Thank you for asking! SRB's or solid rocket boosters have a couple of inherent problems that liquid fuel motors do not.
There are many complaints that launching humans on a single SRB is flawed and should NOT be a man rated launch vehicle. No one has dared do this
yet... no space agency has launched a human on a single solid booster. No one has been foolish enough to try either.
Some reasons are related to what are known as "Criticality One" mafunctions or an event causing 100% loss of vehicle and crew. Some are related to
the nature of SRB burn. Others are related to the lack of throttleability after ignition and yet other are related to the inability of an SRB to
"gimbal" or thrust vector the exhaust stream. Manufacturing consistency is another lesser hassle too.
Lots will dispute my concerns... that's fine. I will put forward a couple of nightmare scenarios that may or may not come to pass if the crew launch
vehicle is powered by a single SRB. OK three scenarios actually. A "part" or off center fire in an SRB burn. An SRB segmental joint burn through ala
Challenger and the hung on the gantry (tower) misfire.
Know this, once ignition of an SRB has been initiated you can't shut it down for love or money. SRB's are pound for pound the most powerful lift per
No one has ever "ridden-out" an escape tower flight other than in some limited and old testing from the early years. And no one has ever been
required to separate from a booster of any kind in the early boost phase let alone at the very high velocities that govern launch as it approaches
MECO and booster sep... Imagine flying along at several thousand knots and getting a "light" on booster failure for whatever reason and having that
dinky little tower "yank" you away from the failing perhaps (explosive) booster cleanly? Nope, ain't gonna happen. Dead astronauts.
SRB's don't gimbal like liquid motors... multi engine liquid boosters have this wonderful Goddard/Braun feature that allows them to steer the rocket
with inputs from gyros should an engine fail or not produce the thrust results expected... astronauts lives have been saved by this feature. A single
SRB with an inconsistent burn, say off center, that is, burning outward from the hollow core unevenly (non symetrically) can and does produce an off
center thrust moment that craft like the shuttle compensate for very well using their gimbal'd main liqiud motors. Such a luxury is NOT present in
SRB's so far... a bad burn off- center or even worse a "chunk-out" could make a launch vehicle have some wild flight dynamics... corkscrewing and
or tumbling come to mind.
The big fear is an explosion during or after an SRB segmental joint "burn-through" and the ignition of the solid fuel strata (think of stacked
hollow cylinders with an outer metal flanged shell filled with solid explosives and a burn aperture in the center) between segments in an uncontrolled
manner leading to burn through and an unanticipated new thruster plume poking out the side of the rocket. SRB's are fine for unmanned launches and
are even acceptable in combination with liquids like on the STS and Delta IV's, but to put humans on a single SRB is just asking for trouble in ways
that have already been proven fatal.
I really like NASA. This launching method I'm not so impressed by, and hope they change it before the program goes too much further. They even chose
liquids for the "hardware" launch vehicles. Maybe they'll decide to do the same on the crew launch vehicle too.
[edit on 27-5-2006 by V Kaminski]