3.9" Ejection Charge Baffle calculation (verification)

[Michael L]

I have LOC's ejection charge baffle system that I want to install into my LOC IV. It's design uses a full coupling and I don't have that kind of room to spare. If the math is right I can cut the coupling down to 130-ish mm (don't hate on the metric, it's just a number that uses the same standard as the imperial measurement. I switch back and forth all of the time. Born and raised in TX so if I can do it anyone can)

38mm tubes in the baffle assembly.
(Pi*D2)/4 gives me exit and inlet area of the tubes - 1,134mm
Pi*D gives me circumference - 119.4mm
1,134 / 119.4 = 9.49mm. 9.5mm, close enough

9.5mm represents spacing of the exit / entrance of the tubes from the baffle plates that roughly equals the exit area of the tubes. So, in theory, no drastic increase in backpressure or baffle chamber pressure. In theory. Butted end to end (100mm) + 9.5mm the shortest I could make the assembly 110mm ish. Coupling ID is 98mm. Area of the end is 7,543mm x 110mm length = 829,726 cubic mm of volume.

But what does that do to residence time? And how much time is needed to cool the ejection charge down? The full coupling is 205mm. Naught from naught, carry the naught, that's not quite double the volume (1,546,308 cu mm for those following along at home) of the cut down baffle at it's smallest. If I triple the spacing distance to 30mm for an O.A.L. of 140mm The volume is 68% of the full coupling (1,056,020 cu mm). Is that enough and how do I know that it's enough?

 

[Steve Shannon]

I have LOC's ejection charge baffle system that I want to install into my LOC IV. It's design uses a full coupling and I don't have that kind of room to spare. If the math is right I can cut the coupling down to 130-ish mm (don't hate on the metric, it's just a number that uses the same standard as the imperial measurement. I switch back and forth all of the time. Born and raised in TX so if I can do it anyone can)

38mm tubes in the baffle assembly.
(Pi*D2)/4 gives me exit and inlet area of the tubes - 1,134mm
Pi*D gives me circumference - 119.4mm
1,134 / 119.4 = 9.49mm. 9.5mm, close enough

9.5mm represents spacing of the exit / entrance of the tubes from the baffle plates that roughly equals the exit area of the tubes. So, in theory, no drastic increase in backpressure or baffle chamber pressure. In theory. Butted end to end (100mm) + 9.5mm the shortest I could make the assembly 110mm ish. Coupling ID is 98mm. Area of the end is 7,543mm x 110mm length = 829,726 cubic mm of volume.

But what does that do to residence time? And how much time is needed to cool the ejection charge down? The full coupling is 205mm. Naught from naught, carry the naught, that's not quite double the volume (1,546,308 cu mm for those following along at home) of the cut down baffle at it's smallest. If I triple the spacing distance to 30mm for an O.A.L. of 140mm The volume is 68% of the full coupling (1,056,020 cu mm). Is that enough and how do I know that it's enough?

The goal of a baffle isn’t to cool the ejection charge down. That would cost you in pressure.
The result of an ejection charge ignition is gas and hot particles. It’s the particles which burn holes in your chute, not so much the gases. The hot particles, being solid and therefore much more dense, have more momentum and travel in a straight line. A baffle system only has to intercept those particles by interrupting their straight path of flight.

 

[Michael L]

The goal of a baffle isn’t to cool the ejection charge down. That would cost you in pressure.
The result of an ejection charge ignition is gas and hot particles. It’s the particles which burn holes in your chute, not so much the gases. The hot particles, being solid and therefore much more dense, have more momentum and travel in a straight line. A baffle system only has to intercept those particles by interrupting their straight path of flight.

Ok... good point on losing pressure if it cools down (PV=zNRT). So it's more of a filter than expansion chamber... Thinking out loud, if the flow changes direction then the heavier stuff falls out. If the ends of the tubes are on opposite sides of one another, rather than offset, the total direction change would be 360 degrees. Even if they are180 degrees out (around the bulkhead) some of the charge will go into the other tube and act as a bypass.

I guess what I'm looking for is can the coupling be too short, sticking with the idea that the space between the tube and the opposite bulkhead shouldn't constrict the flow in any way.

Since this is just a non-stop experiment anyway, I'm going to cut 2" off of the coupling, which leaves 6" of coupling and the ends of the tubes are past one another, ie everything has to change direction to get out. That'll give me the room I need for the drogue and recovery harness.