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AO-40 Burns



Gang

I'm sure I wasn't the only person surprised when Karl said that the first
burn would be done with the 400N motor. (Previous publicity had said that
there would first be several months of burns with the electric propulsion.)

So I asked Viktor, OE1VKW; he was a prime consultant on the propulsion
planning. After the 70cm problem arose, Karl consulted him by phone for
different options. Viktor's (three) replies to me are below and he has
kindly allowed me to share these with the -bb. Thanks Viktor.

Begin quotes:

With two very different motors on board the satellite on a GTO, 
and the goal to achieve a highly elliptic, highly inclined orbit 
of 16 hours period with the apogee somewhere above the northern 
hemisphere for at least 10 years and may be a total lifetime of 
more than twenty years, and if possible, guarantee a proper decay 
in order to leave no rubbish in space, the number of alternatives 
to be considered is pretty high. In addition, there are other 
constraints, like sun angle for the s/c attitude required for 
the burns, power budget, link budget for the intermediate orbits, 
propellant consumption etc. etc.

The major inclination change can only be done with the 400N motor.
The plan is, to do this inclination change at apogee at an
argument of perigee of around 270 degrees. This seems to be
the most economic alternative to achieve a new argument of
perigee of around 355 degrees and an inclination, such that the
argument of perigee will very slowly decrease, reaching 
180 degrees not before 10 years have passed.

There is a trade-off between the height of apogee of the
intermediate orbit and the delta-V required for the 
inclination change. If one chooses a higher orbit, some
MMH/N2O4 propellant can be used for achieving this higher orbit.
Whether one does this before using the arcjet motor, in between
the burns of the arcjet or afterwards, causes just very
subtle differences.

The advantages of a first burn of the 400N motor are:
We get a precise calibration of the performance of this motor
and can plan the inclination change more accurately.
Second, with a higher orbit, the changes in argument of perigee
and RAAN (primarily due to the oblateness of the Earth but 
also the influences of the Moon and the Sun), are decreasing.
This eases the management of the arcjet burns, since 
attitude changes are required for compensating the
changes of argument of perigee and RAAN.

Sorry for such a long description, but it is a very 
complex problem.
			Vy 73 de Viktor OE1VKW

I had delivered a set of alternatives, depending on the height of
apogee for the inclination burn (58000 km to 100000 km).

The higher the apogee, the less propellant for the 400N motor is needed.
Therefore we can use some of it already for increasing apogee height.
Whether we start with the biprop or the arcjet is in principle
irrelevant - except for the sun angle. The advantage of getting
a first boost of about 120 m/s is, we have then already slower changes
of argument of perigee and RAAN. This facilitates the management
of the arcjet burns. We have left about 1000 m/s for the 
inclination change and therefore we have to go for an apogee
between 70000 and 80000 km, where  Karl sees no problems
with communications or accuracy of attitude.
	Satisfied ? ;-)
	Vy 73, Viktor

For a higher and longer orbit, the power budget is better, since
more time is available for loading the batteries for the 
(maximum) one hour burn of the arcjet. And with a constant
attitude, the losses due to the geometry of the orbit around
perigee are reduced.

	73, Viktor
End quotes.

Hope this is useful. Also hope they tell us before the burn; watch for tank
pressurising.

73
Richard W L Limebear G3RWL
g3rwl@amsat.org
FOC # 1188

          So many beautiful girls ..... (sob) so little time
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