Dual Deploy- Chute placement

[Scott C]

More of a curiosity question but what are the Advantages/Dis-Advantages/Pro's/Con's, on having the drogue chute in the payload and main in the booster and vice versa? First time setting up a dual deploy but sticking with the "Traditional" setup.
Just trying to learn all that I can.

 

[Jmhepworth]

Having the drogue in the booster and main in the payload is most common. That lets you use the motor eject for backup.

 

[Zeus-cat]

People have "reversed" the chutes for dual deploy, but the "traditional" setup works best unless you have a reason to do it the other way. Motor backup has saved more than a few rockets when the electronics don't deploy the laundry.

 

[afadeev]

People have "reversed" the chutes for dual deploy, but the "traditional" setup works best unless you have a reason to do it the other way.

It's all about optimally utilizing the space in the airframe that is available to you.
Backup motor deploy is a good extra safety measure, and the chute you would prefer to expose to motor eject (if motor eject is available, or delay is long enough) is the smaller drogue.

However, you might be working with a relatively large main chute that needs to fit into a small diameter high-performance airframe.
And may you are also doing HED (head-end deployment == chute in the nose cone).
If the main doesn't physically fit into the nose cone, then you may have little choice but to switch things around and put main into the booster section (if it has more space). But then, you really don't want motor ejection, as it will defeat the intent behind dual-deploy.

Motor backup has saved more than a few rockets when the electronics don't deploy the laundry.

Agreed.

 

[cerving]

I think the primary reason is that you're a lot more likely to get a fouled main chute with the main in the bottom... I think Teddy (OneBadHawk) explained this awhile ago.

 

[manixFan]

Hmm, or perhaps it's better to have weight forward than backward. Main chutes are heavier, so putting it near the motor case shifts the CG in the wrong direction. Plus the amount of room available in the fin can is heavily affect by the choice of motor, while in a payload bay the space is constant.


Tony

 

[Handeman]

I think your chance of less damage with a failed main occurs when the drogue deploys from the booster. The rocket will fall faster than desirable, but usually flat. A decent rate of 50 - 70 mph (75 - 100 ft/sec) is much better than a 200-300 mph ballistic return which is possible with just a nose cone trailing behind a fully assembled ballistic rocket.

Now it is possible to get a ballistic return with the traditional dual deploy, especial when flying drogueless. I've seen quite a few. Usually the main opens, everything stays together, the fin can doesn't snap off the shock cord when it finally hits bottom and everyone congratulates the flyer for a successful dual deployment flight. In a few case, the booster fell through and fouled the main, or the main shredded, or the fin can broke loose when it hit the end of the shock cord and fell on it's own.

Either way, bad things can happen, even to nice flyers!

 

[QBrandt]

I'm firmly in the camp of "Use whatever setup makes sense for the project."

For example, this SAC project (8.8 lbs of payload to 10,000 feet):

Left to right: white nose cone (main chute), red switch band/avionics bay, black body tube (drogue chute), red payload bay, white booster.
This is a HEDD configuration, but the point would stand if there was another tube between the nose for a chute. No motor eject.

Imagine if the drogue were in the nose. On inflation of the drogue, there is a whole lot of mass in the back end that suddenly goes from freefall to descent supported by drogue. All of the shock from that travels through the avionics bay, which is only kept in place by friction, shear pins and the collective will of everyone present. Sounds like a really good way to simultaneously dual deploy.

Compare that to drogue in body, main in nose. On inflation of the drogue, there is still a whole lot of mass in the back, but the shock doesn't go through the avionics bay. There is still a risk of the inertia from the nose doing the same thing, but significantly reduced because it is so much lighter.

 

[JoePfeiffer]

Another reason (and my own reason for doing it) to put the drogue in the aft section is on a bad flight with lots of horizontal velocity at apogee you don't zipper the tube.

 

[Scott C]

Thanks for all the input so far!

 

[Banzai88]

Another thought about conventional placement is that this way the rocket separates the upper 1/3rd-sh of the airframe at apogee, thus spoiling the aerodynamics that all but eliminates the possibility of a ballistic trajectory. With only the nose cone coming off, I've seen a fair few ballistic trajectories........which is unsafe if for no other reason than the unpredictability of the occurrence and flight path. The weight of the aft section is often enough to go ballistic and drag the nose cone/recovery harness/drogue down with it, flapping along behind. If that happens, it's often because the drogue is orders of magnitude undersized and doesn't provide enough drag to spoil the aerodynamic orientation of the fuselage/fin can.

What if you got the launch angle incorrect? What if it weather cocked or thrust vectored strangely due to a cracked nozzle upon launch? What are the winds aloft and how will they adjust the ballistic flight path? Too many variables for our expected/planned safety margin, that's why they're so dangerous.

Ballistic trajectories are notorious for building enough descent speed that should the main deploy nominally, there's often enough speed to destroy the main parachute.

Conventional layout also helps ensure, with proper drogue selection and recovery harness length, that the main parachute compartment has the spacing away from any other airframe parts to ensure a clean deployment into clean air...and thus more predictable inflation of the main parachute.

If the drogue is sized properly, the main harness will extend fully and the fin can will NOT come to a sudden stop at the bottom, stressing the harness material and the mounting point.

HED is a special case. In HED configuration, the main is often contained in the nose cone, and the nose cone/main/av bay is often purposefully ejected at apogee with sufficient force to FULLY extend the recovery harness, thus ensuring that the main recovery gear starts with the best chance of a clean air deployment.
Also with HED, special care is (or at least it should be) paid to the drogue size, since there's essentially 'unlimited' room for a drogue that would otherwise be considered 'oversize' in a conventional deployment to ensure that there is enough drag from the drogue to spoil any aerodynamics that would lead to a ballistic trajectory.

As with anything, the simulations are free. Make a decision, fly it, and adjust based on empirical performance.

 

[Jowayen]

Compare that to drogue in body, main in nose. On inflation of the drogue, there is still a whole lot of mass in the back, but the shock doesn't go through the avionics bay. There is still a risk of the inertia from the nose doing the same thing, but significantly reduced because it is so much lighter.

I've thought about this very question.
The conclusion I came to was this same scenario that QBrandt describes.
Which airframe component do you want to stress the least?
Keeping (or attempting to keep) the shock off of the av bay seems to be the way to go IMHO.