On 19-10-03 17:23 , David Spain wrote:
I have an update for you Niklas.

Thanks!

Here's a fellow (smallstars) who has proposed an artificial gravity
system based on the same physical principles you rely on but uses a
third Starship as a cargo and axis vehicle that holds an bi-directional
extruding truss system to attach the remaining two crewed Starships. The
rigid structure allows each Starship to pivot on its own pitch axis to
align them for a short Raptor burn to initiate and terminate rotation.
Then they can pivot so that the pitch axis aligns with the rotational
hub to provide the artificial gravity within the Starship for the months
long journey outbound and inbound. At either destination the crewed
Starships detach from the hub for landing. The hub ship also lands
robotically for refueling and reuse.

See:

https://www.youtube.com/watch?v=3CRiJTJikjk

I had a look, and of course the basic idea is the same -- rotation of
two Starships linked nose-to-nose, but sufficiently far apart to avoid
Coriolis problems. After my initial posting I have also found other
presentations of the same idea, some even with wires -- but those are
usually connected to the Starship's nose, not its legs, so they assume
that a Starship can stand the resulting longitudinal tension.

I am totally unconvinced by the video's arguments for a rigid truss
connection, instead of a cable or wire, and many commenters seem to
agree with me (but I didn't read all 1798 comments :-) ).

If the RCS is not powerful enough to spin up the system, even with a
cable one can certainly use the main engines to spin it up, either with
the cable initially loose and then (smoothly!) tightened, or by making
the wire fork near the Starship into two tails, with one tail connected
amidships or at the aft end of the Starship, and the other tail
connected to the nose. By adjusting the lengths of the two tails one can
switch the StarShip from the tangential pose, where the main engines can
be used to speed up or slow down, to the radial pose.

However, it seems to me simpler and more efficient in terms of delta-v
to have an extra-long wire and to use the RCS to give the two Starships
just a little rotational velocity with the wire fully extended, and then
reel in the wire until the desired centripetal acceleration is reached.
Such a wire could even be tapered to reduce its weight, because the
tension on the wire is low when the Starships are far apart and
increases as they come closer.

The recent Starship design change that replaces the combined
fins-and-legs with separate fins and legs means, of course, that my
original suggestion of connecting wires to the bottom of the aft fins is
no longer valid. I believe SpaceX still plans to stack the Starship on
top of the Superheavy booster by crane with a wire to the nose of the
Starship. However, probably the nose lifting-point is currently designed
only to stand the dry weight of the Starship, plus crew and cargo, and
would then have to be strengthened to stand also the weight of the
propellants left on board for the Mars-Earth-Mars transits.

--
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
. @ .