What's "Bernoulli Lock" and why is it called that????

[MetMan]

First time I've heard this term. It looks like the rocket gets stuck on the pad and burns up...

 

[kenobi65]

What is it?

https://www.rocketreviews.com/cgi-bin/flattips/glossary.cgi#107

Why is it called that?

I imagine because it's caused by the Bernoulli Effect.

 

[jflis]

A number of sources found:

From www.hpr.org.uk: 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.

 

[sandman]

Jim,

So what you are saying is lower pressure at the base causes the higher pressure to force down on the model holding it in place?

In other words...Benoulli sucks!

Sometimes when the answer is simpler it's easier to understand.

Example:

Years ago I taught a Powder Puff auto mechanics course for the local ladies. I was totally frustrated trying to get them to understand the 4 cycle of a 4 cycle motor.

Intake, Compression, Ignition, Exhaust.

It just didn't make sense to them...then it hit me!

Suck, Squeeze, Bang, Blow

The ladies seemed to understand THAT with no trouble at all

To this day...18 years later when ever I see one of the ladies from my old class...they tell me they still remember that!

 

[cdma77]

Originally posted by sandman
Jim,

So what you are saying is lower pressure at the base causes the higher pressure to force down on the model holding it in place?

In other words...Benoulli sucks!

Sometimes when the answer is simpler it's easier to understand.

Example:

Years ago I taught a Powder Puff auto mechanics course for the local ladies. I was totally frustrated trying to get them to understand the 4 cycle of a 4 cycle motor.

Intake, Compression, Ignition, Exhaust.

It just didn't make sense to them...then it hit me!

Suck, Squeeze, Bang, Blow

The ladies seemed to understand THAT with no trouble at all

To this day...18 years later when ever I see one of the ladies from my old class...they tell me they still remember that!

Suck, Squeeze, Bang, Blow

Every man's fantasy women !

 

[rstaff3]

My Archer experienced such a lock. Toasty.

 

[JStarStar]

Any rocket i have where the base of the body sits close to the pad, I always use a clothespin to create a prop to raise the rocket up 4" or so to prevent possible Krushnicing.

It effectively shortens the launch rod a bit, but I'm not as worried about that as i am of the tail end of the rocket getting barbecued if the infamous Krushnic Effect strikes.

 

[North Star]

I lost my Shadow Composites 'Sprint ABM' to this
It was the first one in Britain (ser # 21) which made it even more annoying.

It just sat there and cooked until the D12-3 went pop. I should have considered this effect as the Sprint is a wide based cone and I used a large, flat, circular pad protector - with the rod comming up throught he middle. This was a legacy of using my good photo tripod in the early days and really wasn't needed on my later pad set-up.

Won't do it again.

 

[hokkyokusei]

Originally posted by North Star
I lost my Shadow Composites 'Sprint ABM' to this
It was the first one in Britain (ser # 21) which made it even more annoying.

Pics of better days:

 

[North Star]

Thanks for that Hokky .... wiping tear from eye

Nice sunny day at Baildon ..... don't seem to get them any more either ..

 

[hokkyokusei]

I think we had a nice sunny day _last_ year.

IIRC those pics were taken when we flew the 40th anniversary alphas.

 

[North Star]

Originally posted by hokkyokusei
I think we had a nice sunny day _last_ year.
IIRC those pics were taken when we flew the 40th anniversary alphas.

We had lots of sunny days last year ... but I never managed to match one up with flying at Baildon.

You're right about the Alphas (wonder what happened to the 'World Tour' ones?) It was 20th July 2000 . What a beautiful evening that was .. and an excellent 'debrief' at "Dick Hudson's" afterwards.

 

[hokkyokusei]

Originally posted by North Star
You're right about the Alphas (wonder what happened to the 'World Tour' ones?) It was 20th July 2000 . What a beautiful evening that was .. and an excellent 'debrief' at "Dick Hudson's" afterwards.

The last _I_ heard of them was a guy in Belgium had them. I hassled him for a while and then he stopped answering my emails. Shame really - cool idea for a project.

Ah, Dick Hudson's - those were the days.

 

[powderburner]

Does anyone still have a copy of the plans for that Sprint model?
(the one that hokkyokusei posted pix of)

 

[Hospital_Rocket]

I have an unbuilt kit. I'll see if the "plans" are od any value.

The big thing is that it is made by rolling a printed flat stock into a cone. so yo would need the artwork which is what makes it so gorgeous.

There is also a flameproof inner lining.

There is a review on EMRR - check under Shadow Composites.

https://www.rocketreviews.com/reviews/oop/sdw_sprint_abm.html

 

[powderburner]

I have seen that review, and I have seen a few scattered photos of these models on the net. I was hoping to see what the construction looked like, don't care so much what they did to decorate their version of the outsides. I understand the NC is a nicely turned piece of hardwood to get that point.
I will cover your copy-machine costs if you would make me a copy of your kit plans. PM me if you are interested

 

[adrian]

Originally posted by JStarStar
Any rocket i have where the base of the body sits close to the pad, I always use a clothespin to create a prop to raise the rocket up 4" or so to prevent possible Krushnicing.

A clothes-pin won't prevent the Krushnic effect, which is a different thing. Bernoulli lock, as previously explained, is when the exhaust from a wide-bodied rocket too close to a flat deflector goes out sideways rapidly, causing a reduction in pressure under the rocket as per the Bernoulli Principle. Yes, Bernoulli sucks. This is where the clothes-pin helps. So does an angled deflector.

The Krushnic effect is when the motor is too far up the body tube, so the body tube itself acts as an expansion chamber. The exhaust fills it, loses pressure, and thus loses thrust. No amount of clothes-pins will help here!

A detailed treatment of the Krushnic effect is given in NAR Technical Review, volume 1, which also explains how vent holes can prevent the effect.

 

[North Star]

Originally posted by adrian

The Krushnic effect is when the motor is too far up the body tube, so the body tube itself acts as an expansion chamber. The exhaust fills it, loses pressure, and thus loses thrust. No amount of clothes-pins will help here!

A detailed treatment of the Krushnic effect is given in NAR Technical Review, volume 1, which also explains how vent holes can prevent the effect.

Unfortunately you get a shot at both with the Sprint the motor is right up inside the cone - so much so that you need extension wires for the igniter

Oh Also - the flameproof paper (twice the weight in the earlier models like mine) isn't flameproof enough to withstand cooking by a D12.

I did a brief summary of the construction for our webmaster Hokky to put on our site; https://www.northstarrocketry.org.uk/

Look down the left side to 'Projects' and the link leads to the Sprint.

 

[hokkyokusei]

Originally posted by North Star

I did a brief summary of the construction for our webmaster Hokky to put on our site; https://www.northstarrocketry.org.uk/

Look down the left side to 'Projects' and the link leads to the Sprint.

I'd forgotten about that! Here's a direct link in case anyone needs it:
https://www.northstarrocketry.org.uk/projects/sprintabm/sprintabm.html
Same review is also on EMRR:
https://www.rocketreviews.com/reviews/oop/sdw_sprint_abm.html

P.S. I thought I wasn't meant to be the the web master any more?

 

[BobH48]

Originally posted by sandman
Jim,

So what you are saying is lower pressure at the base causes the higher pressure to force down on the model holding it in place?

In other words...Benoulli sucks!

I had this happen on a scratch built cone rocket with a launch lug rolled from cardstock. I had thought that maybe I had rolled the launch lug too tight.

It was the first launch of the day and it just sat there and smoked when I hit the launch button.

When I took the cone off of the pad, the heat from the blast had melted the solder on my controller clips and they fell off and were sitting on the blast deflector. I had two bare wires and no clips so I was done launching for the day.

 

[JStarStar]

Originally posted by adrian
A clothes-pin won't prevent the Krushnic effect, which is a different thing. Bernoulli lock, as previously explained, is when the exhaust from a wide-bodied rocket too close to a flat deflector goes out sideways rapidly, causing a reduction in pressure under the rocket as per the Bernoulli Principle. Yes, Bernoulli sucks. This is where the clothes-pin helps. So does an angled deflector.

The Krushnic effect is when the motor is too far up the body tube, so the body tube itself acts as an expansion chamber. The exhaust fills it, loses pressure, and thus loses thrust. No amount of clothes-pins will help here!

A detailed treatment of the Krushnic effect is given in NAR Technical Review, volume 1, which also explains how vent holes can prevent the effect.

You are correct of course, I transposed the two "rocket stays glued to the pad" thrust effect phenomena in my head. All i remembered, use a clothespin to get the rocket base several inches off the deflector to give those gases room to escape.

Old age is hell, I tell ya.

 

[Stymye]

from what I understand,the krushnik effect will not work on a tapered bore like the sprint abm. not sure will have to read up on it

be very carefull about openly distributing plans of the shadow composites stuff(speaking from personell experience on that one)
it's after all not yours to distribute.

 

[Micromeister]

Well then... I'm not sure if it was the Bernould or Krushnic effect that held my Floatation Duration entry to the launch pad but one of them did had to throw the model in the pond to put out the styrofoam flame the D12 is recessed about 8" inside this 16" cone. Flys just fine when stood off the deflector on two cloths pins.

Heres the BAD liftoff

 

[Micromeister]

and heres the good lift-off