rocket returned decent rate of 17ft/s with the 36".

I flew it this weekend and Alt3 gives a final decent rate of 31 ft/s! I watched the chute fully open and descend so no issues there. So what gives with a 31 ft/s rate????
 

John Beans

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Rocketman never replied to my inquiry concerning the Cd of his 36 and 48" parachutes. I used Mikey D's figures from post #10 in this thread, ie 1.34 for the Rocketman 36" chute. On my Madcow DX3 SD (44.6oz with no motor) Open Rocket returned decent rate of 17ft/s with the 36".

I flew it this weekend and Alt3 gives a final decent rate of 31 ft/s! I watched the chute fully open and descend so no issues there. So what gives with a 31 ft/s rate????
Can you post the flight graph?
 

John Beans

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Great to have data like that.
Yep, that looks like 31 fps. You can pinch and zoom out and focus on the section after Chute Release lets go, then, use the crosshairs (hold your finger down on the graph to get crosshairs) and get the data for actual points).
But my eyeball test from this graph looks like AltimeterThree pegged it correctly at 31 fps.
My guess is that the rocket (with empty motor, paint, etc) is heavier than the sim. Did you actually weigh its landing weight?
Or, like you were wondering, perhaps the Cd of the chute may not be what you thought it was.
 

Buckeye

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Rocketman never replied to my inquiry concerning the Cd of his 36 and 48" parachutes. I used Mikey D's figures from post #10 in this thread, ie 1.34 for the Rocketman 36" chute. On my Madcow DX3 SD (44.6oz with no motor) Open Rocket returned decent rate of 17ft/s with the 36".

I flew it this weekend and Alt3 gives a final decent rate of 31 ft/s! I watched the chute fully open and descend so no issues there. So what gives with a 31 ft/s rate????
Are you certain that the rocket weighs 44.6 oz on descent? Weight should include everything in the motor minus the stuff that burned (propellant, delay, BP).

There are only 2 variables at play (unless some crazy cross-wind lift is going on): mass, and aero drag of the chute/rocket combo as it is falling. That latter is rather hard to determine precisely.
 

dhkaiser

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I did not weight the landing weight, however I did weight carefully the empty, no motor weight. That included the Alt3, CR and BRB900 along with the 36" chute. And yes it cool to have the data.
 

dhkaiser

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OK the empty weight is 44.6oz. The motor weights in at 12.6oz, Aerotech figures. OR gives the correct weight with motor, 44.6 + 12.6 = 57.2oz. I am stumped.
 

Banzai88

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Then the chute is NOT of the CD you suspect that it is.
 

dhkaiser

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Then the chute is NOT of the CD you suspect that it is.
I must agree, looks like I should have used a 48" rather than the 36". Working backwards the CD for the 36" is .4 rather than 1.34.
 
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