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Premature Deployment

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Marsman

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Over the weekend, my first rocket with an altimeter (54mm in diameter with a G76G-7) suffered a premature deployment of both the drogue and main outputs. From the ground, the boost was arrow straight until suddenly it stopped in the sky and the parachutes came out apporximately 2-3 seconds into the flight. I'd guess that it was right near motor burnout. The rocket landed perfectly a few hundred feet downrange.

Onboard was a brand new RRC2 mini with a single vent port (5/64"). My question is what would cause such a premature delpoyment? Both the drouge and the main popped at nearly the same time, and the motor backup fired on the way down. After landing, the altimeter beeped out 300 ft- the altitude that the main was set to deploy. However, the rocket was higher than that, checking the RockSim predictions it should have been between 430-630 feet (which sounds right).

I'm laughing at everyone who overbuilds right now, as this simple paper rocket with the lightweight tubes from Red Arrow deployed at 200+ mph on the way up and suffered no damage whatsoever- no ripped parachute, zipper, or chiped paint.

EDIT: Vent port was 5/64" in diameter
 
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hognutz63

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What size of rocket and did you have your mach delay set?
 

Marsman

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2" diameter and 75.5" long. No mach delay, as top speed was only going to be ~200 mph (.24 Mach)
 

madmax

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Was it windy that day? Single pressure ports are apparently more prone to a wind gust changing the pressure in the bay and thus spoofing the altimeter into thinking it was at apogee. Try drilling three holes with the same area as your single hole. Other than that, I can't think of anything else besides Mach delay not being set that would cause a premature deployment. And you already said it wasn't even going to get close to that speed.

Try emailing MissileWorks, maybe they can shed some more light on this problem. Let us know what they say if you do so.
 

MarkM

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Try drilling three holes with the same area as your single hole.
To be clear, he's referring to total area. Meaning the area of all three of the holes together should be equal to the area of your single hole. Although others do use one hole, I NEVER do..

Are you sure the deployment was due to the altimeter firing? Or could the altimeter fired, but on decent well after the premature deployment occurred?

If so there are two other reasons that could cause premature deployment.
1) Drag separation; probably not likely, but possible. More likely is #2
2) Did you drill a vent hole in main parachute bay? Even though the rocket did not reach a high altitude, if the pressure inside the tube could not equalize with the outside pressure and pressure built up in the main parachute bay, this could easily cause the nose cone to blow off and deploy your main. I've seen this happen a few times even at relatively low altitudes & on slightly larger diameter rockets than yours. So it's possible. Smaller diameter rockets are more susceptible to this since there's less tube volume so the pressure can build up more quickly.
 
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Marsman

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Good tips about the 3 holes, although it wasn't windy.

It was definitely the altimeter.

I've never had drag separation issues, and the couplers were pretty tight anyway- so I don't think that this is likely at all.

As for the vent port in the main parachute compartment, I guess this is possible but highly unlikely. I've gone much higher and faster than this flight, and never had that problem on other flights. As I understand it, the ascent has to be very rapid in order to cause a large enough pressure differential to pop the nosecone or separate the rocket. This rocket wasn't fast (~200 mph) or high (400-600 ft.) when it deployed.

I've emailed MissileWorks so we'll see what they say.
 

mikec

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2" diameter and 75.5" long. No mach delay, as top speed was only going to be ~200 mph (.24 Mach)
How much did it weigh? Guessing 14 oz dry, that sims to more like 380 mph, still not close to mach.

Since I just bought an RRC2 I'm kinda interested. :)
 

MarkM

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Good tips about the 3 holes, although it wasn't windy.
Any kind of air turbulance or air flow irregularity over that single port could cause issues (not just wind). That's why 3 or 4 ports is typically best. That way if one port is blocked or air flow irregular over it, the other ports will compensate and normalize out the irregularity.
 

mikec

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A couple of other possibilities occurred to me: either there was a brief interruption of power during ascent (battery clip moved during thrust, for example) or the altimeter didn't initialize properly when turned on (was it beeping continuity the way it was supposed to?)

I'd try a simple suction test to see if it behaves as expected.

Also, since the minimum launch detect altitude is 250 feet, I'm wondering if setting deploy at 300 feet might have some kind of boundary condition problem if the sensor is a little noisy. Most people deploy at 500 or even 700 feet.
 

MarkM

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either there was a brief interruption of power during ascent (battery clip moved during thrust, for example)
Doubt it. If this happened, there's a high likelihood the altimeter would never have fired because it would reset and be completely confused where it was. This type of problem typically results in lawn darts from no deployment or extremely LATE deployents - not early ones.

Also, since the minimum launch detect altitude is 250 feet, I'm wondering if setting deploy at 300 feet might have some kind of boundary condition problem if the sensor is a little noisy. Most people deploy at 500 or even 700 feet.
Doubt this also. I use a barometric altimeter that has the same altitude detect (albeit a different brand of altimeter) and I've set deployment at 300ft several times with no issues. As long as the rocket exceeds the 300 ft for deployment, it should not be an issue as the altimeter detects that's it has gone past the set altitude, reached apogee, and decended back to that altitude.
 

mikec

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Doubt it. If this happened, there's a high likelihood the altimeter would never have fired...
Some altimeters appear to fire the main immediately if they power-on reset so that the deployment still happens even if the drogue firing causes a reset because of excessive current. I think that's why the RRC2 manual tells you "Never cycle the altimeter power switch off, then immediately back on (allow at least 10 seconds)."

Anyway, I'm just throwing out possibilities here. Since the altimeter thought that apogee was at the main deploy altitude it seems like something similar to my suggestions could have happened.
 

madmax

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How old is your "brand new" RRC2 mini? I seem to remember reading about a firmware problem with the first run of units. What the problem was I can not recall though.
 

bobkrech

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This is a reloadable motor. Did you put BP in the motor as a backup ejection. If so it is possible that the motor ejection BP charge went off and started the chain of events.

Bob
 

Diosces

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I'm thinking the same as Bob, if you also had motor ejection charge it could have went off prematurely. I've had a few of those cause premature ejection which in turn cause electronic deployment.



I'm laughing at everyone who overbuilds right now, as this simple paper rocket with the lightweight tubes from Red Arrow deployed at 200+ mph on the way up and suffered no damage whatsoever- no ripped parachute, zipper, or chiped paint.

EDIT: Vent port was 5/64" in diameter
Just because your 'paper' rocket survived a premature deployment does not give cause to deride those who build sturdy. Most of my rockets are overbuilt to withstand the trials and travails of travel, storage, gettng kicked by anteaters, etc.

Anteater.JPG
 

mikec

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I'm thinking the same as Bob, if you also had motor ejection charge it could have went off prematurely.
Umm, the original post said that the motor ejection didn't fire until after the deployment. Unless marsman misremembers the sequence of events (unlikely) this isn't the explanation.
 

Diosces

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Umm, the original post said that the motor ejection didn't fire until after the deployment. Unless marsman misremembers the sequence of events (unlikely) this isn't the explanation.
Not misremeber but mis-intrepret...
I doubt he had shear pins installed, so I'm betting when his drogue deployed it caused a drag separation of the main at the same time.
So possibly he perceived the main deployment charge which fired on the way down as the motor ejection charge firing.
Like I said I've had the same thing happen a few times to me.
 

MarkM

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I doubt he had shear pins installed, so I'm betting when his drogue deployed it caused a drag separation of the main at the same time.
I've had this happen even with shear pins when my drogue charges are little oversized and this could be what happened. But, it does not explain the altimeter reading. However, Rocksim altitudes are often over-predicted and quite possibly your altimeter is correct, Rocksim overestimated, and your eyes thought it went higher than it actually did (I'm a poor judge of altitude by vision).

If your actual altitude was at or near 300ft (meaning your altimeter IS correct in it's measurement), then the drogue and main charge would go off at roughly the same moment. The altimeter would fire the drogue charge detecting the slight pressure decrease as the rocket arches over at apogee and, if that altitude is ~300ft, your main charge would fire at approximately the same time since that is the altitude you set the main to fire at.

Two tests to do:
1) Fly this altimeter in another rocket with an altimeter you are confident works and measures altitude reliably. Use this one without charges just to determine if the altitude measured with yours compares favorably with the other altimeter. Perhaps a friend will let you piggy-back yours in one of this rockets.
2)Choose a bigger motor that you're certain would push the rocket above 300ft and see what happens. Obviously that would be an H, but then you need to be certified to fly that. If you're so confident in your build, do your L1 on it.
 
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falingtrea

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One thing you could do is connect whatever you use for an e-match to the unit and cause a battery disconnect. That way you could see if a power disruption does cause the charge to fire and verify or eliminate one possibility.

(Note, this is a general comment, and not about any specific unit: ;) )
Frankly, any altimeter design that fires the charge after a power disruption is a poor design, in my opinion.
 

bobkrech

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Umm, the original post said that the motor ejection didn't fire until after the deployment. Unless marsman misremembers the sequence of events (unlikely) this isn't the explanation.
Since I wasn't there and we don't have all the information we would like, my speculation is that if the motor ejection charge fired early, it would deploy the main and that would trigger the electronic apogee ejection almost immediately afterwards, deploying the drogue. If the rocket was above 300 feet, the computer would fire the electronic main charge at 300 ft., but since the main was already deployed, you would just hear the pop.

A data dump might provide the required information.

Bob
 

mikec

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Frankly, any altimeter design that fires the charge after a power disruption is a poor design, in my opinion.
I can appreciate that view, but on the other hand firing the charge at the wrong time in flight may damage the rocket, but not firing it will result in a lawn dart, which is more potentially hazardous.

Given that it's hard to predict all possible types of power disruption and their causes, and electronics may not be able to tell exactly what happened independently, it's tough to say exactly how things ought to be implemented (or what the RRC2 really does, I'm just inferring this behavior from the manual.)
 

Larry Curcio

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Some altimeters appear to fire the main immediately if they power-on reset so that the deployment still happens even if the drogue firing causes a reset because of excessive current. I think that's why the RRC2 manual tells you "Never cycle the altimeter power switch off, then immediately back on (allow at least 10 seconds)."

Anyway, I'm just throwing out possibilities here. Since the altimeter thought that apogee was at the main deploy altitude it seems like something similar to my suggestions could have happened.
I absolutely agree - especially since

1) The altimeter uses an external 9v battery, requiring a battery harness;

2) The deployment appeared to occur at burnout, when payloads love to lurch forward because of external drag; and

3) It's the poster's first altimeter flight - so he likely has no experience securing battery harnesses (or omitting them).

-LarryC
 
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