Have SRBs or SSMEs ever launched without a space shuttle?

Question

It seems to me that the April 12, 1981 launch of Columbia (STS-1) was the manned spaceflight with the most untested hardware of any manned space launch. Even the Vostok and Mercury launch systems had unmanned launches before the manned flights. However the Space Shuttle never had an unmanned test flight, and I could find no information about an unmanned SRB launch.

I am well aware of the ground-based testing of the SRBs and the SSMEs. However, were these systems ever flight-tested before the manned flights?

Answer 1

The obvious way to test the SRBs and SSME would be to fly an unmanned Shuttle mission. Any alternative meant developing a one-off rocket just for the tests.
NASA considered doing unmanned Shuttle flights from the beginning.

STS-1 pilot John Young, discussing the topic, in his 2006 interview with collectSPACE:

They wanted to fly the thing unmanned. I went to many, many meetings where they wanted to fly the thing unmanned, but finally the program manager up at Headquarters, John Yardley, he said he wasn't going to come across California with nobody in the spacecraft.

So, we got to fly it manned. It's probably the safe way to do it. We looked at California and there were all kinds of places you can land out there...

Robert Crippen, his copilot, agrees in this Space.com interview:

But John and I were both proponents of us being onboard, because we thought the flight had a bigger chance of success, and it would have been very difficult to make it unmanned on the first one.

On the first four flights, the two-man crew had ejector seats to increase survivability.

Back then, there were rumors that there was only one real obstacle to making an unmanned Shuttle flight: there was no way to extend the landing gear remotely.
The same thread had another post that ties in with that:

In speaking with a number of the 1970s shuttle program guys (Engineering and Ops), there was at least an impression that there was a contingent of astronauts and managers that fought hard to keep an overtly unmanned flight option off the shuttle, despite how simple it would be to build in. This was supposedly done to ensure that pilot astronauts would not be regarded as superfluous later on - if we don't need them to fly it now, why have them fly it later? This would therefore reduce the importance of the pilots relative to the mission specialists. I've seen some documentation in the JSC archives (it may have even been among the "Young-grams" lending some credence to at least the existence of such a faction, if not their reasoning.

In the end, NASA did develop the means to do unmanned missions. The Remote Control Orbiter cabling first flew on STS-121.

Answer 2

The SRB's were ground tested, as were the SSME's. Consider for a moment the scale/scope of a test stand that can hold an SRB full fire test. (The SRB's put out 2.8 million lbs of thrust. To compare, even an F-1 engine was only 1.5 Mlbs).

Ironically the only real standalone launch of an SRB was the Ares-1X launch, well after the Shuttle was ready to be retired.