Of primary concern was the pogo oscillation, which would not only hamper engine performance, but could exert significant g-forces on a crew.
ʌv pɹˈaɪmɚɹi kənsˈɜːn wʌzðə pˈoʊɡoʊ ˌɑːsɪlˈeɪʃən, wˌɪtʃ wʊd nˌɑːt ˈoʊnli hˈæmpɚɹ ˈɛndʒɪn pɚfˈoːɹməns, bˌʌt kʊd ɛɡzˈɜːt sɪɡnˈɪfɪkənt dʒˈiːfˈoːɹsᵻz ˌɔn ɐ kɹˈuː.