<i>> <br />I really wonder if a 100 ton launcher is needed for the Elon Musk dream of manned space travel. Would the extra cost of development be worth it to move from a 50 ton class launcher to a 100 ton class launcher? I think a 50 ton launcher is more than enough and development beyond that should be focused on advanced concepts such as Solar Thermal Propulsion or orbital refuelling if Elon Musk want the best bang for his buck.</i><br /><br />SpaceX isn't going to operate the fuel depot. They fly rockets. Lockheed is going to develop the fuel depot. SpaceX is going to make a ton of money keeping Lockheed's depot full. 100 tons of propellant for sale for marginal cost over 50 tons (from the rocket operator's perspective) is golden opportunity. I'm guessing they'll move to a standard core with a single F1 class ultra-reliable engine, then common-core stage them. <br /><br />SpaceX has positioned itself as the provider that can bring spaceflight to the next level. They have always been explicit about tackling the "first mile" issue, which means they are going to primarily be a launch operator. I can see them getting into advanced upper stages, but not building habitats (or SPS dev, or whatever). If you've followed anything about the Lockheed fuel depot, you know that it is going to require a lot of flights, and SpaceX is going to be able to out-perform Atlas for that profile, if the Falcons work. <br /><br />I agree about doing space development starting from what's currently available. What can you fit in a 20 ton Proton launch? What can you fly on Atlas? That is where the Bigelow-Lockheed capsule is interesting, and their modular lunar architecture. What can you do with commercially available upper stages like Blok DM and STAR? Can you safely do humans-to-the-moon with the same hardware we currently fly for GEO sats?<br /><br />One last question: if you could refuel it onorbit, would it make sense to use one of these engines on a huge transfer stage? That leads to Elon' <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>