Krushnic effect

[fyrwrxz]

Tapping the hive mind here. I just finished machining out of very cool tailcone for a 2.5 body that is only 1.0" long with about a 2:1 rounded taper. The whole idea was to sink an Aeropack retainer cap in in and thread the same after loading. Tailcone and Aeropack to be bonded with JB. A little nagging doubt finally bloomed in the small part of my pea brain not occupied by Albert Ziemer. The exhaust is recessed by about .70" from the outer edge of the tailcone. Exit dims are .70 deep by approx 30mm. My questions are these:
1) will this reduce the effectiveness of any chosen motor?
2) Does this affect what propellant I should fly? I wanted long burn 29mm Cez motors, but now am I doomed to V-max?
3) lacking any other papers I could find that state "...deeper than one body diameter..." is there any research papers out there I can see that documents a deteriorating airflow turbulence problem as depth increases x and thrust goes to 0?
The rear face is flat and 2.0" in dia. and I was hoping the boundary layer would leave in a little better flow than just a big open wound at the back end.
4) should I just open up the back end a little?
Haven't bonded anything yet- I could just chalk this up as another fun day in the dungeon.

 

[New Ocean]

No math or simulations, but under 1 inch doesn't seem likely to cause any problems. If you want long burn motors, the rocket mass is going to dominate here. (The rocket is thin, and that tailcone does not sound very deep.) My only experience with this matter is extreme cases such as fully enclosed rack rockets and 2 stage rockets with recessed upper stage motors. These are smaller (BP) rockets with motors 1 or more diameters into the airframe.

 

[fyrwrxz]

Thanks! I'm starting to feel a little better now...just maybe over-thougtht it a little.

 

[rstaff3]

As I remember, the rule of thumb to avoid the Krushnik effect is to recess the motor no more than on caliber inside the body.

 

[bjphoenix]

Over 40 years ago I did a science fair project on the Krushnik effect. I sure wish I still had all of the results and so forth. I built a little motor test stand with an electric motor, coffee can and springs. It produced real nice thrust/time curves just like you see in the catalogs. But back to the original question I think rs3 is right, you have to go more than 1 motor diameter deep before it makes much difference.

 

[DavidQ]

The Krushnic effect is a plus for rockets that are meant to fly low and slow.

Really low and slow.

 

[rstaff3]

If you are a fan of the Krushnik Effect, you might want to try the Bernoulli Lock. :grin: Get a large diameter rocket (try a 4" er) with the motor recessed in the body (one caliber NOT required), and set it on a flat thrust plate. The motor thrust will create a low pressure area below the rocket and it will be sucked onto the thrust plate. Toasty! (But you will win closest to the pad.)

A photo of my accidental experiment can be found here.

 

[fyrwrxz]

Not to worry, boys- the next one will feature the Kamm effect!

 

[Lentamental]

Bernoulli Lock in effect on this used-to-be beautiful saturn 5. It got stuck to the pad for roughly 4/6 of a second by my shutter count, and melted through the plastic shrouds, making the fins all wobbly and sucking up the majority of the thrust.

 

[rstaff3]

Bernoulli Lock in effect on this used-to-be beautiful saturn 5. It got stuck to the pad for roughly 4/6 of a second by my shutter count, and melted through the plastic shrouds, making the fins all wobbly and sucking up the majority of the thrust.

Ouch! My Archer stuck for the entire burn of the G35. That was the end of the North Coast by Estes plastic ring and the lower BT. Luckily, the fins survived.

 

[jd2cylman]

My NCR Lance Beta stuck to the pad once. I don't recall the motor, but I had a bunch of extended family there watching a couple of launches and the LB was gonna be the grand finale. It sure was! Roasted on the pad and then blew the laundry uke:

Fortunately, I had ditched the plastic rings for plywood and built the entire rocket with Titebond II yellow glue. So, the butt's a little charred and the rocket's is too

Now I use a rail for it instead of lugs that had too much paint in them.
Adrian