Krushnic Effect Story

[rstaff3]

Today, I went to launch my NCR Archer on a G35. I usually bring a stand-off to hold my rockets above the blast deflector, but forgot it this time. So, the rocket sat flat on the deflector. I thought I might get some singeing or exhaust deposits on the rear of the rocket but was lazy and left it as is. BIG MISTAKE!

Instead of going up, the rocket sat there in a large fireball. After the fire was put out, the rocket slid nicely off the rail - no evidence of binding. I was soon reminded that the Krushnic Effect can cause a vacuum that actually holds the rocket to the pad. This appears to be what happened.

Interestingly enough, despite the damage to the end of the body tube, and a melted/warped rear centering ring (they are plastic on these kits), the fins and motor mount appeared to still be solid. I thought of launching it anyway, but decided not to. Repairs are in progress.

Think my post mortem is correct?

 

[MarkABrown]

Wow, that's an interesting story and picture. I wouldn't have thought that that would happen. However, I don't have any better explanation than yours. Hope the repairs go well.

 

[MarkABrown]

Dick,
I kept thinking about your post and looking at the picture you posted. I don't think that it could have been the Krushnic effect holding the rocket down. Here is a link to an article by Tom Blazanin of Tripoli Pittsburgh that describes the Krushnic Effect very well. My reasoning is that based on the photo you attached, air should have been able to circulate around the bottom end of the rocket and prevent the vacuum of Krushnic.

One theory that I've got is maybe you had a nozzle malfunction. If the nozzle had a crack or something that caused the thrust to be vectored to one side, then that would have applied a torque to the launch lug and bind the rocket on the rod. Once the burn was complete, no more torque and the rocket slides off the rod easily.

 

[rstaff3]

Mark,

I'll check out the link. FYI, the engine on the Archer is recessed into the rear a ways (.75"??).

Looking at the casing, there is indeed a crack in the nozzle. However, given that the casing was subjected to a lot of heat it is in general not in good shape. Also, I would think that a hairline crack would have eroded to something larger.

I definitely need to read more about the Krushnic Effect. I only offered it as a thought because someone else mentioned it, and I do remember some discussion about related problems on the pad.

Thanks for thinking about this problem and for the reference.

 

[rstaff3]

Mark, after looking at the link, I think this explanation may fit. Since the motor was recessed, a vacuum could have formed in the cavity, with the exhaust escaping around a small space between the end of the rocket and the blast plate. ???

 

[rstaff3]

Here is a cross-post form ROL (I assumed this was not bad manners since I forward emails all the time).

Sounds like the effect I was thinking of, just had to blame Mr. Bernoulli instead or Mr. Krushnic


"Sounds like a classic "Bernoulli lock". The Archer is 4" diameter which gives a base area of just over 12.5 square inches. The G35 has a peak thrust of slightly over 17# (for an instant) which then decreases to around 12-12.5#. The Archer itself weights 2#, which leaves around 10lbf to lift the rocket... but if the pressure under the base is reduced by as little as 1psi, it will never take off, due to the difference in pressure between the atmosphere and the enclosed volume at the aft end. The high-speed exhaust creates a partial vacuum in a similar manner to a venturi vacuum generator.

HTH,
Scott (McLeod)"

 

[MarkABrown]

Sounds like you may have found an answer. I didn't realize that the motor was recessed. I couldn't tell from the picture (too much charring ).

 

[Milo]

Man... physics giveth, physics taketh away.

 

[rstaff3]

Mark,

Yup at the point the pic was taken, the motor wasn't recessed.

 

[Ez2cDave]

Bernoulli can be a real "Bass-Turd", at times . . . LOL !

Interesting thread here . . . https://www.rocketryforum.com/threads/krushnik-effect.126678/

https://www.ninfinger.org/rockets/ModelRocketry/Model_Rocketry_v02n02_11-69.pdf

Dave F.

 

[0011001100]

Wow an 18 almost 19 year old thread... how long did you have to look for this thread just to bring it back??

 

[Ez2cDave]

Wow an 18 almost 19 year old thread... how long did you have to look for this thread just to bring it back??

The subject matter of the "Krushnic Effect" is still a valid topic of discussion, as is the "Bernoulli Lock", and the supporting material is the most current I could locate . . .

Actually, I was reading about intentional thrust reduction techniques and the link to this thread came up during my searches.

Dave F.

 

[neil_w]

I think 18 years might be a new record...

 

[Ez2cDave]

I think 18 years might be a new record...

I accept that challenge . . . LOL !

Dave F.