[FFT] Canted engines

it is not the angle, but that you need the line of thrust to intersect with the center of gravity.

so, in theory, you could angle them out to 180 degrees, but I don't think that would be safe.

Please notice that in the deuce the fins are VERY large. I have a hunch that to test stability with this, t you need to toss out some regular rocket assumptions and design it so that it will be stable even if the angle of attack is in excess of 10 degrees from the line of thrust if one motor fails.

gaaaa, I cannot think of a better way to say that.

I forget the exact name of the entry, but a while back someone did a Descon rocket that turned out to "fail-safe" in a partial cluster failure due to its canted thrust lines... it would make a safe (if spiral-ish) ascent in an "engine-out takeoff" situation.

-dave w

I might be wrong about this but I believe that the "line of thrust through the CG" is a safety feature as opposed to a requirement. After all, a standard cluster violates this by definition, since the individual lines of thrust are parallel and therefore can't all pass through the CG.

Assuming 0 degrees means no cant at all, anything from 90 (right angle) up to 180 (pointed down) would be fairly uninteresting as the rocket wouldn't leave the pad. :^)

Ideally, yes, but are all standard cluster (no cant) designs stable under partial ignition?

I just checked my nearly completed Deuce's Wild, and it does appear that the motors point nearly through the CG, though that is of course somewhat dependent on motor choice.

OTOH, on the Tour de Deuce, there has been at least one case of a single engine flight which layed over into the ground. But the TdD Deuces were built stronger thant he standard version so maybe the CG is way off, making them unstable on a single engine.

Or maybe when the DW is on one engine, thrusting through the CG, it IS "stable" but flies on an arc due to the effective cant of the fins. Personally, I think this to be the case, and that it would be the case for any partial ignition canted cluster.

would be safe.

180 (pointed down) would be fairly uninteresting as the rocket wouldn't leave the pad. :^)

Nah, you just turn it upside down and fly it backwards. And you get an easy rear deployment that way too :-)

I remember from my youth the one Estes book dealing with clusters mentioned that maximum separation between motors shouldn't exceed 10% of the total length of the rocket. This was a safety redundancy against misfires in a cluster.

Chuck

but flies on an arc due to the effective cant of the fins. Personally, I think this to be the case, and that it would be the case for any partial ignition canted cluster.

For the one scratch built rocket I have it's been flown twice as a cluster. Both times the outboards didn't all light. Both times it flew arrow straight. All the outboards are canted towards CG.

Chuck

bit eimer wrote: ...

my DW did this, the one time I only got 1 motor lit. (launcher battery trouble)

Hmmm, that's pretty interesting. And surprising: if you think about just one motor thrusting (up through the CG), the whole rocket would tend want to tilt at a slight angle to the line of thrust. And that's exactly what it would do in a vacuum (fly exactly along the line of thrust). In atmosphere, the tilting would be partially compensated by the air flow rotating the rocket back to straight. Nevertheless, there would have to be some residual tilt and that tilt would align the fins at an angle to the motion of the rocket, causing it to arc.

Of course, this is all rocket science, and since I'm not a rocket scientist, but just a plain old engineer, someone else should chime in and tell me where I'm wrong.

...10-minute tea break...

Rereading your note, however, I realize you're talking about the Good Ship Manatee which has a 24mm central and 3 18mm outboards. I suspect that if one works out the composite thrust vector for the case in which one outboard fails to light, it won't that much off of vertical simply because of the dominance of the central motor. Interesting exercise.

...another 10-minute break...

Lets see, according to your plan the outboards are at 14 degrees. That means the vertical component for each outboard is 0.97, while the horizontal component is 0.24. If one assumes a D12 along with 3 C6's, the total vertical thrust would be 12 + 3 * 0.97 *6 = 29.46. The horizontals are self-cancelling.

But with loss of one engine, the vertical thrust drops to 12 + 2 * 0.97 * 6 = 23.64. The horizontal component becomes 0.24 * 6 = 1.44 (in the direction of the dude motor). So the effective thrust angle (off vertical) will be arctan (1.44/23.64) = 3.5 degrees. Probably not measurable with the eye.

In this case, I'm not surprised that your rocket flew straight. Now, what happens if you fly it ONLY on the canted outboards (and lose one).

Vertical thrust is now 11.64, horizontal is the same at 1.44. New angle is

7 degrees.

And if you lost 2 outboards (which is actually the scenario I was thinking of for the Deuce: one canted engine firing), then you have an angle (big surprise) of 14 degrees. This, I think, is enough to create the arcing effect from the fin/thrust misalignment.

Arghh, too much brain strain - need to sleep.

dude = dud (duhe!)

I've had a couple (or perhaps 3) flights of a 4x24 cluster where only three motors have lit, but has only been apparent when recovering the rocket - i.e. no noticable effect on flight.

Niall Oswald =================================== Electronic & Electrical Engineering University of Bristol UKRA 1345 -

Several years ago Mark Page wrote an article for the May-June 1993 High Power Rocketry on this topic. As I recall his premise was that the optimal situation was to have the line of thrust going through the CG at all times. However since the CG changes as propellant is burned this is impractical in model rockets. In "real" rockets with gimballed motors this can be done. His compromise was to have the thrust line pass throug a point half-way between the CG and the CP. He built and flew a rocket with 5 motors, 4 canted at an extreme angle and tried different combinations of motors to test out his hypothesis.

Jonathan

----- Jonathan Sivier Secretary, Central Illinois Aerospace jsivier AT uiuc DOT edu NAR #56437 Tripoli #1906 CIA Web Site:

Page: "Remember to always keep the pointy end up."

If all the motors fire the average of their thrust is through the centerline.

Jonathan

----- Jonathan Sivier Secretary, Central Illinois Aerospace jsivier AT uiuc DOT edu NAR #56437 Tripoli #1906 CIA Web Site:

Page: "Remember to always keep the pointy end up."

90! Back in the "good old days" we flight converted Frisbees. One C6 in the center for lift, one HORIZONTAL A3 on the edge for spin. Reliable ignition was manditory!

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

I forget the exact name of the entry, but a while back someone did a Descon rocket that turned out to "fail-safe" in a partial cluster failure due to its canted thrust lines... it would make a safe (if spiral-ish) ascent in an "engine-out takeoff" situation.

This is correct. Well, frankly, there are no "requirements" either way. Also, as someone pointed out, it is not possible to have the motors line of thrust go through the center of gravity as the CG moves as the fuel burns. Your only option would be to gimbal the motors or have some way to bleed mass off of the nose during boost (either very unlikely ...)

Also, in the case of the Tres, the thrust line is not through the CG (not possible wiht that particular design). Rather, you are expected to have experience with clusters and to get it right or risk damaging your model. This past weekend we had a Tres launch with only one motor burning. She came off the pad and proceeded to lay on the ground near the pad till the motor finished burning (very low risk to injury). The very next launch of this model resulted in only 2 motors burining. She had enough lift off the pad to clear the range area by several hundred feet befor lawn darting (engines had already exhausted themselves)

Another important point for clusters in general. An unstable cluster (or any model rocket) does not pose a great risk (far less risk than a model rocket coming in intact and ballistic) as they can never build up much speed. Of far greater risk is a model that is marginally stable, tumbles coming off the launch rod then stabalizes itself while pointed at the crowd. I've seen that a few times and it isn't fun!

jim

Another solution to the assymetric thrust problem is to ensure some rotation. Given some rotation, the rocket will corkscrew or barrel roll, as the off-CG thrust is distributed around the direction of flight. The Estes Corkscrew does by this combining an off-axis single engine and a bent fin. I'm building a Tres now with each motor mount canted a bit at the front, to ensure this sort of spiraling. I'll report to the 'group if it ever misfires engines.

A while back, I had a brain spasm which caused me to swap booster/sustainer motors on a two stage. Most amazing (though scary) flight.

The rocket arced over due to the delay in the boosting motor (at which point I suspected something might be wrong). But then the ejection charge fired and, incredibly, the sustainer motor lit up. So then it was coming in powered at about 45 degrees. At about 15 ft from the ground the booster blew through and popped the parachute. No damage.

"Power dive" flights are FULLY SAFETY CODE COMPLIANT and rarely seen at group launches anyway. Start a POWER DIVE fad! They are truly cool!

Jerry