The animations I have seen show superheavy landing between cradle arms
and the stack rotated such that the extended grid fins are caught by
those arms.

Considering that the grid fins should be very agile and able to move
quickly to manage attitude during the descent phase, is it possible to
have both this agility AND the structural strength needed not not only
support full weight of the Super Heavy but also the G-force at time of
landing?

Also, while the multiple Starship landing attempts showed good software
ability to manage attitude down to near vertical when engines are
supposed to do the final bit to the ground, do we know if this included
control of rotation (roll?) of the stack once vertical?

Since such a cradle landing requires not only that the fuselage drop at
right place, speed, but also rotated so its grid fins are oriented to be
caught by the arms and not touch the tower itself, has there been any
indication that this has been tested with the startship flights? (even
though starship won't use that, just curious of the software for
rotation control was tested on it).

In article <KOPBI.457$al1.203@fx26.iad>, jfmezei.spamnot@vaxination.ca
says...
>
> The animations I have seen show superheavy landing between cradle arms
> and the stack rotated such that the extended grid fins are caught by
> those arms.

Any of those animations "official" SpaceX videos? If not, I'd take them
with a huge grain of salt.

> Considering that the grid fins should be very agile and able to move
> quickly to manage attitude during the descent phase, is it possible to
> have both this agility AND the structural strength needed not not only
> support full weight of the Super Heavy but also the G-force at time of
> landing?

Yes.

> Also, while the multiple Starship landing attempts showed good
software
> ability to manage attitude down to near vertical when engines are
> supposed to do the final bit to the ground, do we know if this included
> control of rotation (roll?) of the stack once vertical?

Heavy will go up and down like a Falcon 9 booster. Starship is an
entirely different beast.

> Since such a cradle landing requires not only that the fuselage drop
at
> right place, speed, but also rotated so its grid fins are oriented to be
> caught by the arms and not touch the tower itself, has there been any
> indication that this has been tested with the startship flights? (even
> though starship won't use that, just curious of the software for
> rotation control was tested on it).

No, Starship has no grid fins.

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.

On 2021-06-27 17:02, Jeff Findley wrote:

> Heavy will go up and down like a Falcon 9 booster. Starship is an
> entirely different beast.

However, I was wondering if SpaceX has experience in precicely
controlling roll (is it roll?) so that "arms" would be aligned correctly
to fall on the cradle. Has it mentioned wherher Falcon 9 not only lands
on the X but also with the correct roll?

Thinking about it, there is probably little in common between starship
and super heavy landings in terms of software so probably no attempt at
evaluating roll.

The latest I saw was 4 grid fins, but at 2 groups of finds 60° apart,
and then 120" between groups. So this definitely requires precise "roll"
when landing to have these grid fins aligned with the cradle so that the
2 grid fins from each side end up on the cradle.

I take it final roll alignement would be done by thrusters on fuselage?

Jeff Findley scribbled something on Sunday the 6/27/2021:
> In article <KOPBI.457$al1.203@fx26.iad>, jfmezei.spamnot@vaxination.ca
> says...

>> Considering that the grid fins should be very agile and able to move
>> quickly to manage attitude during the descent phase, is it possible to
>> have both this agility AND the structural strength needed not not only
>> support full weight of the Super Heavy but also the G-force at time of
>> landing?
>
> Yes.

Acceleration relative to the tower should be very close to 0 at the
time of capture, so what G-force are you imagining besides the
no-longer-full weight of the Super Heavy?

[That's a pun, son, since the tanks should be very close to empty at
the time of capture]

That said, Elon has apparently allowed as how some additional hard
points might be needed.

/dps

--
Maybe C282Y is simply one of the hangers-on, a groupie following a
future guitar god of the human genome: an allele with undiscovered
virtuosity, currently soloing in obscurity in Mom's garage.
Bradley Wertheim, theAtlantic.com, Jan 10 2013

On 2021-06-28 02:01, Snidely wrote:

> Acceleration relative to the tower should be very close to 0 at the
> time of capture, so what G-force are you imagining besides the
> no-longer-full weight of the Super Heavy?

Falcon9 still has crush zone in landing gear because some landings are
harder than others. Shouldn't one assume that some Super Heavy landings
might not be perfectly smooth ?

In article <F78CI.863359$nn2.491117@fx48.iad>,
jfmezei.spamnot@vaxination.ca says...
>
> On 2021-06-27 17:02, Jeff Findley wrote:
>
> > Heavy will go up and down like a Falcon 9 booster. Starship is an
> > entirely different beast.
>
> However, I was wondering if SpaceX has experience in precicely
> controlling roll (is it roll?) so that "arms" would be aligned correctly
> to fall on the cradle. Has it mentioned wherher Falcon 9 not only lands
> on the X but also with the correct roll?

Engine gimabling provides roll control.

> Thinking about it, there is probably little in common between starship
> and super heavy landings in terms of software so probably no attempt at
> evaluating roll.
>
> The latest I saw was 4 grid fins, but at 2 groups of finds 60° apart,
> and then 120" between groups. So this definitely requires precise "roll"
> when landing to have these grid fins aligned with the cradle so that the
> 2 grid fins from each side end up on the cradle.
>
>
> I take it final roll alignement would be done by thrusters on fuselage?

Possibly.

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.

In article <XZlCI.49215$8f1.5015@fx23.iad>,
jfmezei.spamnot@vaxination.ca says...
>
> On 2021-06-28 02:01, Snidely wrote:
>
> > Acceleration relative to the tower should be very close to 0 at the
> > time of capture, so what G-force are you imagining besides the
> > no-longer-full weight of the Super Heavy?
>
> Falcon9 still has crush zone in landing gear because some landings are
> harder than others. Shouldn't one assume that some Super Heavy landings
> might not be perfectly smooth ?

Any necessary "cushioning" of a Super Heavy booster would be done by the
catching mechanism attached to the launch/landing tower.

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.

On 2021-06-28 21:06, Jeff Findley wrote:
> In article <F78CI.863359$nn2.491117@fx48.iad>,
> jfmezei.spamnot@vaxination.ca says...
>>
>> On 2021-06-27 17:02, Jeff Findley wrote:
>>
>>> Heavy will go up and down like a Falcon 9 booster. Starship is an
>>> entirely different beast.
>>
>> However, I was wondering if SpaceX has experience in precicely
>> controlling roll (is it roll?) so that "arms" would be aligned correctly
>> to fall on the cradle. Has it mentioned wherher Falcon 9 not only lands
>> on the X but also with the correct roll?
>
> Engine gimabling provides roll control.

Not during the Falcon 9 landing burn -- only the center engine is used,
so gimballing can control only pitch and yaw.

During Falcon 9 landing, roll must be controlled by the grid fins and/or
by thrusters. Judging from the rolling during the landing that failed
(and was "sent to sea") because the grid fins were stuck, both methods
can be used for roll control.

If the Super Heavy uses multiple engines for landing, it can control
roll by gimballing.

>> Thinking about it, there is probably little in common between starship
>> and super heavy landings in terms of software so probably no attempt at
>> evaluating roll.

ISTR that SpaceX will try to avoid using entry burns for the Super
Heavy. If so, roll during the entry phase cannot be controlled by
enginge gimballing. They will probably want to have a certain roll
position before the grid fins can get a grip on the atmosphere, so
thrusters will probably be used there.

On 2021-06-28 15:34, Niklas Holsti wrote:

> During Falcon 9 landing, roll must be controlled by the grid fins and/or
> by thrusters.

In final stage, is there sufficient vertical speed for grid fins to have
any aerodynamic control? Or is that something that MUST be done before
it slows down and hope the roll doesn't change in the last seconds of
flight?

On 2021-06-28 23:47, JF Mezei wrote:
> On 2021-06-28 15:34, Niklas Holsti wrote:
>
>> During Falcon 9 landing, roll must be controlled by the grid fins and/or
>> by thrusters.
>
>
> In final stage, is there sufficient vertical speed for grid fins to have
> any aerodynamic control?

Obviously the grid fins stop working when the vertical speed goes to zero.

> Or is that something that MUST be done before it slows down and hope
> the roll doesn't change in the last seconds of flight?

On the Falcon 9 booster where the grid fins locked into a position that
caused a rapid roll, when the booster slowed down for its ocean landing,
the torque from the grid fins also decreased, and I believe the
thrusters were increasingly able to overcome that torque, and thus the
roll-rate decreased before the "landing".