There is even a "5.0" floating about somewhere! After 3.0 though the ideas moved off into different studies and no longer had the conherence of the original DRM. This tendenct is visible in the end of the 3.0 report which scope some alternative options also (nuclear and solar electric, inflatables).<br /><br />The problem with spin gravity is that it opens up a whole range of unexplored engineering and medical issues. Having it as a "must have" increases the development overhead for a Mars mission. The study assumes that this technology is simple and can be either developed cheaply or is already available.<br /><br />On Borowski et al's studies nuclear studies, the same is true. There are some very optomistic assumptions. It assumes as a baseline that a 950 Isp NTR is possible, that you can light up your NTR in LEO (400 km I think), and that NTRs can be clustered safely (earlier DRMs did this too). Some options discussed by Borowski et al. also assume that LH2 can be stored for years and that multiple NTR restarts over that period are possible. With these assumptions the EDS stage mas is about 62% of what would be possible with a chemical stage. Apologies if these numbers (and following) are a bit out, I am logging in from a conference and away from my usual resources! <br /><br />The problem is that these are <b>assumptions</b>. There is nothing wrong with this, Every study has assumptions, but they may or may not be reasonable. Analysis of any study must include an examination of the assumptions. For example the best Isp from an operating NTR is, as I recall, 920, but from Russian designs of the 80's. This is very poorly documented in the open literature. We must take on faith the assumption that this can be upped to 950 and meet modern safey and reliability standards. Clustering lightly shielded NTRs is a nighmare, apparently, because of neutron cross-coupling. It is an assumption that is problem will be solved or deemed irrelevant. Realistically nobod <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em> Arthur Clarke</p> </div>