Re: [Orekit Users] picosatellite biomagnetorquer experiment - request for answers/ideas/comments

[Luc Maisonobe <Luc.Maisonobe@c-s.fr>]

Le 14/06/2012 13:03, Michael Turner a écrit :
> Dear Orekit list members,

Hi Michael,

> 
> I have a question about Orekit capabilities. Zac Manchester is copied
> on this e-mail because he's making the relevant spacecraft; Kartik
> Madiraju is copied because I've engaged him on related (but somewhat
> tangential) issues related to magnetorquing. Please remove them from
> the CC list if they ask to be dropped.
> 
> I've been contributing to the Orekit wiki, but so far only at the
> level of copyediting -- fixing typos, small grammatical errors, etc.
> I'd hoped to learn more orbital dynamics by osmosis, so that I could
> answer any questions I might have on my own. Unfortunately, I have
> several other projects competing for my time. At this point, I can
> only say that helping with the Orekit wiki has improved my orbital
> dynamics *vocabulary* a little, while also helping me appreciate what
> a complex subject it is.

First, of all, I would like to thank you very much for the work you have
already done on Orekit. Documentation contribution are particularly
difficult to get, so this is really appreciated.

> 
> Orekit might be very useful for one of my current projects. However,
> I'd like to be sure, before committing to Orekit.
> 
> The project involves KickSat sprites - picosatellites launched from a
> Cubesat. For a quick overview, see the Wikipedia article about
> KickSat:
> 
>    http://en.wikipedia.org/wiki/KickSat
> 
> and the video in the original Kickstarter solicitation
> 
>    http://www.kickstarter.com/projects/zacinaction/kicksat-your-personal-spacecraft-in-space
> 
> I have donated enough to KickSat qualify for the launch of a
> customized sprite. I want to make this spacecraft part of a science
> education project here in Japan, and in some other countries if
> possible.
> 
> My specific project involves attaching a lightly magnetized strip to
> the sprite before launch, then measuring and transmitting the amount
> of magnetorquing the sprite undergoes before it deorbits. For two
> reasons, I'd like to be very sure of the theoretical *minimum* magnet
> strength that could cause any measurable change in orientation.

This sound really interesting.

> 
> (1) a magnet that's too strong might be disqualified as potentially
> interfering with other sprites in KickSat;
> 
> (2) I hope to use biologically derived biomaterials (bacteria or algae
> containing magnetosomes) to make the strip a magnet; the maximum
> achievable strength might be quite low.
> 
> Because these sprites have a very high ballistic coefficient, and will
> be ejected from the Cubesat at around 300 km (?), they will deorbit
> fairly soon (weeks?). So it's important to be able to filter out any
> attitude changes that are due only to increasing atmospheric drag.

Does KickSat provides some attitude-related information in its telemetry
? Without attitude control, it will probably be tumbling and both drag
force and radiation pressure will change a lot in amplitude and
direction, so geometric information would be really important to
evaluate all the forces and torques.

Will it also be possible to get some orbit measurement for your specific
sprite (or at least the swarm globally) once it is launched ?

> 
> Orekit appears to have models for atmospheric drag (though not much
> wiki documentation yet on this?), and even the beginnings of a
> tutorial about geomagnetic field modeling. In exchange for help with
> questions, I would be more than happy to help flesh out any Orekit
> documentation that already exists on these two topics, as needed. I
> would also be happy to contribute to the Orekit documentation any
> tutorial examples that come out of my own project.

The current Orekit surface force models are more oriented towards center
of gravity motion (i.e force) rather than attitude (i.e torques).
Implementing complete torques models would imply evaluating forces on
each facet individually, which could be done by a specialized version of
the BoxAndSolarArraySpacecraft class. As KickSat seems to have a very
regular shape (basically a single flat board without any appendages
except for you additional strip), this should be simple but getting the
absorption/reflection coefficients for the various components may be
difficult. We would also need the inertia matrix and center of gravity
position. So we could probably set up some model, but to be fair I am
not sure they would be realistic.

If you have specific questions, do not hesitate to ask them on the list,
we will try to help as much as we can.

best regards,
Luc

> 
> Regards,
> Michael Turner
> Project Persephone
> Tokyo
>