please help me interpret this flight graph

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billdz

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Hi,

I had a successful flight yesterday of a Darkstar Jr. on an H550. It was carrying a Telemetrum, which produced the attached graph. It shows that the rocket flew to 2731'.

I assume that the downward spikes in the red and blue lines at 13 seconds indicate ejection? If yes, that means ejection was near apogee. Are the spikes normal or do they suggest I used too much powder?

The rocket had a Jolly Logic Chute Release set to open at 400', and visually it appeared to deploy correctly. The graph shows a spike on the blue line at 63 seconds, altitude about 270', followed by a slight slowdown of descent speed and landing at 73 seconds. Is it possible this is inaccurate? Visually, the rocket appeared to slow down greatly once the chute opened, and I'm pretty sure the rocket was in the air for significantly longer than 10 seconds with the chute open.

Does the graph reveal any other useful info?

Thanks,
Bill
 

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blackjack2564

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Looks like main fired at 500, took awhile to open at 275. That's where you have a big spike, probably chute opening shock load.

10 seconds from 275 to ground is decent rate of 27ft per second. Right in ballpark for many flights.
 

John Beans

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Yep, everything looks pretty good. You were descending at 50 fps prior to inflation, and it took 225 feet of descent after release before inflation occurred. That could probably be improved, depending on how you folded your chute and shrouds. It totally depends on conditions during descent which are a little random, but I shoot for 50 - 150 feet of descent during inflation, so you're a little above that . Every now and then I see a graph where the chute just pops open right at release, but that's too much to expect every flight. Inflation time/distance is a measure you should start noting down, as you tweak how you fold and bundle your chutes to get that down to 50-150 feet.
Nice flight!
 

billdz

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Thanks for the replies, the CR was set to deploy at 400', not 500'. The Telemetrum was set to deploy the main at 500' but I was not using electronic deployment, so ignore that Main line on the graph. The 125' is right within John's range.

Guess we can lose track of time when chasing a rocket. The wind was brisk, I saw the chute open, then the rocket appeared to brake and start traveling almost parallel to the ground, would have bet I was chasing it for more than 30 seconds between the time the chute opened and the rocket touched down.

@tHoagland - 37 G, Mach 0.5
 

EeebeeE

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Actually, your main looks like it took a relatively short time to inflate. Keep in mind that final descent rate is reached after inflation...not at it. My guess is your chute popped right open and it took about 100 feet or so for it to slow down to the final descent rate.

As far as your initial question about BP usage, only ground testing is really going to show you what you need. But in this case your shock cords held as did all your linkages and most importantly your chute, so in my opinion you didn't use too much.
 

John Beans

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Actually, your main looks like it took a relatively short time to inflate. Keep in mind that final descent rate is reached after inflation...not at it. My guess is your chute popped right open and it took about 100 feet or so for it to slow down to the final descent rate.
That's a common misconception. The time it takes to slow is the tiny curve of the elbow where it's transitioning from one slope (descent rate) on the graph to the final slope (final descent rate). Almost no distance at all!

I know people think that for heavy rockets you need hundreds of feet for them to slow down, but you don't. No matter how heavy the rocket is, you'll see a straight line slope before the parachute opens, which means that it's at terminal velocity and is NOT slowing down or speeding up. Then the parachute opens and almost immediately it reaches a second, slower terminal velocity where again it's not slowing down or speeding up.

The reasons for choosing a higher release altitude include:
1. You will have more time to spot it if you lost sight of it
2. You want it to drift farther (as in, back to you from upwind)
3. You are unsure about how long it will take to release and inflate (smart when you first use it)

The WRONG reasons to choose a higher altitude would include:
1. You felt you needed to give it more time to slow down (takes no time)
2. To give more time for an air raid warning (never direct your flights to land near the crowd)
 

billdz

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A decent rate of 27ft per second is OK? Read somewhere it should be 20 or less.
 

John Beans

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A decent rate of 27ft per second is OK? Read somewhere it should be 20 or less.
Depends on how much room for chute you have. With delayed deployment, you can use a bigger chute and not worry as much about drift. Big chute, low opening, soft landing (such as 12 FPS).
 

billdz

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NAR Level 3 Certification Requirements 2.3: "The descent rate at touchdown must not exceed 20 feet per second for any component weighing in excess of eight ounces."
 

rharshberger

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As it was not an L3 cert those requirements can be taken as a recommendation. I have seen 20 fps recommended in NAR documents before only there it was stated as a recommendation no a requirement.
 

thomas

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The question is, what is the velocity and the acceleration?
The acceleration seems to be directly from the sensor.
But the velocity seems not to be the integral of the acceleration (negative acceleration after 65s but increasing velocity) or has the sensor orientation changed?
It seems not to be the derivative of the altitude (constant slope).
Is this just a lag from the Kalman filter?
 

billdz

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Seems there is no real consensus on maximum descent speed, other than "it depends" on conditions and the hardness of the landing spot. Saw this on another thread:
***
A landing velocity of 11 feet/second is the same as a drop from a height of 1'10.9/16" with no parachute(assuming no drag).
A landing velocity of 14 feet/second is the same as a drop from a height of 3'0.9/16" with no parachute(assuming no drag).
A landing velocity of 17 feet/second is the same as a drop from a height of 4'5.7/8" with no parachute(assuming no drag).
A landing velocity of 22 feet/second is the same as a drop from a height of 7'6.1/4" with no parachute (assuming no drag).
....
As for speed for which nothing breaks, it depends on the construction materials of the rocket, the hardness and nature of the LZ, the wind, and the momentum of the rocket.
A fiberglass or carbon composite rocket might be able to handle 30 fps on a grassy or soft surface, whereas landing on a rocky or frozen field at 15 fps might cause damage.
....
I know for a fact that a Darkstar Jr can survive a 50fps descent [at a sod farm].

https://www.rocketryforum.com/showthread.php?66183-Safe-decent-rate

Interestingly, while Open Rocket gives you a warning if deployment speed is too high, it apparently does not give a warning if descent speed is excessive. In a simulation, I reduced the size of the chute to 6", which produced a descent speed of 262 f/s, and still got a green "no warnings" check.


 
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