On Sun, Aug 20, 2023 at 06:22:26AM -0700, Keith Lofstrom wrote:
> Russia/Roscosmos surprised the world by launching their
> Luna 25 robotic lander on August 10, three weeks after
> Before Luna 25, Russia's "most recent" Luna 24 mission
> landed on the Moon on 19 August 1976, 47 years ago.
> Perhaps Russia did not want to be upstaged by "backwards"
> India ... however ...
I wrote an extremely speculative, long, and WRONG "theory"
of what might have happened, early this morning.
I am glad I did not send it, launching a raft of bullshit
into the blogosphere.
What happened to Luna 25?
Here are some details of the near-lunar landing plan:
In Russian, but the numbers are readable. With some
effort (and google translate) the text is also readable,
but the diagram and the numbers tell an understandable
Calculating simple Kepler orbits, plus my extensive
background in measurement, management wishful thinking,
and the relentless accumulation of errors in "open loop"
control systems, helps me speculate some plausible
explanations for the Luna 25 failure.
The diagram shows Luna 25 in a 100 km altitude circular
lunar orbit. Then a delta V at apolune into an 18 km
perilune 100 km apolune elliptical orbit.
This is discussed by "tolis" in reply #95 in this
Tolis calculates an apolune delta V of 16 m/s to reduce
perilune from 100 km to 18 km altitude. If the delta V
was accidentally extended by only 4 m/s, perilune is
subsurface - CRASH!
More importantly, a few small errors in the ACTUAL orbit
altitude and the ACTUAL delta V can lead to far more
than a 4 m/s apogee insertion error.
Presumably, this delta V was made with the same rocket
engine used for circular orbit insertion from an
elliptical trans-lunar trajectory from Earth. A
formidable machine, capable of km/s delta V. What's
a meter per second compared to that?
The mission might be done with three separate maneuvers:
(1) circularize to lunar radius with an apogee burn,
then (2) drop into lunar orbit with another, then (3)
circularize to 100km lunar orbit with a third delta V.
It might be cheaper and faster (but far more risky) to
PRECISELY launch towards a PRECISE point 100 km above the
Moon, then with a PRECISE 3-axis delta V (trans lunar
injection, TLI) to a 100 km radius circular lunar orbit.
I count at least 3 precise 3D vectors above (two delta Vs
and one precise point in space), which is 9 numbers that
must be close to perfect. A more leisurely approach would
relax and separate the constraints, but take longer, and
land on the Moon after Chandrayaan-3 did ... thus
missing the political/propaganda target.
("When do you want us to launch the shuttle, Boisjoly?
The mission timing suggests "beat India" and "hurry hurry"
thinking drove the decisions, acceleration of a program
that might have succeeded a year or two later, with more
time to think and plan and measure and calculate.
Chernobyl thinking, STS-51-L thinking, though thankfully
with no lives lost. Cheap-fast-good, pick any two.
Delta V starting with Earth launch to trans-lunar ellipse,
delivered by the launch stages of the Soyuz-2 Fregat
launcher? 10,920 m/s . A tiny error in that velocity
(ppm) results in HUGE displacements in lunar arrival
radius (many kilometers). And HOW would the Russians
measure velocity and distance? They do not have a
worldwide space radar and communication network like
the US does, just one radar ship in the Pacific for
launch trajectory measurements from the Vostochny
Cosmodrome. Although the Earth will rotate Russia's
space radars below the Luna-25 trajectory a few times
on the way to the Moon, those measurements (plus the
large angular azimuth variance of the radar dishes)
will add much uncertainty to the lunar arrival radius.
Accuracy demands MANY radars, and triangulation.
In peacetime, I'm sure the US would gladly provide
Roscosmos with more accurate tracking data, and Russia may
rely on that data to plan and execute their space missions,
but not while insane Putin threatens the world.
Thus, it is likely that Luna 25 arrived at the Moon
with excessive position and velocity error. The nice
diagram is a goal, not a bedrock reality.
Translunar injection is approximately 3000 meters per
second. While the Luna 25 MISSION GOAL is a 100 km
altitude circular lunar orbit ... how would Russia
verify this? They could use Kepler's laws and orbit
period and doppler shift on the received radio data,
but that process may not be well-calibrated for a
POLAR lunar orbit over the mass-lumpy Moon.
More errors will creep into the mission.
If Luna 25 uses radar altimetry, that will be problematic
over the very rough mountainous terrain near the lunar
south pole. The time-spread of the radar returns will
degrade altitude measurements and calculations.
I worry about this happening to Chandrayaan 3 as well.
Some list members are ISRO friends. I hope they will risk
those friendships to provide some "pessimistic" oversight.
I'd rather ISRO delay the landing by a month, rather than
fail on schedule.
Future lunar operations near the poles will REQUIRE some
kind of lunar GPS, both orbital and surface beacons.
Until we (meaning all of humanity and all lunar mission
planners) have that, lunar exploration will be an
UNNECESSARILY imprecise and risky business. It would be
a WIN-WIN if the US and India collaborated on designing
and deploying such a system, sharing the ENCRYPTED
results with the peace-loving nations of the Earth.
Speak softly, and carry a big DATA STICK.
More important to us, SSPS deployment will be far more
reliable if we have long-baseline beacons for determining
position and velocity. Huge SSPS constellations, with
very high mass and ENORMOUS collision cross sections, is
a Kessler cascade candidate that makes playing with matches
while standing in a pool of gasoline seem "relatively
prudent". While the fate of Luna 25 and Chandrayaan-3
may seem off-topic to SSPS discussions, all these systems
REQUIRE a firm bedrock of caution and near-perfect
real-time measurement and control to reliably succeed.
Perhaps that's just me, an electronic measurement system
designer who adds ppms, not percents. My colleagues who
design sixteen terabit flash memory chips think in parts
per quadrillion; I'm a sloppy amateur compared to them.
Sadly, I am an insanely obsessive control freak compared
to the politician-appointed bureaucrats who oversee space
mission planning. Perhaps far more "control freaky" than
many of the "visionaries" on this list.
Disciplinarians are party poopers. Sorry about that.
But ask yourself: "Do we want to party? Or PRODUCE?"