Disk Parachute Protector

[Jeff Lassahn]

I'm playing with a new style of parachute protection that I can use in small rockets with very limited chute packing space.

It's basically a very minimal piston made from a balsa and paper disk that sits between the motor and parachute. It uses way less space and weight than a baffle, and doesn't sneeze tissue all over the launch range. The disk is attached to the shock cord and fits very loose in the body tube (maybe 1mm of gap around the whole edge).

The idea is that most of the bad stuff the might damage the parachute travels in roughly a straight line from the motor, so we don't need an exact fit, just something to block the initial impact of evil death particles.

I flew my first test this morning, and it went pretty well.

The disk absorbed most of the gunk, but a little leaked past and is visible on the edge of the nose. Pretty similar to some flights I've done with wadding. The parachute is completely clean and undamaged.

This is 12 inch chute, flown in a quicky custom built BT-55 rocket with a B4-4 motor.

 

[David_Stack]

Interesting. Do you have a way to replace the disk when it is 'worn out'/degraded due to exposure to the ejection charge? (or will that require replacement of the shock cord itself)

 

[BABAR]

I think you will need more of a cylinder than a disk. A disk can rotate in the body tube and allow blow by.

Quality Competition Rocketry (QCR) Used to see some foam cylinders that had flameproofing on one end, worked pretty well.

 

[Back_at_it]

Interesting idea but I’m wondering how you keep it from rotating in the body tube and allowing partials to get past it. That is unless it’s tight down around the top of the motor mount.

I played around with an idea some years ago with a piston style system in a BT50 sized rocket. It was made up of a 1 inch dowel rod that I sanded to fit the inside diameter of the BT50 tube. It was approx 3/4 of an inch in length with Kevlar line attached to the bottom screw eye and an elastic shock cord from the top to the nose cone.

worked really well until the lower Kevlar line burned thru on the 4th flight. I scrapped the idea at the time but have wanted to revisit it.

 

[rocketace]

I have started doing similar to protect my fire blankets. I use some old fire blankets or extra flame proof material to cut out small circles that I put in the tube first (I still use some dog barf) then I put my recovery burrito in. This protects my larger fire blanket so I just have to replace the small disk.

 

[Louis Wu]

I've built one of these (Chute Spool) and looks like it would serve as a 'protector' as well..

 

[neil_w]

I've built one of these (Chute Spool) and looks like it would serve as a 'protector' as well..
View attachment 435617

Ooh, keep us posted when you test it. I'd love to see video of the deployment.

 

[Scott231]

Check out the Petal Protectors from DinoChutes. Available in 2.6, 3, or 4"

 

[Jeff Lassahn]

I flew a different model with a very similar disk yesterday and had a completely different experience.
I got a significant amount of gunk blowing past the disk, and some parachute damage.

Looking at the blast pattern of gunk left over, it's pretty symmetric, with the main splot in the center of the disk like previously, but also significant gunk on all sides of the body tube and some on the top centering ring of the motor mount.

The symmetric pattern on all sides of the body tube makes me think the problem isn't the disk rotating -- in that case I'd expect most of the blow-through on one side. So something else is going on.

My working theory is that the gas produced by the ejection charge isn't itself damaging; both the gunk I can see in the tube and the parachute damage are caused by small particles of incompletely burned ejection charge material. (There's also a brownish region in the center of the disk that may be actual heat damage from either hot gas or radiation coming directly from the top of the motor but I think that's very contained) The disk design is based on the notion that the gunk travels in roughly straight lines, so there just needs to be a barrier that blocks all those straight line paths.

That theory matches the blast pattern on my first rocket pretty well, but it doesn't really explain the second one. Some possible explanations:
1. maybe there's a lot of variation in ejection charge behavior, and this last flight my charge generated a lot more of some kind of material that I wasn't seeing much of in previous flights.
2. the nose on this new model is a lot looser than the previous one, maybe the pressure wave from the initial ejection blast moved the disk (and everything else) significantly forward before the flying gunk hit.
3. I'm completely wrong in how I'm thinking about the problem and other stuff is happening.

In any case, more experiments are needed. I think I'm going to rig some kind of crepe paper chute wrap which I can inspect for damage after flights, so I can try some stuff without destroying a lot of parachutes.

 

[neil_w]

My working theory is that the gas produced by the ejection charge isn't itself damaging; both the gunk I can see in the tube and the parachute damage are caused by small particles of incompletely burned ejection charge material.

I believe that is generally accepted.