Author Topic: Ethane rather than methane for the CEV?  (Read 3186 times)

Offline CuddlyRocket

Ethane rather than methane for the CEV?
« on: 11/12/2005 08:46 AM »
I was thinking about the current CEV plan that calls for LOX/CH4 engines.  This is apparently primarily based on the fact that most plans for early manned Mars missions call for the production of CH4 fuel for the return craft from combining feedstock H2 brought from Earth with CO2 from the Martian atmosphere.  There has never been a LOX/CH4 engine, so designing and building one for the CEV is seen as one way of obtaining synergies in the development of the architecture for future beyond-LEO manned missions.

CH4 does have other advantages over H2 for lunar missions.  Although not as good a rocket fuel (a vacuum ISP of 386 as opposed to 469), it is a liquid at much higher temperatures - a fuel temperature of about 112K as opposed to 20K.  This is an easier temperature to maintain (to prevent boil off), especially over the proposed six-month duration missions, which translates into lower mass, volume and power requirements for the refrigeration equipment.  (Also, H2 will diffuse through the walls of the fuel tanks much more readily that CH4, which reduces losses further.)

It is also much denser (423 versus 70 kg/m^3), which translates into smaller, less massive fuel tanks.  (As the tanks on the CEV are relatively small to start with, the weight of the tank is a higher fraction of the total tank + fuel weight, so any savings here are proportionally greater.)

Thinking about those advantages, it appeared to me that C2H6 would likely be a superior fuel choice to CH4 for lunar missions. It is only slightly poorer as a fuel (a vacuum ISP of 384), but has a temperature of about 184K, and a density of 544 kg/m^3. (Anybody good at calculating payload mass fractions confirm this?). But this is providing it did not compromise future Mars missions, or at least the gains to the lunar missions outweighed the disadvantages.

So far as the production of C2H6 as opposed to CH4 on the Martian surface is concerned, a given mass of H2 produces four times the mass of CH4, but five times the mass of C2H6, and all the other advantages of temperature and density apply. However, CH4 is easily produced from H2 and CO2 using the Sabatier reaction – a simple, one-step exothermic reaction.  Subsequently converting CH4 to C2H6 appears (to this non-chemist!) to be a non-trivial problem, and I haven’t found anything on directly producing C2H6 from H2 and CO2. Unless someone with greater knowledge can ‘crack’ this problem, using C2H6 seems a non-starter for early Mars missions.

Can a rocket engine design using C2H6 as fuel be relatively easily and cost-effectively converted to one using CH4? History would tend to suggest not. Could cost savings on lunar missions ouweigh the cost of developing two engines? Engine development seems extortionate. And what would any lunar cost saving be?

Anybody got any comments or brilliant ideas? (I’m hoping to learn something regarding the considerations that going into mission rocket design.) Otherwise it would appear that NASA was right to opt for CH4 in the design of the CEV (who’d’ve thunk it :)).