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astronboy

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I have been toying with an idea, but I am not sure if it will fly...

I want to build a rocket with a single off center engine... It is kinda hard to describe.... but imagine a Ranger, but insert only one motor.... will it fly straight?

Now, remove (or just do not install) the other two motor mounts, leaving one motor up against the BT wall.

Will this fly?
 

powderburner

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If your overall configuration is 'kind of like a Ranger' in length, fin size, etc, you should be fine.

Off-center thrust lines are always a source of concern, but if the thrust vector is anywhere close to the vehicle center of gravity you may not even notice any difference. And an extra-long connection between you and the launcher doesn't hurt either.

An equally big question is: what do the fins look like? If they are fairly large you will have plenty of aerodynamic stability. If they are also asymmetric, you may be able to sort of 'balance' the aero drag components around the thrust line, minimizing any out-of-trim effects.

I love playing with odd configurations, especially those with motors in unusual locations. I have found that an experienced eyeball will get you a long way through evaluating these things, which is good, because I have never found much technical data addressing asymmetric model rockets. I am working on one of these right now (planning/designing).

Please don't forget to post a flight report after you have a couple under your belt.
 

astronboy

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Thanks powderburner, this is the type of info I was looking for at least the theoretical part.

My conceptual design is actual a retro looking rocket, with an asymetrical tailcone, hence the off center motor.

There are three fins in my design sketches, but they are not equally spaced around the bodu tube. Two are relatively large and are directly opposite one another, like an airplane's wings are/ The other fin is smaller, and rudderlike, but is is on the motor side of the body tube.

I was intuatively feeling (and scientifically hoping) that the two large fins would help with stability against the tendency of the off center motor to turn the rocket. Additionally, the 'rudder' fin will ad some drag to the side where the single motor is, thus adding some frag to that side, and leaving no 'fin drag' drag on the side with no fin.

I am not familiar with thrust vectors... I had not intended the motor to be canted or anything, just a BT20 mounted to one side of a BT-60.

I guess at this point I should build a boilerplate model, and work from there.....
 

powderburner

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A vector is just a line. In the case of a thrust vector, it is a line through the centerline or axis of the motor, where the thrust is assumed to be acting (ignoring the small perturbations and large deviations in the real world, caused by things like debris passing through the nozzle or deposits on the throat or nozzle). For most model rocket designs this is right up the centerline of the rocket. Sounds like that for yours, at least the thrust vector will be within the vertical center plane and will not be pointing left or right.

If you design the motor mount to point straight ahead, then the offset location of the mount (say, 1 inch) will be the same all the way forward through the length of the model. Let's assume for this example that the center of gravity is on the main longitudinal axis and not above it or below that axis. As the thrust vector passes the center of gravity, it will still be 1 inch away. So you will have a de-stabilizing torque (or moment, in engineering-talk) of (N lbs max thrust)x(1 inch) = N inch-pounds. The duration of this de-stabilizing or upsetting moment will be the burn time of the motor. After that, your model will cruise straight ahead as normal, stabilized by the fins. So the critical period is really the fraction of a second during peak motor thrust, when your model might still be on the launch rod?

So you could side-step much of the problem by using a pair of launch lugs, spaced up and down the length of your bird, to hang onto the launch rod/rail, together with a reasonably long launch rod. With your largest fins oriented perpendicular to the expected direction of your thrust-induced pitch-up, they should also help a great deal to dampen any pitching motion.

Didja get all that?
 

teflonrocketry1

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teflonrocketry1

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BTW: RockSim version 7 does a good job of simulating asymmetric designs, even complex ones with side pods, tube fins, ring tail fins and fins on fins. If you need help, or want me to run a few simulations for you, get me all the details either in a post or PM.

Bruce S. Levison, NAR #69055
 

jflis

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it's not unpresidented, though your design approach may be extreem, it can work with *over* stable rockets.

one example that comes to mind is one of the Estes space shuttle kits (the one with the external fuel tank). The motor was off center in the fuel tank, but in that case, it was *centered* along the verticle CG.

jim
 

astronboy

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Thanks again Powderburner. Lemme seee if I got this right...so to incredibly simplify, the thrust vector is the imaginary line drawn vertically through the motor's center.

If my motor is mounted to the side of a BT60, I will have a thrust vector that is .45 to one side of the cg as it passes vertically through the rocket.

OK... As this will be a BT-60, designed to fly on a 18mm motor, I should be OK with a 36 or 48" rod. As the motor is offset, I have the option of running the LL internally for added support during max thrust.

The big 'wing' fins should assist in stabilizing during the rest of the flight.

Teflon: I have Version 7, but have not yet attempted a sim of this offset design. I will give it a shot!! (BTW I Love that offset ring rocket of yours!!) Thanks!!

Thanks guys. I am feeling better about this design, so I will go ahead and build a boilerplate to test the basic design. If that flies well, I will go ahead with a full scale model. (The design is actually the model of a cartoon based rocket.... I do not yet want to let the cat out of the bag as to what it is...)
 

gerbs4me

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this is a interesting concept, I might have to try this:)
 

SwingWing

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The Estes Tomcat has an off-center engine mount to help balance those large draggy wings. You may want to copy the tomcat (off)-centering rings.:rolleyes:
 

Micromeister

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another example of offset thrush lines is the Pam Am Space clipper Orion from 2001. Flyes very well!
 

pjbatglen

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While I was following this thread it was stated that he needs to make a "boilerplate model" and test it. I don't know what that means. I looked in the glossary but it is not on the list.
I ran a seach here on TRF and came up with six threads. But those threads only state that some will need to build a boilerplate model. Can some tell me what a boilerplate is?

thanks
 

Micromeister

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Yep! I've been working of a Scale model from that very photo for awhile.. It's truely the second coolest rocket I know. but a very difficult scale model subject:)
 

dtomko

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The Navajo design reminded me of a picture on George Gassaway's site of a shuttle boiler plate model:
http://members.aol.com/GCGassaway/gcghome.htm
look at about the middle of the page; there's a shuttle attached to a body tube with two fins on the opposite side. It appears that the engine mount sits above the body tube, but it's hard to tell from the small photo.
"Boiler plate" refers to a model that is a test version, one without all the final detail, or a rough out of the design.
Drew Tomko
 

powderburner

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Usually, it just means that you build a full-scale model with representative parts, to check for function and stability and rough-order-of-magnitude performance, etc. This is for new designs, especially stuff that doesn't fit into simulations well.
You don't spend any time/effort on painting, cosmetic stuff doesn't matter, you might use a salvaged NC or piece of BT from the junk box----you get the idea.
Flying a boilerplate model is a way to see if your idea works (or stinks) before you sink all your dreams and labor into a beautiful model.
Some guys build their models without painting or decals, and fly them several times before they bother with fancy finishes. I guess that's kind of a similar approach.
 

Micromeister

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good explaination Powder!
It's a very useful tool building Scale models, some folks build two identical models at the same time, one to be the boiler plate flight test the other to end up as the finished product. I find if I do this I end up with two models I wish I had built differently:) I build the one model and take it to primer, than flight test it with all the "Major" details attached. if all goes will.. on to finishing. If we have an Oop's I usually find as least one or two things I wanted to change or constuct in a different fashion anyway. Even with a total crash, we learn somethings about the models behavior.
 

astronboy

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Yup!! You guys have it!! I want to build a boilerplate since my final design contains an asymetrical tailcone. Nuthin really fancy, just something like a V2 tailcone with a scoop cut out of it. Before I have carve the little bugger (and create decals, AND make the other 'scale' little detail bits), I want to be sure that this puppy will actually fly, and not chase it's tail or make a large arc ending in a small heap o' paper and balsa bits.
 

teflonrocketry1

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Originally posted by astronboy
Teflon: I have Version 7, but have not yet attempted a sim of this offset design. I will give it a shot!! (BTW I Love that offset ring rocket of yours!!) Thanks!!
astronboy,

My offset ring design is sort of a "boiler plate" for a design I finally decided not to make:
Picture a scale model of a 44 magnum handgun that flys like a rocket. The grip would be two asymmetric plates attached to the main body tube (barrel) and connected at their tip edges. A fin above and in the middle of these would be the trigger. I was planning on making the "gun sights" into launch lugs. I planed on using rear ejection to eject the casing (I mean spent motor) with a parchute for recovery. I was hoping to impress the NRA members of the NAR with this design. Since the scale model of a handgun might look too much like the real thing; I scrapped the design. I think it would invite trouble that I don't want. Can you imagine entering something like that in an NAR scale model competition!

How about posting a RockSim file of your design?

Bruce S. Levison, NAR #69055
 

powderburner

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Teflon, I don't think there is anything wrong with a scale 44 mag (as long as it is a Dan Wesson), and I don't think anyone would get scared by it!

You could even do a little ring-fin stuff to model the cylinder?
Or, if you build the cylinder as a solid, you could eject some streamers out of the exposed chambers

I did something similar a few years ago. I made up something inspired by an old episode of 'Chicken Man' (on the radio, back in the 60's). My rocket/gun was called the Geshtunkena-Ray Gun. The off-center grip was only one fin on mine. It flew badly. I thought about adding ballast to the front, and using a bigger motor, but then again it was a stupid project to begin with. It is probably in the bottom of a box somewhere in the garage . .

You need to give your magnum a try!
 

Micromeister

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Teflon:
I'm sure your choice of subjects would get lots of good whoo's and Aaaahs. in an Odd-Roc competition but wouldn't qualify for any "Scale" or even "Sport Scale" competition under current NAR pink book rules. Not because its a Handgun, but because it is not a scale model of a rocket, space vehicle or jet.
I think you should build it.. could be kind of neat! We've had flying golf putters, a Bowie Knife, Bircks and outhouses & tanks fly before in funny meets:D
 

Juerg

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You just have to observe two things and your flight will be straight:
- design the rocket to have a <u>2 caliber stability</u>
- point the thrust line on the <u>midpoint between CP and CG</u>

That's just what we did on the Ariane project, ( http://www.argoshpr.ch/Ariane4/Ariane4_e.htm ), there we had K550's and J350's as far as 1 foot off center!
The flight was perfectly straight!

Juerg
 

powderburner

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Actually, you can have BOTH these things:
"-design the rocket to have a 2 caliber stability
- point the thrust line on the midpoint between CP and CG"
and you may have an unstable rocket for many other reasons:

--non-symmetric nose cone (especially one pointed off-center)
--fin (or fins) glued on crooked
--fin (or fins) with non-symmetric airfoil
--fin (or fins) not spaced equally around circumference of body tube
--canted motor (with thrust line pointed properly, per above) not aligned with fins
--one fin (maybe two) significantly longer than the others, putting the 'center of drag' off of the longitudinal axis of the model

This last one is probably the culprit in my Ray-gun Rocket disaster. But it did make for some spectacular in-flight stunts.

As far as locating motors off of the model's centerline:
"we had K550's and J350's as far as 1 foot off center!"
I am sincerely happy that worked for you. However, it also has its share of hazards: motor ignition/thrust peak must be timed VERY closely or you will have significant pitch-over forces. Spread motors will theoretically work fine if you can (1) light them all, (2) light them all at the same time, and (3) if they are balanced in thrust/impulse across the longitudinal axis of the model.
 

Juerg

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it also has its share of hazards: motor ignition/thrust peak must be timed VERY closely or you will have significant pitch-over forces. Spread motors will theoretically work fine if you can (1) light them all, (2) light them all at the same time, and (3) if they are balanced in thrust/impulse across the longitudinal axis of the model.
That's true for motors that are installed parallel to each other and to the rocket axis, but that is not true if the rocket is configured as I said (2-caliber stability <u>and</u> thrust focussed to midpoint between CP and CG). With this configuration it doesn't even matter even if one motor is out, because resulting momentum and aerodynamic reaction will cancel each other out and you will still have a close to vertical flight.

Juerg
 

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