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AO-40: Raising the perigee by testing the arcjet



Raising the perigee by testing the arcjet
----------------------------------------- 

We are approaching the 270/0 mark and it is planned to test the ARCJET
soon. 

The first test will be done without electrical power by blowing only
cold gas to check-out all systems and raise the perigee by some hundred
kilometers. 

Concerning the arcjet performance, we have a number of uncertainties.
The most important one is the actual mass of AO-40. If we just blow gas,
we can expect an Isp in the order of 1000 m/s. With the arc burning, it
will be at least 4000 m/s, probably more like 4500 m/s. With 500 kg s/c
mass we could get out of the arcjet a total delta-v of 500 m/s. On the
other hand if the arc does not work, we would get only in the order of
100 m/s. 

For each 100 km we want to raise the perigee, we need a delta-v of about
7.25 m/s at apogee. 

If we assume a s/c mass of around 500 kg, just blowing gas would require
a mass expenditure of 3.6 kg to raise the apogee by 100 km (out of
50kg+). On the other hand, with the motor burning, we would need only
around 0.8 kg; some 10h burning time with 1000 W power consumption.
Assuming that without arc the mass flow regulator (MFC) would still
result in a mass flow of 20 - 30 mg/s, then we are talking about a total
blowing time of 30 to 40 h. 

Due to the spacecraft mass (assuming it is still 500 kg), we can't make
a big change in inclination, but we can still achieve a 16 hour orbit.
This would much improve the current visibility conditions. 

The required delta-v for achieving a 16 hour orbit with different
heights of perigee (without any inclination change) are: 

 500 km  77 m/s 
1000 km 120 m/s 
2000 km 200 m/s 
4000 km 350 m/s 

In case the arc doesn't work and we want a 16 hour orbital period, the
height of perigee should not be increased beyond 750 km.


Summary:
---------

1) For the first test we do not need to worry about the ArcJet high
power electronics.

2) We can blow gas for several hours around apogee, which we could never
do with the arc burning and not having the solar panels fully deployed. 

3) The thrust will be much less as with the ArcJet burning, but given
that we can blow gas for a much longer period of time each orbit without
worrying about the batteries, we can actually raise perigee faster this
way. 

4) We need to thrust (gas only ) at 0.1 kg/hour for 40 hours (4 kg NH3)
to raise perigee by 100 km. 

5) This thrust level is 60% of the TMFC (mass flow regulator) or 27
mg/s. 

6) If we want to raise perigee by 200 km, we could thrust for 4 hours
around apogee for 20 orbits and get there. 

7) We would consume 8 kg of our 53 kg NH3 in the process. 

8) We can theoretically outgas for 8 hours around apogee with the mass
flow regulator set to 95% and raise perigee about 200 km in 10 orbits. 

9) We can still achieve a 16 hour orbital period (without any
inclination change).


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