<font color="yellow">Can you explain why, In layman's terms please.</font><br /><br />Shuttle_Guy summarizes well in one sentence ---- /> less structural weight, therefore more efficient.<br /><br />By using a "cryo" nitrous oxide (actually "cryo" anything), it reduces the density, hence volume, of the its oxidizer tank. <br /><br />A smaller volume tank means either a smaller diameter, or a shorter, rocket. It reduces the cost of fabricating the rocket. If you keep the same size tank, then you can load MORE propellant, therefore can burn the rocket LONGER, thus the claimed higher delta-vee.<br /><br />But its Isp does suffer a little bit because the oxidizer comes into the combustion at lower temperature (at "cryo" temperature), the combustion must first "raise" the cryo to a high enough temperature before this oxidizer is ready to "react" with fuel. At the end, you do get a slightly lower combustion temperature, so a lower Isp. This is a chemistry thing.<br /> <br />In the case of a first stage launch vehicle, the benefit of denser propellant out-weighs the benefit of a slightly lower Isp. Thereby making it a more efficient stage.<br /><br />This is NOT NEW, by the way. It's the same argument why LOX/ RP-1 is a better propellant combination for a first stage than a LOX/LH2. Compare Atlas V and Delta IV and see how "small & short" the Atals V first stage is?<br /> <div class="Discussion_UserSignature"> </div>