With the latest updates to X-Plane for iPad, you can now purchase the Apollo lunar lander game inside X-Plane.
A quick note: if you lose your preferences (and thus X-Plane thinks you have not already bought Apollo) you will not be charged a second time if you click "buy it" again. When you go to repurchase Apollo, X-Plane will notice that you already have bought the upgrade and will simply re-enable the update.
Hopefully in our next patch we will make it clear in the UI that there is no double-purchase. The upgrade to Apollo is a "persistent" in-app purchase - once you buy it, you own it forever, as you would expect. (And because the iTunes store has a copy of your receipt, you don't lose it even if your iPad or computer's hard drive gets wiped out.)
Also, the latest update should fix crashes on iPads that were running for a while. X-Plane was very close to the memory limit so iPads that had been running for a while wouldn't have enough free RAM for the sim. The new patch is a little bit leaner to work around this problem.