Baffling Observations About Size and Girth

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mh9162013

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I've been tinkering with baffles and have been conducting some tests. Yesterday I did 8 ground tests of my removable baffle system. the system worked perfectly in all 8 tests. But only in 7 tests did the plastic parachute not get damaged.

In tests 1, 2, 4, 5, 6, 7 and 8, the parachute survived the test without even getting warm. The disposable pieces of wadding I used to cover the parachute barely had any charring on them. Test 3 was a different story.

The engine worked as it should during the thrust and coast phase. But when it ejected, it sounded different and only the nose cone popped off; most of the shock cord and the entire parachute and wadding stayed in the rocket.

When I removed the wadding and parachute from the rocket, the wadding looked heavily charred, just as it would if I did not use a baffle. And of course, many parts of the parachute had melted. After inspecting the engine used for test 3, I think I have a theory as to what happened. Let me start with a picture of the clay cap from the A8-3 engine used in test 3:

A8 3 Clay.jpg

It's not the best picture, so maybe this will help:

A8 3 Clay 3.jpg

Looks like only a small part of the clay cap broke up for the ejection gasses/heat to escape. I'm sure many of you have seen something like this before. If so, did you have a partial or full ejection of your nose cone and recovery system?

In all other 7 test engines, the clay cap was almost fully gone or at least 50% of the clay cap was gone. So here's what I think happened.

I think that because only part of the clay cap opened up, the initial part of the ejection charge wasn't strong enough to eject the nose cone, parachute, shock cord, and wadding. Instead, only the nose cone popped off (not that strongly, now that I remember). And with the parachute still stuck in the main body tube, but hot ejection gasses still coming out of the engine, all the heat built up around the disposable wadding and parachute. But because the recovery system was loosely fit into the main body tube (a good thing, right?), there wasn't enough of a "seal" to build up enough pressure to push out the recovery system. Instead, all the heat built up in the main body tube.

So what does this have to do with baffles? As has been mentioned before by others, baffles work by creating a longer path for ejection gasses and bits to travel before they get to the recovery system and other sensitive bits inside the rocket. But I think there's more to it, because there's way more hot gasses and burning bits ejected from a typical model rocket engine for the baffle to handle and fully cool down.

I think what's happening is that the baffle is providing a small delay in the time it takes for the damaging hot gasses and bits to hit the recovery system. This delay is just enough for the creation of ample pressure to eject the recovery system and nose cone using gasses and bits of a nondamaging temperature. But the baffle can only do so much for so long. If the recovery system hasn't left the main body tube "on schedule" what's getting spewed by the baffle will be just as hot and damaging to a recovery system as if there was no baffle to begin with. This is new to me, although I imagine some of you out there already knew this.

This explains several launches I had from a few months ago where I used several 13mm A10-3T mini engines in a BT-50 rocket (no baffle was in that rocket). The nose cone would pop off, but everything else remained in the rocket body. This was because I had so much tightly backed into the main body tube. But when launching that same rocket with the A8-3 engine, everything worked as it should. I think the bigger diameter of the A8-3 engine allowed more pressure to build up more quickly, which was enough for the recovery system to properly eject.

TL;DR: size and girth matter. And what makes baffles work is that they actually only work for a very short period of time. But this delay is just enough to protect the recovery system, but still allow proper ejection from the rocket.

Girth matters when it comes to engines, but length matters when it comes to baffles. The longer the baffle, the more cool gasses are inside and the more time it takes for gasses to flow through the baffle. This provides a greater delay in when the damaging hot gasses make their way to the recovery system.
 

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I've been tinkering with baffles and have been conducting some tests. Yesterday I did 8 ground tests of my removable baffle system. the system worked perfectly in all 8 tests. But only in 7 tests did the plastic parachute not get damaged.

In tests 1, 2, 4, 5, 6, 7 and 8, the parachute survived the test without even getting warm. The disposable pieces of wadding I used to cover the parachute barely had any charring on them. Test 3 was a different story.

The engine worked as it should during the thrust and coast phase. But when it ejected, it sounded different and only the nose cone popped off; most of the shock cord and the entire parachute and wadding stayed in the rocket.

When I removed the wadding and parachute from the rocket, the wadding looked heavily charred, just as it would if I did not use a baffle. And of course, many parts of the parachute had melted. After inspecting the engine used for test 3, I think I have a theory as to what happened. Let me start with a picture of the clay cap from the A8-3 engine used in test 3:

View attachment 534444

It's not the best picture, so maybe this will help:

View attachment 534445

Looks like only a small part of the clay cap broke up for the ejection gasses/heat to escape. I'm sure many of you have seen something like this before. If so, did you have a partial or full ejection of your nose cone and recovery system?

In all other 7 test engines, the clay cap was almost fully gone or at least 50% of the clay cap was gone. So here's what I think happened.

I think that because only part of the clay cap opened up, the initial part of the ejection charge wasn't strong enough to eject the nose cone, parachute, shock cord, and wadding. Instead, only the nose cone popped off (not that strongly, now that I remember). And with the parachute still stuck in the main body tube, but hot ejection gasses still coming out of the engine, all the heat built up around the disposable wadding and parachute. But because the recovery system was loosely fit into the main body tube (a good thing, right?), there wasn't enough of a "seal" to build up enough pressure to push out the recovery system. Instead, all the heat built up in the main body tube.

So what does this have to do with baffles? As has been mentioned before by others, baffles work by creating a longer path for ejection gasses and bits to travel before they get to the recovery system and other sensitive bits inside the rocket. But I think there's more to it, because there's way more hot gasses and burning bits ejected from a typical model rocket engine for the baffle to handle and fully cool down.

I think what's happening is that the baffle is providing a small delay in the time it takes for the damaging hot gasses and bits to hit the recovery system. This delay is just enough for the creation of ample pressure to eject the recovery system and nose cone using gasses and bits of a nondamaging temperature. But the baffle can only do so much for so long. If the recovery system hasn't left the main body tube "on schedule" what's getting spewed by the baffle will be just as hot and damaging to a recovery system as if there was no baffle to begin with. This is new to me, although I imagine some of you out there already knew this.

This explains several launches I had from a few months ago where I used several 13mm A10-3T mini engines in a BT-50 rocket (no baffle was in that rocket). The nose cone would pop off, but everything else remained in the rocket body. This was because I had so much tightly backed into the main body tube. But when launching that same rocket with the A8-3 engine, everything worked as it should. I think the bigger diameter of the A8-3 engine allowed more pressure to build up more quickly, which was enough for the recovery system to properly eject.

TL;DR: size and girth matter. And what makes baffles work is that they actually only work for a very short period of time. But this delay is just enough to protect the recovery system, but still allow proper ejection from the rocket.

Girth matters when it comes to engines, but length matters when it comes to baffles. The longer the baffle, the more cool gasses are inside and the more time it takes for gasses to flow through the baffle. This provides a greater delay in when the damaging hot gasses make their way to the recovery system.
Interesting line of thought.

I wouldn’t think that the issue of trapping gases would be critical in actual in-flight use. Even the minimal airflow into and through the tube would get to work on displacing and cooling the ejection gas. I would guess that this phenomenon is unique to a static-airflow ground test.

I would be very interested in testing this hypothesis via wind tunnel, or just even in front a strong fan. Gluing a grate into the tube to stop the cute ejecting while still allowing the nose to pop off, then running a test while subjected to airflow not greater than desired deployment velocity, may yield an interesting result.

Running the test at varying angles to airflow or even while the rocket is hooked up to a rig that could simulate various forms of tumbling flight would provide a useful data set.

I would also be interested in the effects of a baffle designed to completely reverse gas flow instead of just block a straight-line path through the tube. Something like these Qualman baffles, just tailored to your specifications for removability and simple production.

https://brshobbies.com/qualman-rocketry-ejection-baffle-for-bt80/
 
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