Will it fly? Fun with improbable aerodynamics.

The Rocketry Forum

Help Support The Rocketry Forum:

Jeff Lassahn

Well-Known Member
Joined
Jul 5, 2020
Messages
420
Reaction score
518
Location
Portland OR
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?
 

Attachments

  • shroom1.ork
    1.6 KB · Views: 8

Zeus-cat

Well-Known Member
Joined
Mar 14, 2009
Messages
4,819
Reaction score
1,127
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).
 

Jeff Lassahn

Well-Known Member
Joined
Jul 5, 2020
Messages
420
Reaction score
518
Location
Portland OR
So does anyone know what the range of variation that still works is?
Will this fly?
Screen Shot 2020-10-06 at 7.52.06 AM.png

Or this?
 

Attachments

  • Screen Shot 2020-10-06 at 7.52.49 AM.png
    Screen Shot 2020-10-06 at 7.52.49 AM.png
    11 KB · Views: 4

heada

Well-Known Member
Joined
Jan 18, 2009
Messages
5,072
Reaction score
2,467
Location
Indianapolis, Indiana
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.
 

Bamraam

Active Member
Joined
Aug 12, 2020
Messages
34
Reaction score
33
Location
Scarborough, ON
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?
 

Jeff Lassahn

Well-Known Member
Joined
Jul 5, 2020
Messages
420
Reaction score
518
Location
Portland OR
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.png Screen Shot 2020-10-06 at 9.33.44 AM.png Screen 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.
 

heada

Well-Known Member
Joined
Jan 18, 2009
Messages
5,072
Reaction score
2,467
Location
Indianapolis, Indiana
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.
 

jrap330

Retired Engineer, NAR # 76940
Joined
Jan 25, 2020
Messages
773
Reaction score
247
Location
NJ
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?
 

Y3kankerous

Well-Known Member
Joined
Nov 8, 2021
Messages
75
Reaction score
62
This is fantastic! I've been thinking about forward facing cones quite a bit and some ways to overcome their seemingly inherent instability...
 
Top