Comments on 'First Interstellar Missions, Considering Energy And Incessant Obsolescence'

摘要

I note with interest the recent paper [1] by Millis. In particular I wish to highlight the discussion on the first launch of a Colony Ship being at least 200 years away due to the total energy expenditure in comparison to an average space shuttle launch. The analysis needs reworking due to an incorrect assumption. The author assumed a velocity increment of just 42 km/s using a rocket with an exhaust velocity of 9,000 km/s. This is far from optimal for the assumed velocity increment. Rockets achieve a maximum energy efficiency when the exhaust velocity is approximately 5/8 of the velocity increment i.e. a mass ratio of 4.92. Hermann Noordung [2] derived this as long ago as 1929, while Les Shepherd's seminal article, "Interstellar Flight" [3] published in this journal, also makes use of this theorem. Thus the actual velocity increment for the 4.8E18 J scenario assumed by the author of reference [1] is 500 km/s for a 25,000 tonne Colony Ship.