Just catching up with the B52 forum and came across this. Somebody asked if the X15 could be seen under the wing of the B52 prior to launch....
Well I have not tried it yet, but I guess you could utilize the 'Delay' parameter in the 'GUN' section if you are using the rockets as the fire method. If you add in a suitable figure like 300 it will be visable for 5 mins before it launches After you press the fire button.

<Gun>
          <Type>RocketLauncher</Type>
          <Name>X-15c</Name>
          <Desc>X-15c Air launched test vehicle</Desc>
          <HudString>X-15c</HudString>
          <Pix />
          <ConeAngle>0.005</ConeAngle>
          <TargetRange>2500</TargetRange>
          <MaxGForce>10</MaxGForce>
          <Delay>0</Delay>           <FxFire />
          <Sound

You would of course have to ensure that the actual position was roughly correct under the wing.

Just a thought..

Pete