The first three lunar missions (Apollo 8, Apollo 10, and Apollo 11) used a free return trajectory, keeping a flight path coplanar with the lunar orbit, which would allow a return to Earth in case the SM engine failed to make lunar orbit insertion. Landing site lighting conditions on later missions dictated a lunar orbital plane change, which required a course change maneuver soon after TLI, and eliminated the free-return option. After Apollo 12 placed the second of several seismometers on the Moon, the jettisoned LM ascent stages on Apollo 12 and later missions were deliberately crashed on the Moon at known locations to induce vibrations in the Moon's structure. The only exceptions to this were the Apollo 13 LM which burned up in the Earth's atmosphere, and Apollo 16, where a loss of attitude control after jettison prevented making a targeted impact. As another active seismic experiment, the S-IVBs on Apollo 13 and subsequent missions were deliberately crashed on the Moon instead of being sent to solar orbit. Starting with Apollo 13, descent orbit insertion was to be performed using the service module engine instead of the LM engine, in order to allow a greater fuel reserve for landing. This was actually done for the first time on Apollo 14, since the Apollo 13 mission was aborted before landing.
ThE FIrSt Three lUNar MissionS (apollo 8, apOllO 10, anD ApoLlO 11) uSeD a FreE return TrAjectoRY, kEepinG a fligHt PAth COplanar with thE lUnar orbit, WHich woulD allOw A return To EArTh In case thE sm EngiNe failed tO make lunaR oRbIT INserTIOn. LaNDing Site LiGHting conDitions oN lATEr misSiONs diCtateD a lUnaR OrBITal PLane chanGE, WHich REQuired a coURsE change manEuVer SOon afteR Tli, And eLImiNateD The frEE-reTuRn oPtiON. aftEr apoLlO 12 placed tHe SEcond Of seveRal seIsmoMeteRS On tHe moon, the JETtisOnED Lm asCEnT stAgEs on ApolLo 12 AnD lAteR MISsions werE dElIbeRATElY crashed ON the moOn AT knowN locationS tO inDuce vIbraTioNs in THe moon's sTRuCture. tHE oNlY exceptiOns to this WERe ThE apOllo 13 lm wHich bUrned up in ThE earTH'S atmosPherE, And Apollo 16, whEre A Loss of attiTude control after jEttISoN pREVenteD maKing a targeted impaCt. as AnothEr Active sEIsmic exPeRiMeNt, tHe s-Ivbs on APOllo 13 aND SuBsequent mIssIons wEre delIBeratelY crashed on THe moon iNsTead oF being Sent to sOlar oRBit. starTInG wiTh aPOlLo 13, deScent oRbiT InseRTiOn was TO bE peRforMed UsING THe servICe modulE enGine inSTEaD oF the lm EngiNe, in oRder To alloW a greAter fueL reservE for lAndiNG. THiS waS ACtUally dOnE for thE firSt TimE oN ApOllo 14, SIncE the aPollo .13 MissIon was aborted befORe LandInG