Is it the case that, if used for propulsive landing (which is speculative at this point), each of the Super Draco engines would be turned on and then left on -- presumably, with variable non-zero thrust -- until landing has completed? It's not obvious to me that they wouldn't need to be throttled down to zero. Or is that not relevant so long as the pressure from the helium remains on throughout engine usage?
Musk says it's still on the table as a backup plan "pending NASA approval". Different commenters on /r/SpaceX have greatly varying estimates about how realistic that is.
It's possible that the change to burst discs will not affect the feasibility of propulsive landings, depending on how the system is pressurized and used.
During a propulsive landing, the Dragon capsule would test the SuperDracos while still at a reasonably high altitude; this way, in the event of a failed SuperDraco, the parachute backup can be used. (The landing burn for Crew Dragon would start only a few seconds before landing, far too late to deploy the parachutes if the engines don't perform properly.)
That said, this is old information, and many things could have changed since then.
Throttling an engine down to zero would not happen during propulsive landing until the craft is on the ground. Reducing thrust to zero would result in violent rotation of the craft, and the available thrust would be well below the weight of the craft at minimum throttle. At this point in time the helium lines would be fully pressurised so there’s no way to repeat the anomaly.
After landing there would be a process to make the craft safe by releasing all pressurised gasses, and either sealing or burning off remaining propellants. With no pressuring remaining there is no way to repeat this anomaly.
There will of course still be issues with the residual hypergolics since they are highly toxic and the propellant tanks will need to be decanted: whether than happens before or after crew recovery is another discussion.
as long as there is a positive pressure differential between the helium lines and the tank, all is well. Also, there's no valve for the water hammer effect to destroy now :)
