The space shuttle main engines did fire at liftoff while the SRBs were firing. They may be a bit hard to see at launch but the exhaust mach diamonds are clearly

The space shuttle main engines did fire at liftoff while the SRBs were firing. They may be a bit hard to see at launch but the exhaust mach diamonds are clearly visible e.g. in [0]. They were much more clearly visible in the 7-ish seconds before liftoff, after the SSMEs had been ignited but before the SRBs go off. [0] https://upload.wikimedia.org/wikipedia/commons/d/d6/STS120LaunchHiRes-edit1.jpg https://upload.wikimedia.org/wikipedia/commons/d/d6/STS120La...