"The FSW satellites used a 5.9-inch thick heat shield made of oak. During reentry, friction caused the wood to burn and char, leaving behind a layer of charcoal." "Though the FSW series began in the mid 1960s, it only made its first flight a decade later." https://vintagespace.wordpress...

Here's how to build your pet door.

Buy one of these for $115
http://www.allflex-boulder.com/OemModule.htm
It has TTL serial output that sends the number from the tag in your pet when it's within range.

Here's a pet door made from an automotive window motor, but doesn't use RFID to trigger it.
http://www.parallax.com/html_pages/resources/custa pps/app_doggy_door.asp

Add your favorite microcontroller (a Basic Stamp would do the trick).