what causes main chute at apogee when doing dual deploy?

[Dave S.]

What causes main chute to deploy at apogee when doing dual deploy?

Usually you would just want the drogue to come out at apogee, but I have read a few times of the shear pins on the nose cone shearing and the main coming out at apogee when it wasn't supposed to.

 

[Hobie1dog]

That's usually only happens when the main and drogue wires get reversed and put on the wrong terminals

 

[Titan II]

https://www.rocketryforum.com/threads/inadvertent-main-deploy-at-apogee.161998/

 

[David_Stack]

Usually you would just want the drogue to come out at apogee, but I have read a few times of the shear pins on the nose cone shearing and the main coming out at apogee when it wasn't supposed to.

If the shear pins securing the nose cone to the payload tube are shearing at drogue deployment that could be an indication that the drogue charge was larger than actually required. As a result the payload and nose cone reach the end of the drogue recovery harness traveling at a velocity sufficient to cause the nose cone shear pins to be cut.

Many here on the forum advocate that the drogue charge needs only to be large enough to separate the payload from the booster, and get the drogue out into the air, the recovery harness does not need to be fully extended by the drogue deployment charge.

 

[sriegel]

I had apogee charge and motor backup go off at the same time on a flight. Rather violent jerk at the end of the harness caused the nose shear pin to break and the nose quickly wiggled off and released the main.

 

[Voyager1]

What causes main chute to deploy at apogee when doing dual deploy?

Usually you would just want the drogue to come out at apogee, but I have read a few times of the shear pins on the nose cone shearing and the main coming out at apogee when it wasn't supposed to.

If it is normal DD, then it's either NC shear pins shearing due to vigorous drogue deployment, altimeter fault or deployment charge wiring.

 

[tfish]

If you're using dual electronics..and they are different brands. The could be firing closure together then you think, or have them setup at.
Took me 3 or 4 flights and video to figure it out.

About 1:23 min in this video..


Frame by frame shows a second flash in the drogue bay..just before the rocket is fully separated.

Tony

 

[thequick]

Too dig of a drogue charge add/ or the harness is too short

 

[manixFan]

Another potential factor is using too short of a shock cord for the drogue. Even if the ejection charge isn’t overly strong, too short of a cord does not allow enough energy to dissipate. Then the shock when the payload section reaches the end of the cord can break the shear pins. (Basically the same as using too strong a charge.) This happened to me several times and simply increasing the length solved it. Using a lesser ejection charge didn’t produce reliable results.


Tony

 

[Dave S.]

To give you more background:

I am working on my 1st dual deploy rocket and the AV bay coupler is not that tight in the booster tube. Normally I would just put some blue tape on the coupler to make it more snug and prevent drag separation. The rocket is 4" fiberglass, dual deploy, and I have OneBadHawk Kevlar shock cords that are 25 and 30 feet long.

I was thinking of putting two of the smaller shear pins on the booster/AV bay coupler connection (instead of blue tape). Then traditionally, rivets to connect the payload tube to the AV bay and shear pins to connect the nose cone to the AV bay coupler. Ground testing before flight of course.

Does that make sense? I was trying to think of anything that could go wrong.

 

[Antares JS]

Make sure the ends of your av-bay are properly sealed. If ejection gas from the drogue gets into the av-bay, it can cause a pressure spike that your altimeter reads as being at or below main deployment altitude. This happened to me on one rocket a couple of times before I realized what the issue was.

Shear pins are also a good idea since your rocket is fiberglass. Fiberglass parts tend to be smooth and the coefficient of friction between two pieces of fiberglass isn't all that high. I always put shear pins on all separation points of dual-deploy fiberglass rockets.

 

[Dave S.]

> Make sure the ends of your av-bay are properly sealed. If ejection gas from the drogue gets into the av-bay,
> it can cause a pressure spike that your altimeter reads as being at or below main deployment altitude.


Does anyone use RTV or something similiar to make for a better seal around the AV bay lids? Mine seem ok, but a better seal can only help. Maybe I am thinking about this too much?

What about plastic caps on screw posts for electrical connections that are on the outside of the AV bay lids? I was thinking of ptotection from BP gas and electrical insulation. A quick link attached to the shock cord could flop over and short out the electrical connection to a charge - it would never fire.

 

[conneryc]

In my case, there was 4lbs of weight in the nosecone. The inertia of that weight was enough to overcome the shear-pins holding the nosecone on when the drogue was released and inflated. So instead of the main coming out at 800' and landing on the field, it came out at 5000' and I had to walk a mile to retrieve it. In this case, ground testing the charges couldn't predict this occurrence. So add more shear-pins, ground test again and then go fly it again

 

[thequick]

Lots of great advice. I switched to shear pins to keep the main from deploying early after friction fit with tape proved very unreliable. Call me a slow learner. Okay, sometimes a long walk on a clear sunny day isn’t all that bad

 

[Titan II]

Does anyone use RTV or something similiar to make for a better seal around the AV bay lids?

Not here.

A quick link attached to the shock cord could flop over and short out the electrical connection to a charge - it would never fire.

Consider skipping the quick links and use knots.

I also use shear pins on all rockets, at all separation points, regardless of body material.

 

[Rocketjunkie]

Main coming out at apogee can be caused by many factors. The most likely are:
In fiberglass rockets, 4" and above, I usually use 3 #4 on the main separation point and 2 #2 at the apogee separation point. You still need to use enough tape on the couplers to snug up the fit until the joint slowly slides apart under the weight of the rocket. You do not want a sloppy fit as the jerk when the drogue deploys will shear the apogee pins. For cardboard rockets I use masking tape for a friction fit. You want the apogee separation point to be tight. The ejection charge will separate it even if you can't pull it out yourself. Use plenty of BP. Better a little too much than not enough. I always make my apogee shock cord very long, typically 50'. The main doesn't have to be very long, I usually use about 15'. This is nylon, Kevlar puts a lot more load on the attachment points.
Separation at motor burnout is caused by negative g's when the motor burns out. A snug slip fit helps here also and usually eliminates the need for shear pins. On my 6" and 7.5" fiberglass rockets, I use the 2 #2 at apogee and 3 #4 at main. When using a long burn motor there is a good chance of an arcing flight where the rocket is moving pretty fast even at apogee. In these cases I double the main pins to 6 #4 and leave the apogee alone. (Think N1000 or M750 or my Rx N800.)

 

[bad_idea]

What about plastic caps on screw posts for electrical connections that are on the outside of the AV bay lids? I was thinking of ptotection from BP gas and electrical insulation. A quick link attached to the shock cord could flop over and short out the electrical connection to a charge - it would never fire.

I am using plastic caps for the terminals of an avbay I am working on now for exactly those reasons, primarily corrosion protection but also to mitigate the potential for a shorted deployment connection. As @Titan II says, it's best to move away from quick links here (and this design doesn't use them near the avbay), but I use one at the chute, and there's always a slim possibility of the one on the main slipping backward through the rest of the recovery bundle under boost and coming to rest on the forward bulkhead.