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  #1  
Old 09-10-2010, 04:10 AM
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Default Aerodynamics for rockets with forward mounted motors

I've been successful with my adherence to principles of aerodynamics and Rocket stability in my kit and original builds. Of all my collected literature (the classic tech reports) I can't seem to find references to the stability of rockets with forward mounted motors and testing such designs while still on the drawing board. I immediately think of Robert Goddard's first attempts, bottle rockets and many boost gliders I have built with forward motors.
It appears more is going on than I can fathom. With the motor mass so far forward wouldn't weathercocking be a huge issue.?

How would one accurately determine the fin area needed, optimal CG in relation to CP to keep from "overstability".

What am I missing? Want to try this but get it right the first time. Looks like it will be fine, would like the math to prove it.

Understanding Cones is another - the Point, Bug and Vulcan, still working on those.
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  #2  
Old 09-10-2010, 06:43 AM
Rocket Doctor Rocket Doctor is offline
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Your best resource for information is by going to the Estes site (www.estesrockets.com) and click on education, under that, click on publications, there is a long list of publications that are available. I think this would help you out.
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Old 09-10-2010, 12:41 PM
shockwaveriderz shockwaveriderz is offline
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I once had a copy of a research report on the aerodynamics of pyrotechnic skyrockets; rockets at the top of a stick.....(lost in a disk crash long ago) and if I remember correctly the distance between the cg and the cp of the skyrocket determined the stability of the rocket.

If I also remember correctly, the longer the stick (up to some point) resulted in a more stable rocket as both the cg and the cp were then behind the center of thrust of the rocket.

The original Carlisle Mark-I consisted of a forward mounted rocket engine (ala Goddard) with 3 "stilts" and small fins at their base. What your diagram shows basically the same thing, except the ring fin tube acts as fins.

Members of the ARS in the1935-38 timeframe used pyroctechnic core burning skyrockets as replaceable solid propellant black powder rocket engines in their experimental testings. SOme of these designs were forward engine mounted with multiple tubes and fins and such trailing behind the engine. Some of these "pre-model rocket" model rockets even had functional parachute recovery systems.

You might consider a solid rod within a sliding tube on the stilts that you could "tape up) before flight to vary the length of the stilts to get the right/best stable combination.

Perhaps Rocksim could model this design?

Ring tail fins have about 2x the normal force as their flat plate cousins if I again remember correctly.

I'm sure there are others here that are much better versed in rocket stability than I that could offer some guidance on this subject.

see this on page 18:

http://www.ninfinger.org/rockets/Mo...01n03_01-69.pdf



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Old 09-10-2010, 12:56 PM
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As with any free-flying ballistic object the CP/CG relationship is key. It's nice to have over 1 caliber of stability (CG forward of CP), but even 1/4 caliber is sufficient for a long rocket.

Rockets with forward propulsion can have a traditional stability calculation done if it is a tube and fins rocket. There is some interference drag with the thrust and rocket body, so there is a thrust loss compared with a rear mounted motor. It's around 10-15%.

Dynamic stability is your real question. That is achieved by near 1 caliber static stability, a longer length vehicle, whether stick, tube or other, and by not much stuff resembling fin area near the front and some limited amount of stuff resembling fins near the rear.

We used to sell a kit called "Push-Pull"-tm which we are contemplating re-releasing which is in principal one motor taped to the top of a stick and another taped to the bottom. During thrust the dynamic stability associated with the thrust streams provide dynamic stability and the rearward motor is in more turbulent flow so it tends to act more like a fin than the top motor which at least enters laminar flow and transitions to a bow shock with symmetrical flow over it.

After burnout the device becomes dynamically unstable (barbell) before terminal velocity is achieved.

Those factoids and details ought to be indicators for the design you are contemplating.

The term "overstability" is actually best shown by a C egglofter. It needs larger than normal fins to correct for normal wind gusts and flight speeds with the increased nose mass and standard rocket length (short lever). Correcting this is typically achieved by both increasing fin span and area. It could also be achieved by more than doubling the tube length. Usually for a contest the mass is critical so the trade off toward fins works better. It is minimized by also employing a piston launcher to increase departure speed.

Jerry
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Old 09-10-2010, 03:01 PM
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Quote:
Originally Posted by Jerry Irvine
We used to sell a kit called "Push-Pull"-tm which we are contemplating re-releasing which is in principal one motor taped to the top of a stick and another taped to the bottom. During thrust the dynamic stability associated with the thrust streams provide dynamic stability and the rearward motor is in more turbulent flow so it tends to act more like a fin than the top motor which at least enters laminar flow and transitions to a bow shock with symmetrical flow over it.

After burnout the device becomes dynamically unstable (barbell) before terminal velocity is achieved.



Interesting concept. Does this design have more or less drag than a clustered and finned model? I am thinking of an application to NAR Cluster Altitude events...


Bill
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Old 09-10-2010, 06:17 PM
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The event it would be best suited for is predicted altitude.

It might be a candidate for superroc since the tension by disproportional thrust might allow a considerably longer rocket.

Even with a nose coned Push-Pull-tm the lack of reasonable coast phase makes any finned rocket superior for altitude events.

But Push-Pull -tm is cooler!

Jerry
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Old 09-11-2010, 01:10 AM
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Quote:
Originally Posted by shockwaveriderz
I once had a copy of a research report on the aerodynamics of pyrotechnic skyrockets; rockets at the top of a stick.....(lost in a disk crash long ago) and if I remember correctly the distance between the cg and the cp of the skyrocket determined the stability of the rocket. -SNIP-
According to All About Rockets and Jets by Fletcher Pratt, the Chinese discovered that if a rocket's stabilizing stick was at least 7 times as long as the rocket itself, it didn't need any feathers. (The first rockets used arrows as stabilizing sticks, but because their feathers were burned off by the motor exhaust, the Chinese made the feathered sticks longer and longer until they found that the arrow feathers were no longer necessary.)
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Old 09-11-2010, 10:46 PM
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Quote:
Originally Posted by Jerry Irvine
The event it would be best suited for is predicted altitude.

It might be a candidate for superroc since the tension by disproportional thrust might allow a considerably longer rocket.

Even with a nose coned Push-Pull-tm the lack of reasonable coast phase makes any finned rocket superior for altitude events.

But Push-Pull -tm is cooler!

Jerry



No coast time.

<smack on the forehead>

I don't know how I missed that...you did say that it went unstable at motor burnout.


Bill
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Old 09-12-2010, 09:21 PM
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With the "rocket on a stick" design, does the stick act like a fin? I had the impression that even though it is often very thin, the length and profile of the fin provides enough restoring force to bring the rocket back into a straight trajectory? Or is this an overly simplistic explanation?
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Old 09-13-2010, 08:24 AM
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Quote:
Originally Posted by Mark II
With the "rocket on a stick" design, does the stick act like a fin? I had the impression that even though it is often very thin, the length and profile of the fin provides enough restoring force to bring the rocket back into a straight trajectory? Or is this an overly simplistic explanation?


It uses gravity gradient boom stability as well as induced drag from high AOA stick angles. An extreme example of that is the Ace Squid which used multiple gravity gradient boom stability which damped each other out and made for a smooth stable flight.


This is an Ace Squid about 2.65" diameter.


This is an Ace Squid flying with an F motor.

Note that recommended motor list includes motors from Estes, Aerotech, Composite Dynamics, Crown/SSRS, and Enerjet.

Jerry

Last edited by Jerry Irvine : 09-13-2010 at 08:45 AM.
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