Will it fly? Fun with improbable aerodynamics.

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Jeff Lassahn

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So, the fins are supposed to go on the back end, and they're supposed to be parallel to the thrust axis, right?
So, how about this, will it fly?
shroom1.jpg
OpenRocket says no, or course.
Potential flow math seems to say yes.
The "Base Drag Hack" says yes, but is it still "base drag" if it's at the front of the rocket?
Is it a "base" effect if most of the unbalanced force is actually on the front surface, not the back?
Is it a "drag" effect if the torque is coming from the center of lift on the disk moving sideways?

So what do you think? Will it fly?
 

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I have at least 4 rockets like this. They all fly well. They don't go high, but they are crowd pleasers.

Three of mine are from NewWay Rockets and the fourth is the Semroc Jupiter B (which looks suspiciously like the Lost in Space saucer).
 
Lots of flying pyramids out there. The flat disc type does have a limit to the distance between the disc and the thrust but I don't know what that limit is.
 
I have at least 4 rockets like this. They all fly well. They don't go high, but they are crowd pleasers.

Three of mine are from NewWay Rockets and the fourth is the Semroc Jupiter B (which looks suspiciously like the Lost in Space saucer).

Wasn't the Lost in Space saucer called Jupiter?
 
According to OpenRocket, all of these designs have CPs very far forward, but that's probably not accurate. OpenRocket doesn't handle blunt ends well. The effective CP of a flat plate is weirdly far behind the object itself.

Playing with a slightly more sophisticated fluid dynamics model (which I still don't really trust for these cases):
Screen Shot 2020-10-06 at 9.32.47 AM.pngScreen Shot 2020-10-06 at 9.33.44 AM.pngScreen Shot 2020-10-06 at 9.34.28 AM.png
For the reported CPs on these, more negative is farther rearwards (i.e. more stable) and the diameter of the cone base is 0.5 (so a CP of -2 is behind the center of the object by four diameters).
So these simulations think the flat plate is the most stable with about 10 diameters of margin, the rear facing cone is next with about six diameters and the front facing cone is worst with a bit over one diameter of margin. Basically it's saying a flat front contributes more to stability than a flat rear because flow separation limits the effect on the trailing edges. But the way this model estimates flow separation is complete garbage, so YMMV.

If you really believe that 10 diameter number for the flat plate, then you would expect this to be stable:
Screen Shot 2020-10-06 at 9.48.17 AM.png
So ... yah, sure.
 
The flat plate creates a large negative pressure zone behind it (what is known as base drag) and so that's what moves the CP far aft. That negative pressure robs performance significantly. When you don't have that empty space behind the plate, you don't have such a negative pressure zone. That's why a tailcone can give enhanced performance, it reduces that negative pressure. Your CFD examples mostly show this.
 
I have at least 4 rockets like this. They all fly well. They don't go high, but they are crowd pleasers.

Three of mine are from NewWay Rockets and the fourth is the Semroc Jupiter B (which looks suspiciously like the Lost in Space saucer).
Alias Jupiter B...do you NOT read the description?
 
This is fantastic! I've been thinking about forward facing cones quite a bit and some ways to overcome their seemingly inherent instability...
 
I finished reading the RAIS thread a couple of weeks ago, very interesting and a methodology I would aspire to - I have got as far as a GDS front with a coupler so the pseudo-fin or intake part can be changed over on the same motor mount (to trial different shape/area inlets as well as making it easier to change out toasted ones), plus then an open fronted cylinder with some slot on rings with different internal and external fin arrangements...
There is a slowly developing forward facing cone idea in my building box which would be GDS-ish but because of the forward facing-ness it I think it could also have a RAIS-like contribution. In the same vein as one of the posts on your original thread about a GDS-RAIS hybrid I suppose. I have a nice light plastic cone which is only mildly tapered to give this some chance, will update as and when I get anywhere with it! Also keen to see outcomes of other peoples (careful) experiments along these lines.
The key issue with GDS that I think I have come up against a bit is that it wants the motor-exhaust velocity to be high compared to the model-through-air velocity, preferably a lot higher I think, esp if the stability is already marginal. This doesn't work that well with typical BP motors because they all have big take off thrust spikes. I'm trying to get on to low impulse long burn APCP (Klima) motors (starting with B2) to get past this with one or two small/simple models, like the simple Black Arrow I have made and started to trial, fingers crossed....
My estimation is that a forward facing cone may need some very strong ducting/induction stabilisation however this is done because not only is it unstable CP-wise but is also has a strongly positive delta (frontal SA offset)/delta (theta) characteristic for small angles of attack, ie the cone will always increase its veer and spin with even a tiny initial offset (positive feedback), whereas the conventional point-forwards cone (ie a nosecone!) has the opposite characteristic where it will tend to self-correct for small aberrations.
The other part that I think may be difficult for a cone is getting the change in momentum of the air (whether ducted or inducted) to be far enough from the CG to get a decent moment, without which I think the stabilising effect is lost.
 
One possible way that occurred to me to wildly and inadvisedly (definitely unnecessarily!) incorporate all this in one model, which would mainly have the aim of looking unusual and possibly surprising (my main motivation in model rockets at the moment) is an open ended hourglass shape, with a motor mounting at the waist and a set of ducts+ports so the front ram intakes from the inside of the fore cone, and out-lets over the outside of the aft cone. The GDS intakes from around the outside of the fore cone and out-lets as an expansion cone into the centre of the aft cone. The GDS part would stabilise during the initial high thrust then the RAIS stabilises once its moving (i can't see any way it won't go unstable in the coast phase!). The intake ducts would cross-over quadrants to counteract the dSA/dtheta effect. Differences in sizing of the two cones (starting off with a large one at the aft...) and the ducts could be use to test what is actually going on and relative constibutions to (in)stability.
Whether these effects could collectively be made strong enough to make it go in a straight line? Who knows!! But it sounds like a challenge....
1656597628075.png
 
Just to help visualisation, here is the "simpler but probably even more unstable" version of a GDS-ish+bonus-ram air-intake cone that I mentioned above. I hesitate to use the word "stabilised" anywhere in relation to this! Strangely, I think this could be made to appear as conventionally stable design (CG ahead of CP) but would still spin like a top because of the dSA/dtheta effect (realise this will have a real name in the real world if its actually a real thing).
1656598558445.png
 
Related to forward facing cone... something I just built last week and flew yesterday... 🙂
IMG_20220629_161407470_HDR.jpg

Edit: This was done mostly as a fun experiment for an upward facing cup rocket, which I've been wanting to do for years. I've taken extra measures for stability since I want it to work first and then try it without the extending fins or nose sticking up.
 
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Yes! Thats about the size of it! I await with interest to see what happens without the fins! Your forward facing cup is much nicer than the one I have in the build box, which is an old plastic pint glass... but if it works then i will paint all but the top 1/4 inch black and call it "GDS is good for you"! Even better that it will sit the right way up on the rod.
At the risk of taking this all too far (but this is an improbable aero thread...), the addition that I would like to see eventually is a central triple canted/rotated cluster where all three motors fire thru the hole in the back of the cone so it spins axially via reaction force, then there is a counter-rotating spiral fin on the inside of the cone surface to oppose this and create a vortex down the centre-line. Imagine the exhaust trail! It might even take off spinning in one direction then change and spin in the other as the thrust balance shifts from thrust to ducting which could be resolved and used to calibrate the angular forces.
But more likely it would jump off the rod then briefly but wildly spin on all three axes at once....
 
Related to forward facing cone... something I just built last week and flew yesterday... 🙂
View attachment 524953

Edit: This was done mostly as a fun experiment for an upward facing cup rocket, which I've been wanting to do for years. I've taken extra measures for stability since I want it to work first and then try it without the extending fins or nose sticking up.
(Hmmm...I thought I had posted this reply earlier.)
I do envision a version closer to the forward cone and similar to Y3's second design. It would be without a nose cone, but motor mounted up to (or above) the lip of the cup and just partial fins inside the cup (center omitted to allow exhaust gasses through). In my mind, this should work and look like a regular cup from the side view.
 
(Hmmm...I thought I had posted this reply earlier.)
I do envision a version closer to the forward cone and similar to Y3's second design. It would be without a nose cone, but motor mounted up to (or above) the lip of the cup and just partial fins inside the cup (center omitted to allow exhaust gasses through). In my mind, this should work and look like a regular cup from the side view.
That would be an amalgamation of ducted intake, tractor motor, reverse cone (part spool?), possible gds and internal fins (including center ring fin).

...and a cup! 😁👍

I'll see if I have time next week to build one...shouldn't be too hard.
 
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Don't forget that the affect of base drag goes up dramatically as acceleration increases. The higher the acceleration, the lower the pressure zone of the toroidal vortex that is created by the skirt. The lower the pressure, the higher the effective pressure is at the low pressure boundary layer pouring over the skirt. This is what causes base drag. Let us not forget that CP is a shortened abbreviation for Center of AERODYNAMIC pressure. We are all taught that the CP is the point on the windward side of the aerodynamic body where the SURFACE area is equal forwards and aft. That's not totally correct and there are caveats with it as well.

I would think that the long versions above would likely be more stable opposed to the shorter as the shorter has the ability to affect the rotational moment at a faster rate where as the longer version has a longer lever arm and therefore will have a slower moment.

We should really have a campaign to model and build some of these really strange shapes. Get a batch of Qjets and some foam board, dowel rods, and tape and see what happens.
 
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