I always question authority in matters of orbital mechanics. IMO the "experts" are typically more concerned with what you <b>cannot</b> do in space than in what you <b>can</b> do.<br /><br />So I wrote some software to figure out things for myself. It was a big project, but when things like this come up, I can quickly determine for myself the difficulties.<br /><br />It turns out the experts were right in this case <img src="/images/icons/laugh.gif" /> Mercury is very expensive in deltaV. The most fuel efficient transfer, the Hohmann transfer, takes about 110 days. It varies quite a bit because Mercury's orbit is quite eccentric. For one particular alignment, the trip takes 105.82 days and requires a departure dV of 7.4 km/s and an arrival dV of 14.4 km/s. These are large numbers.<br /><br />For that example, even if you were able to drop the stage which gives you the departure velocity, you find it is nearly impossible to build a vehicle to do the job.<br /><br />The mass fraction of the vehicle you need to build to arrive at mercury can be found by:<br />mo / mf = e ^ (dV / Ve) = e ^ (dV / (g * Isp))<br />mo= mass of vehicle fully fueled<br />mf = mass of vehicle after burning the fuel to achieve orbit<br />g = 9.81 m/s^2<br />Isp = 450 s for a really kick-ass rocket engine<br />mo / mf = e ^ (14400 / (9.81 * 450)) = e ^ (3.262) = 26.1<br /><br />Which means that the mass of your propellant needs to be 25.1 times as much mass as the rest of the vehicle. This is technically possible, but rather ridiculous in practice. Especially when you consider that the specific impulse is for LH2/LOX engines, any other propellant combo would be much worse, and LH2 needs really big tanks.<br /> <div class="Discussion_UserSignature"> </div>