What's this about? Rocket engines, of course!
Specifically the RS-25, which was recently successfully test fired for 500 seconds in testing for the SLS (Space Launch System).
There was some minor controversy a few years ago as to what would power the SLS, solid-fuel or liquid-fuel rockets. Ultimately the Utah delegation attempted to mandate the use of SRB (solid rocket boosters) in the initial design. According to New Scientist, this is not the best option.
Practicable doesn't mean "existing," or even "practical." It means capable of being done, or used. Besides, there are downsides to solid rockets that make liquid-fueled alternatives more attractive.
Claiming that solid rockets are necessary for a heavy-lift launcher is obvious nonsense. The US’s previous heavy-lift launcher, the Saturn V, used no solid rockets and lifted a bigger load than the new launcher is required to carry.
What’s “practicable” depends very much on who analyses the problem. Usually the devil is not in the details, but in the assumptions [emphasis original] .
One key assumption involves the engine used to power the launcher’s first stage, which is used to push the rocket off the ground. The launcher will probably weigh 2000 tonnes or more at takeoff (the Saturn V weighed about 3000). That requires a lot of thrust, which can be attained with either a few big engines, or a large cluster of smaller ones.
For one thing, shutting solid rockets down in an emergency is difficult. Contrary to popular belief, it’s not impossible, but it’s a somewhat violent process and that creates its own problems. (Solid boosters were chosen for the shuttle on the assumption that they could be shut down on command, but the decision wasn’t reconsidered when that proved to create impossible structural loads.)It's important to plan for when things fail. A proper FMEA should (and probably will) be done by NASA when they select the next iteration of the booster stage.
For another, while solid rockets can be fairly reliable if you spend enough money on them, when they do fail, their failures are often catastrophic. (Ailing liquid-fuel engines, by contrast, usually just shut down when they fail – this happened several times on the Saturns.)