I was delighted to see a post from remcook elsewhere, I didn't know if he was still with us. So I did a search on his posts to see how long he's been back, and discovered this thread, which I missed before . . .<br /><br />So I'm not trying to one-up anybody, but . . . in the interest of accuracy . . . you know how it is around here . . <img src="/images/icons/smile.gif" /><br /><br />It's not that un-simple . . . <br /><br />The burn to raise the orbit actually slows you down. A higher orbit has less velocity, so <i>less</i> kinetic energy. Weird, huh?<br /><br />It took me a long time to figure this out, but look up the "vis-viva equation". It shows, not so transparently, that when you raise an orbit, you increase the potential energy by twice the amount that you decrease the kinetic energy. <br /><br />Of course it is true that you cannot discontinuously change the potential energy, but that's not required to increase the total orbital energy. The potential energy of the bound orbit is increased, but you do not see the greater distance from the orbited body until after periapse. (It is not the same thing as a block of matter statically held in a gravity field.)<br /><br />But to the question at hand, the reason the burn at periapse is more effective is it minimizes gravity losses. <br /><br />Anytime you burn your spaceship's engines in a gravity field, some of the energy is "wasted". The losses increase as a function of the time of the burn, the angle from the local horizontal, and decrease as the radius of the orbit during the burn. A periapse burn is going to have essentially zero angle, and we are asking what's the difference between periapse and apoapse for the same burn time. So what matters is the orbital radius. It's a minimum at periapse, so that's when you want to do the burn.<br /> <div class="Discussion_UserSignature"> </div>