From: jfindley@cinci.nospam.rr.com   
      
   In article ,   
   jfmezei.spamnot@vaxination.ca says...   
   >   
   > On 2021-03-01 08:39, Greg (Strider) Moore wrote:   
   >   
   > > The risk of an explosion comes from the fuel already on-board,hydrazine. It   
   > > doesn't need air.   
   >   
   >   
   > Is it fair to state that unlike Merlin engines of a landing Falcon 9,   
   > the landing thrusters don't have fancy high energy turbines that break   
   > apart in an uncontained fashion when the landing doesn't go well?   
   >   
   >   
   > In the Perseverance case, my understanding is that they ran the engines   
   > till rey ran dry and then let the lander fall down.  So there wouldn't   
   > be much fuel left.   
   >   
   > And in the case of hydrazine, assuming the intended to land it, so there   
   > would be expectation of some residual fuel, how explosive is a hydrazine   
   > engine?   
      
   The better question is, how explosive his a COPV that's pressurized to   
   run a hydrazine engine?  You'd have to run it to depletion (i.e. zero   
   pressure) in order for there to be no chance of an explosion.  That's   
   exactly what they did with skycrane.  They rendered it essentially inert   
   before it hit the ground as far away from the rover as possible.   
      
   Attempting to land so that at touchdown you have exactly zero propellant   
   in the tanks would be risky.   
      
   Any way you look at this problem, attempting to land the skycrane   
   increases risk to the rover.   
      
   > If only one tank ruptures, is it fair to state that other then the   
   > environmental aspects of dangerous substance, it is otherwise fairly   
   > stable until/unless it contacts with the other one?   
      
   Either of the propellants released on its own is highly reactive and   
   would contaminate the area.  Again, increased risk to the primary   
   mission of the rover.  You don't need it roving over a contaminated   
   area.   
      
   > And finally, if you get to a point where the lander can gently deposit   
   > the rover on the ground (mission objective), wouldn't it be fair to   
   > state that having it fly away and land wouldn't have very different odds   
   > compared to having fly away, hover till out of gas and then crash ?   
      
   No.  Flying away, depleting its propellant, and then falling to the   
   surface as far away from the rover as possible is still better.   
      
   > Isn't the high rish portion the re-entry interface and then slowing down   
   > for gentle landing of rover? Once you are overing over ground, isn't   
   > most of the risk gone?   
      
   Nope.  Until the skycrane, with its pressurized propellant tanks, flies   
   far away and depletes its propellant, there is still non-zero risk to   
   the rover.   
      
   Jeff   
      
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