I built one a couple of years ago to fit Estes BT-50 (or similar) body tubes. I used a freescale MPX2100 pressure sensor

http://www.freescale.com/files/senso...00.pdf?pspll=1

The problem you may face is the output is analogue and the 16f84 has no ADC. I would recomend a different PIC, which as a 10-bit (how accurate do you want it?) ADC and also internal osc. Because of small size and weight, you will want to keep the number of components to a minimum.

Also the relationship between pressure and altitude is non-linear (and also depends upon temperature, but the MPX2100 compensates) so I used a look-up table to convert from pressure to height. Another consideration is the output, mine had a serial output (which requires laptop and hyperterminal!) which I replaced with a LED that flashed the maximum height, I have also seen a buzzer used inplace of the LED.

If you want to log the flight in stead of apogee - use a really fast PIC!

For more info try asking your local rocketry club - for me in the UK, I would start with UKRA.

www.ukra.org.uk.

Hope this is helpful