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A press release by the SatRevolution consortium says that it plans to send cubesats "as small as 50 kg" to Mars to "conduct a variety of valuable science." So that means orbiting, not just a flyby like MarCO-A and MarCO-B on 2018 Nov 26.

50 kg is a twentieth the mass of previous Mars orbiters. How could something this lightweight attain Mars orbit? The required delta V is about 3 km/s (there's no Oberth effect shortcut for an electric thruster).

Deep Space 1's ion thruster achieved 4.3 km/s from 74 kg of xenon propellant, or 0.058 km/s per kg. Dawn was slightly less fuel efficient: 11.5 km/s from 247 kg, or 0.046 km/s per kg.

At 0.058 km/s per kg, a Mars orbiter would need 3 / 0.058 = 51 kg of fuel, more than the mass of the entire proposed spacecraft.

Have thrusters improved so much since those launches that a 50 kg all-up weight is plausible?


Related buzzword (not yet a tag): tankage fraction.
18-page analysis of this kind of propulsion for orbit transfers: http://www.umich.edu/~peplweb/pdf/AIAA-96-2973.pdf

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  • $\begingroup$ Is that the one using the solid iodine propellant instead of xenon? Could be wrong. $\endgroup$ – Magic Octopus Urn Oct 12 at 1:48
  • $\begingroup$ I hadn't heard what kind of thrusters. Sounds intriguing, though. $\endgroup$ – Camille Goudeseune Oct 12 at 2:37
  • $\begingroup$ It may not be this one- but I know that was a newer paper about the feasibility of solid iodine propellant as a stand in for Xenon at a small loss in ISP. Would suprise me if someone was trying to test it- seems like it mitigated a lot of storage issues at the cost of complexity in the thruster itself. $\endgroup$ – Magic Octopus Urn Oct 12 at 3:19
  • $\begingroup$ Atmospheric braking is not mentioned in this press release, nor is it discarded, still its an option. $\endgroup$ – qq jkztd Oct 12 at 12:28
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The amount of propellant required to achieve a certain delta-V is dependent on the ratio between the starting and ending mass of the spacecraft, according to the Tsiolkovsky rocket equation; a given thruster and fuel supply will get you more delta-V on a smaller spacecraft and less delta-V on a larger one. That is, 0.058 km/s per kg is not an inherent property of DS1's ion thruster.

Furthermore, 3 km/s is what you need to get from Earth escape to Mars orbit; if the launcher can put the spacecraft on a Mars intercept trajectory, the spacecraft itself needs only about 1.5 km/s of delta-V to get into orbit.

By the rocket equation, I think you need only around 3 kg of propellant with a 25 km/s exhaust velocity ion engine for a 50 kg spacecraft to obtain the required delta-V to get into orbit.

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  • $\begingroup$ 1, Thanks for catching my oversimplifications. 2, A pity the MarCO's didn't have a few extra kg to become orbiters themselves! $\endgroup$ – Camille Goudeseune Oct 11 at 20:58
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    $\begingroup$ The fixed mass of the thruster and, especially, its power supply does tend to dominate the mass of propellant at such small sizes; a chemical thruster might be more capable in a 50kg spacecraft despite having only 1/10 the specific impulse. $\endgroup$ – Russell Borogove Oct 12 at 3:57

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