Baffles VS Pistons.... What's Your Choice? And, Why?

[gldknght]

I've used the pistons that come with all the Pml quantum tube rockets I've built over the years. Lots of flights, no fails due to the piston. I use a vegetable brush duck taped to a long 1 inch dowel to clean inside the tubes every third or fourth flight. And religiously check the piston fit before every flight. Having said that, I know quantum tube is more stable when it comes to weather changes, and have never used a cardboard piston in a cardboard/paper tube.

The baffle idea bothers me. I don't see how the gas flow through the baffle cools enough to avoid melting the parachute, and even if it does, I expect the soot and such from the ejection charge still gets all over the recovery gear.

The "labyrinth" baffle system that Aerotech uses seems to work well, but I haven't flown those enough to get that steel mesh really dirty. Will that mess eventually plug up?

 

[Jim Hinton]

I've used the pistons that come with all the Pml quantum tube rockets I've built over the years. Lots of flights, no fails due to the piston. I use a vegetable brush duck taped to a long 1 inch dowel to clean inside the tubes every third or fourth flight. And religiously check the piston fit before every flight. Having said that, I know quantum tube is more stable when it comes to weather changes, and have never used a cardboard piston in a cardboard/paper tube.

The baffle idea bothers me. I don't see how the gas flow through the baffle cools enough to avoid melting the parachute, and even if it does, I expect the soot and such from the ejection charge still gets all over the recovery gear.

The "labyrinth" baffle system that Aerotech uses seems to work well, but I haven't flown those enough to get that steel mesh really dirty. Will that mess eventually plug up?

Ejection charge covers from the motor(s) seem to be the main hazard to airflow through the 'labyrinth'. As long as I pick out that debris, it works pretty reliably.

 

[DaHabes]

Thanks. Had planned to put a baffle in the zipperless coupler of the fin can in a dual deploy design, less because it was needed than for expeience building it. Thinking now I'll leave the baffle out and just put my faith in a Nomex blanket for the backup motor eject for the drogue. Since I was going to use a blanket to protect from the apogee-fired charge anyway, the baffle was sort of illogical and just there to be there in this design, but I tend to try out a lot of things on the same rocket - not always necessary or optimal - just to gain experience. Will skip it this time around.

My only concern with using Nomex is if you are using the AV-Bay as primary for the drogue and motor ejection for the backup, the blanket needs to protect from both directions, which isn't always achievable depending on parachute and airframe sizes. A baffle for the motor eject side eliminates that issue.

 

[DaHabes]

I've used the pistons that come with all the Pml quantum tube rockets I've built over the years. Lots of flights, no fails due to the piston. I use a vegetable brush duck taped to a long 1 inch dowel to clean inside the tubes every third or fourth flight. And religiously check the piston fit before every flight. Having said that, I know quantum tube is more stable when it comes to weather changes, and have never used a cardboard piston in a cardboard/paper tube.

The baffle idea bothers me. I don't see how the gas flow through the baffle cools enough to avoid melting the parachute, and even if it does, I expect the soot and such from the ejection charge still gets all over the recovery gear.

The "labyrinth" baffle system that Aerotech uses seems to work well, but I haven't flown those enough to get that steel mesh really dirty. Will that mess eventually plug up?

It's not the heat that affects the parachute as much as the burning embers. The baffle captures all of those. Also realize that thermodynamics is your friend. The ejection charge creates high pressure because the BP (or Triple seven, or whatever you are using) is tightly contained. As a hot gas expands, it cools, so by the time it works it's way through the baffle, it's already cooled down significantly.

 

[gldknght]

It's not the heat that affects the parachute as much as the burning embers...

That can't be completely accurate. I've had Estes type rockets come back with the parachute melted from heat, not holed, as it would be from burning embers or sparks.

 

[DaHabes]

That can't be completely accurate. I've had Estes type rockets come back with the parachute melted from heat, not holed, as it would be from burning embers or sparks.

I never use plastic parachutes anymore. Rip stop nylon, always. Top Flight Recovery has 9" chutes. TFR and Dinochutes both have 12" and 15". Buy one each, move between LPR rockets.

 

[gldknght]

I never use plastic parachutes anymore. Rip stop nylon, always. Top Flight Recovery has 9" chutes. TFR and Dinochutes both have 12" and 15". Buy one each, move between LPR rockets.

A good idea, but doesn't really address the problem of hot gasses hitting the parachutes. Rip stop nylon will melt also.

 

[heada]

The space above the baffle is not a vacuum. There is air there. The baffle prevents direct flame and burning particles. It then allows the pressure from the ejection charge to compress the air in the parachute area. Thus the nose and parachute are ejected but not damaged by flame or burning particles. Plastic parachutes should be fully protected by a working baffle.

 

[Back_at_it]

I use both nylon and standard Estes plastic chutes and have never had an issue with any rocket that uses a baffle. No holes, no melting.

if it doesn’t get hit directly with burning partials or the giant flame that shoots out the top of the motor which the baffle prevents it will be fine.

 

[DaHabes]

That can't be completely accurate. I've had Estes type rockets come back with the parachute melted from heat, not holed, as it would be from burning embers or sparks.

Relative to using baffles, heat isn't an issue because of the baffle and my above thermodynamics discussion point.