Re: Probably Stupid Idea for Saving Hubble
From: Andrew Nowicki (andrew_at_nospam.com)
Date: 08/16/04
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Date: Mon, 16 Aug 2004 19:05:19 +0200
Andrew Nowicki wrote:
> It would take about one year to haul the Hubble to
> the ISS orbit by a small, economical ion thruster...
Henry Spencer wrote:
> Uh, no -- you need a battery of sizable ion thrusters and some very large
> solar arrays to get it there in reasonable time. Ion thrusters have some
> nice capabilities but they are not magic, and Hubble is big and heavy.
__________________________________________________________________________
DATA:
Fakel SPT-140 thruster specs: (from John Schilling and the Web):
power consumption: 4.5 kW
thrust: 280 mN
ion velocity: about 18 km/s
thruster mass: 6.8 kg
power processing unit mass: 13.7 kg
propellant management assembly mass: 3.9 kg
xenon propellant consumption: 16 mg/s
photovoltaic solar array mass: only 20 W/kg
(only 20 W/kg because of the Earth's shade)
Hubble's mass: 11,110 kg
Hubble's orbital velocity: about 7.5 km/s
orbital plane change: 23 angle degrees
time to complete the orbital plane change: 1 year = 3.2x10^7 seconds
__________________________________________________________________________
CALCULATIONS: (x = multiplication)
V = velocity change due to the orbital plane change.
V = 7.5 km/s x (23/90) = 1.9 km/s
>>From the rocket equation we get mass ratio of 1.11.
Total mass of xenon propellant consumed in one year =
16 mg/s x 3.2x10^7s = 512 kg.
Mass of photovoltaic solar array for one thruster =
4.5 kW / 20 W/kg = 225 kg.
Total mass of one thruster and its photovoltaic solar array =
6.8 kg + 13.7 kg + 3.9 kg + 225 kg = about 250 kg.
Suppose that we use three thrusters.
The dry mass = 11,110 kg + 3 x 250 kg = 11,860 kg.
The wet mass = 11,110 kg + 3 x 250 kg + 3 x 512 kg = 13,396 kg.
Mass ratio = 13,396 kg / 11,860 kg = 1.13. (good enough)
__________________________________________________________________________
CONCLUSION:
The total mass of the ion thrusters, their photovoltaic power supply, and
their xenon propellant equals 13% of the Hubble's mass. This confirms my
opinion that the thruster would be small.
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