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If a cubesat is launched as secondary payload to LEO, what ways are there to put it in a higher orbit, for example outside of the van Allen belt to make a radiation experiment? The ISS launches cubesats and larger minisats. Can it help a sat leave LEO?

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    $\begingroup$ Related: Has any CubeSat flown with an active propulsion system? $\endgroup$ – Chris Jul 8 '15 at 12:37
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    $\begingroup$ @Chris Yes, I've heard that explosive rocket fuel isn't welcome in cheap secondary launcher payloads. That's why solar sail might prove to be a good idea even short-term commercially. $\endgroup$ – LocalFluff Jul 8 '15 at 12:48
  • $\begingroup$ More radiation => More protection => Heavier payload. Might be an issue for a 10x10x10 satellite $\endgroup$ – Antzi Jul 8 '15 at 16:03
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Engines are under development for CubeSats. For example there's the Electrospray (colloid) thruster which is effectively a low-thrust ion drive.

An alternative is to launch from the ISS with a catapult (eg railgun/coilgun) or a small single stage solid rocket. This would give an elliptical orbit, so only part of the time would be spent at the target altitude.

(Note that at the moment the CubeSat Requirements Document prohibits propulsion systems on CubeSats though. Presumably this would include ion drive as well as the more obviously hazardous solid rockets. So launching self propelled cubes directly from the ISS is really only a theoretical possibility at the moment.)

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    $\begingroup$ Also note that there may be limitations that may not allow you to use a propulsion system in your CubeSat. See point 6.2.5 of this NASA spec $\endgroup$ – neelsg Jul 8 '15 at 11:54
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    $\begingroup$ @neelsg "6.2.5. CubeSat shall not contain propulsion systems." :-) $\endgroup$ – LocalFluff Jul 8 '15 at 12:50
  • $\begingroup$ Thanks folks, I'll edit the answer to include this - it's a definite prohibition at the moment. :) $\endgroup$ – Andy Jul 8 '15 at 13:02
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    $\begingroup$ One other problem, since you mention high impulse launch catapults, is that momentum imparted on deployed satellites has to come from somewhere. In case of launching from ISS, that would be at the cost of ISS' own orbital energy, so it would have to perform orbital reboost to compensate. Launch and reboost would also result in a push on the station, so you'd have to secure and halt microgravity experiments. TL;DR About 1 m/s deployment and up to 64 N (48 x 1.33 kg for NanoRacks launcher) force is OK-ish, compared to ISS' 390 kN & 7.66 km/s. More mass and/or higher ∆v impulse,... not OK. $\endgroup$ – TildalWave Jul 8 '15 at 15:41
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    $\begingroup$ @RussellBorogove Actually, at such small impulse that current ISS small satellite deployers achieve, you can select if you wanna raise apogee or perigee. Back of an envelope calculation says that you'd get only about 1 km change at maximum, so less than ISS' orbital eccentricity. I know ... too geeky but I thought you might appreciate the irony. :) $\endgroup$ – TildalWave Jul 8 '15 at 18:24
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As long as there are ways to change its velocity, the CubeSat can move to a higher orbit.

However, the CubeSat deployers onboard ISS are most likely spring-driven and will not provide much delta-V for the job. If they equip ISS with a railgun and somehow find a way to deal with the reaction, it will certainly launch a CubeSat to orbit higher than LEO.

Using ion thrusters on a CubeSat will probably be more easier than the a railgun method.

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