# Canted engines — pointing inwards

#### MetricRocketeer

##### Member of the US Metric Association
TRF Supporter
Hi TRF colleagues,

Here is a theoretical question pertaining to canted engines. A presentation at the recent vNARCON 2023 got me thinking about this.

Let's say that you have two engines on exactly opposite sides of the center line of the longitudinal rocket axis and equidistant from the center line. And let's say that the two motors were both canted away from this center line at, oh, 25 degrees for each motor. Then the engine thrust of the two engines would equal the cosine of 25 degrees, and therefore the thrust would now be about 90.6 percent of the full thrust. So far, so good.

What if the two engines were canted, not away from the center line, but canted towards the center line. Would the resulting thrust be calculated the same way — that is, the cosine of 25 degrees?

Stanley

##### When in doubt... build hell-for-stout!
TRF Supporter
Ideally.. the motor cant is such that the motor c/l's point approximately through the rocket c/l.

#### neil_w

##### OpenRocketeer
TRF Supporter
If I understand what you are proposing (nozzles close to each other, fronts further apart), then if all motors don’t light it will be Bad.

##### Current host of Thumper
TRF Supporter
What if the two engines were canted, not away from the center line, but canted towards the center line. Would the resulting thrust be calculated the same way — that is, the cosine of 25 degrees?
Yes, the thrust would be reduced exactly the same. That orientation is however a very bad idea as the posts above note.

Edit to add: the intersecting exhaust of such a set up might lead to some additional forward thrust, but it wouldn't be simple to model. As a baseline, you could go with the assumption that it's the same as canting them away from center. Just as a matter of fight safety, I wouldn't try canting the motors towards center without having a large number of motors involved, far more than two.

Edit again: the stricken part was clearly nonsense, as Steve points out below. Brainfade.

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#### SolarYellow

##### Well-Known Member
TRF Supporter
Ideally.. the motor cant is such that the motor c/l's point approximately through the rocket c/l.

The approximation is that the motor centerlines should point through the rocket center of mass, so if one doesn't light, the other one is pushing straight against the mass of the rocket. Thus, it would not cause rotation of the rocket. That is what would happen in a vacuum, with the rocket moving in the direction of thrust with no rotation. However, in air, with the fins working to stabilize the rocket aerodynamically, it would still tend to rotate away from the push of the motor due to the lateral air velocity. So to get a straight flight on one offset motor, the cant would have to intersect the centerline of the rocket farther back than the center of mass, likely somewhere between the CG and CP. Someone better at modeling the stuff than me would have to calculate where. I believe it would be a moving target throughout the rocket's flight, as the CG position, acceleration and airspeed will all vary substantially.

##### Current host of Thumper
TRF Supporter
I believe it would be a moving target throughout the rocket's flight, as the CG position, acceleration and airspeed will all vary substantially.
Airspeed variation alone would make it a moving target. The rule of thumb I've seen - which seems reasonable to me (without my having tested it with deliberate asymmetric thrust) - is to aim roughly halfway between CG and CP.

##### Oddroc scum. Mindsimmer.
TRF Supporter
I sense the dark side. A flying ring of death with inward canted tractor motors. Or a silly inward cant on a more traditional rocket, with the motors in back...where they SHOULD be! Thrust lines not angled between the CP and CG! Bullocks!

Such abominations are more Giarandola like, more pyrotechnic, more South East Asia let's do some wagering on super large examples at a crowded festival. RSO frightening machines not worthy of sanctioned events. Considered unnatural and punishable by inquisition. Burn the witch! Give me science, not necromancy!

Purge inward canting from your mindsim, it can only lead you on a dark path...to fun! Totally sick dude! Blast the Jedi.

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#### KenECoyote

##### Well-Known Member
TRF Supporter
Edit: I'm answering assuming you mean engines canted like this: \/ since canted the other way (towards the centerline like this: /\ ) is the common way and has been done for years with the Fliskits Dueces Wild.

The way I understand how one canted motor works in something like the Dueces Wild is like one person paddling a canoe or kayak only on one side (vs. alternating sides)... you need to paddle out and away in order to go "straight ahead" (with the canoe angled towards the paddled side) since the propulsion is offset. If you paddled in and back, you'd just be doing sharp circles.

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TRF Supporter
V for Vendetta!

#### mjennings

##### Well-Known Member
As said above the reduced is the same. It's basic vector break down of a force at an angle. I'm not sure that the two (or more if we go full dark side here) exhaust plumes intersecting would gain back any of the thrust. While they would work on each other and turn the plumes down, that turn is going to induce more turbulence and energy loss.

##### Oddroc scum. Mindsimmer.
TRF Supporter
The Soviet R2 (I think that was the designation) an upgraded V2, had those weird inverted side pods. It is in Alway's book.

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##### Oddroc scum. Mindsimmer.
TRF Supporter
As said above the reduced is the same. It's basic vector break down of a force at an angle. I'm not sure that the two (or more if we go full dark side here) exhaust plumes intersecting would gain back any of the thrust. While they would work on each other and turn the plumes down, that turn is going to induce more turbulence and energy loss.
A ring of inverted canted motors built around a central launch lug. The ring is a crown of thorns and the motor plumes are focused at the bottom of the lug where you will find the bound heretics. Who isn't up for a good burning at the stake these days?

Flight card RSO notes: Rejected due to excessive morbidly.

How about the same basic design but with a nice Guy Fawkes Night theme!

A Toy Story "Sid Special" with "Falling with style" recovery. You could burn up the full line of Toy Story figures over the life of such an oddroc abomination.

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#### georgegassaway

##### Well-Known Member
TRF Supporter
Such abominations are more Giarandola like, more pyrotechnic, more South East Asia let's do some wagering on super large examples at a crowded festival. RSO frightening machines not worthy of sanctioned events.
Actually, that basic design by itself is pretty neat.

The problem is as implemented using a steel tube for the horizontal "engine", which has holes drilled into the underside, at various points to cause it to spin, and with nuts welded over those holes to reduce exhaust erosion (I saw enough videos years back to understand how the heck those clever bizarrely unique engines work, it's not a cluster of engines). Sady, those horizontal steel tube engines often go "Boom", with the multiple hand-fuse lighting persons too close by. The worst part of it is simply having ANYONE nearby, rather than use remote electrical ignition, and also crowds way too close if the flight goes awry.

I have considered making a model rocket version, using a Hula Hoop. But it would use a cluster of E6 engines (8 second burn), spaced apart on a horizontal beam, and angled appropriately for spin vs thrust, and likely also an electric powered "pre-spin" launcher to ensure enough gyroscopic spin as it took off.

Anyone reading this, can feel free to do the same thing, but be sure you have reliable ignition for E6's.

Examples below, for those who do not know. If the pics veer too much into "promoting" a very unsafe activity (not my intent), moderators feel free to edit out the photos.

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#### MetricRocketeer

##### Member of the US Metric Association
TRF Supporter
Hi everyone,

Understand, please, that I am not intending to cant motors in the way that I proposed. I was asking this just as a theoretical issue.

Thank you all for your informative responses.

Stanley

#### Steve Shannon

##### Well-Known Member
TRF Supporter
Yes, the thrust would be reduced exactly the same. That orientation is however a very bad idea as the posts above note.

Edit to add: the intersecting exhaust of such a set up might lead to some additional forward thrust, but it wouldn't be simple to model. As a baseline, you could go with the assumption that it's the same as canting them away from center. Just as a matter of fight safety, I wouldn't try canting the motors towards center without having a large number of motors involved, far more than two.
Intersecting exhaust does nothing to the thrust of the rocket. Once exhaust is outside of the nozzle it has no effect on the rocket. The mass has already left the rocket.

#### rocketgeek101

##### Well-Known Member
Is there a benefit to canted motors? I've noticed that on some orbital rockets the engines do appear to be canted slightly.

##### Oddroc scum. Mindsimmer.
TRF Supporter
Hi everyone,

Understand, please, that I am not intending to cant motors in the way that I proposed. I was asking this just as a theoretical issue.

Thank you all for your informative responses.

Stanley
Good, good! Stay on the straight and narrow path of the Jedi. Do not be influenced by the ruffians in the peanut gallery. Stay clear of the restricted area of the Hogwarts library and the Secrets of the Darkest Art! Purge your mind of impure thoughts regarding inward canted motors. Traditional 3-4FNC rock!

#### Steve Shannon

##### Well-Known Member
TRF Supporter
Is there a benefit to canted motors? I've noticed that on some orbital rockets the engines do appear to be canted slightly.
There can be. In a cluster canting a motor can help reduce the moment caused by being offset from the centerline, but it’s not as simple as pointing them directly at the Cg. They must be pointed at a location along the centerline such that the restoring force the fins create combines with the moment to allow a straight flight. That will differ as thrust changes and is greatly affected by the location of the Cp. If you could get it right you theoretically could fly with a single offset canted motor.
For large space rockets with bells extending downward I would suspect that the beginning cant is calculated to avoid that moment. The bells are probably on gimbals to allow steering, but you want to start out straight to minimize the corrections needed.
Perhaps there’s also an issue with shock waves reflecting off flat surfaces below the nozzle and back into the engines and causing combustion instability problems for liquid engines. I could understand how canting the engines reduces that, but I am speculating.

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##### Oddroc scum. Mindsimmer.
TRF Supporter
Is there a benefit to canted motors? I've noticed that on some orbital rockets the engines do appear to be canted slightly.
I don't know about real rockets but canted motors in model rocketry let you fly funny looking oddrocs.

Pull it up and don't burn the payload!

##### Oddroc scum. Mindsimmer.
TRF Supporter
There can be. In a cluster canting a motor can help reduce the moment caused by being offset from the centerline, but it’s not as simple as pointing them directly at the Cg. They must be pointed at a location along the centerline such that the restoring force the fins create combine with the moment to allow a straight flight. That will differ as thrust changes.
Blinding me with Science!

##### When in doubt... build hell-for-stout!
TRF Supporter
Intersecting exhaust does nothing to the thrust of the rocket. Once exhaust is outside of the nozzle it has no effect on the rocket. The mass has already left the rocket.
Ladies and Gentlemen... The Mass Has Left The Rocket....

#### Funkworks

##### Low Earth Orbit, obstructing Earth's view of Venus
What if the motors were canted inwards, but long enough to cross each other and be outwards again. This might be useful in a design where you want a motor to point away from a payload, but long enough for a longer flight. Since motors can't intersect, you'd get a spiral, but spirals can be made to work.

##### When in doubt... build hell-for-stout!
TRF Supporter
Worth noting... a rocket with multiple canted motors can still have a successful flight even if one of the canted motors doesn't ignite. The fins stabilize the flight, but the offset thrust makes the rocket fly in a "crab walk" orientation.

##### Oddroc scum. Mindsimmer.
TRF Supporter
Worth noting... a rocket with multiple canted motors can still have a successful flight even if one of the canted motors doesn't ignite. The fins stabilize the flight, but the offset thrust makes the rocket fly in a "crab walk" orientation.

Flis kits: The Hallmark of good rocket design!

##### Oddroc scum. Mindsimmer.
TRF Supporter
What if the motors were canted inwards, but long enough to cross each other and be outwards again. This might be useful in a design where you want a motor to point away from a payload, but long enough for a longer flight. Since motors can't intersect, you'd get a spiral, but spirals can be made to work.
Much better than purposely burning your payload for some kind of twisted enjoyment!

Set your payload for maximum smoking. A barbecue rocket hauling up a meaty delight!

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#### Steve Shannon

##### Well-Known Member
TRF Supporter
What if the motors were canted inwards, but long enough to cross each other and be outwards again. This might be useful in a design where you want a motor to point away from a payload, but long enough for a longer flight. Since motors can't intersect, you'd get a spiral, but spirals can be made to work.
I apologize if I misunderstand. The length of the motors has no effect on the thrust (from a physics perspective, not from a solid motor propellant perspective).
Why wouldn’t you want the motor to point towards the payload? By definition that’s what you’re trying to move. Everything else is just the overhead necessary to move it. And unless you mount the motors perpendicular to the axis of the rocket and directly towards each other there’s always a resultant thrust that acts through the payload.

##### Current host of Thumper
TRF Supporter
Intersecting exhaust does nothing to the thrust of the rocket. Once exhaust is outside of the nozzle it has no effect on the rocket. The mass has already left the rocket.
You're right of course. I was having an evening brain fade.

Thinking about it though, there might be some effect on expansion ratio akin to that of an aerospike. If so, it would likely be a negligible effect.

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