at.algebraic topology - What is $TC(Sigma^infty Omega X)$?

The TC spectrum, at a prime $p$, of this is the homotopy pullback of a diagram

$S^1 wedge (Sigma^infty_+ Lambda X)_{hS^1} to Sigma^infty_+ Lambda X leftarrow Sigma^infty_+ Lambda X$

after $p$-completion. Here the left-hand map is the $S^1$-transfer from homotopy orbits back to the spectrum and the right-hand map is the difference between the identity and the "$p$'th power" maps on the loop space.

This is in Bökstedt-Hsiang-Madsen's original paper defining topological cyclic homology, in section 5.

ADDED LATER: This doesn't really work on the space level, because they don't have all the structure necessary. They have the $F$ maps, but not the $R$ ones which only come about from stable considerations. Spaces with a group action really only have one notion of "fixed points," namely the honest fixed points of the group action.

However, the associated equivariant spectrum of $Lambda X$ is built out of spaces like

$$Omega^V Sigma^V Lambda X = Map(S^V, S^V wedge Lambda X_+)$$

where $V$ ranges over representations of $S^1$. This has two "fixed-point" objects for any cyclic group $C$: there's the fixed points, which is the space

$$Map^C(S^V, S^V wedge Lambda X_+)$$

of equivariant maps. There is also the collection of maps-on-fixed-points

$$Map((S^V)^C, (S^V wedge Lambda X_+)^C)$$

which is called the "geometric" fixed point object, and it accepts a map from the ordinary fixed points. The fact that $(Lambda X)^C cong Lambda X$ implies that you can interpret this as a map $(Q Lambda X)^C to (Q Lambda X)$ where the latter uses an accelerated circle. These maps give rise to the $R$ maps in the definition of $TC$, and they definitely rely on the fact that you're considering the associated spectra.