The Krushnic Effect and the Bernoulli Lock

Earlier in the year I built a rocket based on an old Centuri design. The rocket was never produced but the design was published in the early seventies. The rocket used something called Uni-Body staging. Instead of having a fully developed second stage the rocket had a small pod that would eject and use streamer recovery. I had an issue with the sustainer stages motor placement. In order for the pod to work the motor was recessed into the tube. Unfortunately it was recessed just enough that it caused the Krushnic effect

“Krushnic Effect: A very dramatic phenomenon where your rocket makes a tremendous amount of noise and smoke but doesn’t go anywhere! This happens when the motor is recessed into the body tube by more than one tube diameter. If so recessed, the cylindrical volume below the motor forms a secondary expansion chamber which allows the exhaust gasses to expand below atmospheric pressure before leaving the rocket. Surrounding air aspirated into the exhaust stream causes turbulence which negates much of the thrust, along with creating the characteristic roar. A multi-stage model that ejects its booster motor, but not the airframe, is a perfect example. Very damaging; it almost always destroys the lower body tube beyond use. Named for Richard Krushnic, the rocketeer who characterized the effect in the late ’60s. “

More on the Krushnic Effect(PDF)

Just recently I purchased the recently released Semroc version of the old Centuri classic the Point. A fellow rocketeer commented that I should watch out for the Bernoulli Lock. Not knowing what that was I had to do some investigating.

“Bernoulli Lock: A phenomenon similar to the “Krushnic Effect” (q.v.) where the rocket seems to be “glued” to the pad at liftoff. This afflicts larger, flat-bottomed rockets launched too close to pads with flat blast deflectors. The exhaust gasses escape at great speed through the small annular space between the rocket and the pad creating a venturi which generates a low pressure region at the base. This pressure deficit can be significant, and if it is greater than the thrust being generated by the motor, the rocket won’t go anywhere! This is quite possible as a 2″ dia.rocket has, potentially, over 45 lbs (200 N) of “suction” available to hold it back, while a 3″ rocket has over 100 lbs (460 N)! The old Centuri “Point” was an infamous Bernoulli locker when launched from an Estes Porta-Pad with its perfectly matching round blast deflector.”

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One Response to The Krushnic Effect and the Bernoulli Lock

  1. Ken says:

    Great definitions for both effects.
    I once saw Bernoulli Lock combined with Composite Motor Chuff due to proximity to a flat deflector. The motor would thrust and BL woud suck it down, shockwaves bounceing off the deflector would snuff the flamefront in the motor core and the rocket would rebound several inches upward, motor thrust would resume and the BL sucked it down again. The cycle repeated half a dozen times over several seconds before the rocket finally took off. It was a 4″ airframe with a J or K Blue Thunder. The Blue corona of flame during lock was priceless.

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