I just finished a scratch build and it’s a zipper less design with a BT-80 tube. Generally, does a baffle lessen the pressure of an ejection charge? I’ll be using mainly AT OR QUEST E engines. I weighed the existing charges and they vary from .6 grams to around .65. An E35. had .7. I upped all the charges to .7. Thoughts?
Ejection pressure with a baffle
- Thread starter Fireball
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If you are worried, do a ground test using a spent single use motor or reloadable hardware with an appropriate amount of BP...
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In my experience...
Baffles do reduce the effectiveness of the BP..
If you think about it the Baffles job is to reduce the amount of heat..getting to the recovery gear..To me heat means hot, gasses and pressure.
I have crashed a rocket by installing 2 stainless steel scrubbing pads on to the shock cord....and NOT ground testing my charges again.
like T-Rex says..ground test it..
let us know what you find out...
Tony
mh9162013
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In my experience, baffles work by lengthening the time it takes for the damaging heat to get to the recovery gear. Unless there's leakage in the MMT tube/system, I don't think most baffles work by reducing the overall pressure of an ejection charge (although it may very well reduce the overall force).
Instead, it spreads out that force over a longer period of time. So by the time gasses reach the recovery gear that's hot enough to damage it, it should already have been ejected from the rocket.
mh9162013
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I have definitely noticed this as well, as I noticed a relatively high rate of motor ejection in my minimum diameter rockets that use a baffle.
So just to clarify my earlier post, I believe there's an increase in pressure behind the baffle, but not in front where the recovery system lies.
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What kind of baffle design?
Baffles trap hot, solid particles from hitting your parachute. Firey chunks of stuff burn your chute, not hot air.
Generally, no, the pressure does not change. What may change is the path of least resistance for the pressurized air to escape. Back in the day, LOC sold a baffle that was like a mini chain link fence rolled into a cylinder and jammed into the motor mount tube. Instead of ejecting the nose cone, the charge blew out the motor instead, bending the Kaplow clips out of the way.
mh9162013
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That's not always true*, at least in my experience.
When testing my removable baffle system, the parachute would somtimes get stuck and stay in the main body tube. I used 1 sheet of disposable wadding for these ground tests and they kept the parachute from any burning bits exposure. However, the parachute still partially melted where it was touching the disposable wadding.
* I am assuming the words "burn" and "damage from heat" are interchangeable.
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I use the same baffle in my MPR rockets, 41mm to 66mm in diameter. The standard amount of BP in 24mm and 29mm reloads works just fine. In fact, I cut open a bigger hole in the rear bulkplate and stuff a wad of Chore Boy in the baffle coupler (ala Aerotech's "Cooling Mesh"). That works really well in stopping hot particles.
I have had no issues, btu I do echo the idea of ground testing. It will make sure your charge is large enough but will not damage the baffle. Start low and work high.
No doubt that baffles reduce the pressure after the baffle and increases pressure before it. The design of the baffle will determine how much pressure is loss. I've never done and actual testing but I would imagine that of all the designs I've seen and used, the plate style would have the least pressure drop. I can say that I've never had an ejection failure with a plate style baffle that wasn't the motors fault.
I think you are wrong about this. The baffle doesn't affect the pressure because it's in the way. The pressure on one side will equalize with the other side and only the speed of the pressure wave is affected not it's amplitude. The pressure above the baffle will equalize with the pressure below in milliseconds.
The issue I see with lower ejection charges strength at the nose cone when using a baffle isn't the pressure being transferred through the baffle, it's the baffle cooling the gases, which in turn, reduces the overall pressure from the gases generated by the charge causing a lower peak pressure on both sides of the baffle. I suspect a baffle with SST wool to stop particles will cool the gases and reduce overall pressure more than a baffle that just redirects the gases in S turns to capture the particles.
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If the parachute doesn't eject than all bets are off. For a correctly-ejecting parachute, which doesn't stay in the body tube getting cooked, the hot particles are the culprit, as far as I understand.
That slight delay is more time for ejection gasses to escape elsewhere (like out through the engine and nozzle). I don't think it's a big difference, the time is very short. And I've not observed any problems ejecting through a baffle.
It's not a big enough difference to matter, but there should be slightly less pressure at the nose cone.
I'm not sure there is much delay. Since the ejection charge is pressurizing the whole body tube, including the air in the BT, above and below the baffle. Depending on how much volume is between the baffle and the charge, there could very well be enough pressure in the BT to eject the nose cone before any of the hot ejection gases even go through the baffle. It's when that nose cone comes off where there is a big pressure differential between the open BT and the point the hot expanding gases from the ejection charge are at. It's after the nose cone comes off is when you get a large amount of gas flow from high to low pressure.
Of course, all that happens in milliseconds and that heat susceptible chute and shock cord is sitting right in the flow of those hot gases escaping out the open BT. That's when the baffle really comes into it's own.
Well I had time today to do alittle testing with the ejection charge. I measured charges in AT and Quest e-20s and E35s and they varied between .62 and .7 grams. I backed off the first test to .55. Worked perfectly. No heat damage and no baffle damage. I didnt feel like rigging up and electronic initiator so I just used a fuse...works fine. Thanks again for your advice guys
Yes it can depending on how the baffle is bullt. They need to be very easy flowing. A good test is to blow in the end of the motor tube and see how the NC ejects. It should fly out very easy, if It doesn't or you have to blowing air ontk the tube super hard for the nosecone to come out it could cause issues if you have a week ejection charge. Even with that size BT I just experienced that very thing, and had to remove a baffle.
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Which is why I use LOC or Blue Tube on everything MPR and up.
