1. Are there better ways to build a stave?

2. 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.

3. 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

4. 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

5. 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

6. 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)

7. 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…

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

9. 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

10. 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

11. 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

12. 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: e+04mm^3 = 20.2cc = 37.12g of CFRP (rho=1.838g/cc)

13. 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

14. 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) *(102/120)= 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.

15. 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

16. 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)

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