JF Mezei wrote on Wed, 18 Apr 2018
03:24:19 -0400:

On 2018-04-17 20:05, Jeff Findley wrote:
In article ,
says...

https://www.space.com/40313-spacex-r...y-balloon.html

By "party balloon" are we talking helium or Hydrogen filled balloons
that will "lift" the stage to slow its descent? (and I assume provide
drag initially).


Think about the physics. It's going to be almost all drag initially
(not much to displace at high altitudes) with gradually increasing
lift as it drops lower.


Is this a case of planning bouyancy such that it is massively positive
initially to slow down descent, but as it drops into denser atmpsphere,
bouyancy dimonishes and they plan it such that it gets to just a tad
below neutral bouyancy for a gentle touch down with the engine firing to
stop vertical speed just bfore touching ground?


That would sort of violate the laws of physics. Note that the ship
they're talking about using to 'catch' it is intended to catch free
falling objects (like fairings).


Are we talking langing legs and fins?


No.


Or is this going to be dead weight falling back help by a balloon and
truly fall on some mattress factury like when Batman and Robin had
engine problems in a helicopter?


That, except it's more of a 'net' than a "mattress factory". Did you
actually read the article, Mayfly, or is it that you've forgotten what
it said in the time it's taken you to write this. Again, think about
the physics. A balloon greatly increases the drag, which I'd bet is
the primary slowing method. I wouldn't be surprised if the balloon is
designed to be 'parafoil-like' at low altitudes so it can be steered
somewhat.


How would de-orbit work? From the point stage-2 is no longer needed for
payload, isn't it firmly in orbit and coudl stay there for very long
time? I assume it would require de-orbit burn?


They already do a commanded deorbit burn to drop the stages in the
Pacific so they're not cluttering up orbital space. They would just
target the reentry point closer to land instead of out in the Pacific
and stick a 'catcher' ship out there. This is all described in the
article, which you have apparently forgotten the content of in true
Mayfly fashion.

I would expect this, like fairing recovery, to have a pretty low
chance of success.


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw