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?
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
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.
(replace "spambait" with "merlinus" to respond directly to me)
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!
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.
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.
Western New York Sailplane and Electric Flyers
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
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
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.
Thomas Koszuta wrote:
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
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
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.
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?
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.
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.
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.
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
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.
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
American's today are far less capable of independent thought and/or
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
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
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.
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
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
Darrell D. Mobley 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
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