In article ,
says...

JF Mezei wrote:

On 2017-09-23 13:33, Fred J. McCall wrote:

down. I think it's funny that NASA lets astronauts ride on Soyuz,
which relies on power at landing on dirt for them to survive, yet is
being obstructionist to safety certify Dragon V2 for fully powered
landings on dirt.


A rare instance where I agree with you.

But being devil's advocate: At the time NASA started to purchase seats
on Soyuz, it had already proven itself and it ability to land. Also,
they had no choice since Soyuz was also the escape pod in case of emergency.

And during post Columbia stand down and after permanent shutdown of
Shuttle, NASA had no choice either.


True, but doesn't address the issue. Why not let SpaceX prove powered
landing?

Higher (perceived) risk for returning payloads. NASA doesn't want
valuable commodities like EMUs being turned into garbage upon high speed
impact with land. On the other hand, splash-downs have been used by all
other US capsules with only a handful of notable "anomalies", so
management is more comfortable with what they (think) they know.


Had SpaceX insited on doing land landings for dragon from the get go,
NASA's reaction might have been to not bring back important cargo until
after a few flights when the capsule had proven itself.


Just how much cargo do you think comes back down? Most cargo vehicles
are used to dispose of trash by doing uncontrolled reentries.

Absolutely true, which means that cargo which *must* come back to earth,
like EMUs, can *only* come back on Dragon. No other cargo craft
departing ISS has this capability besides Dragon.

Perhaps in a few years we'll see Dreamchaser returning cargo on the old
shuttle runway at KSC, but they're far from orbital flight. They're
just now resuming (space shuttle Enterprise style) test flights.


But because the option to land on water remained available, NASA saw
this as a much simpler way to get commercial cargo going, and once this
happens, it is easier to just insist on it continuing.


True, but what about all the flights that aren't bringing anything
back? Why obstruct development?

Because there are payloads which absolutely need to be returned intact.
With the space shuttle, this was a given capability with the MPLMs.
Post space shuttle, only Dragon has this capability, so it's a quite
valuable capability.


Note: for researchers, landing on land has HUGE advantages as they can
get to experiments much faster.


To some extent, but not all that much.

Agreed. Due to schedule constraints (astronaut time is a precious
commodity), most completed experiments surely end up in storage for
weeks or months before being returned on a Dragon.

Question: for a "newbie" company like Space-X, and considering the
USA
does not have vast areas of undevelopped land that is flat and easy to
land on, how difficult is it to fairly precicely target a landing site
and what are the implications if re-entry isn't perfect (as has happened
on Soyuz a few times) ?


If you're coming down under power (as opposed to Soyuz which comes
down on parachutes and just uses power to 'soften' the landing enough
to be survivable), you're not going to 'miss' by more than a handful
of meters.


Agreed. Dragon capsule is designed to provide lift, so it can steer
itself during descent, even when the Super Dracos aren't firing. This
is quite similar to Falcon 9 first stage landings where the Merlin
engines only fire briefly for boost back, reentry, and landing. The
most recent Falcon 9 first stage landings have literally all been right
in the center of the X. Their landing accuracy appears to be within
mere meters of the center of the landing pad.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.