  Emerging studies are geared toward exploring new methods of nuclear rocket
propulsion to provide more efficient space transit beyond Earth's orbit. One
method is to employ a Fission Fragment Rocket Engine utilizing fissionable
layers embedded in a low-density aerogel. A quantitative understanding of
particle attenuation is essential for developing a functional prototype that
permits fission fragments to escape the layers and contribute to specific
impulse rather than being attenuated and generating waste heat. In this
investigation, the MCNP code was used to theoretically analyze the attenuation
of alpha particles from Am-241 sources within aerogel materials. Simulations
were conducted on aerogels with various densities and compositions. These
simulations aimed to predict the expected intensity of alpha particles reaching
a detector. CR-39 was employed as a Plastic Nuclear Track Detector to assess
particle attenuation by the aerogels. The threshold areal density of atoms was
found to be around $10^{ 20}$ atoms/$cm^{ 2}$ for the three materials studied
in this project. Using a 0.22 mm thick SiO$_2$ aerogel with a density of 90
mg/$cm^{ 3}$, which exceeds the threshold, nearly all alpha particles were
attenuated. Conversely, employing a 1.6 mm thick graphene aerogel with a
density of 12.5 mg/$cm^{ 3}$ resulted in an average attenuation of 32.3%.
