Krushnik Effect

This is a spin off from a thread titled "C6-0 meltdown" in Propulsion where the Krusnik Effect was mentioned along with a link to an old NAR report explaining the phenomenon.

From the last paragraph in the report:

"There is much more to be done in the
study of the Krushnic effect. Specifically,
there is a marked lack of accurate data on
the loss of thrust as a function of tube
diameter and length."

Posted by Handeman in C6-0 meltdown:

I don't know how common this is with the C6-0, but I've had several D12-0 motor "blow the nozzle" on me. Usually this works out with very little damage to the rocket, but that would depend on the pad design.

The last one I had was a CHAD staged D12-0 to a E9-6 in a D-Region Tomahawk. It was on a 1010 pad with an angled blast plate so no damage was done to the rocket, but I took quite a ribbing from fellow fliers. The D12-0 burned on the pad with no thrust and then lit the E9-6. Since that was burning through the D12-0 case, it had no thrust either. Six seconds after it was done burning, the ejection charge went off and got a big laugh from the crowd since the rocket hadn't moved the whole time. It did create a pretty impressive cloud of smoke though.

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So with that in mind and for "scientific" reasons :wink: I decided to try and duplicate Handeman's event. I static tested an E9-4 (with the ejection charged removed) for a baseline and to see how well it compared to the published thrust curve. Later I removed the nozzle from the spent tube and cut the tube down to the same length as a D12-0. This was taped to a new E9-4 as described by Handeman and static tested. The results show a significant loss of thrust as Handeman noted. I must admit I was surprised by how much. :eyepop: Learned something today.

Steve G

Are you going to experiment with different lengths to further characterize the effect? I would be curious to see the result.

Probably, any particular length in mind?

Steve G

The rule of thumb is to recess the motor no more than 1 caliber into a tube. I'd like to see:

Min diameter with 1, 2 and 3 calibers
~2x min dia with 1, 2 and 3 calibers
etc

May be more resources than you would want to invest.

May be more resources than you would want to invest.

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Yeah that's about 40 bucks in E9s, but I have some AP Ex grains in 29mm I'm willing to burn in the above specs if that would suffice.

Steve G.

You could certainly use any motor for such a test. C6's would be a lot more economical too. If I had to pare the tests, I'd say min diameter with a recess of 1 cal and 2 cals.

OK a couple of packs of C6-0 it is. To be continued.

Steve G

Thanks, this info will be most useful.

Here is something of interest to those who like to think about how to stuff motors way up in there while minimizing Krushnic:

https://www.apogeerockets.com/education/downloads/Newsletter379.pdf

There is a recent R&D report on the NAR site

(I tried to upload the report as an attachment but the upload failed, perhaps for size)

Great reference, thanks!

One of my last rocketry endeavours of my youth was to do a science fair project on the Krushnic Effect. I hypothesized that the reduction in thrust was caused by the body tube acting like an extension of the nozzle. So, I recorded the total impulse of rocket motors fired at various depths inside tubes and compared the results with calculated values based on the motors having larger nozzles.

The test stand I used was interesting. I had the motor fire against a spring. As it compressed the spring, it moved the lever of a linear potentiometer. The variable resistor was connected to a simple 555-based timer circuit which produced from zero to a few pulses per second as the resistance changed. The output of the timer was connected to the "=" key of a four-function calculator. Before firing the motor, I would press "C" then "+" and "1" on the calculator's keypad. Then I would ignite the rocket motor. The timer would begin pressing the "=" key and the calculator would display the number of pulses. The display on the calculator would be a number which was proportional to the total impulse of the motor.

I thought my project was pretty good (for a ninth grader) - though my final report wasn't anything like John Dunbar's.

-- Roger

I'd have run the voltage from the pot into an op-amp based integrator and used a voltmeter to read the proportional-to-total-impulse result, but your way was clever and certainly had more visual appeal.

jqavins said:

I'd have run the voltage from the pot into an op-amp based integrator and used a voltmeter to read the proportional-to-total-impulse result, but your way was clever and certainly had more visual appeal.

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At that time I always had to make do with whatever parts I had on hand. Later, I built a more traditional test stand with a pen drawing a time-thrust curve on a piece of paper mounted to a rotating coffee can. I don't think I used it for a science fair project, though. It was just one of those things a nerdy kid did.

-- Roger

Yeah, I just figured if you could get your hands on a 555 you could probably get a 741. But it's not important.

TopRamen said:

Here is something of interest to those who like to think about how to stuff motors way up in there while minimizing Krushnic:

https://www.apogeerockets.com/education/downloads/Newsletter379.pdf

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That's essentially what I am playing with down in the Scratch Built sub-forum.

The two questions that I am trying to answer are:

1. Can I use the concept of Gas-Dynamic Stabilization to enhance the stability of a small-finned rocket?
2. Can I use some of the principles of Gas-Dynamic Stabilization to deeply recess the motor, shifting the CG forward whill minimizing added weight?

Parts should arrive this weekend, so I can begin playing with it.

Screaminhelo said:

That's essentially what I am playing with down in the Scratch Built sub-forum.

The two questions that I am trying to answer are:

1. Can I use the concept of Gas-Dynamic Stabilization to enhance the stability of a small-finned rocket?
2. Can I use some of the principles of Gas-Dynamic Stabilization to deeply recess the motor, shifting the CG forward whill minimizing added weight?

Parts should arrive this weekend, so I can begin playing with it.

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There are a few military style rockets that I'de like to build that would require substantial nose weight, but using a series of holes in the airframe to mitigate Krushnic and let me place the motors higher would solve that. Plus, I think the holes in the airframe would look cool, even though they would not be a "Scale" feature. If I want true "Scale", I'll seek out real missiles on the black market. For models that I can fly, I just care if they look cool and fly straight. I won't be entering any competitions.
An aluminum soda can can be used to line the inner wall of the airframe where it will be exposed to heat, bonded to the wall with a high temp epoxy.
I'm still just in the thought stages though.
Can't wait to see what you come up with Bill. Keep us posted!

#2 has been answered, kinda. Here's one example https://www.spacemodeling.org/JimZ/eirp_10.htm There have been others. As I remember someone did this with a long pyramid/tetrahedron at LDRS/BALLS.

This example shows that, with venting, you can recess the motor a lot. What has never been demonstrated to my knowledge is whether the actual venting adds to stability (other than by the motor weight being moved forward). I've see people discuss whether venting high speed air can augment thrust. Search YORF for that discussion.

The article published in PoF 379 hints that the pseudo-fin effect can be used to help small fin rockets but does not go into any sort of detail on the subject. I have a plan in mind to try this out as well as mitigating the Krushnic effect.

Picked up a 3 pack of C6-0 Estes engines and static tested today with the motors recessed 1 and 2 calibers. Results indicate about a 15% loss of impulse at 1 caliber and a 37% loss at 2 calibers.

Steve G