Concerning the discussion up the thread a bit about nozzle designs... do any of the factors mentioned apply to cold-gas thrusters like some attitude control jets? How about thrusters where a monopropellant like hydrazine or hydrogen peroxide is decomposed across a catalyst bed? Do those concerns not apply so much to units that tend to be that small?
They apply to any gas thruster if the mass flow is more than a mere trickle. Regardless of whether the gas is produced via combustion or not, it will expand into the area of low pressure (e.g., the vacuum at one end of the nozzle). The flow will still choke at the narrowest point.
Quote from: Herb Schaltegger on 03/05/2009 02:27 amThey apply to any gas thruster if the mass flow is more than a mere trickle. Regardless of whether the gas is produced via combustion or not, it will expand into the area of low pressure (e.g., the vacuum at one end of the nozzle). The flow will still choke at the narrowest point.Is the cold gas in a cold gas thruster cold enough that the nozzle cannot convert much thermal energy to (macroscopic) kinetic energy, or does the gas get appreciably colder and faster as it expands through the nozzle?
Basic physics - when gas expands, it cools.
I'm exactly as dumb as I look. I'm still trying to figure out why a Geo sat that's a half a degree off the equator (like MSAT1) needs less fuel for station keeping. It's Sband, made for low gain ground antennas so it doesn't have to stay perfectly still.
MSAT 1 isn't drifting. MSV took it off the equatorial plane on purpose because they were running out of fuel and they say it takes less to stay in place if you're out of place......uh....sort of. Now that I bothered to look it up, it's actually 2 1/2 degrees off.
Quote from: nomadd22 on 03/05/2009 08:31 pmMSAT 1 isn't drifting. MSV took it off the equatorial plane on purpose because they were running out of fuel and they say it takes less to stay in place if you're out of place......uh....sort of. Now that I bothered to look it up, it's actually 2 1/2 degrees off. Okay, so it's nearing end of life and they're just letting inclination go, it would seem. That's pretty typical. The satellite might still generate some revenue for a while, or sometimes the "inclined" sats are simply used as placeholders to hold an orbital slot until the operator can get a replacement launched.
OK. It might be simpler than I thought. Maybe the fuel savings comes from maintaining longitude but letting latitude get sloppy.
I'd thought they put it where it was on purpose.
But, it's more than just a placeholder. There are many, many customers using that thing, from sailboats to large ships. (Like mine) We just bought a mess of replacement radios on MSV's promise that the new sats would be compatible. I can even get internet access with them at a blistering 4800bps.
That's cool! I wonder how much inclination they can tolerate while still providing continuous coverage.
Most MSAT remotes are autotracking marine or mobile antennas or low gain antennas that you just point in the general direction, so the sat can afford to move around some. It's mostly Ku band birds who might have neighbors on the same frequency and remote, fixed high gain antennas that have to be really precise. We're just starting to get marine Ku band antennas now. Suckers cost a fortune because they have to maintain half a degree precision in rough seas.
Intuition (and simple physics) tell the satellites should be accelerated by the positive anomalies, hence causing an oscillatory motion between two points centered in the gravity well: the strongest gravity source. Are people somehow disoriented by the graphical representations of the geoid, thinking the wells are the "lowest" points in the figure? Or am I missing something entirely and being ridiculously stubborn?