Fancy was a HTP powered 21inch anti-surface torpedo for submarines and surface ships. The project was abandoned on safety grounds and the technology was sold to Sweden which has managed to make very successful incident free use of it. HTP could have solved many of the UK torpedo problems in the period.
Pentane was either a heavy 21 inch air dropped ASW torpedo for fixed wing aircraft. It seems to have been aimed at the Gannet and Shackleton and was considered too heavy for the Wasp; this is purely my speculation but I wonder whether the Bristol Type 192 hunter killer ASW chopper that was cancelled in 1957 was also intended for this weapon or a dedicated submarine launched anti ship version of the Mk 20. Sources differ. However I am of the opinion that it was the former, mostly on the basis that more sources state it was and it turns up in relation to the Shackleton Mk3.
There is a great article about UK (and other) Torpedo development from the Journal of the Royal Naval Scientific Service dated March 1972 on the Defense Technical Information Center website. It covers Pentane and others in quite some detail and is very much worth a read if torpedoes are your thing. The article is part 3 of a series but I have not had time to look for the others yet. I am putting it in this thread because it was Pentane I was looking for when I found it but I would understand if the mods want to put it somewhere else!
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD595842&Location=U2&doc=GetTRDoc.pdf
Parts 1 and 2, littered with various experiments and concepts, vcan be found here starting on page 33:
http://www.dtic.mil/dtic/tr/fulltext/u2/595766.pdf
