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OK, guys. No one really liked my Mars mission architectures based on nuclear thermal rocketry. So, over the last couple of years, I have worked out a one-stage reusable lander, and all-chemical transit vehicles. It's pretty much similar to my nuke stuff, except I cannot afford 16 landings, only 6, and I really had to get creative to get that done in a credible fashion. And, I have added a Phobos trip and the start of a base to the mix! I posted it today over at "exrocketman". Too big to try to bring here.
I have no idea what "active material" means. The article cited gives no hint, either.
The 30% of an atm compression is not what is required for life support. It is what is required to avoid oxygen pre-breathing to purge nitrogen without getting the bends. That is an artifact of 0.7 to 1 atm of ordinary air. You could go to just under 0.5 atm at 40% oxygen, and avoid nitrogen purge, at a moderate fire danger risk, and a still-unknown hazard to pregnancies.
All that is required of MCP is 20-25% atm compression. 15% if you are really acclimatized to high altitude living. Webb achieved that 25% level with panty hose material in the late 1960's.
I have to ask why this is still a research issue half a bloody century later. It ought to be prototype test issue by now.
GW
"Withstanding a temperature" almost has no meaning at Earth entry conditions. That's a concept from earthly furnace and engine experiences. A rough rule of thumb for entry says the heating rates behave as if the air temperature in degrees K is numerically equal to the speed in meters per second. It's not really that high, it's ionized instead, but the energy content of the air behaves like that (the more energetic the "air", the more it overheats whatever it touches).
It's in an afterburner or a ramjet combustor that you actually "withstand" a real gas temperature of 4000 F. That's a different kind of heat transfer problem entirely, more like what's actually in the heat transfer textbooks. That entry stuff is a specialty application that really isn't in there.
Most of the entry heat shield ablatives are pyrolyzing at a material surface temperature near 3000 F (the organic binder types), some nearer 4000 F (the carbon-carbon composite types because that's about the sublimation point of graphite). The detailed heat balance of this process is not easy to analyze. There are no simple models that really work well.
Refractories like shuttle tile or metallic shingles reach an equilibrium temperature where heat radiation by the T^4 law (and any conduction into the interior) just balance the convective input from the oncoming stream. At the stagnation point that convection is proportional (more or less) to velocity^3. What complicates this is that none of these materials are "black" or "gray" emitters, all have very strongly spectrally-dependent emissivities, and they are not the same dependencies at high material temperature that they are at room temperature.
Below about 10 or 11 km/s velocities, radiative input from the shock layer adjacent to the heat shield is relatively unimportant. Above that speed, it quickly dominates, being more-or-less proportional to velocity^6. The thing about that equilibrium material surface radiating temperature is that the material must withstand being that hot, and must also withstand the temperature gradient into the material, too. Lots of stuff cracks when the gradient is too steep.
The historic capsule heat shields were silica or carbon phenolic, very dense, very heavy. PICA and PICA-X are low density variants of this same basic ablative material concept. I'm less familiar with Avcoat, but it is also a more-or-less simple ablative, just not good enough for the windward side of most spacecraft today.
I don't know a lot about the metallic shingles, but the low-density ceramic that was shuttle tile is an alumino-silicate processed to achieve very high void space inside the material. It melts near 3300 F, but there is a solid phase-change at 2300 F that causes shrinkage cracking on cooldown. You have to stay under that, if you expect to fly more than once. The re-radiation process requires "black" high-emissivity color on windward surfaces, while "white" low-emissivity works on leeward surfaces. It's just not up to the job near stagnation, which is why the shuttle had carbon-carbon ablatives for its nose cap and leading edges.
GW
Lace-up is slow but very effective. That's what they used in the earliest pressure-breathing rigs (1940's) and partial-pressure suits of late 40's and early 50's (a real misnomer if ever there was one).
In the more-developed partial-pressure suits of the mid-50's, they went to an inflatable tube that tensioned and relaxed the non-elastic fabric. The compression they achieved was very uneven, and hands and feet were unprotected. But it was good enough for a 10 minute fall bailing out from 70-80,000 feet. Medically, 80,000 feet is hard vacuum.
I would suggest combining that inflatable idea (but reversed) with elastic fabric. Build it so that it requires inflation pressure to relax the elastic grip by increasing girth, not decreasing it. That way it is fail-safe when out in vacuum. Relaxed grip should make donning and doffing much easier than the panty-hose problem of the late 60's.
There is absolutely nothing wrong with the gel bag way of coping with otherwise-intractable anatomical geometries. That's how you "tailor" the suit to fit its wearer, more so than the suit itself. Fit is essential to achieving even compression, that's the downside with MCP. There's genitalia, breasts, and small-of-the-back problems to address with gel bags.
BTW, if the suit tears, you don't have to suffer much bruising at the tear location. Just give the torn spot a tight wrap of "Mach 1" tape. Any compression beats no compression at all.
You know what they say about duct tape and bailing wire. That's the band-aid kit you use until you get back to the shop.
GW
We still have a long way to go before we understand how to successfully implement a closed-cycle life support. The wastewater in your transfer vehicle needs to be used as your solar flare radiation shield. Only in a one-way mission would you land that material on Mars. And you will need it as fertilizer for farming, even without a closed cycle ecology.
The water problem points out exactly why the first base needs to be sitting on top of a massive buried glacier, for easy ice mining. Those are mostly thought to be somewhere around +/- 30-40 degrees latitude, I think, but there is as yet no ground truth regarding the existence of massive buried ice there. Equatorial bases apparently will lack even the possibility of such a water resource.
Ground truth: you gotta drill. Deep. No other way is known. Considering what kinds of landers and rovers we'll send before men actually go, we'll not have the ground truth about buried massive ice. The men will have to get that data when they go.
Think they might need to take a drill rig with them? Think maybe they need to visit multiple possible base sites with that drill rig on that first mission? Think maybe we pick the best site based on ground truth findings, and establish that base on that first trip, before they fly home? I do.
Do that on the government first mission, and the commercial guys will go there. The government won't go twice. Politics of money will forever prevent that. It's not very likely the government alone will even mount that first mission. But without government help, not even the most visionary commercial guys will undertake the first landing mission. This has to be "bootstrapped" just right, or it'll never happen.
GW
That's very good news, Midoshi. Keep us posted.
GW
Exoskeleton suits: there are lots of jobs better done by remote teleoperated equipment. There are some jobs where eyes on site with full field of view works better -- and that's where an exoskeleton suit comes in handy when the forces exceed what a human can do. Hard to name specifics, but you have to be prepared for both eventualities, since unplanned things have such a nasty habit of happening.
Vacuum-protective underwear (MCP done "right"): this frees up people physically to do a lot more, both in terms of agility, and in terms of fine motor skills. For one thing, you can doff the compression gloves and work barehanded for many minutes, as long as thermal injury is not an issue. Human applied force levels can be higher, when unimpeded by a resistive gas balloon suit. Done with stretch fabrics, it would be difficult to don, as it was in the late 1960's, but you can leave it on whether inside or outside. All you have to remove going inside is the helmet, oxygen backpack, and your outerwear. You sweat right through it for cooling. I am assuming we can do a better job now of garment design than the 1969 model.
GW
I looked around in 2 or 3 places on the internet and confirmed what I heard on the TV news last night. The Spacex site has the latest tweets posted. The rocket worked fine, including the second stage restart. The satellite is in the intended geosynchronous orbit. Congrats to Spacex.
GW
Yep, it's always tough, even for a lot acceptance test on a production motor. A lot of rocket guys tend to be a bit superstitious around test time, especially us old solid propellant missile guys. We would often joke about sacrificing a virgin to the test gods, or at least a reasonable facsimile thereof. None of the gals ever volunteered, though. Even so, most of the live firings went very well.
GW
Spacex website says the abort cause was "oxygen in ground side TEB/TEA". Somehow or another, there was unintended oxygen in the igniter line somewhere, which I suspect is a very serious issue. I don't know exactly how they are doing this, but TEB/TEA has been used a long time. It was the afterburner igniter for the SR-71's engines.
Anyhow, today their site says they have resolved all the issues. There will be a launch attempt today (Tuesday) during a window about dinner time Eastern, with a backup day tomorrow. Cross your fingers.
Their site does say cleaning out all the turbopump machinery took a lot of time. I would suggest they talk to XCOR about piston pump machinery for some future "Merlin 1-E". Reusability is as much about lower labor costs as it is about avoiding production costs.
GW
The perchlorate that is technologically useful as a solid propellant oxidizer is ammonium perchlorate (AP), not any of the metal salts. AP is an ocean evaporite product, but it is nowhere near as common as the nitrates, like sodium or potassium nitrate (those are "saltpeter" for black powder).
In the US, there is an AP mine in Utah. That's where the industrial plant that processes it into rocket-grade AP is located. There used to be two plants there, but one blew up. AP is a mass-detonating monopropellant explosive hazard.
The other useful solid oxidizer is ammonium nitrate (AN). It has to be synthesized from ammonia and acid. AN is a less effective oxidizer than AP, but still far better than the saltpeters and similar. AN is also a monopropellant mass-detonating explosive hazard, but it is nowhere near as sensitive to initiation as AP.
GW
A lot of folks use an igniter that is simple TEB (tri-ethyl borane) or TEA (tri-ethyl aluminum) injection. No real hardware required, other than the injector line. The stuff explodes on contact with air though, so handling is a bit more stringent than hydrazine, or even nitric acid.
GW
I think the exoskeleton idea would be great for stuff like loading dock and heavy construction work. In that sense, the sci-fi movies got it right.
I think we really do need an MCP suit, but I think the concept should be revised to "vacuum-protective underwear" with a pure O2 breathing helmet and tidal volume bag, rather than the traditional "everything for any use" idea. You wear whatever outer clothing is appropriate to the job you are doing, over your "vacuum-protective underwear" MCP suit. Every job is different, just like here at home. In fact, the outer clothing can be exactly what we wear here at home, boots and all.
I have some body-compression/breathing-pressure requirements for MCP designs analyzed and posted (that include the water vapor displacement effect) over at http://exrocketman.blogspot.com, in an article dated 1-21-2011, and titled "Fundamental Design Criteria for Alternative Space Suit Approaches". These compression recommendations are surprisingly low to some (but not me), they ignore the nitrogen-blowoff decompression often required from higher-pressure habitation atmospheres, but they do fall within the reach of MCP compression garments made with materials available since 1969.
I based this analysis on effective wet-lung partial pressure of O2 equivalent to that at 10,000 feet altitudes here at home. Even-lower values would work, but not very effectively for unacclimatized flatlanders. There has to be a habitation atmosphere compatible with no-decompression MCP suit use at these lower compression levels, and with health safety for inhabitants that are reproducing successfully. I just don't know what it is. Yet.
There's not much time left to prove out these ideas, if the first trip will really take place circa 2030. The smart thing to do is establish a base on that first trip. The commercial guys will follow quickly, once that is done.
GW
Re Terraformer in post 197 (hey I'm agreeing with you!): Miscarriages are not (anecdotally) noticeably higher in Boulder CO or even in the Andes. There is acclimatization, including genetic changes, associated with the higher populations (15-20,000 feet above MSL). But, the fact that the Ptot is way lower up there while the percentage (20.9% O2) is unchanged means it is partial pressure of O2 that is far lower up there.
That in itself tells you that partial pressure is NOT the entire story as regards human health. The real world is almost always more complicated than what we can write down in simple science equations. Midoshi's recovered post is quite good, but I'd bet we'd better consult some medical schools about this before deciding upon an atmosphere for colonists who can have babies. And we'd better do the research. Time is short, the smartest thing to do is to plant a base on that first mission. The commercial guys will follow ("if you build it, they will come").
GW
I'm sorry, I have to both agree and disagree simultaneously with RobertDyck. Partial pressure of oxygen (or any other constituent) is the product of total pressure and percent (by volume, same thing as mole fraction) of the oxygen (or other component). Those two must both be considered to find the partial pressure of the component. No way around that. And that's where I disagree with him. It's just not one number.
As for partial pressure of O2 for breathing purposes, that's what drives osmosis across the alveoli. There is a displacement effect for both water vapor and for CO2, but the CO2 is essentially neglectible, because the numbers are so small. The water vapor pressure is not, because it is always 100% humidity inside the alveoli, and the equilibrium vapor pressure of water is significant at body temperatures. At human body temperature (37 C), the equilibrium partial pressure of the water vapor displacement is 47 mm Hg, regardless of the partial pressures of all the other constituents. And there's no way around that, either.
The net effect of that is that lower total pressure dry atmospheres must have higher percentage O2 content, just to get the same partial pressure of O2 inside the lungs, by that 47 mm Hg. No way around that.
The 17 psi partial pressure of O2 in the Apollo-1 (old name Apollo-204) fire was also the total pressure of the dry atmosphere being used in the capsule, because it was straight dry O2, no other components. That's 879 mm Hg oxygen partial pressure which also equals the total dry atmosphere pressure. Inside the wet lungs, there was essentially 47 mm Hg water vapor pressure and 832 mm Hg oxygen partial pressure. Those two add to 879.
That's where I agree with RobertDyck: at either 879 mm Hg in the capsule, or 832 mm Hg inside the lungs, there was more than enough oxygen to make almost anything "ordinarily flammable in air" into something almost explosive, which is typical of any oxygen-enriched atmospheres with total pressures anywhere near 1 atm.
Chemical combustion reaction global rates are more-or-less proportional to oxygen and fuel partial pressures raised to some exponent that typically falls between about 0.2 and 1.5. (Depends upon what global reaction rate model you want to use to use as an approximation for something fundamentally more complicated.)
So, yes, fire is a serious concern, including atmospheres like 0.5 atm at 60-40 O2-N2, which is an O2 partial pressure of 0.3 atm, richer in O2 than sea level air. It ain't just lungs drying out.
And there's doctors that cry out about damaging effects to the fetus if the O2 percentage is too high, even if total pressure is somewhat lower. That seems to be a partial pressure thing, but no one knows for sure. And THAT is my point. Until we do know, then 21% O2, 79% N2, at 0.6 to 1 atm total pressure, seems to be the "safe prescription" if pregnancy is involved, and with respect to fire dangers as well. Until we know better, there is no way around THAT.
As for EVA suits, I have posted some stuff on what is really needed, ignoring the N2-blow-off decompression effect. Based on partial pressure of O2 inside the wet lungs, with water vapor displacement effects included, I get a minimum compression or breathing pressure of 20 to 25% of an atmosphere, if you feed pure, dry O2 to the breathing helmet. I posted that analysis long ago over at "exrocketman", in an article dated 1-21-2011. I intended it as a design requirement for MCP suits, but it applies to gas balloon suits, too.
GW
Re post 187 above: a transit vehicle needs to fairly spacious, or the crew won't arrive in good mental condition. There should be enough space for a prep room adjacent to the airlock, in which the astronauts can prep and dress at reduced pressure/enriched oxygen relative to the rest of the hab. That's the N2 blowoff needed before going into a pure O2 suit at 20 to 35% of an atmosphere breathing pressure. The rest of the hab could be 21-25% O2 at about 0.7 to 1 atm total pressure. Doesn't have to be Earth air in composition, but if it's close, fire dangers are reduced, and so are hazards to pregnancies. It certainly need not be full sea level pressure, either. Could be as low as 0.5-0.6 atm. (The pregnancy thing applies more to a base or colony, but all the air composition and pressure considerations still apply to base or transit vehicle).
GW
Does anyone know which perchlorate salts they are talking about, when they say "perchlorate"? Some are more useful than others.
All are poisonous if concentrated in the water, so water as ice or liquid in the porosity of salty dirt is not usable for life support. You need to find "massive ice" whose origin was frozen surface water, so that it will be fresh (free of perchlorates or any other salts).
As I already said, arctic (and antarctic) pack ice is freshwater ice, in spite of the salty ocean from which it formed.
GW
It's been found that the Hindenburg would have crashed and burned even if she had been filled with helium. The hydrogen just made the fire brighter. The real fire was her doped skins which were actually a pyrotechnic material. The dope was nitrate base (well known, even infamous, for being extremely flammable) with a pigment that amounted to a weak thermite explosive (aluminum and iron oxide, but at the wrong ratio to be fully explosive). These skins were not electrically grounded to the aluminum frame, allowing ignition by static electrical discharge. That information was in insurance records from 1937 kept secret by the zeppelin company. These were finally opened in the late 1990's, and more than one investigator has now seen them. They (the owners) knew then what caused the crash.
Static discharge (any friction source, not necessarily anything to do with the thunderstorm in the vicinity) started a skin fire on the upper aft fuselage near the base of the upper vertical fin. The heat from that fire burst the hydrogen envelopes inside, one-by-one, as the fire propagated forward around 20-to-30 m per second. The slow tail-first crash confirms the one-by-one envelope burst pattern moving forward. The two-digit burn rate in m/s confirms the fire was the skins, not the hydrogen. Pre-mixed hydrogen in air only burns about 1.3 m/s laminar flame speed at 1 atm pressures, and this wasn't pre-mixed, much less turbulent.
As for Curiosity, static discharges really derange electronics, often fatally. This has been known for a long time now. These discharges are quite common on structures immersed in dusty winds, sometimes at lightning strength. If these discharges cause surges that reach the electronics, then the rover is at risk. But, I'd have a hard time believing they didn't provide protection in their design. This isn't the first rover those guys have built. It's just the first nuclear-powered one.
GW
PVC/plasticizers? I dunno. I'm not that good with the plastics and elastomers. Metals, wood, and organic composites I have experiences with. But I do know a little about aircraft fuel bladders, and the bladders used in liquid-fueled missiles. Those are various rubbers, including both a polyurethane and neoprenes. They work with what we consider to be "room temperature liquids". Under military specs, that's as bad as -65 F to 145/160 F.
If the Mars ferry used NTO-MMH or similar "storable liquids", the tank bladder materials we have right now could be part of the balloon tank we are talking about here. I'd use the tank bladder material as an inner liner layer, with a "rubberized canvas" outer layer, perhaps of neoprene and kevlar, or even just cotton. It would probably need an outer fabric shell with some distributed heating elements built-in, so that it could be maintained warm enough to fold up for stowage without cracking.
If we use cryopropellants, then we'll need to develop a combination of presently-unknown materials that would work very cold. Most of the elastomers that we have undergo glass transition at too high a temperature even for LOX.
I don't know much beyond that, so I cannot judge whether a cryo-propellant balloon tank could be developed in time for a Mars mission in the 2020's or 2030's.
GW
If the payload fraction is big enough, losing some of it to reusable ascent balloon tanks could well be worthwhile. Elastomers will be hard to come by on Mars, at least initially. Does anyone know if there really is a suitable elastomer for LOX and LCH4? It doesn't have to bend while cold, just not spontaneously crack.
As for N2O4 and MMH, those are room-temperature storable, and we already have elastomers used as fuel bladders for them. The difference between that scenario and mine is we dispense with the surrounding metal pressure tank and run the bladder "naked" as a balloon. I'd bet they need to be composites: essentially rubberized canvas, to control shape.
Tanks like that would need to be pressurized somewhat, much like the stainless steel balloon tanks on Centaur. Then you withdraw propellant with a pump as you add pressurant gas to the expanding bubble on top of the propellant inside.
XCOR is demonstrating that the propellant pump can be positive-displacement piston technology, which has a far longer potential service life, as nothing is on the "hairy edge" the way turbine blading has to be.
I don't see why this reusable ferry can't be built almost right now, and simply used to make a bunch of landings on the first trip. It should go on the first mission, and be left there for subsequent missions and base personnel to use.
GW
Hi Bob:
I had considered external airframe insulation, and I have used it inside a burner to include a low-pressure gradient nozzle. It would do what fiberglass wool does at temperatures higher than glass can stand. Had never considered rocket nozzle bell extensions, but it might work.
Proposing to the government is a difficult process at best. Do you know someone in DARPA we can approach with this, someone who can guide us through the proposal minefield? And, the small business minefield is different from the big corporate minefield for sure, but just as lethal. That's why we need a contact inside DARPA.
GW
Hi Josh:
To answer your question, I don't remember now where I read about pregnancy risks vs "air" composition and pressure. It was about the time I first started looking into MCP suits and found that old flight surgeon's page about his late 1960's experiments. It was people like him (not him specifically) voicing concerns. But it makes sense, because all sorts of seemingly-trivial insults can induce abortions and miscarriages.
The numbers I have based on populations living at altitudes agree quite closely with your 55 Kpa figure for something resembling Earthly air. I had not heard of the 1.2 ratio governing nitrogen for getting into a spacesuit without pre-breathing O2, but that neatly explains the older 1/3 of an atmosphere O2 relative to 1 full atmosphere of air. That 1/3 of an atmosphere compression is what has been holding back MCP suits.
I ran some quick numbers on 55 Kpa in the hab air. 0.543 atm total for 0.114 atm O2 and 0.429 atm for N2 (excluding argon, call it "synthetic air if you like). Using the 1.2 ratio rule, the pure O2 suit has to be at least 0.357 atm, just about the old NASA compression requirement. That's 36.2 Kpa, or 271 mm Hg, which is beyond what MCP can achieve (200+---ish mm).
The old experiments say 190 mm Hg worked, and my altitude-equivalent numbers say 170 mm Hg would work, even accounting for water vapor displacement in the lungs. None of those are compatible with the 1.2 rule using 55 Kpa air in the hab. You would have to use a higher oxygen/nitrogen ratio in the hab to match an MCP suit and the 1.2 rule, and the doctors start screaming about high oxygen during pregnancy. 0.243 atm O2 and 0.300 atm N2 for 0.543 total pressure, or 45% O2-55%N2 mix at 55 Kpa. That's just about like the hospital 40% O2 that the doctors warn about.
Question: why does everybody in the hab have to breathe the same "air" that EVA astronauts breathe? Is that assumption not driving us into an impossibility here?
GW
To a certain point, it's not about the pressure. There are acclimatized populations as high as 20,000 feet on Earth. But most folks from nearer sea level do poorly above 10,000 feet. That's why they put oxygen on airplanes. But, there are no populations at all above 20,000 feet. Must be a reason for that! That's air, and it's obvious that 55 KPa is OK in every way for acclimatized populations. Most of us are not so acclimatized. And that shows up in the regional genetics, I might add.
The things I have seen for colony proposals often are custom-composition atmospheres at very low pressure. I know from the MCP spacesuit work in the late 1960's that pure O2 would work just fine for functioning adults at only 180-190 mm Hg. But, composition is important long term, and both pregnancy and child growth are very sensitive to a lot of things that functioning adults are not sensitive to. Composition is one of them, or so I have been led to believe.
You can only go so low with 21% O2 79% N2. Maybe 55-65 Kpa. To go lower, it ain't gonna be air. And that's definitely a problem.
Plus, you're already in trouble trying to cook with boiling water above 10,000 feet.
GW
This thing lost my response just as I finished typing.
OK, try again: I had in mind a sturdy, high-inert fraction capsule-shaped Mars ferry, just a lot smaller. It has conventional internal tanks for only the rocket-brake landing. The ascent propellant goes in very-low-inert (5%-ish) balloon tanks tied externally to the nose "somehow". Don't know how yet, or even that an elastomeric cryo-tank balloon is possible. But if it is, there's a huge advantage to it.
I don't see why such a balloon tank need be discarded after ascent. It is not that much weight to carry around. Warm it up and then fold and stow inside the capsule for the descent. On the surface, get it back out, reinflate it, and reload propellants for the next ascent. We're probably talking about multiple tanks of the same form factor, in a cluster. Might need a mast sticking out of the capsule's nose, I dunno.
Control during retro-braking descent is by simple attitude thrusters. Just use bigger ones to offset the larger disturbing forces. I'm not talking about the whole descent, just the final phase where you come out of hypersonics at local Mach 3 (0.7 km/s) around 5-10 km altitude. Thrusters like that could also do the de-orbit burn that starts the descent.
Retro thrust during descent is by the ascent engines, firing through open ports in the heat shield. If you seal the engine compartment, you stop the entry slipstream throughflow, other than a very minor pressurization transient as you descend. It would be easy to provide an offsetting gas flow into the compartment, if heat-sinking isn't enough to handle the pressurization intrusion of slipstream gas.
All just wild guesses.
GW
Hi Bob:
Not as sure as I'd like to be.
My stuff simply "felt like" styrofoam, and middle-of-the-range for commercial styrofoam is near .03. My stuff could be as dense as shuttle tile, I suppose, and still feel like commercial styrofoam. I just don't know. It is far more damage resistant, as the unreinforced stuff (like NASA's stuff) shattered to pieces almost instantly when I drove that little combustor into rich instability.
The reinforced stuff survived dozens of instabilities and hours of burn at the hairy edge of meltpoint. I had shrinkage cracking post-burn, but in that application, cracking was not objectionable. It didn't go all the way through. I suppose without proof you could ride it through entry with surface cracking, but I don't like the notion of hypersonic fluid shear tearing at the edges of cracks like that. Flow speeds were very subsonic inside that little combustor.
I think the density depends critically upon the curing process. The maker lists a much higher density for the material, but my molding process was quite different from what most folks do. I had a lot of metallic confinement, and I cured above the boiling point of the water vehicle in the potting compound, instead of below. I think the steam wormholing its way out is how I achieved low density in what is otherwise just a slather-on ceramic pipe insulation potting compound.
Metal shingles would be a lot stronger, but would have a conduction path "open" into the interior of the vehicle. That would require backside cooling or heat-sinking to go through re-entry. That's how they were going to do it on the old X-20 with Inconel skins. I suspect the X-33 would have functioned similarly. The advantage of the low-density ceramic (mine or NASA's) is that you cut off that conduction path. The backside can be uncooled aluminum.
The advantage of mine over NASA's is actually threefold: (1) larger panels that are way easier to install, (2) redundant retention (bondline plus mechanical retention of the reinforcing fabric), and (3) non-exotic commercial materials in common use for decades now. You do have to install it integral with the hull plating panels, as simple bolt-up items to the framing.
The biggest question would be process controls during panel cure to ensure an acceptable low density. I hit on that by accident when I did this back in 1984. I don't forsee a problem with that, but a spec does have to be defined.
GW