  We present the simulation tools developed to aid the design phase of the
Cassegrain U-Band Efficient Spectrograph (CUBES) for the Very Large Telescope
(VLT), exploring aspects of the system design and evaluating the performance
for different design configurations. CUBES aims to be the 'ultimate'
ultraviolet (UV) instrument at the European Southern Observatory (ESO) in terms
of throughput, with the goal to cover the bluest part of the spectrum
accessible from the ground (300 nm to 400 nm) with the highest possible
efficiency. Here we introduce the End-to-End (E2E) and the Exposure Time
Calculator (ETC) tools. The E2E simulator has been developed with different
versions to meet the needs of different users, including a version that can be
accessed for use by the broader scientific community using a Jupyter notebook.
The E2E tool was used by the system team to help define the Phase A baseline
design of the instrument, as well as in scientific evaluation of a possible
low-resolution mode. The ETC is a web-based tool through which the science
community are able to test a range of science cases for CUBES, demonstrating
its potential to push the limiting magnitude for the detection of specific
UV-features, such as abundance estimates of beryllium in main sequence stars.
