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Mars Penetrator Probes, Brilliant Balance Masses & Other Ideas
JRehling
post Feb 5 2021, 06:53 PM
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There was a report on the failure of Deep Space 2 which was inconclusive… there were many possible points of failure. The report came out in 2000, and included the possibility that the ground was rockier than was designed for. I think the subsequent findings of Phoenix (the landing site of which was not as poleward as MPL's) may mean that ice was closer to the surface than anticipated and perhaps purer than anticipated. The unexpectedly high abundance of ice may have caused the failure, but then again, there were many candidate causes.

If anything, I would imagine that hitting relatively pure ice would be advantageous because it reduces the unknowns you have with a mixture of soil and rocks, but only if the ice was what you designed for.

Significant point: The intended depth of the DS2 penetration (0.3 to 2m) was less than that required by Insight's heat probe. If you land in an area that is, like the Viking 1, Viking 2, Pathfinder, or Spirit landing sites, the stochastic distribution of rocks in the soil creates an element of randomness to the penetrator design. If hitting a rock is fatal to the outcome and the surface has about 10% rock by area, that's a high probability of hitting a rock somewhere in 3m of penetration.
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mcaplinger
post Feb 5 2021, 07:20 PM
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Any penetrator discussion should start with a reading of https://www.lpl.arizona.edu/~rlorenz/penetrators_asr.pdf

QUOTE
Because the success of emplacement and operation depends inevitably on uncontrollable aspects of the target environment, unattractive failure probabilities for individual vehicles must be tolerated that are higher than the typical ‘3-sigma’ (99.5%) values typical for spacecraft. The two pathways to programmatic success, neither of which are likely in an austere financial environment, are a lucky flight as a ‘piggyback’ mission or technology demonstration, or with a substantial and unprecedented investment to launch a scientific (e.g. seismic) network mission with a large number of vehicles such that a number of terrain-induced failures can be tolerated.



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Disclaimer: This post is based on public information only. Any opinions are my own.
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HSchirmer
post Feb 5 2021, 09:46 PM
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QUOTE (Art Martin @ Feb 4 2021, 08:29 PM) *
My guess is creating the penetrator with a heat probe inside it is not as much the issue as ensuring that there's a way to transmit whatever data it found though electronics and power circuits capable of surviving the trauma of the penetration. The thing would basically look like an arrow where the pointed head would naturally seek down (essentially a lawn dart). Have we done such a task on Earth before?


Yes.
The US Air Force currently has high g-force logic chips for "bunker buster" munitions which not only survive slamming through multiple meters/yards of rock & reinforced concrete, but also use accelerometers to count how many floors they've punched through, and then detonate at a specific floor number.
These chips COULD be repurposed so a Mars probe's ballast slams into the ground, penetrates X-meters deep, records the details about the strength of underground rock layers in non-volatile memory, then transmit that to a rover. With a bit of engineering, I'd expect that primary rover could position smaller "weather stations" with big unfurling solar panels that talk to the embedded "lawn dart" and then broadcast surface and at-depth temperature and vibration data.

"The HTSF (Hard Target Smart Fuze) designated the FMU-157/B, is an active decision-making accelerometer-based fuzing system capable of counting layers and voids (floors), as well as calculating distance traveled. When the weapon reaches the pre-determined floor it tells the bomb to explode. The HTSF is compatible with a variety of penetrating warheads."
https://www.globalsecurity.org/military/sys...ons/fmu-157.htm
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Tom Dahl
post Feb 6 2021, 12:30 AM
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Indeed, the Viking capsule had a carefully-controlled offset center of gravity. Here is a diagram showing the overall layout of components within the lander capsule with an indication of the 1.85 inch offset CoG (this is in one of my Google Photos albums with many other historic Viking diagrams and photographs). The decelerator (parachute) canister and mortar support truss was aligned with the capsule's offset CoG, rather than with its geometric centerline. As far as I know, there was no "dead-weight" ballast on Viking.
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stevesliva
post Feb 6 2021, 03:01 AM
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QUOTE (HSchirmer @ Feb 5 2021, 04:46 PM) *
The US Air Force currently has high g-force logic chips


I've said this about solid state electronics for Venus, and I'll say it even more so here... the "chips" aren't the hard part. The hard part starts with what you solder/otherwise attach them to.

Airplane black boxes are solid state now for the recording media. I suspect they didn't need to do much to the silicon at all. And probably not much for the package. And they don't have to worry about the board. And you're right, there's probably tech for that too. I'm just being semantically ornery about "chips."

QUOTE
Paradoxically, one of the biggest problems is probably making them heavy enough


Is depleted uranium right out in these RoHS days? Incredibly enough it was used in older 747s... they use tungsten now. And W<Pb, but RoHS, probably. Make part of it out of DU, and it'll penetrate like an antitank round, and maybe be heavy enough.
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rlorenz
post Feb 6 2021, 04:02 AM
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QUOTE (JRehling @ Feb 5 2021, 01:53 PM) *
There was a report on the failure of Deep Space 2 which was inconclusive… there were many possible points of failure. The report came out in 2000, and included the possibility that the ground was rockier than was designed for.


I was on the DS-2 science team, responsible for interpreting the impact accelerometer record to determine the hardness and layering of the ground. We even did 200 m/s airgun tests at a range in New Mexico.

Of course it cannot be excluded, but I never liked the 'hit a rock' failure scenario. Environment causes (terrain, atmospheric factors [especially Jedi'd away as 'turbulence']) are all too convenient for engineers... The loss of both DS-2 probes is less easy to explain this way, and the associated loss of MPL all suggested a common cause, and I suspected some sort of separation failure. Even on the day of landing, a JPL engineer (Cook?) dismissed that, noting that there were redundant pyros. But redundancy doesnt save you if there's a common failure mode (everything too cold, a missing command, whatever). As the years have gone by, I've heard stories about cold-welding on umbilical connectors that could have caused some issues. I guess we'll never know.
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rlorenz
post Feb 6 2021, 04:14 AM
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QUOTE (Phil Stooke @ Feb 4 2021, 06:59 PM) *
The impact sites of the various MSL balance masses were imaged by HiRISE after the landing. Big dark splats. Presumably the new ones will be as well. There might be a bit of science from that, extending the range of impact effects of objects of known mass and velocity.


Yeah, maybe. In theory the two impactors are of known mass and velocity. But there is the small matter that there were four craters... (see Bierhaus' 2013 LPSC abstract) - did the cruise stage partly survive entry, with two large fragments? Or did it burn up and the two balance masses broke in half ...
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Art Martin
post Feb 6 2021, 05:05 PM
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QUOTE (mcaplinger @ Feb 5 2021, 12:20 PM) *
Any penetrator discussion should start with a reading of https://www.lpl.arizona.edu/~rlorenz/penetrators_asr.pdf


What a fascinating read. Thank you.
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JRehling
post Feb 6 2021, 07:11 PM
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Ralph's piece is indeed a great read, and it's the first paper about planetary exploration that I've ever read mentioning people dropping things off the top of a football stadium.

For the heat probe objective specifically, would the summer ice cap be an appealing target? The martian regolith holds arbitrarily many differences from Earth, but H2O ice is a known material, and the rock problem (even if moderate elsewhere) would be near zero on the ice cap, right?
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rlorenz
post Feb 7 2021, 04:08 AM
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QUOTE (JRehling @ Feb 6 2021, 02:11 PM) *
For the heat probe objective specifically, would the summer ice cap be an appealing target? The martian regolith holds arbitrarily many differences from Earth, but H2O ice is a known material, and the rock problem (even if moderate elsewhere) would be near zero on the ice cap, right?


There's a few points to this. The 'ultimate' goal of HP3 was to measure the planetary heat flow (well, to measure the crustal heat flow at one point, from which the global heat flow might be estimated... there's more that could be said about that..but let's skip that today..) You do that by measuring the local thermal conductivity (which MP3 can still do, at its shallow depth) and the temperature gradient.

But, there are additional gradients imposed by transient events, by the diurnal cycle, by the annual (seasonal) cycle, and by longer-term variations (global dust storms on some years not others.... perhaps even a Little Ice Age signal, if the LIA was caused by solar variations rather than e.g. volcanism on Earth). The 3-5m depth goal of HP3 was driven by the need to get under the annual heat wave to pick up the underlying gradient, and you need a big enough depth range that the measurement errors on temperature itself do not degrade the measurement (e.g. at a 10K/km gradient not untypical for Earth, a 10m borehole gives you a 0.1K temperature difference, so if your measurement error is 0.02K, you can be 20% off, but if you could drill a 100m hole, you'd be within 2%...) . The annual wave depth is predicated on an assumption of conductivity - specifically the e-folding penetration depth of a wave is (kappa * tau)^0.5 where kappa is the thermal diffusivity and tau the timescale. For regolith, kappa is maybe 1E-7 m2/s , for solid ice, maybe 1E-6 (depends on temperature). So in solid ice, the thermal wave penetrates deeper.

(I published a paper a while back showing that if the LIA were caused by a solar luminosity variation, it could affect the HP3 heat flow retrieval. You can see a LIA signal in Greenland ice cores, down to 50-100m IIRC, which would correspond to a few meters in regolith)

So yes, from a penetration mechanics point of view, maybe an ice sheet has the prospect of better homogeneity (no rocks - although surface textures on glaciers and ice sheets can in fact be rather forbidding). But from the geothermal heat flux measurement standpoint, you get a much poorer measurement for a given depth in solid ice than in regolith.

There is also the question of how to attain that depth. I don't think a self-hammering drill like HP3 could penetrate solid ice effectively. A kinetic penetrator (e.g. DS-2) similarly would only go in 0.5-1m into ice, far too shallow to do heat flow. So to do an ice cap drilling mission (and that would be cool, and I think there have been Discovery/Scout/DSMCE proposals) you'd need a conventional drill or a thermal drill, a much more complicated proposition.

In this context, HP3 was a relatively inexpensive thing to try.

Recall that heat flow has been done on the moon, albeit with astronauts drilling a hole and emplacing a probe with the temperature sensors. Yet even that effort yielded only 2 measurements out of 4 planned (Apollo 13 never made it to the moon; Apollo 16 a cable broke). Space is hard.

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