Is the LRASM virtually undetectable? It is the context that is important. The F-22 is generally considered to be an order of magnitude more stealthy than the F-35. According to the AWST pictorial, the S-400 has a engagement range of 34 kms for the f-35 as opposed 21 kms for the F-22. This is a function of their RCS difference
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The LRASM's RCS is classified and so I don't know how stealthy it really is. Given that the F-22 is at least one order of magnitude lower than the F-35, it is not inconceivable that the LRASM has two order of magnitude or even three lower than the F-35. Translated this means the following engagement range :
2 X = 11 kms
3 X = 7 kms
At this range, the engagement window is extremely small. The prospect of collateral damage on the target is very high even if the LRASM is destroyed prior to impact.