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Re: arcjet firings

Ronald Long wrote:
> I have read your proposal to use the arcjet for many months before the 400N
> motor is fired. I am curious about how this will work. Perhaps if you have
> time you might post something to amsat-bb discussing some of the questions
> I ask below.

Dear Ron,

I will try to answer your questions, but you figured out already all the 
important things.

> What I am wondering is how much orbital change is accomplished with a
> single one hour firing. Will we be able to know right away whether the goal
> was accomplished, or will it have to be fired each orbit for say 10 or 100
> orbits before the change is measurable?

With the initial 650 kg total mass and around 0.09 kg/hour NH3 at 4750
you get about 0.66 m/s (4750 * 0.09/650) deltaV for a one hour burn.
The velocity at perigee will be nearly 10 000 m/s, the increase is
not very dramatic, BUT height of apogee will increase by about 45 km !
The accuracy of ranging is a few km, so the command stations will be
able to check each single one hour burn of the arcjet.

> Which Keplerian parameters are most sensitive? 

The primary goal in the initial phase is to increase height of apogee, 
with constant height of perigee this means increase of mean motion.
Due primarily to the oblateness of the Earth, but also due to
influences of the Moon and the Sun, the argument of perigee and RAAN
(longitude of the ascending node) are changing. Therefore the attitude
of the spacecraft has to be adjusted, either at every perigee pass by
a few tenth of a degree or at some greater intervals correspondingly.

> The firings are at perigee but that mainly changes the apogee,
> is that correct?

Yes, correct.

> I understand that the 400N motor produces a high thrust, but only for a
> short time. With the fuel capacity of P3D how many seconds would the 400N
> motor operate if it was fired only once to fuel depletion?

196 kg of MMH/N2O4 will be consumed in 1470 seconds, 24.5 minutes. 
> I have found that the arcjet consumes 24mg/S of ammonia. So with 52kg, it
> can run for 600 hours.

Correct. That means also about 600 ignitions - to be controlled and
checked by our command stations !
> I have looked at various published information comparing the 400N and
> arcjet motors, but don't really understand the numbers.
> For example the 400N motor has a Delta-V 1150m/S and the arcjet Delta-V is
> 450m/S. Does this mean that if the arcjet ran for all 600 hours it would
> accomplish 40% as much (450/1150) as the 400N could if it ran until all the
> fuel was used up?

Roughly your figures are OK. The deltaV depends on the ratio of the
propellant used and the current total mass - which changes with the
use of propellant. So if you use the arcjet propellant first, the
deltaV of the arcjet will be smaller than when you use it after
the burns of the 400 N motor, when the total mass is 196 kg less.
But there are also other considerations. The major inclination change 
has to be done with the 400 N motor, it wouldn't be feasible to do it
with the arcjet. Second, although the very same 400 N motor has
worked perfectly at a recent Jupiter mission, you want to pressurize
your propellant tanks only shortly before usage, in case there are
any leakages. Just to be safe. 

I hope, I have answered all you asked - let's hope that everything goes
as planned, our command stations have a tremendous task ahead !

	Vy 73, Viktor OE1VKW
Viktor W. Kudielka    
Internet: viktor.kudielka@ieee.org,oe1vkw@amsat.org,oe1vkw@oevsv.at
URL: www.oe1kib.ampr.org/~oe1vkw/ 
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