Apart from using the same base metal (Aluminum), CMC's and conventional alloys like 2195 (Aluminum Lithium) aren't related to each other. Their precipitates are different and their mechanical properties are markedly different.
]]>Aluminum-lithium is a bit stronger at a bit higher temperature than the more standard alloys, and the penalty you pay for that gain is crummier fatigue life. (I've seem aluminum-titanium proposed as stronger when hot, as well.) Which is part of why they talk about re-flying first stages 10 times, not 100 times. The other part is that entry stresses are very high, so the number of repeat exposures before fatigue failure happens is just inherently low. With aluminum-lithium, that limit is a bit lower yet.
Even if they were using standard old 2024, 5052, and 6061, below Mach 2 in the stratosphere (or Mach 1.5 at sea level), the heated air is cold enough not to hurt the aluminum. Which means the shocked air is a coolant to dilute the effect of hot rocket gases as it mixes. That mixing is very incomplete, by the way. You've got big turbulent eddies all over the place, some effectively big globs of rocket gas, others big globs of shocked air. That just the nature of high-speed flow around non-streamlined shapes.
Water injection for entry cooling was to have been tried in the 1960's experimentally on board the X-20 Dyna-Soar, except that it was cancelled as the first 3 examples neared the end of the assembly line. The main heat protection was black metal re-radiating lateral skins, with reinforced carbon ablative leading edges and nose cap, and some sort of active cooling most likely added to the windward belly skins. This was a late 1950's design. The vehicle was a hypersonic boost glider suborbitally, or a spaceplane to low Earth orbit. It was to have been launched by the Titan-III.
GW
]]>The ceramic matrix composite Aluminum alloys are not butter at 420K. They're between 4 and 10 times stronger than conventional Aluminum alloys at 350C (623K) after 10,000 hours of exposure. Some of the precipitates are replaced with nano-particle Alumina Oxide, amongst others. Dependent upon the exact alloy in question, it forges and casts like the conventional Aluminum alloy analogues. However, acceptable feed rates are slower because the material is somewhat harder (up to 40% harder, again, dependent upon the alloy in question), so you have to use carbide tools for machining. I paid about as much for the material as a higher end 300 series stainless steel. It's not cheap, but not mind-blowingly expensive. The stuff I purchased is about .1g/cm^3 denser than the comparable analog alloy, but that varies with the percentage of ceramic (Al2O3, SiC, or TiB2; they call it nano) mixed into the base alloy powder. I bought some bar stock for engine case studs, in case you were wondering. The best part is that it achieves that strength without any heat treatment process. Their metallurgist said they did some heat treatment testing and the alloy becomes stronger still, but he thought that not having to heat treat the product to achieve substantially better strength at elevated temperature was good enough and a major selling point (no heat treatment process to screw up or add cost to the product).
]]>There's lots of heavy turbulence, so these rocket plumes mix with the shocked air behind that wave. It is likely that the flow right adjacent to the stage has a higher percentage of rocket gases, while the flow out nearer the big shock wave has less. But who knows what the "right numbers" are? I sure as hell don't.
Now, the rocket gases are pretty close to 3000 K chamber, and thus post turn-around shock for mixing. For all stage flight speeds above about 3000 m/s (3 km/s), the shocked air behind the big stage shock is at or above 3000 K. That's the old entry rule-of-thumb talking. So for higher speeds, the rocket gases are effectively an injected coolant.
Falcon first stages aren't flying that fast. About 3 km/s is the staging velocity which is more-or-less oriented downrange, and the first recovery burn kills that and more, to get a lower flight velocity back toward the launch site.
It picks up some speed due to gravity descending toward sensible air around 150-200 kft altitude. The entry burn slows that "impact" into the air, but speeds are well under 3 km/s, so the rocket gases are heating-up the shocked air behind the wave. Effectively the shocked air is a coolant for the rocket gases, which by themselves are way-to-hell-and-gone too hot for aluminum structures.
It's a transient, and that's the saving grace. At very low pressures high up like that, heat transfer coefficients (and heat rates per unit area) are low, so it takes significant exposure time to overheat the aluminum. There's just not quite enough time for the damage to occur: that's how they're surviving entry with aluminum rocket stages.
Did you notice that these two and the landing burn are all separate burns? There's time to cool off between burns, washed only by shocked air. While hot, it's a lot cooler than rocket gases. Maybe 600 K in the stratosphere at Mach 3. And maybe 390 K at Mach 2 in the stratosphere. Hotter when the air itself is warmer than stratospheric, though, which is 217 K, compared to 288 K at sea level. On a standard day, that is.
Aluminum starts turning to butter structurally once it soaks out to about 420 K. It's cooling by radiation externally, and by convection internally to the vapors and fluids inside the tank. Between burns, it only sees the hot shocked air for heating. And below Mach 2 in the stratosphere, exposure to the shocked air is tolerable.
Lots of words. Too many. A schlieren would be far less voluminous, but doesn't tell you about the heat transfer.
GW
]]>But GW can slap me up if necessary.
]]>Luckily we did not need to make use of it...
An estimate that 5 falcon 9 heavies strapped together would achieve the same goal as a bfr and due to the refueling I think we need to think something smaller in the order of 2 or 3 when scaling in falcon 9's. at a 2 x we would be at 60 plus and at 3x we are 90 ton ranges which should be a more fitting level of payload sizing.
They do not need to compete with Nasa and its sls for payload as that is just dumb...
Keep driving cost down and people to orbit up Space X....
]]>The white ones had low thermal emissivity, and did not re-radiate heat efficiently. The black ones had high thermal emissivity, and did reradiate well. That's why white tiles (and later white thermal blankets) could be used on the lateral and leeward surfaces, where entry heating inputs were lowest. The windward side tiles had to reradiate a lot more heat, which is why they had to be black.
The nose cap and leading edges saw higher heating still, pretty much stagnation heating. This was beyond what the black tiles could do, and stay below the 2000 F danger point restriction for phase change. That's why they were the slow ablative carbon-carbon composite. 5 or 6 flights, and you replace them. Very expensive.
By the way, vulnerable as the fragile tiles were, the foam strike that destroyed Columbia was actually to a carbon-carbon leading edge piece. That stuff will go hotter than low-density ceramic tiles, but it is very brittle in its behavior under impact, and so is just about as vulnerable as the tiles.
The ceramic tile/carbon-carbon system is still flown today on X-37B. Something very much like it is on Dreamchaser, if I understand correctly. Also if I understand correctly, the heat shield on the BFS second stage vehicle is the same PICA-X "limited-reusable ablative" that flies on Dragon. By "limited reusable", I mean it can be flown more than once if the entry environment is less than maximally stressful. That might be around 4 returns from Earth orbit before replacement, but only once for the Mars entry plus an Earth return entry from direct interplanetary trajectories.
I've made a low-density silicate material myself, one time about 30 years ago. I made it from commercial fire curtain cloth and pipe insulation paste, and utilized steam wormholing during cure to achieve enhanced void space. My application as a burner liner was interior, not exterior, so I had positive retention even it it cracked some. I used it right up to its meltpoint, and hardly ever below 3000 F. My stuff was tougher and cheaper than NASA's stuff, but not quite as high on void space, although comparable.
GW
]]>The problem with the shuttle is that the ceramic tiles were actual tiles, not a unitary structure. That gives you a lot of failure points.
That was done because the high purity silica would expand when hot. A layer of felt separated tiles from aluminum skin, and cracks between tiles gave them room to expand. It worked, the first ever reusable heat shield, but fragile. They replaced white upper tiles first with thermal blankets, then advanced thermal blankets. Far more durable vs vibration from SRBs, and strikes from chunks of ice filled foam. Black tiles were slightly more durable, and less susceptible to ice-filled foam strikes. But the real problem was the foam. If they built the piloted fly-back booster with aircraft skin over the insulation, as originally planned, they wouldn't have had a problem. Even a polymer film wrap over the foam would have solved the tile problem. But that would add weight. There's a reason they removed the paint.
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