Krushnic Effect—- can I get away with 1/2 body tube diameter recess?

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BABAR

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doing a nongap two stage rocket with a short booster, basically booster is length of the motor.

I am using a REAR thrust ring in the booster so the booster will not eject out the back, it’s minimum diameter 18mm motor, BT-20 tubing.

the motor will be recessed into the rear of the booster tube by 9 mm (1/2 body tube inside diameter.). The FORWARD end of the motor “nests” into the sustainer a corresponding 9 mm, so the sustainer motor will also be recessed 9 mm into the sustainer tube.

normally only 9 mm “nesting” would be borderline to act as a coupler, but since I am taping the motors together (I,e., no gap) I think the booster sustainer junction will be solid for the boost through upper stage ignition.

Both motors will be friction fit.

the sustainer has a coupler forward of the fin can that will be taped at launch, i can disconnect the Sections and shove the sustainer motor out from the inside with a dowel.

my understanding is that Krushnic Effect doesn’t kick until the motor is recessed greater than 1 body tube diameter.

so at 1/2 diameter, am I good?
 

BABAR

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Maybe I can punch some small holes in the sides? They are gonna be pretty tiny.
 

CalebJ

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That sounds like approximately the inset of the Estes Checkmate. Maybe worth a look at that one to see - if so, I'm sure you're fine.
 

Winston

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From that NARAM report's Table 2 (restated as a percent of motor tube ID inset) with a 19mm diameter motor in 19mm ID motor tube:

26% (5mm) of motor tube ID inset = no effect on Total Impulse (It)
43% (8.1mm) of motor tube ID inset = 17% reduction of It ***
100% (19mm) of motor tube ID inset = 22% reduction of It

*** closest to your 1/2 diameter inset case.
 
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Sooner Boomer

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These tests were done with an external tube that has the same ID as the OD of the motor.

I'm curious what would happen is you started varying the external tube (inner) diameter. I'm thinking of cone rockets, in particular, where the engine might be up inside the cone, rather than at the base.

Second bit of inquiry along same thread; what happens when you start varying the base diameter of rockets, going from minimum diameter to 10x or more of the diameter of the motor.

Bit of trivia: the Tiamat missile (first attempt at guided missile) had the rocket motor buried inside the body of the missile. They were unsure how the changes in CG would affect stability.
 
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Sooner Boomer

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The original question:

so at 1/2 diameter, am I good?
I think it will be fine. I have a rocket, a much modified Fat Boy, that ended up having the motor mount centering rings located about an inch up inside the body tubes so I could re-enforce the bottom of the body tube with fiberglass. Other than getting sootier than normal, it flies well.
 

Winston

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These tests were done with an external tube that has the same ID as the OD of the motor.

I'm curious what would happen is you started varying the external tube (inner) diameter. I'm thinking of cone rockets, in particular, where the engine might be up inside the cone, rather than at the base.

Second bit of inquiry along same thread; what happens when you start varying the base diameter of rockets, going from minimum diameter to 10x or more of the diameter of the motor.
Intuitively, because of the less restricted air flow path from the base of the rocket to the area surrounding the exhaust plume, I think the Krushnic effect may be less in an apex motor driven cone airframe.

However, related to your second question, there is then a larger base drag to consider which is a reduced pressure area itself, so that may have a negative effect by further over-expanding the already over-expanded exhaust plume of an Estes BP motor regardless of better airflow access to the motor plume.

Thinking of doing a Sprint? I was, too, using a series of consecutively smaller 3D printed interconnected rings at stations along its length, then wrapped in a cone of cardboard.
 

Arsenal78

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The now oop Pro Series Nike Smoke motor mount sits about 2" inside the body tube which is 3" diameter and flies fine.
 

BABAR

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Very helpful and encouraging, thanks.

I am still a coward, I drilled a bunch of vent holes for “insurance.”
 

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