Thanks for replies. John Farley, especially. I flew Mustangs countless years ago and I recall we were warned about the shock of high altitude bale-outs and your reply John now bears that out. Where I am uncertain, however, is the TAS/IAS question.
If the shock of opening is dependant on the volume or pressure of air particles filling out the canopy, then at high altitude it would be the IAS that counts, wouldn't it? If not, I am at loss to understand where TAS comes into it at high altitude. If you jump at say 40,000 ft and open the chute at 230 IAS vertical speed (if you had a speedo on your body), then would not the opening shock be the same as opening the chute at 230 IAS at say 10,000 ft?
The porosity of the silk at high (cold?) altitudes is something I never thought about. And just to confirm part of my original question and that is assume that temperature and oxygen aspects are a different story -so ignore their effect on the pilot.