[pfflier]

Ignition does occur after the missile leaves the launch tube. There was an inertial sensor that sensed the deceleration after ejection from the launcher. When that measurement reached a certain threshold, the main engine ignited. That is essentially the same sequence the Trident and other SLBMs use except they travel through a steam/gas bubble to the surface.

When you watch the launch videos of the M-X you can see that the deceleration is noticeable.

The plume that follows the missile out initially is the ejection gasses (mostly steam). The pads acted like wadding in a musket barrel keeping the gasses behind the missile.

[SaveFerris]

Yeah that’s what my coworker indicated - deceleration-triggered main(s).