Canted engines — pointing inwards

[Steve Shannon]

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.

That’s a good point. My response over-simplified the situation.

 

[Funkworks]

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.

Miscommunication. Imagine 2 motors making the letter X. Now attach a nose cone on top and a payload below.

I’m on a smartphone so this the best rocket I can draw right now:

^
[]
X
[]

The X represents 2 intersecting motors somewhere midway in the body tube. Extend the arms as long as “needed”.

I’ll come back later to explain if still not clear.

 

[Steve Shannon]

Miscommunication. Imagine 2 motors making the letter X. Now attach a nose cone on top and a payload below.

I’m on a smartphone so this the best rocket I can draw right now:

^
[]
X
[]

The X represents 2 intersecting motors somewhere midway in the body tube. Extend the arms as long as “needed”.

I’ll come back later to explain if still not clear.

Sorry, I must just be slower than usual today. I can visualize the rocket; I just don't understand what effect that would have. You'll still have a resultant vertical thrust acting on the payload. The horizontal components would serve to spin the rocket, as you earlier said. I just don't understand what else it would accomplish.

 

[bad_idea]

That’s a good point. My response over-simplified the situation.

Actually I think you were entirely on-point with what you said. I wouldn't call it over-simplifying to ignore speculative aerospike-like effects that would be negligible and obscure without very deliberate engineering. There's always an extra layer of complexity to anything, and it's not oversimplyfing to ignore a lot of it for practical purposes.

 

[Rex R]

So if you are running a pair of c6s canted, would that make them 'slant 6es'?

 

[BigMacDaddy]

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.

Is it possible that you would get something like a Krushnic Effect if you had too many engines (more than 2 I am guessing) pointing inwards since they would all be fighting to draw in oxygen?

 

[BigMacDaddy]

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.

https://www.rocketryforum.com/threads/okturbo-at-naram-63.173915/

This is my limited experience with the P-38... Once the rocket is going fast enough it can be stable. However, a single canted engine tends to turn the rocket quite a bit before the fins have enough CP impact to straighten things out.

 

[Steve Shannon]

Is it possible that you would get something like a Krushnic Effect if you had too many engines (more than 2 I am guessing) pointing inwards since they would all be fighting to draw in oxygen?

No - rocket motors do not draw in oxygen. They carry their own as the major part of their propellant.

 

[lakeroadster]

This is my limited experience with the P-38... Once the rocket is going fast enough it can be stable. However, a single canted engine tends to turn the rocket quite a bit before the fins have enough CP impact to straighten things out.

@curtisheisey 's One motor Deuces Wild upscale is discussed in his report...

BUILDING AND FLYING AN UPSCALE DEUCE’S WILD! AND HIGH-POWER CLUSTERING TECHNIQUES

 

[Funkworks]

Sorry, I must just be slower than usual today. I can visualize the rocket; I just don't understand what effect that would have. You'll still have a resultant vertical thrust acting on the payload. The horizontal components would serve to spin the rocket, as you earlier said. I just don't understand what else it would accomplish.

Well outward canted motors can only be so long until they touch at the top. But it looks like if one accepts a spiralling flight, the motors can be longer.
Is it useful? .
Can a good looking design be made with criss-crossing motors.
Has anyone seen or done this before.
Is it worth thinking about.

Criss-crossing motors. That's my out-of-the box idea for today. Not all of them are pretty, but I was taught not to care.

And now back in the box I go.

 

[Rocketjunkie]

@curtisheisey 's One motor Deuces Wild upscale is discussed in his report...

BUILDING AND FLYING AN UPSCALE DEUCE’S WILD! AND HIGH-POWER CLUSTERING TECHNIQUES

My upscale Deuces fly well on 1 motor if it has enough thrust. I had only 1 M1419 light at NSL and it didn't get high enough for the chutes to open. However, it flew straight.
This is my 54 mm version on only 1 J410.

 

[Igotnothing]

Actually I think you were entirely on-point with what you said. I wouldn't call it over-simplifying to ignore speculative aerospike-like effects that would be negligible and obscure without very deliberate engineering. There's always an extra layer of complexity to anything, and it's not oversimplyfing to ignore a lot of it for practical purposes.

Speculating on aerospike-like effects: Look at a cross section of an aerospike nozzle. The the mass flux has momentum through the "throat", and the vector of that momentum would have a \/ much less sharp than the spike \/ itself. The effect then, of the spike is to re-direct the flux more downward. The annular-cone shape of the sum of the flux vectors is jamming gasses into a tighter and tighter volume as it descends the spike - and reacts the spike upwards. This gas momentum convergence well below the annular nozzle somewhat offsets the lack of divergent nozzle cone on the outside, and this is why aerospike nozzles were conceived for propulsion in a near-vacuum environment.

As the gasses descend the spike they are also rapidly expanding outward and slowing, so there is no point in making a spike bigger than the flux can support - it would be like having a huge divergent cone behind a small throat.

Some spikes are truncated (to save weight and materials), and a bubble develops there that acts like the missing portion of the spike - keeping the flux flowing into a converging/accelerating mass of gas - even though there is no physical rocket part there. Eventually, gasses meet gasses, and there is no rocket part against which to accelerate, whether metal or pressure bubble, and so, no additional thrust.

Your two converging bell nozzles might have some aerospike-like "effect" if there were a physical extension of the "bells" in their own \/ against which to react the gasses. Maybe something like the Sea Dart (amphibious jet) keel except directly between the nozzles. Of course, the drag against those gasses so far from the throat needs to be considered, plus the added weight. Again, is our bell too big for the motor?

 

[georgegassaway]

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.

Yah don't say......

Tractor Helix Development Thread here: https://www.rocketryforum.com/threa...ront-engine-rocket-that-spins-project.169137/

Video of 11 flights. Uh, 10 flights and an oops.
Well, two oops. One of which did leave the pad, eventually.
Dang it, make that three oops, lotta learning going on early......