they state 40 mile range what do you suppose that the range would be with a something in the way?
the XStream website states """Indoor/Urban range: Up to 1500 ft (450 m)""" so that would be your worst case scenario.
i presume the non LOS issues arise when your launch site is out of sight of the bunker where you sit?
what about using several of these modules to relay the signal to the launch site?
look at some of the XBee mesh modules for this as i *think* they can be used as repeaters with little extra work.
lower frequencies will stand up to objects in the way better than high frequencies but at a cost of lower datarates.
search DigiKey for low frequency RF modules.
the only other thing i can think of is build a directional antenna to improve range. obviously you would need to track the rocket with the antenna.
i know there are lots of instructions for 2.4GHz directional antenna on the internet from people trying to improve WiFi range but at 900MHz you might need to buy a book on the subject.
the only other option i can think of is getting yourself a Ham radio licence so you are not bound by maximum power levels (or be a rebel and ignore the restrictions) and look into adding an RF amplifier to an existing RF module.