[AR] Re: ion drive de-orbiting option (was Re: Cubesats orbital grenades ?)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 17 Dec 2017 17:19:39 -0500 (EST)
On Sun, 17 Dec 2017, John Dom wrote:
Smart-1, a 1 m^3 m volume complex surveying probe once made a slow
transfer from Earth to lunar orbit using solar/ion drive propulsion. It
worked. Regarding a cube sat de-orbiting application, such low impulse &
long duration propulsion is an option: no urgency.
Unfortunately, ion propulsion systems tend to be complex and heavy, and
they use a lot of power and put out a lot of heat, even though they use
very little fuel. They are seldom the best way to impart small velocity
changes, because the added hardware dry mass and the complications imposed
on the spacecraft design outweigh the fuel saving. For small delta-V,
chemical rockets or lower-Isp electrical rockets are usually simpler and
lighter.
The fuel advantage also isn't as great as you might think, because
low-thrust orbit changes are inefficient -- for example, a slow spiral
down from an 800km orbit to a 300km orbit requires about twice the delta-V
of a high-thrust impulsive burn that lowers perigee to 300km (after which
air drag will bring the apogee down quickly). Yes, you can fire an ion
thruster in pulses, once per orbit, to avoid the inefficiency... but that
takes even longer than the slow spiral, and all the starts and stops
themselves tend to waste some fuel.
And there may actually be some urgency, if your spacecraft is ailing and
you want to deorbit it while it's still controllable. People seldom want
to deorbit an operational (i.e. non-experimental) spacecraft that *isn't*
misbehaving in some way, or likely to misbehave soon.
Henry
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