Bob,
Nathan at PSAS made a python utility to make spoof liquid engine .eng files
for open rocket. You can find it here:
https://github.com/psas/liquid-engine-analysis/tree/master/delta-v-estimations
also as a jupyter-noebook if you want to look it over to get an idea of how
it works:
https://nbviewer.jupyter.org/github/psas/liquid-engine-analysis/blob/master/delta-v-estimations/fake-liquid-motor.ipynb
Ad astra
Erin Schmidt
On Sun, Dec 30, 2018 at 2:52 PM William Claybaugh <wclaybaugh2@xxxxxxxxx>
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> wrote:
OpenRocket shows Mach 0.75 for the 10 second burn and 0.92 for the 15second 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 forliquids. 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 ofstability 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?
------------------------------
*De:* Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
*Enviado:* domingo, diciembre 30, 2018 05:44
*Para:* arocket@xxxxxxxxxxxxx; rebel without a job
*Asunto:* Re: [AR] Re: Fin stability and flutter margin
OpenRocket shows Mach 0.75 for the 10 second burn and 0.92 for the 15
second burn. The tanks are small and heavy but I'm not trying to push
the envelope for this first launch. I'll check out FinSim. The Baxter
rail at FAR is 60 feet long so there should be plenty of velocity off
the rail.
The component and tank layout worked out favorably with a CG variation
between full and empty of only 2.8 inches and it moves slightly forward
as it burns. 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". 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.
I have RasAero so I'll try it as well.
Thanks for the reply!
-Bob
On 12/29/18 10:14 PM, rebel without a job wrote:
(Disclaimer: engineering school dropout)normal atmospheres, flutter is mostly an interaction between aerodynamic
Flutter:
What’s the velocity range you’re looking at? For those of us operating
(speed) forces and inertia. Every aerostructure has a flutter speed where
that interaction becomes resonant. Generally speaking, probability of
flutter increases with higher aero forces (speed), lower damping
(stiffness) and longer moment arms (fin CG further away from joint)
likely won’t be moving fast enough to flutter an Al fin. I’m guessing M1.5
At a first order of analysis, something that propellant to mass ratio
at max? 1/8th of an inch of Al moving at under M2 isn’t going anywhere,
assuming that the fin to case joint is good and stiff. People regularly hit
M1.5 with 1/8” plywood fins bonded with wood glue to an internal motor
tube. You can hit M2 with 3/32” G10 fins surface mounted with nothing but a
solid fillet of RocketPoxy.
programs out there. You’re well within or better than the mainstream of
If you want to prove it to yourself, there’s FinSim and other similar
mass, speed, and materials used in garden-variety HPR.
half the force at twice the distance is the same arm. Fins further back is
Stability:
In general, you’re on the right track. It’s just a righting arm, and
better.
though because your boundary layer is thicker than you might think, and you
You gain less than you might think putting shorter fins further back
want the fin to stick out into clean(ish), laminar(ish) air to provide that
righting arm. Cutting fin span by 20% might cut effective area by twice
that.
caliber of the base, at least one caliber of span, and at least 2-3
The generally accepted practice is: trailing edge of your fin within a
calibers of root edge. Take a look at fiberglass rocket kits in about your
size range.
transonic range.
There’s two points you want to be looking at: off the rail, and the
speed. Generally you want 1-2 cal of stability. Too little and you’re
Off the rail the figure of merit is ratio between your speed and wind
unstable, but too much and you’ll weathercock into the wind. You can make
off the rail less worrisome by going faster. Since your T/W is fixed, solve
this by having a sufficiently long rail.
you’re going to have max-forward CP and max-aft CG, which means your
Transonic range: The bad news is that as a biprop pilot at burnout
stability margin will decrease as you fly.
to plot CP vs speed or time. Unfortunately it won’t plot CG vs time, and in
At this point your CP is going to move forward. OpenRocket allows you
the case of a liquid, CG moves back (bad) over time. While OR is generally
useful, in this case you might want to override your CG to max aft (no
fuel, worst possible) and ensure that you have 1.5 cal stability throughout
the flight.
time as you’ve put into this rocket, it’s best to have two opinions. For a
Trusting simulations:
You may also, time permitting, sim with RasAero as well. With as much
third opinion, take a look at a Madcow or Wildman rocket in about your size
range and see if your fin size and placement is generally similar. Those
are dead in the middle of the envelope and make a good sanity check.
wrote:
Feel free to contact me off list.
On Dec 29, 2018, at 7:44 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
found but I'd appreciate a sanity check. The rocket will be 14 feet long, 6
I've sized the fins for my 250 lbf biprop based on common guidelines I
inches in diameter and will weigh about 51 lb empty with a propellant load
between 11 - 16 lb (10-15 second burn). I was planning on using four
aluminum trapezoidal fins with a 10 inch root chord, 4 inch tip chord, 6
inch height, and 0.125 inches thick. From the flutter calculations in
Apogee Newsletter 411, this should provide a 50% flutter margin at burnout.
Is that reasonable?
increments but I was considering configuring them for somewhere between 2
The fin mounts allow position adjustment of a few inches in 1 inch
and 3 calibers of stability. Is that a reasonable balance between stability
and tendency to weathercock? I plan to fly out of FAR on the Baxter rail
but I'm not sure what the typical winds are there.
difference between a taller fin closer to the CG vs. a shorter fin farther
When placing the fins to achieve a target Cp, is there a practical
back? I would think the aerodynamic moment would be the same either way for
a given set of conditions . Shorter fins farther back would be preferable
from a weight and drag standpoint.
over a wind range of 0-15 mph.
I'm using OpenRocket for simulation and the results seem reasonable
Thanks,
-Bob