Were the Space Shuttle's Main Engines ever off while the Solid Rocket Boosters were firing?

Question

I always though the Space Shuttle needed a combination of its solid rocket boosters and its main engines to remain stable while launching. The following diagram found on Wikipedia seems to agree: after liftoff, the Shuttle tilts backwards to get its center of mass in line with the direction of thrust of the SSMEs. By the time the SRBs run out, the Shuttle and its drop tank are properly aligned for that configuration. Some time later the SSMEs are cut off, and the drop tank is separated. This concludes the "launch"; what remains are orbit insertion burns, doing what they went up for, and returning.

When I recently saw someone with a T-shirt depicting a Space Shuttle launch with the SRBs firing, but the main engines not showing any exhaust, I thought that was an error in the drawing. However, I've since come across this picture, which shows a launch with the SRBs very much active, but no visible exhaust plume from the main engines:

Is this simply an issue with the relative brightness of the exhaust of the big barrels o' fireworks vs. the exhaust of the liquid fuel engines, or are the engines effectively off in the picture?

For context, pictures in an answer to another question on this site make it very plausible that the SSMEs simply produce less soot and light. A picture on the NASA website also seems to show SSMEs which are 'on', but not very bright in comparison with nearly burnt-out SRBs.

Answer 1

Hydrogen-oxygen engines produce a relatively faint blue flame, with visible blue-white shock discs or diamonds under certain circumstances, particularly low altitudes where air pressure confines the plume. At high altitude the plume is nearly invisible. You can look at a number of shuttle launch photos and see the different appearances the main engine plume can take on.

In the photo you have there, the exposure is calibrated for the exhaust plumes from the solid rocket boosters, which are a brilliant yellow-white.

The exhaust from the main engines is simply too faint to be clearly visible in the shot (though I believe the pinkish tint seen between the right wing and the right SRB is it). You can see the blue-white glow inside the nozzles; the engines are clearly running. They'd be deeply shadowed otherwise.

The shuttle's launch profile does include the so-called "throttle bucket", where the main engines are turned down to about 68% of rated thrust for thirty seconds or so, in order to keep the whole stack from reaching too high a speed while still fighting air, but even at 68% that's still nearly a million pounds of thrust coming from those engines.

Answer 2

No, they were never off. Here is my explanation:

• have you ever noticed that the top engine is tilted down? This is us to keep the center of thrust more or less aligned with center of mass of the space shuttle. As the SRBs produce 83% of the thrust at lift-off, a counter-force was needed to go straight-up. With shuttle engines off, I am not sure the whole thing would stay under control and not flip upside-down.

• the SSME exhaust is water vapour as this is an oxygen+hydrogen reaction (but you may not drink it). Combine the exhaust speed and pressure and you obtain a hardly visible flow, and the picture in OP can mislead you as the output flame is aligned with SRB exhaust (which is VERY visible and bright). A questionable comparison would be your pressure cooker: the steam is invisible just after the nozzle.

Answer 3

Offered as a supplement to the other answers - here's a frame from an SRB mounted camera (post-separation) showing the SSMEs running in the absence of SRB plumes. As stated by others, there's no real visible SSME plume at this altitude, just a glow inside the nozzles.