I have crafted it one time and one time I have added this in my inventory. I dont know but I cant fly with this. The Engine is on (green text in chat say this to me) but with double jump it doesnt work on both SSP / SMP.
I have this problem too 
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Because it's very advanced tech
No cheap craft recipe.
Yep, but this need very long playing Minecraft. 
41 diamond for GraviChestPlate and 10 diamonds for Ultimate Lappack (normal). I count diamonds, and this approximate numer of diamonds. 41 diamond is a very huge size for this.
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