Saturn V Fire? TIP

On the Estes Model Rocket Facebook page - 

Mark Hansen posted a picture of an Estes Saturn V that never got off the launcher. 

The tail end is on fire! There might also be a hole burned into the thin Estes metal deflector.

There is reasons why the instructions say to elevate the rocket 8" above the blast deflector. 

From Rocketreviews.com:

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. Not to be confused with Suction Lock.

Bernoulli Lock

A phenomenon similar to the Krushnic Effect 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.

With the wide body and recessed engine - This doesn't always happen. But why take the risk?

Raise your (4" diameter, not the smaller plastic version) Saturn V, 8" above the blast deflector. And, you should have a longer launch rod or rail.

I experienced this in the early 1970s with a Centuri Point. 

The base of the Point is round almost matching the diameter of my round Estes blast deflector plate. The model never left the launcher and caught fire.

On my more recent Semroc Point, notice the base is raised above the blast deflector. In the original instructions, Centuri did not mention raising the rocket above the deflector. But, Centuri blast deflectors were angled, not flat.