Recessing motors... how far? =advanced=

[benjarvis]

Dear all


Having already flown a ducted cone rocket a good few times, I'm now looking at a non-ducted cone design.

Wanting to avoid the crushnick effect, but also wanting to avoid having to use the 10lbs of nose weight I've just calculated I'll need to make this stable... how far can I recess the motors inside a conical airframe?

IIRC the basic rule for tubes is 1 caliber, but obviously that is hard to apply in a tapered airframe.

Also, what sort of thrust loss will there be and is the loss linear against how deep the motors are recessed?

If for example, I had a BIG cluster of 13 motors that took up a large amount of the ID of the cone, their nozzles about 14" up inside the rear of a cone that was 14" in diameter at it's base and about 8ft long.... am I gonna loose LOTS of thrust, or should I be ok?


Don't wanna make a BIG expensive mistake on this.


Cheers all!


Ben


PS: would venting the aft skirt even a little (for example drilling a ring of small 1/2" vent holes around it) make any difference? Is there some point at which that'd help? eg, when the surface area of the vents excedes the surface are of the exit planes of the nozzles or something?

 

[Stymye]

Is it possible for a "cone " to produce the Krushnik effect due to the taper?
I know bernouli lock is a possibility with cone rockets(like the Centuri point),

either way,cool,are you building a Sprint abm?

 

[powderburner]

Originally posted by benjarvis
how far can I recess the motors inside a conical airframe
what sort of thrust loss will there be
is the loss linear
If for example, I had a BIG cluster .... their nozzles about 14" up inside the rear of a cone that was 14" in diameter ...... am I gonna loose LOTS of thrust?
would venting the aft skirt even a little make any difference?

wow, Ben, you don't ask for much, do ya?

I have not read the report by Krushnik in a looong time but I don't remember it being quite this detailed. IIRC he limited his work to a few more basic configurations.
How far can you imbed the nozzles? **Probably** would be OK to go 1/2 of the local BT diam. Probably pushing your luck to imbed a full 1.0 diam, at least thrust-loss-wise. Probably also pushing your luck as far as roasting the insides of your BT (or even setting your rocket on fire?).

What sort of...........
Is it linear............
--I am not sure that anyone has that much info.

Would venting help? If the venting is simply holes through the BT to open a passage for ambient air, maybe not much help there. Again, I am unaware of anyone with hard data in this area.
My aerospace engineering training would lead me to expect that you probably need vents/inlets sufficient in number and size to enable your rocket to ***smoothly*** scoop in external air and deliver it to the region of the nozzles such that the inlet flow maintains good pressure and velocity, and probably needs to be sized on the order of 1.0 to 2.0 x the nozzle base area to be able to 'break' the Krushnik recirculation. (Note: that is based on intuition, not analysis.)

Actually, that last qstn is a very good one. That would make a great research project.

Anyway, by the time you add some major inlet features, your rocket will not look very 'scale' anymore?

 

[benjarvis]

Originally posted by powderburner
wow, Ben, you don't ask for much, do ya?

I have not read the report by Krushnik in a looong time but I don't remember it being quite this detailed. IIRC he limited his work to a few more basic configurations.
How far can you imbed the nozzles? **Probably** would be OK to go 1/2 of the local BT diam. Probably pushing your luck to imbed a full 1.0 diam, at least thrust-loss-wise. Probably also pushing your luck as far as roasting the insides of your BT (or even setting your rocket on fire?).

What sort of...........
Is it linear............
--I am not sure that anyone has that much info.

Would venting help? If the venting is simply holes through the BT to open a passage for ambient air, maybe not much help there. Again, I am unaware of anyone with hard data in this area.
My aerospace engineering training would lead me to expect that you probably need vents/inlets sufficient in number and size to enable your rocket to ***smoothly*** scoop in external air and deliver it to the region of the nozzles such that the inlet flow maintains good pressure and velocity, and probably needs to be sized on the order of 1.0 to 2.0 x the nozzle base area to be able to 'break' the Krushnik recirculation. (Note: that is based on intuition, not analysis.)

Actually, that last qstn is a very good one. That would make a great research project.

Anyway, by the time you add some major inlet features, your rocket will not look very 'scale' anymore?

No, I've done PROPER intakes on a cone...

https://www.bits.bris.ac.uk/niall/gallery/EARSMarch2005/IMG_0411


This IS (as guessed) a scale Sprint, so intakes of a size likely to have much effect would be a no-go.

I'm not worried aboat toasting the inside of the rocket, some clever materials can take care of that... just wondering which side of the envelope to push really


At one end I could take the route of only recessing the motors 0.5 - 0.75 X the body dia and putting so much nose weight in the front that the density of the vehicle starts getting VERY serious indeed (looks like I'm gonna need about 0.8X the weight of all the loaded motors in the nose tip to keep it stable).

Or I risk some pseudo-ducted bodge and recess the motors a little more and accept it being slightly off-scale in some way (perhaps some slots around it, holes, removeable pannels or such like)... and get away with a lighter and less horrifically dangerous rocket.


Hmmm...


I guess this is one of those 'someone has to just try it' type projects


Ben

 

[nialloswald]

How about a downscaled test version with Estes motors? I don't know how representative it would be, you could even build a simple model of the lower part of the cone and static fire it perhaps.

Of course a flight test would be much more fun

 

[Daedalus]

Originally posted by benjarvis
I'm not worried aboat toasting the inside of the rocket, some clever materials can take care of that... just wondering which side of the envelope to push really

Ablative coatings on the inside, rather than on the outside of the real thing

I would think Nials idea of an Estes scale test model is the way to go though.

 

[nialloswald]

I would think Nial(l)s idea of an Estes scale test model is the way to go though.

Not to mention giving Ben an excuse to build a small, but silly cluster

 

[teflonrocketry1]

Ben,

A NAR technical report on the Krushnic effect by Lindsay Audin is available here: https://www.nar.org/pdf/TCR1.pdf

I have been working on the simulation of shapes like cones and saucers; the "Bernoli Lock" phenomenon is very important in these designs. I now believe that this dynamic base drag effect is mainly responsible for the aerodynamic stability of these designs. While conventional theory places the CP of a cone 2/3 the length from the tip; dynamic stability suggests a CP about 2 the diameters behind the base! I can provide you with more details and even a create a simulation of your design if you are interested (RockSim versions 6 and 7). The theory behind my simulation is based on wind tunnel testing of spool rocket designs and has yet to be proven with a true cone rocket.

Try using grill or engine paint on the base and inside of your rocket to prevent it from scorching.

Bruce S. Levison, NAR #69055

 

[powderburner]

One of the factors that should be in your favor would be the divergent shape of the rear of a cone. IIRC, the bulk of the work done on Krushnic (spelling?) effects has been done on constant-diameter body tubes. The flared configuration of the rear of a Sprint-type shape might give you a little less recirculation effect and a bit more thrust (for a given amount of nozzle inset).

 

[benjarvis]

Thanks all for your help and suggestions...


I only yetserday saw how recessed the motor is in the Shadow Sprint model... WOW!!

The model is about 4.5" - 5" dia at the base, and the base of the D motor is at least 6" - 7" inside the base of the cone!


That bodes well for my 0.8X the diameter recessing.


Not sure if I'll have the time/cash to do this project quite as soon as hoped... but it should be do-able I think.


Ben