Krushnick Effect

Greetings!
I'm desinging a high performance rocket whhere there is a CofG advantage to locating the motor well into the body tube. My question is: does the
Krushni(c)k Effect kick in suddenly as the motor is positioned further into the body tube or does its effect start as soon as the engine is recessad at all and get worse and worse as the engine position is moved up the tube?
BW
Halam
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Halam Rose wrote:

I don't think it is well understood. The air flow that causes base drag, and indirectly the Krushnic Effect, varies with speed, diameter, engine thrust, etc. There are some startling base drag effects that are not intuitive.
Perhaps you can avoid the issue by eliminating the aft part of the body tube that would extend past the nozzle. :-) In other words, instead of moving the motor in an attempt to move the CG forward, use swept fins to move the CP aft.
--
Steve Humphrey
(replace "spambait" with "merlinus" to respond directly to me)
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Halam Rose wrote:

If you're designing a rocket with the motor well into the body tube it ain't going to be "high performance", maybe high maintence, but not high performance.
DSC
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Relax! Just use a composite motor. New data indicates that composite motors,unlike BP motors are NOT subject to the Krushnik Effect!
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I think you're pulling his leg! The AT Sumo is known for this. I have seen more than a couple of Sumos where the motor mount gets pushed up several inches into the body tube during thrust and doesn't get too far from the launch rod. But the flame exiting the Sumo is awesome looking! Daniel

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just did. I mix rocket propellent professionally. and to reiterate: Composite fueled model engines are NOT subject to the Krushnick Effect!!!!!!!!!!!
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I don't know why the fuel of the motor would make any difference. This is simply a case where the effective nozzle changes from the nozzle included with the motors to the entire airframe, causing over expansion.
There is a nice, old writeup here. http://www.ninfinger.org/~sven/rockets/ModelRocketry/Model_Rocketry_v02n02_11-69.pdf
I would have to believe that there is some effect other than the propellant composition at play if you are not seeing the problem with composite motors.
--
Tom Koszuta
Western New York Sailplane and Electric Flyers
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Excellent link to that issue, I have to say after looking over that magazine that it's clear we are moving backwards. Could you imagine a similar level of technical articles appealing to a large section of today's youth?
Last month I picked up a rocketry magazine at the hobby shop, the large Mars Lander was on the cover and caught my eye but I read the whole issue in about an hour and can't say I took away anything new. It seemed to be more of a social page- here's a pic of so and so at this launch, and here's another picture of so and so at this other launch.
Just compare the articles and writing in the 1969 issue posted and the new magazine and it's clear we are afloat in a generation of youth that would rather plug in a controller and play than build, test, and experiment much less understand how to.
One of the car magazines had an article about a builder that put together a three wheeled car using an Indian Motorcycle engine, drove it a few thousand miles with his brother and kept it all these years. He gave it to Jay Leno so it could be restored as he was getting on in years; what struck me was the fact that he was 15 years old when he began his project.
I'm 40 and a parent of two sons, I've purchased nice tools for them to accompany every holiday so they'll have a good collection to begin adulthood with. I also involve them in just about every home and car repair, they know the best things sometimes require patience and effort, but damn if some days I don't shake my head struggling to offer them options for activities that aren't dumbed down for their age level.
Sorry for the rant, just my 2 cents.
Oh and the Krushnick Effect- has nothing to do with propellent and everything to do with the nozzle interface with the slipstream.
Mike Doyle
Thomas Koszuta wrote:

http://www.ninfinger.org/~sven/rockets/ModelRocketry/Model_Rocketry_v02n02_11-69.pdf
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Mike,
You are SO correct. Modern day rocketry really is re-inventing the wheel in so many areas. The best sources of information are the old journals, magazines, and reports. The MIT reports of the 1960/1970s are some of the BEST reading for information. The Model Rocketeer magazine, and old Centuri/Estes reports are great also.
You have to realize that MOST of the people who fly model rockets (LPR - HPR) are NOT rocket scientists. Yes they are well meaning and motivated people, but they don't have letters in front of their names from Cal Tech, MIT, Berkeley, Stanford, ect.. Most of the hobbyists come from well grounded back grounds that lend themselves to non-scientific pursuits. This isn't a bad thing, I just point it out because you have realized what many have realized and caused them to subsequently leave the hobby all together: There are REAL limits to what can be done in model rocketry. The next logical progression is to REAL space flight systems.
One of the major model rocketry selling points to the consumers, military, and government, was that model rocketry gave young boys a head start on the careers of tomorrow (circa 1960 something). In many cases this was true and did result in kids who flew model rockets in 1965, then sitting in a Space Shuttle seat and launched in 1990 something.
Early rocketry was taken far more serious in its educational and scientific importance, and less about colored-sparkly exhaust flames. Times have changed.
Yes, there is TARC, but face it, there was a time, when 'TARC' like meets, clubs and organizations existed all over the country (circa 1950s/1960s). Young minds were designing, and building rockets that today either wouldn't be allowed to fly or require special permits; and yes they were dying too! Frankly, I just asked the reader of this posting to go over to the ninfinger.org site and view the MR magazines of the late 1960s. In these, there are calculus differential equations, log-log graphs, Computational Fluid Dynamics studies, and more scattered throughout the issues. Also, try and get a hold of the Popular Mechanics magazines of the 1950s and 60s to see teenagers designing and SUCCESSFULLY flying ZnS, LOX, and solid propellant rockets. It was a different time. Some feel it was bad for personal safety, while others feel that it was a time of great learning and discovery that had benefits in hobby, military and scientific rocketry.
Today, all you need is a credit card and you too can launch your 'heart out'; that's not a bad thing, just an observation. You can by GPS Flight Computers. You can buy Hybrid systems. You can be active in the hobby without having to be 'brainy' with rocketry. This is a good thing, as it opens the excitement and fun of rocketry to the masses.
So then when you see the magazines of today as 'social photographic commentaries' between the hobbyists, you are correct. It's more about being seen or having your prized project recognized and appreciated, then it is about finding out something new about aerodynamic flight, stability, or fuel efficiency. Now that is NOT to say that these things aren't going on, its just that those types of scientific endeavors have been pushed to the back ground, and 'recreational rocketry' has been pushed to the foreground. It's still a good thing as more people enter the hobby; this will lead to spin-offs into profession rocketry and technology for our citizens.
And yes, the Krushnick Effect has to do with the mass flow and how it behaves as it moves out of the nozzle and into the atmosphere. A rule in motor design is that the reactants down stream must not know what is happening upstream. You actually can embed the motor well into the rocket and still get good performance. You just have to know what you are doing and understand what is going on. If you design the rocket correctly, you can actually... well I will leave that up to someone who might want to use it as a science project, or research thesis to do. Jetex planes have their motors recessed several calibers up into their 'tubes/airframes', now how do they get they models to fly? Well I know how, but I think someone might want to do the research and find out :P
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What a relief to see your post, I half expected a lynch mob after I hit the "post message" button. I was fortunate enough to have parents that kept an active subscription to Popular Mechanics throughout my youth along with a well worn set of encyclopeidas.
It's not a case of glorified memories about how things were better then with regards to youth hobbies, it's the truth with direct evidence to prove it. Technical hobbies present a learning process that can't be received any other way.
I cringe when see some of the work college engineering students have entered in robotic or solar powered racing competition. It's clear that the emphasis is placed in the wrong areas in some curriculums; if after all you're training the next generation of engineers they should be able to work as a team and complete a task successfully, creatively, and on budget. Instead we see rudimentary design flaws that never complete the task or cross the finish line, and should never be present at this level of education.
It's getting cliche but this social experiment of not keeping score, absentee parents, and constant electronic entertainment cannot end well. It seems socially acceptable to "have two left thumbs" and no clue how anything works these days; well it isn't on my watch.
I'm the neighborhood geek and I take some good natured ribbing about it, but guess who gets called first when there's a technical question? For that I'm grateful.
Mike Doyle
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snipped-for-privacy@gmail.com wrote:

I totally agreed with your post. That was the first thing that came to mind when I looked through the Model Rocketry issue in the link... that you won't see that level of technical writings in today's mags. Not to mention that it seems everything that takes "skill" has to be dumbed down for kids... can you say RTF?
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wrote:

Sure I can imagine it. Well, maybe a small section of today's youth, but probably the most significant section.

True, but let's not short change the social issue. The most important thing that goes on at a model rocket contest is not the competition, but the exchange of ideas and discussions that happen when people come together. In this regard, large sport launches may have overtaken contests. Many people still say that what they like about this hobby is the people that they meet. Now it would be great if the magazine had the picture captioned, "So and so explaining the Krushnick Effect", and if the text presented the main points of the discussion. The sad fact is, professional journalists are not drawn to rocketry events and if they were, they would not be "working". So basically some rocket geek takes a few pictures, that he can't remember much about, and sends them into the magazine, and the editor does what he can with them.

The magazine content is reflection of the submitters, and editors, and it is not a reflection of a generation of youth. I suspect the new generation of whiz kids are familiar with the content (or lack thereof) of sport rocketry magazines and walk right past that flashy cover to grab the latest issue of Make.

I feel your pain.

Alan
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Alan Jones wrote:

While that statement is true, the cause and effect isn't defined. Magazine content is driven by readership who votes with their dollars so it's not as innocuous as stating the Editor decided to create a mag full of pictures and captions, the readers decided through proxy.
The social exchange of ideas that takes place at launches is excellent, it doesn't require validation through photos however and there isn't a reason it can't take place in a rocketry magazine too.
Something good has been lost, I don't know how to define it but it seems prevalent everywhere. At a time when information exchange has never been easier, everything has been simplified to the point of loss.
Mike Doyle

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I think it might be *because* information is so easy to exchange now. In previous years, if you wanted to learn about something like the Krushnick effect, you would have to take a trip to the library(college library normally), research it and actually learn about it. Now you can do a 3 second search on google and have the answer handed to you. No real learning involved. If it can't be 'learned' in a quick search of the internet, its too difficult to learn.
As for kids understanding science, R&D and the like, go look at some of the projects going on at magnet schools in some of the bigger cities. It may not be rocket science, but some of them are pure R&D. I remember seeing one in the early 90's where the student (16 or 17) was performing protien analysis...things major bio-chem companies are paying top dollar for now.
-Aaron
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I'm only 40 but this discussion is making me want to utter the phrase, "when I was a kid".
Yes there are excellent science programs in some areas, my comments are directed at the masses. I would argue that the general overall level of understanding in practical physics, and mechanical aptitude- things children can learn by direct hands on activities has declined.
It's not just rocketry, just about any hobby that requires patience and the development of a level of skill in assembly has seen a decline. It may not show in sales numbers because they have been supplanted by RTF and ARF products, but youth today is less likely than their predecessors to cobble a plaything together out of household or kit items.
As a parent I'm not waiting for the situation to correct itself, so we suppliment science education at home whenever possible. The leap from merely doing to actually understanding is an important one, that is best cultivated with a healthy dose of opportunity and time.
Mike Doyle
Aaron wrote:

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I completely agree: Americans today have been 'dumbed down' by a teacher's unions that cares little about the education of the students, and more about electing the liberal democrat of their preference to political office. If you have been living under a rock, then you wouldn't know of the scandle that has rocket the National Education Assosiation and their use of union dues to bank roll the campaigns of Democrats.
American's today are far less capable of independent thought and/or action.
Mike in 30 years, your child WILL benefit from what he/she is doing now. They will be middle to upper income+, while most of her peers will be middle to lower class- ... its the old fable, 'The Grasshopper and the Ant'. Looks like you are raising an Ant! Good for you and your child!
People don't be STUPID! Don't let the left-winged led socialist educational system in this country, raise your child to be a tax paying dummy!
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snipped-for-privacy@gmail.com wrote:

Enter the age of "consumer rocketry".
Liken it to UseNET 20 years ago -- Prior the AOL-ization of consumer computer users, there were people who actually knew how computers worked, and therefore discussed them in terms of actual knowledge.
Hobby rocketry has evolved in the same way the Internet has since AOL and Microsoft enabled regular people to flock onto the medium. Most fliers today are more interested in simply "flying" as opposed to knowing "why" their rockets fly. The "cool factor" significantly outweighs the "why factor" in the hobby today.
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Darrell,
You are on target ... as usual I might add. Hey, buy back ROL and reverse it back into a USABLE site once more! PLEEEEEEEEEEEEEZE :)
The 'cool factor' is cool, its just that I feel that bigger is better isn't ALWAYS the way togo. I remember back in the 80s and 90s, where models kept getting bigger and bigger. There was a lot being learned back then. Now, ANYONE with a big enough bank roll, can just BUY a big 2 story tall rocket!
And that brings up another thing that had been on my chest for a few years:
Ky Michaelson, BOUGHT his flight to space. He didn't formulate the propellant. He didn't design the booster. He didn't design and build the flight computers or telemetry units, or the flight transmitters or ground receivers. All he did was gather the talent together and bank roll the affair. And after what 2-3 attempts you are bound to get it right eventually :)
I use this a PRIME example of what money can buy you today.
But because he throws free Super Bowl parties (bribes) and gives away a lot of 'samples', his ass is kissed... as it should be! :)
I have met in my years so many talented and gifted people who unfortunately didn't make millions of dollars in Hollywood jumping off buildings as stuntmen. If they had the money Ky has made, they would be orbiting micro satellite for fun by now...
I know I know, how DARE I speak such things of the 'GREAT ONE'! Bad Lunarlos!
Darrell D. Mobley wrote:

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snipped-for-privacy@juno.com wrote:

Why, thank you! But why would I buy ROL back when there is a perfectly suitable alternative at http://www.rocketryplanet.com ? If you haven't taken the time to review the site, please do. I don't think you will be let down.
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Darrell D. Mobley wrote:

I visited the site, and bookmarked it.. very nice, easy to navigate around, and its full of information. Brent really should take notes from your site :)
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