Are there better ways to build a stave?

Background The basic stave design has been unchanged for sometime. And represents the standard way a honeycomb panel would be built The LBL pixel I beam stave motivated consideration of an alternative stave structure The goal would be to remove the core material, and instead use temporary void fillers instead of a core.

What is “wrong” with the present stave design? Will have large thermal margins when looking at 130nm ASIC, there is no obvious way to efficiently reduce thermal performance. The skins are not yet smooth, so we need a lot of glue to stick down the module The core is a large part of the cost of the stave and mechanically does very little once the stave is complete The present design is not optimised for torsional stiffness which would be useful for edge and end mounting

What an alternative stave might look like Several tubes of CFRP running the full length No core, the support needed to mount modules would be temporary No edge close outs they are built in End close outs are unclear Fibres are all at ±15° to honour 25mm bend radius

Initial structure – no thought to layups 200mm long All laminates +15,-15 0.14 degrees twist in 200mm – 0.85 degrees twist in 1.2m – one corner sits 1.5mm high

Better laid up structure Changing the skin layup so the skins and tube counter should improve things Tubes are +15,-15 degrees Skins are -15, 15 degrees Max to min deflections are similar to above but as can be seen below most of the motion is in the last few cm. Looking at just the centre 10cm and scaling we get a twist of 2mm on one corner on a 1.2m stave Strange effects can be seen in the last 20mm of the stave which might be hiding other effects (next page)

Better laid up structure #2 Stave is now 500 mm long Total twist is 1mm, 0.52° which will scale to 2.45mm or 1.2° for a full stave. Thus the effects shown on the previous slide are real, and are not end effects. So the more logical layup mat not help – perhaps I have made a mistake…

Maybe I got confused… lets look at the parts. Outer tube #1 Outer tube #2 Centre tube Skin

So.. Skin #1 Outer tube #1 Centre tube Skin #2 Outer tube #2 The skin moments clearly dominate… It might make more sense to balance the two opposing skins and the outer two tubes. Skin #1 Outer tube #1 Centre tube Skin #2 Outer tube #2

Results 500mm stave with central tube omitted Again we get small anomalies at the ends. Ignoring these we get 600 microns out of flat along a 1.2m stave 0.31° degrees of twist along the length Also ran without the central tube which should come out flat (or very close) 500mm stave with central tube omitted 500mm stave with all tubes in place Note: the constraints are slightly different in this, thus the bulges at the left end

Two central tubes So asymmetric laminates work, but a single unbalanced central tube causes an issue – one solution would be a double central tube with cancelling forces. Inner tube #1 Total twist of stave is 0 – local deformations are either end constraints or local deformation due to end of structure Inner tube #2

How good is it? Deflection of ~260 microns with 1000 mm kg/sec^2 => 9.81N load on a 500mm stave This is achieved with: 2.0261713e+04mm^3 = 20.2cc = 37.12g of CFRP (rho=1.838g/cc)

Comparison with standard stavelet Bending Stiffness I think your load is wrong… 1000mmkg/s2 sounds like 10N to me not 1?? Assuming I’m right.. /PL = 260/10/500 = 0.052 L^2 = 250,000 Equivalent stiffness to normal stave Note… this HAS to be about right as you’ve got fibres at +/- 15 running down the length so the properties of the face sheet (in that direction) will be similar to ‘normal’ staves

Logic behind bend measurements I have 2 15° plys, 50microns thick – the pair resolve to the equivalent 96.6microns The top skins of the tubes are similar and cover 68mm of the width – again 2 15°plys Stavelet skin width = 120mm So we have 96.6*(68/120) + 96.6*(102/120)=136.85 equivalent 0° fibre Stavelet 4 and 5 have 2 ply of 65 micron skins = 130 of 0° fibre We could expect that the hollow stave in FEA would be 95% as stiff as the stavelets as built in Liverpool Thus we should expect it to fall much in line with the stavelet tests to date as shown on slide 13 – which it does.

Stavelet 3 & 4 Masses Component Stavelet 3 Stavelet 4 Difference Mass (g) Mass of glue used (g) Total Mass (g) Facesheet 1 20.14 2.88 23.02 20.36 2.9 23.26 0.24 C-channels 4.15 27.17 2.21 25.47 -1.7 Pocofoam bases 12.73 39.9 13.54 39.01 -0.89 Mass lost from milling -3.64 36.26 -3.3 35.71 -0.55 Glue in Pocofoam base 1.62 37.88 1.52 37.23 -0.65 Cooling tube 37.83 75.71 37.82 75.05 -0.66 Pocofoam Lids & glue 11.02 1.91 88.64 12.2 1.5 88.75 0.11 CF Honeycomb 7.06 1.87 97.57 7.69 3.4 99.84 2.27 Z=0 closeout 1.3 0.05 98.92 1.53 0.35 101.72 2.8 Z=500 (4) closeout 5.01 1.04 104.97 5.9 107.62 2.65 Mass lost from grinding -1.23 103.74 -2.19 105.43 1.69 Glue from side 1 dip 1.54 105.28 6.79 112.22 6.94 Facesheet 2 19.79 2.09 127.16 20.26 2.43 134.91 7.75 Mass lost from clean up -0.08 127.08 -0.04 134.87 7.79 Final Build 114.08 13 115.98 18.89

Traditional Stavelet mass = 20.14 facesheet mass #1 4.15 c channels 19.79 facesheet mass #2 40.08 total (ex glue) 6.29g glue (HC to skins + C channel to skins)

Next steps Stiffen skins so they are more comparable with standard stavelet Look at thermal properties Do proper material comparison