Scott Ferrin wrote:

It didn't use special fuel (although they wanted to use a fuel with
boron added at one point). I *think* part of the reason for the
differences would be related to the compression ratio of the engines
and that they were straight turbojets rather than turbofans. The
XB-70, Blackbirds, and Mig-25 all used low pressure engines I *think*
because at high speed they compressed the air so much to get something
to work with that by the time it got to the engines they couldn't take
the temperature of compressing a ton more. ISTR the J93's compression
ratio being around 9 to 1 whereas something like the F100-129 is up
around 32-1 or 36-1. Then again they though the could get the
Crusader III up to 2.9 with a J75 so who knows. All of this is just
my opinion from what I've read over the years so hopefully someone
will weigh in who knows a lot about it.

IIRC, the XB-70 engine had bypass ducting similar to the
engines on the SR-71.
--
--Matthew Saroff

I'm not an actor, but I play one on TV.
Check http://www.pobox.com/~msaroff, including The Bad Hair Web Page