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."
<http://spacecraft.ssl.umd.edu/akins_laws.html>. 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 giveif
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,
interestany 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
whichis
in using fin tabs to assure spin at a specific rate (9-10 hertz),
carefullyappears--to me--to be cheaper and more reliable than trying to
straightalign the fins to just the correct offset. Aligning them dead
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.