[HN Gopher] The damage to lunar orbiting spacecraft caused by th...
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The damage to lunar orbiting spacecraft caused by the ejecta of
lunar landers
Author : belter
Score : 75 points
Date : 2023-05-23 16:39 UTC (6 hours ago)
(HTM) web link (arxiv.org)
(TXT) w3m dump (arxiv.org)
| moralestapia wrote:
| Funny, didn't seem to be an issue when the landers came from a
| different country ...
| gs17 wrote:
| The paper models a 40 t lander. That's roughly Blue Moon. The
| sum of all Chinese probes landed on the moon is far less than
| that, and I wouldn't be surprised if the sum of all Soviet Luna
| landers was also less than that.
| shagie wrote:
| As noted, the paper is referring to a 40 t lander with a
| single 67 kN thrust engine.
|
| As some other data points, the Apollo landers were 16t (later
| ones a bit more for an extended mission). The ascent module
| had a dry mass of 2445 kg and had an additional 2376 kg of
| propellant (5t).
|
| The lander's descent propulsion system was capable of 10,500
| lbf (47 kN) that could be throttled between 10% - 60% and
| 100%. (1,050 lbf (4.7 kN) and 6,825 lbf (30.36 kN))
|
| The ascent propulsion system was 3,500 lbf (16 kN).
| gs17 wrote:
| And Starship is already 100 t empty, although we don't have
| exact numbers for anything Starship HLS really AFAIK.
| flangola7 wrote:
| Where did they say that the country was a factor?
| ikekkdcjkfke wrote:
| [flagged]
| pmontra wrote:
| So, land as few times as possible to carry what's needed to build
| a landing platform?
|
| Conversely, blowing an engine nearly horizontal to the surface is
| going to shut off the Moon for a while. That's a weapon.
| Etrnl_President wrote:
| [dead]
| SketchySeaBeast wrote:
| > Conversely, blowing an engine nearly horizontal to the
| surface is going to shut off the Moon for a while. That's a
| weapon.
|
| Is it in any way more practical weapon than just lobbing stuff?
| JumpCrisscross wrote:
| Yes. You don't have to aim as you inundate an orbital plane
| with debris.
| masklinn wrote:
| Good ol' kessler syndrome.
| JumpCrisscross wrote:
| > _Good ol' kessler syndrome_
|
| Kessler is a chain reaction. Destruction is caused by
| secondary effects, _i.e._ bits of satellites the primary
| projectile broke hitting targets. This is closer to an
| area denial weapon: the destruction is caused directly by
| the debris blown off the surface.
| jerf wrote:
| It is also not permanent. The orbits of the debris would
| intersect the point where they depart, which is pretty
| close to the engine. Basically the engine would be
| hitting itself with everything it fired.
|
| Therefore, the logical thing to do is to put it on the
| correct side of a mountain, to shield the engine. But
| that would also collect all the debris. So it would
| generally only be in orbit for one orbit.
|
| That makes it more targeted than some of the other
| unfriendly things you can do in an airless planetary
| environment:
| https://news.ycombinator.com/item?id=35862424
|
| (To be honest, I think on any planetary body without an
| atmosphere, long term everyone is going to _have_ to dig
| in to the planet, and to a non-trivial degree, too, not
| least of which is the complete indefensibility of surface
| installations.)
| snovv_crash wrote:
| The expansion of the gas after it leaves the nozzle in
| vacuum would give the particles an additional kick. I'm
| not sure if their orbit would still intersect the engine
| or effectively boost higher.
| Delfwood wrote:
| > blowing an engine nearly horizontal to the surface
|
| I doubt that objects put in "orbit" with a periapsis of almost
| zero (launched from the ground) would stay in flight for too
| long.
| [deleted]
| moffkalast wrote:
| Well lunar gravity varies a huge amount by location, it may
| be possible for some of it to somehow end up in actual
| orbits, especially when it's intentionally done so.
| Robotbeat wrote:
| But by the same token, those orbits will be unstable and
| the particles will quickly lithobrake.
| [deleted]
| adastra22 wrote:
| Or add an atmosphere to the moon. Even a tenuous one would
| prevent this.
| nomel wrote:
| A discussion about that:
| https://space.stackexchange.com/questions/12576/could-the-
| mo...
| belter wrote:
| As mentioned in the link below, the Moon has a tiny
| atmosphere.
| tabtab wrote:
| Small and medium meteorites smack into the moon's surface all the
| time, being it has no atmosphere. I find it hard to believe that
| human-built landers have nearly as much impact on low-orbit grit
| than these meteor impacts.
| dr_orpheus wrote:
| Some of the data from LADEE and Lunar Dust Experiment (LDEX)
| instrument might point to this being the case, but it was more
| along the lines of human-built landers do not have nearly as
| much of an impact as meteor showers themselves (not necessarily
| the impact and resulting dust).
|
| "if LADEE did encounter any lunar soil particles thrown up by
| the final descent of Chang'e 3, they would have been lost in
| the background of Geminid-produced events." [0]
|
| That said, the Chang'e 3 is an order of magnitude (or close to
| two) smaller than the lunar landers they are talking about in
| the study. Also my own speculation is that the more continuous
| thrust of a lander may get particles to higher velocities due
| to the additional time for acceleration in the wake of the
| thrust as compared to the single impact of the meteor.
|
| I struggle to compare exactly how bad the lunar dust ejection
| is though. Most Micrometeoroid and Orbital Debris (MMOD) curves
| are specified as a Flux by particle size (velocity is sort of
| irrelevant, as you assume most of the velocity is from the
| spcecraft itself and most hits are in the direction of travel
| of spacecraft, the ram direction). My suspicion is that MMOD
| flux in a LEO orbit is still going to be far far worse.
|
| [0] https://www.nasa.gov/ames/ladee-project-scientist-update-
| mil...
|
| Edit: The paper talks about flux of particles 10 um and smaller
| of about 10,000 impacts/m^2 during the passes. If we assume
| that this is a sphere of iron (new MMOD fluxes are specified in
| mass, not size) its ~5e-9g. In LEO at 400 km altitude (a little
| above the ISS) the flux of particle this size is ~1000
| impacts/m^2/year. But the paper says smaller than <10 um. And
| at smaller masses the flux increases exponentially to 10^7
| particles/m^2/year at a particle mass of 10^-18 g. So I believe
| my suspicion is correct that most LEO orbits are still worse,
| but its hard to compare apples to apples.
| azernik wrote:
| Landers don't impact hard - they fire rocket engines down,
| which may be much more efficient at kicking up dust than an
| impact.
|
| (I still think this is overblowing the problem, because any
| lander that causes this big of an ejecta problem would also
| badly damage itself. All the designs will put a LOT of
| engineering work into minimizing debris, eg Starship putting
| separate landing engines high up on the vehicle.)
| Sharlin wrote:
| > because any lander that causes this big of an ejecta
| problem would also badly damage itself
|
| Not necessarily because the relative speeds will be very
| slow. Not so in low lunar orbit, where an orbiting spacecraft
| will _slam_ into the ejecta curtain at >1 km/s.
| azernik wrote:
| The paper estimates that if there's LLO debris, it'll be
| starting out with about 1.6km/s of surface-relative
| velocity. Not something you want to get even a small
| percentage of on your landing gear.
| sneak wrote:
| My first time encountering this acronym. I can't wait
| until it's in common use and we have cities and
| communication satellites in and around the moon.
| cubefox wrote:
| I guess you really like 1950s sci-fi.
| Sharlin wrote:
| Oh, that's a good point.
| LorenPechtel wrote:
| The ejecta would go out to the side and not harm the lander.
| I'm just amazed that the ejecta is being thrown hard enough
| to be a threat at orbital altitude.
| adastra22 wrote:
| It's a combination of the moon's orbital velocity being low
| and the exhaust velocity being high.
| azernik wrote:
| That's a simplifying assumption (per the paper). In reality
| it would be a distribution, with a lot _less_ going up than
| to the sides.
|
| Plus then you're dealing with damage to your hopefully-
| smooth landing site.
| icodestuff wrote:
| That doesn't bode super well for Blue Origin's lander then,
| since its engines are on the bottom.
| azernik wrote:
| They're not going to get far without some other method of
| reducing ejecta, doing so is in the NASA requirements.
| _joel wrote:
| yet they've just been awarded a tender for HLS
| sandworm101 wrote:
| Impacts are impacts. Rockets are more like leaf blowers. Drop a
| massive rock into a pile of leaves and few leaves even move.
| Point a leaf blower at the pile and leaves will scatter
| everywhere.
| moffkalast wrote:
| Yeah but throw a car sized boulder into a pile of leaves at a
| few km/s and they'll scatter all the same.
| fnordpiglet wrote:
| I would expect it's the sustained thrust prior to landing and
| at take off that is the difference.
| uguuo_o wrote:
| This problem has been known for a long while. The models used
| here by Metzger have such a large uncertainty and only take into
| account erosion due to shear. As soon as the erosion transitions
| from shear-based (smooth sheets) to bulk (fluidized), none of the
| data extends to that regime. A massive vehicle landing on the
| moon will definitely cause fluidization. To estimate erosion and
| ejecta needs far more detailed numerical methods [0].
|
| [0]https://doi.org/10.1017/jfm.2021.29
| jcims wrote:
| Didn't we largely solve this with the sky crane for recent Mars
| missions?
| ericbarrett wrote:
| Velocity required to reach a 50 km orbit* above the moon's
| surface is only ~1.6 km/sec, and there's no air resistance to
| slow dust particles kicked up by the craft.
|
| For Mars, the orbital velocity is ~3.5 km/sec, thus requiring
| almost 5x the energy for a given mass of detritus (E =
| 1/2mv^2); and while its atmosphere isn't as thick as Earth's,
| it'll definitely cause drag for particles going that fast.
|
| * You don't quite need orbital velocity for a plume to get high
| enough to disrupt an orbiting craft, but it's a handy reference
| point.
| masklinn wrote:
| The lander size under consideration is about 40x that of the
| Martian rover, I don't know that a sky crane would work as well
| without a parachuting stage and atmosphere, and finally it
| seems rather unhelpful for taking off.
| inamberclad wrote:
| I presume this is part of why SpaceX is planning to use
| landing thrusters that are higher up the rocket.
| moffkalast wrote:
| Afaik that's mainly so the plume of razor sharp dust
| doesn't tear their engines to billion tiny pieces. Unlike
| the LEM, they won't be bringing a spare for liftoff.
| _joel wrote:
| I wonder how much Starship mitigates that by having the descent
| propulsion controlled by the top thrusters. I'd assume they use
| the same thrusters from take off too as we've seen the damage
| Raptor 2's can do, let alone 3 and whatever comes after the
| 350bar line.
|
| Also, could regolith be bound or, well, packed down to build
| pads?
| WalterBright wrote:
| Why wouldn't the orbit of the ejecta intersect with the surface,
| since it is in free fall ever since leaving the surface?
| CydeWeys wrote:
| I think this is only in the immediate aftermath of a landing,
| not permanent orbiting debris.
| WalterBright wrote:
| How is it going to get into an orbit that doesn't intersect
| the moon? Any impulse from the surface will hit the surface
| in one orbit.
| zardo wrote:
| The moon has large enough density variations that you can't
| analyze low orbits as though it's just a point mass.
| WalterBright wrote:
| Yes, I know about the masscons, but it seems highly
| unlikely they'll be just at the right point. It's hard to
| believe that any object given a single impulse at ground
| level, in a vacuum, is not going to hit the moon again in
| one orbit (unless it is given escape velocity).
|
| Masscons did perturb the Apollo missions enough that they
| had to switch to a doppler radio to navigate accurately.
| PeterisP wrote:
| The paper models an 'ejecta sheet' of particles that exceed
| the escape velocity of the moon and thus aren't in orbit.
| fnord77 wrote:
| so, low lunar orbit/landings could be DOSed by a couple of well-
| placed impacts of sufficient force?
| [deleted]
| areoform wrote:
| The paper's author Dr. Phil Metzger is such a rockstar. He is The
| Expert on the mechanics of soil erosion by rocket exhausts, and
| writes a lot about the problem in an approachable way,
| https://twitter.com/DrPhiltill/status/1658507854859337737
|
| It turns out that the rate & mechanics of erosion by rocket
| plumes is an unsolved problem that requires a new kind of model.
| To quote from his thread, In the Apollo era the
| thinking was that the rate of soil erosion is controlled by
| conservation of momentum. It turns out this is wrong.
| NASA researcher Leonard Roberts, the first person to research
| this topic, hypothesized that the soil grains steal momentum from
| the gas, which slows down the gas and thus reduces the erosion
| rate. It was this feedback that determined the rate.
| I argued some years ago this has to be wrong because the
| particles achieve their high velocities far downstream of where
| they are lifted off the surface, so momentum transfer does not
| provide feedback to control the rate that grains are lifted.
|
| He's going to be publishing his alternate model soon-ish. Can't
| wait to see what he has come up with.
| belter wrote:
| "...This manuscript analyzes lunar lander soil erosion models and
| trajectory models to calculate how much damage will occur to
| spacecraft orbiting in the vicinity of the Moon. The soil erosion
| models have considerable uncertainty due to gaps in our
| understanding of the basic physics. The results for ~40 t landers
| show that the Lunar Orbital Gateway will be impacted by 1000s to
| 10,000s of particles per square meter but the particle sizes are
| very small and the impact velocity is low so the damage will be
| slight. However, a spacecraft in Low Lunar Orbit that happens to
| pass through the ejecta sheet will sustain extensive damage with
| hundreds of millions of impacts per square meter..."
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