ðə sˈɪmpəlɪst dɪzˈaɪn hɐz ɐ pɹˈɛʃɚɹˌaɪzd wˈɔːɾɚ tˈæŋk wˌɛɹ ðə wˈɔːɾɚɹ ɪz hˈiːɾᵻd bᵻfˌoːɹ lˈɑːntʃ; haʊˈɛvɚ, ðɪs ɡˈɪvz ɐ vˈɛɹi lˈoʊ ɛɡzˈɔːst vəlˈɑːsᵻɾi sˈɪns ðə hˈaɪ lˈeɪtənt hˈiːt ʌv vˌeɪpɚɹᵻzˈeɪʃən mˈiːnz ðæt vˈɛɹi lˈɪɾəl ˈæktʃuːəl stˈiːm ɪz pɹədˈuːst ænd ðɪ ɛɡzˈɔːst kənsˈɪsts mˈoʊstli ʌv wˈɔːɾɚ, ɔːɹ ɪf hˈaɪ tˈɛmpɹɪtʃɚz ænd pɹˈɛʃɚz ɑːɹ jˈuːzd, ðˈɛn ðə tˈæŋk ɪz vˈɛɹi hˈɛvi.
The simplest design has a pressurised water tank where the water is heated before launch; however, this gives a very low exhaust velocity since the high latent heat of vaporization means that very little actual steam is produced and the exhaust consists mostly of water, or if high temperatures and pressures are used, then the tank is very heavy.