# [amsat-bb] Re: Kick motors on Oscars: How does attitude control work?

Burns Fisher burns at fisher.cc
Mon Sep 23 12:46:59 PDT 2013

```Very interesting stuff.  Thanks to everyone who responded.  Using a model
rocket engine has occurred to me :-)  I noticed that the smaller ones (A,B)
tend to peak at 10s of Newtons (presumably to get the model rocket going
fast enough to be stable after it leaves the launch rod) and then settles
down to sub-10 Newtons for the rest of the burn (a second or two).  So it
is quite a whack over a short period for a small bird.  I did not look up
the higher power size.  It would be interesting and not too difficult to do
the math to see what kind of a perigee raise would happen if someone fired
one of these on a 3U cubesat of modest mass in GTO while it was at apogee.

Don, there is enough interesting stuff to keep me watching the list.  There
is also a lot of complaining.  We'd love to have you rejoin AMSAT.  Fox-1,
the upcoming launch, IS an FM bird, but I hope you noticed that the last
AMSAT bird, ARRISat-1 was indeed a linear.  We need to continue to learn
and experiment.

Burns, W2BFJ

On Sun, Sep 22, 2013 at 5:49 PM, Daniel Schultz <n8fgv at usa.net> wrote:
>
> > >I just want to ask a question:  If you have a motor of a few hundred
> > >Newtons, how to you keep the attitude stable during the burn?  For that
> > >matter, how do you get the attitude correct for the start of the burn?
> >
> > Simple question, simple answer: You use the magnetotorquers to point the
> > spin
> > axis in the right direction, check and check again to make sure you got
> > that
> > right, then use the magnetotorquers again to spin up the satellite at a
> > high
> > angular rate (maybe 20 RPM). The angular momentum of the spinning
> satellite
> > keeps it stable while the motor is firing. The motor thrust must of
> course
> > be
> > well aligned with the spin axis, but if the thrust vector is not perfect,
> > the
> > spinning satellite tends to even out the small deviation.
> >
> > One of the recent Cubesats carried high power model rocket engines to try
> > an
> > experimental orbit adjustment. They forgot to spin the satellite and the
> > Cubesat tumbled wildly as a result of the motor burn. Some of these
> groups
> > are
> > really lacking in basic physics knowledge (and we are not talking about
> wet
> > behind the ears students in that case.)
> >
> > Dan Schultz N8FGV
> >
> >
> > _______________________________________________
> > Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
> > Not an AMSAT-NA member? Join now to support the amateur satellite
> program!
> > Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
> >
>
>
> ------------------------------
>
> Message: 7
> Date: Sun, 22 Sep 2013 22:22:11 -0400
> From: Joe Fitzgerald <jfitzgerald at alum.wpi.edu>
> To: AMSAT BB <amsat-bb at amsat.org>
> Subject: [amsat-bb] Re: Kick motors on Oscars: How does attitude
>         control work?
> Message-ID: <523FA5D3.3020001 at alum.wpi.edu>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> On 9/22/2013 4:19 PM, Burns Fisher wrote:
> >   I would not think that electromagnets
> > operating against the earth's magnetic field would have enough power with
> > such a large motor.
>
> The electromagnets had some "oomph"  I seem to remember the engineering
> beacon moving in frequency slightly when they switched on ... I don't
> remember if it was because of a sag on the DC bus, or the magnetic field
> affecting the tuned circuits of the transmitter.
>
> But you bring up important points,  if we are to do orbit adjustments,
> we need to do attitude determination and control in addition to getting
> a motor aboard.  No easy feat in a 3U cubesat!
>
> -Joe KM1P
>
>
> ------------------------------
>
> Message: 8
> Date: Mon, 23 Sep 2013 08:14:44 +0200
> From: "i8cvs" <domenico.i8cvs at tin.it>
> To: "Peter Guelzow" <peter.guelzow at kourou.de>,  "Amsat - BBs"
>         <amsat-bb at amsat.org>, "Daniel Schultz" <n8fgv at usa.net>
> Subject: [amsat-bb] Re: Kick motors on Oscars: How does attitude
>         controlwork?
> Message-ID: <000901ceb824\$34922710\$0301a8c0 at i8cvs>
> Content-Type: text/plain;       charset="iso-8859-1"
>
> Hi Peter,DB2OS
>
> If I well remember in addition with the Magnetorquing ,AO40
> was equipped with a 3 axis X-Y-Z stabilization wheel/EPU
> acting as gyroscopes that never where used except one time
> I remember to have seen on the P3T TLM the wheels were
> tested rotating for a short time at a very low numbar of turns
> ....... or I am wrong ?
>
> Why the 3 axis stabilization wheel/EPU whre never used on
> AO40 ?
>
>
> 73" de
>
> i8CVS Domenico
>
> ----- Original Message -----
> From: "Peter Guelzow" <peter.guelzow at kourou.de>
> To: <amsat-bb at amsat.org>
> Sent: Sunday, September 22, 2013 11:21 PM
> Subject: [amsat-bb] Re: Kick motors on Oscars: How does attitude
> controlwork?
>
>
> > Hi Burns,
> >
> > yes - all Phase 3 satellites use Magnetorquers to control attitude
> > during perigee. They were pulsed by the IHU on-board computer which was
> > running a model of the physics and orbital dynamics to trigger the
> > correct coils at the right moment.   This Timing was synchronized with
> > the Sun sensor...   indeed P3 spacecrafts are Spin stabilized...
> > There was no feedback. Command stations did some calculations based on
> > Earth and Sun sensor data, but once calibrated the system worked quite
> > smoothly and predictable...
> > For the motor burns, the attitude was indeed determined by the Sun and
> > Earth sensors and several times corrected until the perfect attitude was
> > achieved.. this took a few days.
> > Basically a very simple system was simple physics involved...
> >
> > 73s Peter
> >
> >
> > On 22.09.2013 22:19, Burns Fisher wrote:
> > > There has been a lot of discussion about AO-10, 13, and 40 (and maybe
> > > others) with various kinds of apogee kick motors (and inclination
> > > changers etc).  Rather than fanning any flames, I just want to ask a
> > > question: If you have a motor of a few hundred Newtons, how to
> > > you keep the attitude stable during the burn?
> > > For that matter, how do you get the attitude correct for the start
> > > of the burn?  I would not think that electromagnets operating against
> > > the earth's magnetic field would have enough power with
> > > such a large motor.
> > > Obviously it depends on the balance of the satellite relative to the
> > > position of the kick motor, but still...was the balance really good
> > > enough to allow magnetic attitude control?
> > > Was it active (i.e. with feedback)?  Does that imply a rate gyro?
> > > (No MEMS then, I suppose).
> > >
> > > Thanks in advance for the technical history lesson...
> > >
> > > 73,
> > >
> > > Burns W2BFJ
>
>
>
>
> ------------------------------
>
> Message: 9
> Date: Mon, 23 Sep 2013 20:51:19 +1200
> From: don <donmc at xtra.co.nz>
> To: amsat-bb at amsat.org
> Subject: [amsat-bb] Re: Kick motors on Oscars: How does attitude
>         controlwork?
> Message-ID: <52400107.9060000 at xtra.co.nz>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Hi,
>
> After a long absence I decided to check out the AMSAT mailing list again.
>
> It's been on my spam list for some time now, (mainly because of the
> insane FM square collecting posts)but I thought it time to check again.
> Amazingly I now find real technical discussions being carried on, is
> this now normal? could some one reply to the affirmative.... I may even
> find some funds to rejoin amsat if the board now reflects the
> experimental and technical pursuits of real satellite enthusiasts.
> Please no more FM sats. I like "beep sats" and enjoy getting data from
> them.....
> Don.
>
>
>
```