Another brief 'blast from the past'....
Concorde had no APU.... engines were started with compressed air from an external starter trolley.
The two preproduction aircraft and the first two production aircraft (used for certification) had something called an MEPU - Monofuel Emergency Power Unit - that would provide emergency power for about ten minutes after a four-engine flame-out.
Since it ran on hydrazine (which is more like a rocket fuel than something you would like to have around on an airport) the airlines that finally operated Concorde (BA and AF) decided they preferred doing without
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CJ