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Comment by msmithstubbs | Hacker News Reader

> They expect that the smaller amount of lifting will increase satellite lifetime by 50% even after accounting for atmospheric drag.

I'm curious: why will this provide a longer life? Is it the lift burn itself that affects the lifetime (so reduced lift burn is better for the satellite)?

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You can use fuel for station-keeping instead of orbit lifting.

Ah! Thanks.

Wouldn't burning all station-keeping fuel immediately be better than riding through thicker atmosphere for a while, and burning fuel bit by bit?

no, because a modern satellite with ion thruster or very high efficiency (high specific impulse, low thrust measurement in newtons), once it's been ejected from the second stage of a rocket and is above 99.99% of the atmosphere, if you burned all of its stored xenon fuel immediately after launch, would end up in a 45,000 x 450 km elliptical orbit.

If you want to keep a satellite in a mostly circular orbit from 350x350km to 600x600 km you do periodic very small boost maneuvers.

On an unfortunately very theoretical (but apparently already patend-encumbered, from what I can glean from Wikipedia) level there is also the concept of air-breathing electrical propulsion, an ion engine replenished from the the same trace atmosphere that causes the drag the engine is supposed to counter. Basically a solid state propeller that can still work in very thin air.

From my layman's understanding, because the effect of aerodynamic shaping breaks down at very low pressure, exhaust speed would have to be travel speed (TAS) x (total cross section / intake cross section) to keep orbit. Assessing whether that puts the concept in the realm of feasible technology or not is beyond my skills, but at least there seem to be projects working on that question. If it does work out, it would completely change the economics of LEO use.

I'm having a hard time finding good atmospheric density charts that go all the way to orbital altitudes, but this one looks good: http://hildaandtrojanasteroids.net/Atmosphere_model.png

Basically, there's already nothing left at their new planned altitude. In either scenario they'd need to burn fuel in order to compensate for irregularities in Earth's gravitational field, but I assume they've run the numbers internally.

Station keeping doesn't only involve drag; it involves rebalancing the orbital plane when a satellite fails; you also need to provision fuel to deorbit.

The higher the orbit, the more fuel you need to deorbit.

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