Bill,
That's useful information - thanks. Is there anything I can do to
compensate design-wise in an unguided vehicle? The fin positions can be
adjusted in 1 inch increments so would it be worth moving the fins a bit
forward on launch day if the winds are strong?
-Bob
On 12/30/18 1:51 PM, William Claybaugh wrote:
Bob:
At the MTA and I assume at FAR, the launchers are located a little past the crest of a slight rise in the terrain. When winds are strong they effect upon leaving the launcher but when gentler they separate from the ground at the crest of the hill, creating a wind shear 60-100 feet above the launcher depending on conditions.
This means that that in light winds the vehicle will track more or less to the launcher angle until it hits the shear, at which point it will weather cock in the down range direction.
The evidence for this can be found in the hundreds of images of launches at the MTA showing a smoke trail matching the launcher angle until suddenly turning 5-10 degrees downrange at an altitude 5-8 times the launcher length (which is 12 feet in most cases).
Bill
On Sun, Dec 30, 2018 at 11:59 AM rebel without a job <rebelwithoutajob@xxxxxxxxxxx <mailto:rebelwithoutajob@xxxxxxxxxxx>> wrote:
> OpenRocket shows Mach 0.75 for the 10 second burn and 0.92 for
the 15 second burn.
Go for the 10s then. No reason to deal with transonic stresses,
plus the lighter load means a little faster off the rail. With
60ft this might not be an issue at all. At that length you have a
different worry in wind: the faster you go the more force the fins
generate, and those forces are reacted against your launch lugs.
> I wish it was easier in these sim programs to create motor files
for liquids. Ideally you would specify some tank parameters and
depletion rates instead of having to make a fake motor mount in
the middle of the rocket to place the "motor".
Agreed. Hybrids have the same problem.
> OpenRocket 15.03 does plot both CG and Cp though and it shows a
3.0 of stability margin at at start and 3.5 at burnout. At apogee
it drops rapidly to -.75 so I'm not sure what to make about that.
Might be a bit too stable, and you will weathercock and chase the
wind a bit. OR usually shows loss of stability at apogee but it’s
worth looking at the plot to determine the direction of causality:
are you losing stability due to apogee or hitting apogee due to
stability loss?