vectored thrust info request

[wwattles]

I've got a Tres that flies beautifully, and I know that there are various upscale incarnations in progress, but I'm still a little fuzzy on the reasoning behind the vectored thrust.

My basic understanding of it is that the lateral thrust vectors of 3 matched engines in the mount produce a net vector in the horizontal plane that cancels out, leaving only the vertical plane vectors still active.

But do those lateral thrust vectors have to be aimed at a particular point? Obviously, they'd have to be aimed at the same point, but is that point something in particular, such as the CG?

The reason I'm asking is I'm considering doing a Tresified version of the Art Applewhite Qubit, putting 3 18mm mounts into the corners, adjacent to the intersections of the upper panels. All 3 motors would be aimed inward, at the same vertex, and would be thus cancelling their horizontal vectors, leaving only the vertical ones.

The big danger, of course, would be a failed ignition, since that would leave a huge uncancelled lateral force that would throw the thing sideways.

Any thoughts?

WW

 

[edwardw]

From my understanding they have to be aimed at the same point - a point of intersection. When I analyze trusses we call these points the work points. It makes sure there isn't any moment being created that could cause twisting or rotation of the piece. The less cant and consequently the further the work point is away from the motor the more vectored thrust you have going in the Y-Y plane. The less cant you have the less thrust you have in the Y-Y plane. At 45 degrees you get a 50%/%50 in each plane. The less you cant them also will help if you mis-align them because then more thrust is going up and not creating a nice moment that will send yours spinning and stressing out the fins.

HTH,

Edward

 

[jetra2]

I believe that in the case of the Deuce, the MMT's are designed to point right at or near the CG, possibly the CP - not sure exactly. But I guess you could think of this way. Imagine you have a line drawing of the Deuce in 2D - just a flat image. Then you have a line running right through the middle of the rocket, and lines running through the middle of the motor mount's, representing the thrust of the motors. Where these lines intersect the center line is where the thrust is directed to, and the CG or CP is probably right there around that point...

Alright, I'm gonna shaddup now and let the math guys talk about it...

Jason

 

[Micromeister]

As you guys suspected to thrust focus point is the C.G.

 

[jflis]

ok, a couple of items of importance here (It would be interesting to work up a class session for NARCON on this, frankly...)

When using canted motors here is what is happening:
- The off balance (sideways) thrust is canceled (assuming that the angle of the cant is the same and that they intersect a point that is in the *center* of the rocket. The way to calculate the motor power/impulse is to take the cosine of the cant angle times the thrust curve of the motor times the number of motors (also assuming all motors are the same which they had better be )

Now, a couple of important points.

To fly straight, it is NOT necessary that the angle of the cant result in an intersection point at the CG or the CP. The intersection point can be anywhere along the length of the rocket (or even beyond) so long as it is centered along the axis of the rocket.

*IF* however you have a misfire of one of the motors, the model *could* go unstable *unless* the intersection point is at the CG (CP does not matter in this discussion, other than normal CP requirements for stability)

If you have the motors canted such that they intersect at the CG and one or more motors misfire, then the NOW angled thrust is pushing on the CG and the model will remain stable but will fly slightly sideways. There is also a risk of it getting hung up on the launch rod.

Also, due to the nature of solid rocket motors, it is NOT possible to keep this relationship constant during the entire burn because even if you start out with the intersection at the CG, the motors expend fuel (weight) as they burn and the CG moves during flight.

The best bet scenerio is to have the intersection point slightly above the CG so that as the motors burn, the CG moves up, *through* the intersection point. This can be calculated for one motor type and would work fairly well.

The Deuce intersection point is near the CG but not *at* the CG. Experience has shown that, on average, the Deuce's Wild! is stable on only one motor.

The Tres, due to the nature of the design, has an intersection point that is way below the CG and instability is a real possiblity with 1 or 2 motors not lighting (however, with only ONE motor burning, this model will be greatly under powered...)

Sorry to drone on for so long, but one more important point concerning the adaptation of a flying saucer...

No two motors are identical, no two motor tubes are mounted exactly the same, no two motors ignite at the exact same time, etc, etc, etc.... This results in a build up of cumulative error. The CLOSER these motors are to each other, the less the effect these errors have on the flight.

If, however, you have the motors separated by great distance (inches), you create a "moment arm" that results in a multiplication of these errors.

If you wanted to put canted motors in a saucer, this should work fine so long as the ;motors are as close to each other as possible. You will note in FlisKits designs, these motor tubes are so close to each other that they actually *touch* at the top.

Hope all this helps!

jim

 

[scadaman29325]

Wasn't there a guy (Europe somewhere?) that posted a while back about his Deuce that fired one engine then when it burnt out the other fired?

He posted the pics and sure enough just like jflis said you could see it angled to one side but still going straight up, then the other engine fired and angled to the other side and kept going.

Very nice pics! I wish I could find them.

 

[Micromeister]

I remember that thread also

The important thing to gain for that thread is to place a 1/4square of wadding in the forward clay capped end of your Clustered motors. By placing the wadding in the clay capped end of the motors, should one or more of the motors fail to ignite at launch the ejection charge gases form the other motors will not ignite the unlite motor(s) backwards. This procedure should be followed with all clustered model where motors are subjected to the ejection charge gas and particles for adjacent motors. Isolated pod motors need not have this added protection.

Jim: while it is true the cant can be at any angle.. if we are to follow the premise "Safety First" than angling the motor mount to to C.G. Is the option we should be teaching IMHO.

 

[jflis]

John,

I agree in principle (teaching the angling through the CG), as that *is* the safest, but that doesn't make the other "unsafe". If that were true then every cluster model that was *not* canted would fall into that "unsafe" mode as the thrust line does not intersect the centerline of the rocket at the CG (or anywhere, as it is parallel to the axis).

There are many aspects of sport rocketry that are safer than others. Single motors are safer than clusters, alpha's are safer than orbital transports and most anything is safer than a super-roc...

but all those other things are still "safe", just a little closer to the envelope