OK... as long as we're being anal about this.... <img src="/images/icons/smile.gif" /><br /><br />Historically, solids were never referred to as "engines" but rather as "motors", e.g., solid rocket <i>motors</i>, or solid rocket <i>boosters</i>. Often only the pump operated liquid propulsive devices were referred to as <i>engines</i>, e.g., liquid rocket engines, but never liquid rocket <i>motors</i>. <br /><br />Small thrust, liquid propulsive, devices were referred as <i>thrusters</i> and not engines, e.g., attitude control thrusters. Whereas solids are once again referred as <i>motors</i>, e.g., apogee motors. <br /><br />Booster is often referred to the first stage of a <i>launch vehicle</i>. So the SRB is a booster, but since the SSMEs were on during the boost stage, I guess it can be called a booster engines as well. It's actually more like the a parallel burn 2nd stage engine but people use these nomenclature loosely :shrug: <div class="Discussion_UserSignature"> </div>