JF Mezei wrote:

On 2017-09-24 19:27, Fred J. McCall wrote:

No. The only reason there were 'issues' with the Falcon Core is that
the engines can't be deeply throttled. That's not the case with
Dragon V2.

Whatever the reason, it took a number of attempts before they fine tuned
the software to make it work. And one should expect Dragon to have
similar failures at first.


And why is that?


Why is it so hard to safety certify? Because NASA doesn't want to
certify it.

Not hard, But it would take a few flights to prove it, diuring which you
can't return anything, at a time NASA was craving to return stuff.


Of course you can. Nothing requires ALL reentries to use propulsive
landing.


This is why I suggested SpaceX should have insisted the text flight land
on land to do that testing/certification/debugging.


That's a bad plan, because you've just done the very thing you said
was bad and prohibited returning anything until propulsive landing is
safety certified.


Considering it is likely they could do much of the testing/debiugging by
dropping it from 30,000 feet or lower, they could have done this at
faster rate than the space flight rate.


Nope. Things that are different aren't the same.



Capsule is round. Why would it be limited to only 'left/right'
control?


When you have 0 left/right speed, it is easy to use a bit of
aerodynamics at right time to creare left/right cross range.


Poppycock! You have to 'bend' your velocity vector and bending it one
way is just as difficult as bending it some other way.


But when you're coming in at 25,000km/h in forward speed, I would think
that any miscalculation could get capsule to land very far before or
after landing point.


You shouldn't think. It doesn't appear to be one of your core
competencies. The velocity vector isn't 'forward'; it's at a slant
downward. Hence 'reentry'. But it doesn't matter. To change the
landing point you need to 'bend' the velocity vector by applying lift.
Bending it left or right is no harder (or easier) than bending it up
or down. It's just a matter of how much 'lift' you apply and in which
direction.

[Actually you have slightly less authority 'up' and slightly more
authority 'down' because of this stuff called gravity, so landing
'shorter' is a little easier than landing 'longer', but it's not THAT
much harder.]


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn