Jeff Findley wrote:

In article . com,
says...

On 2017-09-24 16:34, Jeff Findley wrote:

From above article:

"The reason we decided not to pursue (powered landings) heavily
is it would have taken a tremendous amount of effort to qualify
that for safety, particularly for crew transport," Musk said.

Ok, so was Dragon gonna do powered re-entry like Stage 1, or be like
Soyuz with lifting re-entry, parachutes and powered "cushion" very near
ground?

No. Parachutes would not have been used for a nominal powered landing.
Parachutes would have been necessary in case of an abort because the
abort burn would have used too much fuel to allow for a powered landing.

I was told it was to be a powered one like stage I, where fuel available
becomes critical. (and validating software a bigger challenge).

It's critical in the sense that you have to budget enough for landing,
but the terminal velocity of the capsule limits the amount of fuel
needed for landing, so it's not hard to make sure you have enough.


Since Dragon V2 has internal fuel tanks and that fuel is pretty much
used only for the Super Draco engines (I think they use cold gas for
OMS now), you come down with virtually full tanks.

So, since it's not as important to Mars as SpaceX once thought it was,

I smell excuse here.

Funny, I saw a clear engineering/management trade-off.


That's because you know something about engineering and management and
Mayfly knows something about taking the trash out for Mom and the
living conditions in her basement.

The Mars plans were announced years after Dragon was designed. I suspect
this is more about SpaceX realising that the refurb costs for salt water
landings aren't as high as thought, so it is more economical to land in
water than to spend the extra money to develop/test land landings.

I suspect Mars landing will be different enough to not be applicable to
Earth landings.

We'll see what has changed with the next update from SpaceX on its Mars
plans. It could be that a (relatively simple) powered vertical landing
has been abandoned in favor of something else a tad more complicated.


I think they may have tripped over the same issue landing anything of
size on Mars has. The air is to thin for aerobraking adequately and
the gravity is too heavy for a pure propulsive landing.

was a management/engineering trade-off and appears to be SpaceX's
call, not NASA's.

Suspect origimally NASA, then SpaceX realised water landings not as bad
as thought.

Possible.


But there is the cost of the capsule recovery force.

Yes. Dragon flies a lifting reentry and therefore has some cross-
range
capability. This was also true of both Gemini and Apollo (and Russian
Soyuz and Chinese Shenzhou).

Does "cross range" refer to only left/right, or also forward/backward?
(aka, a narrow elipse or a full circle as possible landing locations?

Roughly speaking, by rotating the capsule, you can direct the lift
vector anywhere along the circle that's normal to the velocity vector.
So in your terms, yes you can direct the lift vector left/right and/or
forward/backward.


Yep.


--
"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