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7 Oct 2012, 18:56 UTC
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Mf is a MofoThe Tyranny of the Rocket Equation is an excellent article by astronaut Don Pettit written while he was aboard the I.S.S..The foundation of the article is this version of Tsiolkovsky’s rocket equation:Mf = 1 - e-delta v/vexhaustMf is the fraction of the spaceship’s mass that is propellant.Vexhaust for chemical propellant ranges from 3 to 4.5 km/s. Delta V to get into orbit is around 9 km/s.Plugging these in we can see a spaceship must be more than 80 percent propellant. Pettit notes The 3-stage Saturn V rocket was 85 percent propellant and the Soyuz rocket 91 percent.Pettit explains this sort of mass fraction is very challenging and drives up engineering expense:If a vehicle is 10 percent propellant, it is typically made from billets of steel. Changes to its structure are possible without detailed engineering anyalysis: you simply weld on another hunk of steel to reinforce the frame according to what your intuition might indicate. I can easily overload my three quarter ton pick-up truck by a factor of two... Once vehicles become airborne, engineering structures become more serious. Lightweight structures made of aluminum, magnesium, titanium, and composites of epoxy-graphite are the norm. To alter a structure requires ...

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