[AR] Re: dynamic stability
- From: Paul Mueller <paul.mueller.iii@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 21 Aug 2014 13:30:06 -0600
As I understand things, one major aspect of "dynamic stability" is
roll-pitch coupling, where a certain spin rate can cause resonant pitch
response (at the pitch natural frequency), causing the rocket to basically
"cone" in a divergent manner until the angle of attack gets to be too much
and the rocket breaks up due to aerodynamic loads. The original sounding
rocket community avoided this by rapidly spinning their rockets (and
spinning them up quickly) so that the spin rate is well above the pitch
natural frequency. As I understand it, the pitch natural frequency depends
on the mass properties of the rocket and the aerodynamic forces (which
change with speed and altitude), so it can be kind of a hairy problem. I
have RSPro and it does not calculate the pitch natural frequency (at least
not that I have been able to decipher). So it's not much help for me.
But those sounding rockets tended to be high-acceleration (high thrust,
short duration), and misaligned thrust and/or asymmetric drag could really
send them off course. So they spun them up. I think an equally valid
approach for lower-acceleration liquid (or hybrid) rockets is the approach
used by Carmack--slow spin rates (or none at all) to avoid roll-pitch
coupling. Slow spin rates can help mitigate effects of off-axis thrust,
asymmetric drag, etc. so the vehicle still goes basically straight.
RSPro does have the ability to calculate "splash" plots of where the rocket
would be predicted to impact the ground (with no recovery, I believe) with
varying initial conditions, etc. (launch angle, winds at different
altitudes, misaligned thrust, etc.). I heard somewhere that a 50 km or so
flight in the past few years at Black Rock impacted well outside the
predicted "splash" zone so the FAA was reconsidering how to handle this
requirement, but I haven't heard anything more recently. My takeaway is
that computer simulation (RSPro or otherwise) still has a ways to go to be
something that can be relied upon. My opinion only, and maybe things have
gotten better.
Carl, I'd be happy to run RSPro simulations for you. My version is about 6
years old--I don't know if RSPro has been upgraded since then and whether I
could get a free upgrade if needed.
Paul M
On Thu, Aug 21, 2014 at 12:51 PM, Ken Biba <kenbiba@xxxxxx> wrote:
> I have RSPro and we are regularly submitting 100k+ projects.
>
> RSPro does poor job of estimating drag, there are other tools useful for
> estimating drag.
>
> Is this a single stage of multistage airframe?
>
> K
>
> Ken Biba
> Novarum
> Managing Director and CTO
> +14155775496
>
>
> > On Aug 21, 2014, at 10:28 AM, Carl Tedesco <ctedesco@xxxxxxxxxxxxxx>
> wrote:
> >
> > Thanks Richard, but this is for a liquid bi-propellant rocket, which I
> don't believe Tripoli allows.
> >
> > This is for the university rocket project I mentor --- SDSU. I just
> looked into Rocksim Pro and it is $1000 which is more than we can afford.
> Do any Arocketeers have Rocksim Pro and would be willing to help out a
> University team.
> >
> > --- Carl
> >
> >> On 8/20/2014 6:30 PM, Richard Hagensick wrote:
> >> If you belong to Tripoli Rocketry they have a Class 3 committee that
> will do the analysis for you and provide you with the paperwork you need to
> file your permit. They use Rocksim Pro. I submitted mine to the FAA for a
> 150K launch at BALLS this year.
> >>
> >> Richard Hagensick
> >>
> >> -----Original Message-----
> >> From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx]
> On Behalf Of Monroe L. King Jr.
> >> Sent: Wednesday, August 20, 2014 7:44 PM
> >> To: arocket@xxxxxxxxxxxxx
> >> Subject: [AR] Re: dynamic stability
> >>
> >> I don't think Flowmetrics flies under a TRA waiver for starters. The
> one launch I observed was an atlas vernier modification LOX and Hydrocarbon
> at FAR
> >>
> >> As I understand it Class 3 is "P" and above and you can bet they want
> a FULL 3 Sigma 6 degree of freedom analysis.
> >>
> >> AT a TRA event such as BALLS if you within a certain amount of their
> standing waiver for altitude you might get some benefit. But they are still
> going to require 3 sigma analysis (although you might get some help with
> it) and they in-turn are going to hand that to the FAA.
> >>
> >> Last I remember it's 14 points they look at during the analysis I have
> a list here somewhere.
> >>
> >> Monroe
> >>
> >>> -------- Original Message --------
> >>> Subject: [AR] Re: dynamic stability
> >>> From: Carl Tedesco <ctedesco@xxxxxxxxxxxxxx>
> >>> Date: Wed, August 20, 2014 5:24 pm
> >>> To: arocket@xxxxxxxxxxxxx
> >>>
> >>>
> >>> That is what I wondered. From an aerospace engineering point of view,
> >>> dynamic stability looks to see how a rocket will respond when a
> >>> disturbance is imparted on it. CP/CG rules determine static stability,
> >>> albeit still important. So, the Tripoli example you mention suggests
> >>> that they just want to know how the CP and CG change throughout the
> >>> entire flight?
> >>>
> >>> Monroe, do they require they require 3-sigma or dispersion analysis?
> >>> Below is an excerpt of what I was emailed (looks like it was cut &
> >>> pasted from the FAA website).
> >>>
> >>> (a) Class 2---High-Power Rockets. When a Class 2---High-Power Rocket
> >>> requires a certificate of waiver or authorization, the person planning
> >>> the operation must provide the information below on each type of
> >>> rocket to the FAA at least 45 days before the proposed operation. The
> >>> FAA may request additional information if necessary to ensure the
> >>> proposed operations can be safely conducted. The information shall
> >>> include for each type of Class 2 rocket expected to be flown:
> >>> (1) Estimated number of rockets,
> >>> (2) Type of propulsion (liquid or solid), fuel(s) and oxidizer(s),
> >>> (3) Description of the launcher(s) planned to be used, including any
> >>> airborne platform(s),
> >>> (4) Description of recovery system,
> >>> (5) Highest altitude, above ground level, expected to be reached,
> >>> (6) Launch site latitude, longitude, and elevation, and
> >>> (7) Any additional safety procedures that will be followed.
> >>> (b) Class 3---Advanced High-Power Rockets. When a Class 3---Advanced
> >>> High-Power Rocket requires a certificate of waiver or authorization
> >>> the person planning the operation must provide the information below
> >>> for each type of rocket to the FAA at least 45 days before the
> >>> proposed operation. The FAA may request additional information if
> >>> necessary to ensure the proposed operations can be safely conducted.
> >>> The information shall include for each type of Class 3 rocket expected
> to be flown:
> >>> (1) The information requirements of paragraph (a) of this section,
> >>> (2) Maximum possible range,
> >>> (3) The dynamic stability characteristics for the entire flight
> >>> profile,
> >>> (4) A description of all major rocket systems, including structural,
> >>> pneumatic, propellant, propulsion, ignition, electrical, avionics,
> >>> recovery, wind-weighting, flight control, and tracking,
> >>> (5) A description of other support equipment necessary for a safe
> >>> operation,
> >>> (6) The planned flight profile and sequence of events,
> >>> (7) All nominal impact areas, including those for any spent motors and
> >>> other discarded hardware, within three standard deviations of the mean
> >>> impact point,
> >>> (8) Launch commit criteria,
> >>> (9) Countdown procedures, and
> >>> (10) Mishap procedures.
> >>>
> >>> --- Carl
> >
> > --
> > Carl Tedesco
> > Flometrics, Inc.
> > 5900 Sea Lion Place, Suite 150
> > Carlsbad, CA 92010
> > tel: 760-476-2770 ext. 515
> > fax: 760-476-2763
> > ctedesco@xxxxxxxxxxxxxx
> > www.flometrics.com
> >
> >
>
>
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