Just watched a small explanatory video by Bill Nye on the Planetary Society’s LightSail 2 spacecraft (not exactly the same one as the one attached to this post but essentially the same). And by the end some voice in my head was shutting out Bill.
This is what it was saying.
- 1 – That lightsail material in the demo looks ‘crinkled’ as in – the foil is not stretched taut at all.
- 2 – The actual momentum of photons is quite small so a maximal energy received => propulsion produced ratio would be desired. Er=>Pp as close to 100 % as possible.
- 3 – A small force impressed on different parts of an uneven, and not taut, surface might get spent simply rearranging the flatness of that surface with a consequent reduction in actual delivered momentum to the spacecraft itself. Let’s call that Mechanical absorption. So Pp = Er – Ma
- 4 – A stiff surface might reduce losses due to Ma.
- 5 – How to produce a very light, large, stiff surface in space?
Would there be enough delivered momentum to actually stretch a foil sail taut?
And if that was possible would a flexible material like a foil surface not also be subject to mechanical waves due to continuous driving inputs of momentum due to photon impacts?
If the wave length of any induced acoustical/mechanical phenomena approached some integer fraction, or multiple, of a major surface dimension it could result in destructive (or wasteful) harmonics.
In the video Bill says
and so we found that the sales do billow a little bit just like sales on a sailboat
From the video at about 2:20
My guess is that billowing would come from either unspent mechanical vibrations in the structure or induced mechanical impulses unevenly distributed. If the latter then that energy used up in material deformation (billowing) is energy not used for propulsion. Much like the sound coming from a motor represents energy lost from the system.
How do you make a surface that is acoustically immune to momentum sucking vibrations and stiff and light and properly able to absorb as much photon momentum as possible and transfer that into propulsion momentum as efficiently as possible?
There’s a lot of ‘and’s in that question . . .
Please don’t misunderstand me – I’m not criticizing Bill or the Planetary Society or any researchers hoisting lightsail powered craft into space. Bravo to them all – I applaud their efforts immensely.
But I just wonder if the system could be made more effective with better applied materials science from the start.
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