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For GW Johnson re #25
Congratulations on your progress in developing a concept for a vehicle that can do both suborbital and orbital flights.
The little movies you made based upon a suggestion by kbd512 can be played for tonight's Google Meeting audience, if you are able to attend, and if there is an audience.
(th)
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Reposts for materials with respect to mars atmospher that it will see.
Just to make sure, I re-sent the 4 minute "truck train" and 4 minute (suborbital) "rocket hopper" presentations to Tom, Brian, and Keith. I also finished and uploaded a 6 minute "hopper / orbital taxi" presentation, and sent it to the same 3.
The same 3 presentations also exist in longer forms more suitable for a quarter-to-half-hour time slot, but no MP4's have been made of these longer forms.
The most astonishing thing about the "hopper/taxi" study was that the long-range suborbital mission set the need for heat protection, not the orbital mission! In fact, the vehicle could return as all exposed-metal construction from all shorter-range suborbital missions and the orbital mission! Keeping that result in, which is truly significant, is what made the "hopper/taxi" presentation run 6 minutes instead of 5.
Even the long range suborbital requirement for heat protection is rather minimal. Stagnation zone surface temperatures peak at just barely over 1800 F, everything else, including windward lateral surfaces, runs under 1600 F. That makes anything with good strength and corrosion resistance at 1600 F a candidate material. That includes SS316L and Inconel X-750. There are others.
The stagnation zone heat protection could be as simple as a thin sheet of 2000 F-capable metal over mineral wool insulation, mounted locally to the surface, and only in and near the stagnation zone. Pressures are low enough to make any low-density alumino-silicate ceramic composite feasible. Or even simple ceramic fabric blankets or quilts, if the wind shear doesn't rip them away.
GW
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repost
Hot CO2 gas is corrosive to steels. The UKs gas cooled reactors experienced corrosion that was only partially mitigated by methane injection. For Magnox reactors which operated at temperatures ~400°C it was a tolerable problem. But AGRs operated at 600°C and suffered severe boiler corrosion. Stainless steel has lower corrosion rates but is not immune, because CO2 reacts with chromium to form chromium carbide, which then oxides and scales off.
https://iopscience.iop.org/article/10.1149/2.F08212IFHowever, these are problems in metals that are exposed to hot CO2 continuously for many years. The rocket is exposed for a period of minutes out of every flight. It isn't clear if this would neccesitate use of a different metal or if the corrosion rate is tolerable in light of other life limiting factors for the rocket. There is little point specifying exotic metal heat shields if the fatigue life of the airframe is up before this becomes a problem. One additional complication with the Martian atmosphere is the presence of free oxygen ions in the ionosphere due to photolysis.
On top of the corrosion life concerns is the fact that 316SS melts at about 1400°C. Long before it melts, it progressively loses strength the closer it gets to melting point. At high temperatures, alloy steels become vulnerable to grain boundary precipitation that will aggrevate any corrosion problems and may lead to stress corrosion cracking. There is no easy way of quantifying these problems without experimentation because there are too many unique variables. Oxide dispersion alloys have higher service temperatures, because the oxide particles lodge in crystal dislocations and grain boundaries reducing slippage even at higher temperatures. This doesn't increase melting point, but it does reduce the rate of decline of strength with increasing temperature. These steels are expensive and difficult to fabricate.
One could simply accept the fact that a heat shield has limited life and design a shield that can be unbolted from the airframe after x number of uses. That way, you can live with the technical uncertainty and use experimentation to refine the heat shield design in service to zero in on a cost optimum solution. In much the same way, aircraft engineers were able to refine the material science of gas turbine engines, making periodic improvements over many decades. Modularising the vehicle subsystems makes this easier to do.
Alumino-silicate materials would be less vulnerable to corrosion, given that they are oxides already. Aluminium oxides are not vulnerable to chemical reduction in reducing gases. If they were, we would be making aluminium in that way instead of molten electrolysis. The problem with ceramics in this situation is that we have high a material that is both brittle and a relatively poor conductor of heat. This leads to thermal gradients, with different rates of thermal expansion between layers. This would lead to cracking in any solid ceramic tile. Cermets are possible, as are fibre reinforced cements. But a ceramic heat shield is going to be relatively maintenance intensive and requires inspection between flights. You aren't going to have the same problem with stainless steel. Once you have quantified problems like corrosion and grain boundary precipitation, the safe working life of shield is a known variable.
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Just to keep everything in perspective, here I a reference to a "rocket hopper" as envisioned by Robert Zubrin, posted in 2002...
peronally, i can think of better methods of getting things around mars. not that the idea doesnt sound good, but rocket hoppers, like zubrin described, could use methane fuel to hop around the planet quickly, carrying cargo, and making fuel upon landing. these could be used anywhere on the planet, not just where there are heavy winds. nuclear based rocket planes would be even better.
the idea of a windsurfer could be used if you wanted to drop things at different locations across the planet. but i really dont see it as being a replacement for conventional proposals. sorry if i seem to be harsh, im just offering my opinion ???
I found this while looking at one of the topics brought back to life by Mars_B4_Moon.
(th)
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