Mars-Venus Shuttle could work

In theory, perhaps.  The purpose here is not to analyze the merit of the idea, but to explore its feasibility in terms of a few assumptions.

Assumptions:

1. Use the external tanks of the SLS for storing reaction mass collected from Venus' upper atmosphere.
2. Reaction mass of sulfuric acid and carbon dioxide can be recovered from Venus' upper atmosphere.  Sulphuric acid can be heated to remove the water.  The acid can be dumped back into the Venusian atmosphere.
3. The SLS external tanks are not significantly different from the Shuttle's.
4. The tanks can be modified to store propellants indefinitely.
5. A nuclear thermal rocket with an ISP of 1700.  Such a high ISP would be achievable with monoatomic hydrogen.  Monoatomic hydrogen would be obtained from irradiating hydrogen with gamma rays as they are passing through the heat exchanger atomic core.
6. Aerobraking to save fuel
7. Calculations allow for some leeway for moving nuclear engines around for repairs and replacement.

 With these assumptions, I came up with the following spreadsheet calcs using the rocket equations:



The results are a system that can deliver LH2 from Venus on a regular basis----every 22 months.  The LOX tank can be used for Methane/LOX as well as additional LH2.

That's good enough for a regular launch window from Earth, which is a little over 2 years.  Thus, a Mars-Venus shuttle could support the fuel needs of a Earth to Mars shuttle.  This would include propellant for landing.  Maybe even for landing and returning to Mars orbit!