It is enlightening to compare the function of the airplane as the first stage of a rocket with the first stage of the Saturn V rocket. The S-IC weighed 5 million pounds and it got the assembly to an altitude of 200,000 feet and a speed of 6000 mph. The second [and third] stage did most of the delta V. It got the Apollo into orbit and into trans-lunar injection (a bunch more delta V). In short, all the first stage did was to get the second stage up above the atmosphere so that the second [and third] stage could fly like hell.
EDIT: dalke pointed out that my assertion that the second stage reached orbit was incorrect. Thanks and apologies for the sloppy proofing.
EDIT: dalke pointed out that my assertion that the second stage reached orbit was incorrect. Thanks and apologies for the sloppy proofing.