Simming descent rate in OpenRocket

[Charles_McG]

I tune my OpenRocket sims for descent rate as well as ascent. I know the trick of putting in a drogue chute in order to sim dual deploy.

So I've recently just collected my first descent data for a booster. I normally don't put an altimeter in the boost, but really wanted to get a real number for my Talos, since it's the biggest and heaviest thing I've flown to date. It took just a little Cd tuning for the 40" recon chute to get a good match to the measured rate.

But the Terrier sustainer is just screwy. I had to run the phantom drogue up to 18" to get the measures 48fps 'drogueless' descent rate. But for me, the head scratcher is the main chute. It's a 30" Topflight, and I had to reduce the Cd down to 0.55 to get the measures 28 fps 'under-main' descent rate. That seems like a big adjustment, and I'm wondering why.

So how does openrocket handle multiple chutes? Simple addition, assuming they are all working simultaneously? Dropping out the drogue forces when the main deploys?
I'm curious, because I want to switch to a bigger chute, and I'm wondering how to update the sim - just up the diameter? Reset the Cd, too? @neil_w , @JoePfeiffer

I'll check tonight if it was really a 36". and I'm barking up the wrong tree.

 

[waltr]

Interesting results.
I have data (Eggtimer Quantum) for multiple flights on my DD 2.2" LOC IRIS so need to compare descent data to OR sim.

 

[Charles_McG]

As a Bong flier, I really like (need) to have a good idea of landing zone - to avoid the swamp hazards.

So I try to tune my descent rates to be realistic, and I use the MultiLevelWind add-in with data from the Windy sounding forecast to update the sim the night before, and plot the predicted landing area on google maps. I usually do the chute-at-apogee case, too.

The MultilevelWind add-in does seem to work - you can see the difference in the ground track plots.

(36/30)^2 * 0.55 is really close to 0.8 (the default chute Cd) - I need to measure that chute tonight.

 

[waltr]

Ok, my 2.2" IRIS with a 6" drogue, Thin Top flight) chute) descends at -35f/s (-10m/s) (derived from Quantum data.
OR sim says -27m/s. OR chute is 18cm (7") dia with Cd of 0.8.
When the drogue is deployed the booster section is nearly horizontal so adds a lot more drag than just the drogue chute.

With Main deployed (24" LOC) descent rate is measured at -23f/s (7m/s). OR sim is -7.8m/s which match pretty close (chute is 60cm with CD = 0.8). Now the booster section is hanging straight down so is not adding much drag.

Is this helpful?

 

[JoePfeiffer]

So how does openrocket handle multiple chutes? Simple addition, assuming they are all working simultaneously? Dropping out the drogue forces when the main deploys?
I'm curious, because I want to switch to a bigger chute, and I'm wondering how to update the sim - just up the diameter? Reset the Cd, too? @neil_w , @JoePfeiffer

Simple addition. The drag force from each chute (and streamer, if you had any) is calculated and added together. The body tube, shock cords, etc are ignored. For a given chute family, you ought to be able to just adjust the diameter.

The math used for a parachute is simple enough that trying to figure out what's going on is probably easiest to get by just doing it by hand: F = 1/2 * rho * V^2 * CD * A
(rho is the air density).

 

[JoePfeiffer]

Rereading, I wonder if eliminating the phantom drogue would make the main descent more accurate?

 

[Charles_McG]

Without the drogue, the drogueless phase is ballistic.

 

[JoePfeiffer]

Without the drogue, the drogueless phase is ballistic.

Yes, of course, but I'm wondering if it's affecting the accuracy of the chute simulation.

Reducing the diameter of the simulated chute so it and the phantom chute combined have the same area as the real chute would have the same effect as only simming the real chute.

 

[Charles_McG]

@JoePfeiffer ,
Yep. If I set the drogue to never deploy, then I have to set the Cd of the main back to 0.8 to get the measured descent rate.

I also confirmed last night that it's a 30", and not 36" chute.

That tells me how to 'correct' the simmed main.

Is all the chute line information there just for weight estimation?

 

[JoePfeiffer]

Is all the chute line information there just for weight estimation?

I'd have to check to be sure if it's even used for that. It's definitely not used for anything else.