Zach:
I've seen "simple" rollerons used in a sub-sonic HPR and they worked fine to
zero roll.
For my application, the fins may--for 9" and 6" diameter rockets--simply be too
thin to install a slipstream powered gyro mass. In addition, modifying the
current design to include a hinge--whether in the fin or between the fin and
the vehicle (an all moving fin)--seems to me potentially very expensive, not to
mention much less reliable than a solid fin w/ either differential lift or a
tab.
I'm happy to go there if a non-moving solution will not work, but I suspect an
analytical solution to the "tab" problem is lower cost....
Bill
Sent from my Commodore 64
On Jul 17, 2016, at 11:12 AM, Zachary Martinez <znm3m8@xxxxxxx> wrote:
Another option to greatly reduce the spin on the rocket would be to use
rollerons. This would use the same principle as control moment gyros on
spacecraft and help to stabilize the rocket during flight.
If you are set on having ~10hz roll you could use control surfaces on the
fins and have them actively controlled. This would be a very complicated
setup so to simplify it you might want to have cables attached to the control
surface on one side and springs in the rocket on the other. When the rocket
is launched the the tab could be at any angle you choose up to horizontal and
that angle would correspond to zero loading on the spring. As the rocket
speeds up more drag force acts on the control surface and causes a moment
that will make the spring stretch and the control surface more flush with the
fin. This will have an advantage because the high angle in the beginning of
flight will cause the rocket to spin up faster and the low angle at
supersonic flight will allow it to spin at a much slower rate than
traditional spin tabs. I haven't done any analysis with numbers so take this
with a grain of salt. "Engineering is done with numbers. Analysis without
numbers is only an opinion.". I am pretty sure this would work better than
traditional spin fins but someone would need to go through the math and
optimize the spring, start angle, area of control surfaces ect. to see if a
constant spin rate could be achieved after the initial spin up.
Hope this helps,
Zachary Martinez
Aerospace Engineering | Missouri S&T
On Sun, Jul 17, 2016 at 7:59 AM, Wyatt Rehder <wyatt.rehder@xxxxxxxxx> wrote:
You could make your fins airfoils, glue something to the side of the
fin to have them make lift. Wouldn't need to cant the fins in that
case. Or just make your fins airfoils from the start.
On Sun, Jul 17, 2016 at 3:55 AM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
If you assume a spherical cow then there is no fin setup that can give
a constant roll rate other than 0 for any accelerating rocket. An
ideal fin tab or canted fin will spin the rocket through the air at a
constant "thread pitch", nominally with 0 spin rate at launch and
apogee and maximum spin at maximum velocity, though in reality inertia
would slow the spin up and spin down.
On Sun, Jul 17, 2016 at 1:29 PM, William Claybaugh
<wclaybaugh2@xxxxxxxxx> wrote:
Yep, supersonic. Still interested in a non-canted fin solution theory, if
any one knows of one.
Bill
On Saturday, July 16, 2016, Ben Brockert <wikkit@xxxxxxxxx> wrote:
I imagine your rockets go supersonic, so you'd be better off with a
canted fin (or a trim fin at the end of a fin) than a trailing edge
fin tab due to the change in behavior of a trailing edge control
surface in the transonic and supersonic regimes.
On Sat, Jul 16, 2016 at 11:37 PM, William Claybaugh
<wclaybaugh2@xxxxxxxxx> wrote:
I'd be interested in a theory to describe this problem. My own
interest
is
in using fin tabs to assure spin at a specific rate (9-10 hertz), which
appears--to me--to be cheaper and more reliable than trying to
carefully
align the fins to just the correct offset. Aligning them dead straight
and
using a fin tab appears to be much simpler....
Does anyone know of such an analysis?
Bill
On Saturday, July 16, 2016, Ben Brockert <wikkit@xxxxxxxxx> wrote:
Gary Cooper posted this to the FAR facebook page, though he's flying
in northern California:
https://youtu.be/I-FhXvLtk-U
A homebuilt motor that appears quite reliable, and trim tabs on the
fins to get rid of the tiny amount of roll the rocket was doing.