[amsat-bb] Re: Kick motors on Oscars: How does attitude controlwork?

Peter Guelzow peter.guelzow at kourou.de
Mon Sep 23 12:19:14 PDT 2013

Hi Domenico,

Yes,  AO-40  is equipped with magnetically suspended reaction wheels for
3-Axis stabilization.
They were turned on, spun up and functioned very well, even at higher
The 3-Axis stabilization was needed to generate enough electrical power
to "hot" operate the ArcJet/EPU with fully deployed solar panels...
After the Motor incident we ended up in a nicely high apogee, but the
perigee was too low and withing a very short time AO-40 would have
burned up in the atmosphere.
For that reason it was decided not to take any further risks and run the
ArcJet "cold" to quickly raise the perigee..   this goal was successful
achieved and AO-40 was finally in a safe orbit...   but this life saving
maneuver consumed all the Ammonia, if I remember correctly..
There was no immediate need to go into 3-Axis mode and to deploy the
solar cells.
I think there was more concern about what else might have got damaged
and thus I think it was decided to be more on the safe side and stay in
Spin mode..
At least we got 4 more years to operate S-Band....

73s Peter DB2OS

On 23.09.2013 08:14, i8cvs wrote:
> 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 ?   
> Thanks for your answere.
> 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

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