Aerodynamics for rockets with forward mounted motors

[Bob Thomas]

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.

[Rocket Doctor]

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.

[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.

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/ModelRocketry/Model_Rocketry_v01n03_01-69.pdf



Terry Dean

[Jerry Irvine]

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

[Bill]

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

[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

[blackshire]

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.)

[Bill]

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

[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?

[Jerry Irvine]

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.

http://v-serv.com/-upload/CRm.4-83.26.squid1.gif
This is an Ace Squid about 2.65" diameter.

http://v-serv.com/-upload/CRm.4-83.26.squid2.gif
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. :D

Jerry

[soopirV]

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.

http://v-serv.com/-upload/CRm.4-83.26.squid1.gif
This is an Ace Squid about 2.65" diameter.

http://v-serv.com/-upload/CRm.4-83.26.squid2.gif
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. :D

Jerry

This is a cool looking rocket- anyone have more details on this one?

[rstaff3]

This blog post has some references (including one back to Jerry's site). http://rocketdungeon.blogspot.com/2006/01/desert-squids-exist.html#comments

[Brain]

Looks like an engine off a STAR WARS Y-Wing.
Not bad...

-Brain
Aim for the sucker holes.

[blackshire]

The streamer stabilization method (see Figure 23 on page 20 here: http://www.ninfinger.org/rockets/catalogs/semroc570/570semroc28.html ) that is discussed in the technical information section of Semroc Astronautic's May 1970 issue of Astronautic Modeler would make for some wild models (and wild flights, too, if one streamer tore loose from its outrigger during powered flight!). :-)

[Mark II]

It looks to me like streamer stabilization is another form of drag stabilization, but I can't visualize it relying on base drag like other drag stabilized rockets do. One can intuitively grasp its effectiveness as soon as one sees a simple graphic that illustrates the setup, though. But just out of curiosity, what is the aerodynamic process or mechanism that trailing streamers employ to produce the stability, anyway? How do they do it?

[blackshire]

It looks to me like streamer stabilization is another form of drag stabilization, but I can't visualize it relying on base drag like other drag stabilized rockets do. One can intuitively grasp its effectiveness as soon as one sees a simple graphic that illustrates the setup, though. But just out of curiosity, what is the aerodynamic process or mechanism that trailing streamers employ to produce the stability, anyway? How do they do it?They use balanced leverage. The drag force of each of the four (or three) streamers is magnified (with respect to the rocket body) by being transmitted through the "lever" (moment arm) of each outrigger, just like the balance of forces on a see-saw:

Mass X Distance from fulcrum = Mass X Distance from fulcrum

The streamer-stabilized rocket has these forces acting on two perpendicular "see-saws" (whose fulcrums are where the rocket's pitch and yaw axes cross) instead of just one as a see-saw does. That is why the loss of all or most of one streamer from its outrigger could be disastrous, because it would allow the (no longer counterbalanced) force of the opposite streamer/outrigger to make the rocket veer in the direction of the remaining streamer on that "see-saw" (which pivots about either the pitch or yaw axis that passes through the model).

[jharding58]

So how did Goddard (and I suppose by inheritance Flis) do it?

[blackshire]

So how did Goddard (and I suppose by inheritance Flis) do it?"On calm days, with a judicious dose of luck."

A "nose drive" (tractor) rocket is neutrally stable (or nearly so) because the restoring forces are small. They have pendulum stability, but the bottom end of the pendulum, once kicked out of line, causes the forward motor to pull the rocket along in its new orientation. Skyrockets overcome this through their high acceleration, which gives them a strong vector along the thrust line (the wind does make them veer, but at their velocities their dominant motion is along the thrust line).

I saw this at work in heavy, slowly-accelerating stick-stabilized rockets that I built--even slight air movements caused large deflections in their trajectories because the wind vector was a significant fraction of their thrust vector (the wind speed was comparable to the rockets' speed during early ascent). The far ends of their sticks "fish-tailed" quite a bit because they didn't have the aerodynamic "authority" of fins, which produce larger restoring forces.

[Bill]

So how did Goddard (and I suppose by inheritance Flis) do it?


The original Goddard rocket was not stable.

The FlisKits model of it is because the CG is in front of the CP. The rear mounted "fuel tank" is draggy and empty (lightweight.)


Bill