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Re: Further analysis of the last telemetry from AO-40



I view it as giving us little choice.  I don't believe
that one can study the problem and decide that
we can just forget the 400 N motor and use the
arcjet with the huge mass of propellant, oxidizer,
and He that we have on board.  We will have to
bleed it off and then use the arcjet or waste the
arcjet pushing around filled tanks and I think we
can't afford to do that.

We have done a burn that, if we believe element set
#5 (I don't) gave us about  400 meters/sec delta V.
I view this as an upper bound on what we have done.
The reason I don't believe element set #5 is that the
perigee is way too high in #5 given the perigee
we now observe from the radar painting of AO-40
from NORAD.  If would have been nice if we had
a radar painted set before and after the burn. But
if wishes were dollars . . . .   

Remember the burn was basically a Hohman transfer
step.  That is,  we applied the change in velocity along
the velocity vector at perigee to raise the apogee.
You can't lower perigee by burning at perigee, especially
by 250 km!   Therefore the perigee was lower than
we initially thought.

This is going to make the maneuver to raise the inclination
particular dreadful for the control stations.  The pucker
factor will be huge in hoping not to lower the perigee
by a slightly wrong vector at apogee when we change the
inclination.  Therefore the orientation vector for the plane
change burn (at least the first one) will have to have
MEASUREABLE component along the positive
velocity vector so as to not lower the perigee (into
the atmosphere or even upper atmosphere making
its attitude uncontrollable).  We can't wait forever for the
maneuver since we need to burn at apogee, while apogee
is near the equator.  If we decide to raise the perigee first
then the it will take much more energy to raise the inclination
since we will have raised the velocity at apogee.
Are we confused yet?  ;-)

Finally, assuming that we don't trust the 400 N motor, and
we make a plan to dump the prop/oxidizer, we cannot
get anywhere near the planned final orbit with only the arcjet.
It will be a big time compromise.  Furthermore, whose to
say that we can rely on the arcjet to burn hundreds of
times?  I am not saying that we can't, but how do we know
how reliable it is?  Has this motor been flown many times
before on other spacecraft?  NO.  

For all these reasons, I believe we have NO choice but to
decide to roll the dice, figure out what we can with the 400 N
motor/propellant flow assembly/LIU , and go or end up with a
bird that has an apogee at 60000 km in a nearly equatorial orbit. 
Mr. Kudielka has already shown us years ago that these elliptical
are somewhat unstable.
]
(AO-13 re-entered as predicted).  With this high apogee, we are
going to have our perigee bounced around by perturbations.  This
needs to be studied in detail by Mr. Kudielka, Mr Ernandes, etc.
In the end I am sure that the decision will be that we cannot
leave this perigee this low (370 km!).

These guys with their fingers on the buttons have TREMENDOUS
pressures on them to make right choices.  I have just painted
a small picture for you of how complicated just one aspect
of this is.    I for one wish they would avail themselves of
all the help they can get but in the end, they may view design
by committee as guaranteeing five humped camels or
ADA or . . .

Bob

----- Original Message ----- 
From: "Jeff Kelly" <jkelly@BellAtlantic.net>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, December 24, 2000 9:09 AM
Subject: Re: [amsat-bb] Further analysis of the last telemetry from AO-40


> >that we are going to have a serious pucker factor
> >in trying to use the motor again.
> 
> What impact would it have on the design if it was
> decided not to use the motor again?
> 
> Jeff Kelly
> KT2K
> 
> 
> 
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> 

----
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