This is not insider knowledge. It is based only upon what SpaceX has publicly released.
There is some kind of geometrically-shaped shield plate that
protects the front end of the Superheavy booster where the forward dome of the
forward propellant tank is otherwise exposed.
This shield was shed after hot staging with Version 2, but is now integral to the booster stage in
Version 3. It deflects the plumes of the
Starship upper stage engines, from
striking and damaging or destroying that forward tank end, during hot staging.
This shield structure is coupled with an interstage ring
between the stages, part of the
Superheavy, that is “porous”, in the sense that there are openings through
which the deflected engine plumes can exit laterally, in a more-or-less radial direction, during the transient hot-staging event. The force to turn a plume gets exerted upon
that shield.
The Superheavy booster had four grid fins spaced 90 degrees apart, in Version 2.
Plume blast damage had been noted to the grid fin rotated into the way
of plumes, during the booster flip
maneuver, in Version 2. In Version 3,
there are only 3 grid fins, still
spaced 90 degrees apart, but somewhat larger than the Version 2 grid
fins. That leaves a space where there is
no grid fin, which is clocked to align
with the dorsal side of the upper stage Starship, as shown in Figure 1.
Figure 1 – Version 3 Starship/Superheavy Arrangement and
Intended Booster Flip Direction
The design direction of the booster flip is a plane aligned
with the dorsal-ventral plane of Starship,
and the front of the booster is supposed to flip in the direction where
it moves “down” as shown in the figure.
Thus, no grid fin is exposed to
plume blast damage anymore, in Version
3. The side of the booster is less
vulnerable to plume blast damage, being
cooled by the cryogenic propellant vapors inside.
In Version 2, the
torque to flip the booster came from a strong gimbal angle of the 3 booster
engines firing during the staging event to keep the booster propellants settled
into the aft ends of their tanks, where
the suction-feeds to the engines are located.
This is still the case in Version 3,
with some control of the deflected plume force directions added, by one or both of two means: (1) the geometric shaping of the shield and
of the “porosity” of the interstage ring,
can favor a plume deflection force component in the intended flip
direction, and (2) the startup sequencing of the Starship
engines can also produce a temporary plume deflection force component favoring
the intended flip direction.
There are two separate effects going on during the
hot-staging event, which must both be
successful, but which are also linked to
each other in ways that make the successful “solution space” rather small! These are indicated in Figure 2 below, which depicts the conditions as the hot
staging begins, but before the flip
starts. (Drag was ignored.)
First,
the acceleration of the upper stage Starship has to be larger than the
acceleration of the Superheavy booster (before the flip starts), in order not to risk a collision.
Second,
the axial acceleration of the Superheavy booster must be a positive
number large enough to keep the propellants settled, all during the staging event, and all during the flip maneuver.
That last is also complicated by the flip motion possibly flinging
the propellants forward, in only the
forward tank, if the flip is too fast!
The variables available for controlling this event are the
thrust levels used in the booster and upper stage engines, the shaping of the shield and interstage ring
porosity geometries, and the sequence of
ignition of the upper stage engines.
To simplify this first look,
the author chose only a full thrust setting in the 3 booster
engines, and lighting all 6 upper stage
engines at once, but a some reduced
thrust. Guesses were made for plume
deflection force component angles and booster engine gimbal angles. All of this is indicated in the figure.
The author has no actual data for the engine thrust levels
at the staging altitude, or for the
actual masses of either stage at the time of staging. He made reasonable guesses for these, as well,
as indicated in the figure. So
the results are inherently approximate!
Figure 2 – Finding A Thrust Setting For Starship That Keeps
Superheavy Accelerating
It would not be possible to get full thrust in an engine at
the moment of ignition! It is more
practical to ignite at a partial-thrust setting propellant flow rate, stabilize after a split second, and then throttle up quickly a split second
after that, as desired.
The author chose to look at full and half thrust
initially, and then added a 40% thrust
setting, after getting negative booster
acceleration at full upper stage thrust,
and zero booster acceleration at half thrust. The 0.1 gee booster axial at 40% Starship
thrust is in the rough ballpark of other ullage thrust applications. The 40% thrust setting is within the
capability of the Raptor-3 engine design,
so this point really is a feasible thing to do!
As the figure indicates,
the Starship upper stage has the acceleration to leave the booster
behind, at 40% thrust on 6 engines. It cannot throttle up until the flip has
proceeded enough to direct the front end of Superheavy out of the way of the
Starship engine plumes.
Also indicated in the figure, the Superheavy axial acceleration is zero at
half Starship thrust, and only 0.10 gees
at 40% Starship thrust. Starship cannot
throttle down much more than that, and
still leave the vicinity at about 0.4 gees!
Which is exactly why this author says the solution space here is
quite narrow! Screw this up, and the propellants will unsettle, the booster engines will suck vapor, and their turbopumps will explode!
The next step was to look at starting the booster flip. This is illustrated in Figure 3, which presumes the 40% thrust solution just obtained above. Two cases were examined: a booster engine gimbal angle of 10 degrees, and a comparable lateral deflection force generated by shield geometry and/or interstage porosity distribution. The booster mass moment of inertia was approximated by the solid bar formula for rotation about its center of gravity (cg). That cg was simply presumed to be halfway along the booster.
Figure 3 – Starting the Booster Flip Maneuver During Hot
Staging
The results for time-to-clear shown lower right of figure
are amazingly close to what is in the publicly-released videos of these flight
tests (about a second or two to clear)!
Considering how crude all the assumed data is, that outcome is quite remarkable! Letting deflected upper stage plumes
help the gimballed booster engines to start the flip maneuver is definitely beneficial, but it is not an overwhelming effect. That would seem to be the lesson to be
learned here.
That brings up the “spin gravity” effect of the flipping
booster upon the settling of propellants in its forward tanks by engine-induced
acceleration. As depicted in Figure 4, this depends upon which side of the booster
cg falls the free surface in the forward tank.
One must take into account relative tank sizes, and how full those tanks are. A very crude estimate is shown in the
figure, indicating that spin will help
settle the propellants.
Figure 4 – Evaluating Whether Spin of Flip Affects
Propellant Settling
Note that had the LOX tank been forward in the design
instead of aft, this outcome would not
be true!
The magnitude of the “spin gravity” effect depends upon the
rotation rate ω at the time the booster clears the plumes, which is crudely angular acceleration times
the clearance time, for the two cases
shown in Figure 3 above. Those would be 0.139 rad/s for the gimbal only
case, and 0.186 rad/s for both effects
together.
The acceleration in gees at the free surface would be L ω2, where L is the distance between the cg and
the free surface. As measured from the
left as shown in Figure 4, as
long as the free surface is left of the cg,
the spin gravity gees adds to the axial thrust gees. If the free surface falls to right of the cg
as shown in the figure, then the spin
gees would subtract from the axial gees!
There is a coupling between stage layout and “spin gravity” risks.
For the dimensions indicated in Figure 4, the LCH4 free surface is 10 m left of the cg.
So, if only gimballing, one would add about 0.020 gees to the axial
0.1 gees. If faster for both effects
together, one would add about 0.035
gees. The additions would be a bit over
3 times larger for the LOX tank, as the
L to its free surface is larger at something like 33.7 m.
Much more accurate dimensions masses, and propellant-remaining percentages would be
needed to get reliable results! But
these results obtained here do seem to crudely indicate what is going on during
Starship/Superheavy hot staging.
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