Quote from: Impaler on 06/24/2015 09:09 pmSome decade old sour-grapes quotes from Max Faget dose not constitute a counter argument to the fact that EVERY entry vehicle has had a back-shell with thermal protection.Apollo imagery of people fishing out the capsules with leeward side in near pristine condition proves that Faget was right.India's SRE-1 proves that you are wrong.
Some decade old sour-grapes quotes from Max Faget dose not constitute a counter argument to the fact that EVERY entry vehicle has had a back-shell with thermal protection.
Well, I kinda was being pedantic. Sorry Impaler.Sometimes a simplified explanation becomes technically incorrect, but still can be useful. (Full disclosure: I do this all the time with non-technical listeners to get the big picture point across.)
Impaler, let me push back a bit against the premise of what you are pushing. If I understand you correctly, you advocate lower speed reentry, such as from an orbit (both on Mars and Earth) than direct reentry for the MCT, primarily because then one could use metallic TPS rather than ablative TPS (i.e. PICA-X). And that this is practically necessary to ensure high reusability and high flight rates. Do I have that right?But is avoiding ablative TPS really that important in the grand scheme? Doesn't SpaceX intend to rapidly and frequently reuse the Dragon 2, which will surely have PICA-X. What do we suppose is the answer here? Is PICA-X something that can be de-ablated (reblated?) back on to the bottom of a capsule without too much hassle. I'm imagining that there is an inch or so of the material, and half an inch ablates off during reentry (a little more some places, a little less others) and then then additional PICA-X is applied and added to what remains before the next flight - sort of like retreading a tire. Or conversely, the entire backshell of PICA-X is designed and installed as a bolt on module, and a new one will be bolted onto the capsule for each flight (i.e. changing the tires). I ask because I don't know.But in any case, this seems to solve the problem fully, without a great deal of bother. Surely a simpler solution than radically modifying the flight profiles to include orbital insertion on each end, which requires a great deal of additional fuel, don't you think?
First off trying to use your EYE as a after-the-fact calorimeter on a surface that was heat-shielded is a profoundly flawed. You have no idea what surface temperature it reached nor do you have any idea if it is truly pristine, that would take chemical analysis after-the-fact or ideally a temperature probe during the entry itself. ...The rear of this IRSO vehicle looks to be composed of small solar panels, solar panel are covered with glass which has a high melting point, we would not expect this to blacken or char, the forward TPS is non-ablative ceramic tile which would not deposit black char streaks. The shuttle didn't LOOK chared when it returned from orbit but the top of it had TPS of a thinner, lower temperature type, but TPS none the less. What is behind the panels? you have no idea.
Use a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.
Quote from: Robotbeat on 06/24/2015 06:56 pmUse a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?
Quote from: Andy Smith on 06/25/2015 10:43 amQuote from: Robotbeat on 06/24/2015 06:56 pmUse a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?Yes but it would mean sending a lot of expensive Raptors on a one off mission. If inflatable LOX/methane tanks are possible it seems like the more cost efficient solution to me.
Quote from: guckyfan on 06/25/2015 11:55 amQuote from: Andy Smith on 06/25/2015 10:43 amQuote from: Robotbeat on 06/24/2015 06:56 pmUse a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?Yes but it would mean sending a lot of expensive Raptors on a one off mission. If inflatable LOX/methane tanks are possible it seems like the more cost efficient solution to me.That's a good point. Of course if this depot had appropriate fuel levels it could later de-orbit and land for servicing - say every two years following the mars departure window. It is no longer a one off mission, just another part of the reusable infrastructure?
Quote from: Andy Smith on 06/25/2015 10:43 amQuote from: Robotbeat on 06/24/2015 06:56 pmUse a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?Very good point! And a much better idea than mine. Especially a huge first stage with no legs. Because it's big, low drag loss. Because no upper stage, low gravity losses. f9 v1.2-like mass fractions and propellant densification. Raptor's high Isp. All those engines allow you to have ability to throttle way down to prevent crushing your stage due to over acceleration of such a light stage. Just put an aerodynamic fairing on top, and yeah, it should have no problem reaching orbit.But if it's going to be a depot, it might need better insulation than a first stage usually has. That might be a major reason why you wouldn't do this, since you might need to fly it inside a fairing.
Another possibility though would be to retrieve the engines only or keep them as a stock of on orbit spares for replacements (or even ship them to Mars as a pool of spares there).
Quote from: Andy Smith on 06/25/2015 01:05 pmQuote from: guckyfan on 06/25/2015 11:55 amQuote from: Andy Smith on 06/25/2015 10:43 amQuote from: Robotbeat on 06/24/2015 06:56 pmUse a modified first stage as a giant depot. Launch it partially filled (to reach orbit dry, so acting as its own upper stage) on top of another first stage.Given the speculated mass fractions, isn't the actual first stage just about capable of SSTO if it doesn't have a second stage or any payload attached?It may need a pre-launched second stage to dock and act as a shepherd, raising it to its working location - but that should be do-able?Yes but it would mean sending a lot of expensive Raptors on a one off mission. If inflatable LOX/methane tanks are possible it seems like the more cost efficient solution to me.That's a good point. Of course if this depot had appropriate fuel levels it could later de-orbit and land for servicing - say every two years following the mars departure window. It is no longer a one off mission, just another part of the reusable infrastructure?But it would need significant modifications to handle re-entry as it was designed for something like 2.5 km/s re-entry. Another possibility though would be to retrieve the engines only or keep them as a stock of on orbit spares for replacements (or even ship them to Mars as a pool of spares there).
Also because of the intensity of the black body radiation of the earth and its daytime reflection of heat, I see the depot needing far more active cooling than the MCT which will only need to keep its propellant from boiling off near Mars and between Mars and Earth but will not need to keep it cool for long in the 10 radii range of the Earth.
Your original flight cost was calculated correctly (800 passenger flights, 8000 cargo flight, 8800 total at $50M a pop yields the $440B.The cost of the cargo is presently unknown. My gut feeling is that the answer to question "what do you have to pack in order to live on Mars" is quite lengthy, complex and thus expensive.
Yes boiling points are: 252.8 degrees C for hydrogen182.9 degrees C for oxygen162 degrees C for methane. Hydrogen is much harder to keep from boiling off. I worked for a natural gas company and we liquefied natural gas in the summer for winter peeks. Boil off was not that big of a problem on the ground, and space is colder. Tanks on the ground were doubled like a thermos bottle, with a vacuum pulled between the inner storage tank and outer shell. There was about 3' of space between them (1m), so keeping cold wasn't hard, and that is in the deep south.
Quote from: nadreck on 06/21/2015 06:05 pmAlso because of the intensity of the black body radiation of the earth and its daytime reflection of heat, I see the depot needing far more active cooling than the MCT which will only need to keep its propellant from boiling off near Mars and between Mars and Earth but will not need to keep it cool for long in the 10 radii range of the Earth.Not sure this is a problem. Other depot studies have noted this as an issue for hydrogen but methane is quite a mild cryogen in comparison. With solar power a methane prop depot should be able to be zero boiloff anywhere.
And remember a LEO depot will spend roughly half its time above a sunlit Earth that is radiating significantly more than its black body night time amount and that it will cover a significant fraction of the visible area around the depot.
Quote from: ArbitraryConstant on 06/25/2015 02:11 pmQuote from: nadreck on 06/21/2015 06:05 pmAlso because of the intensity of the black body radiation of the earth and its daytime reflection of heat, I see the depot needing far more active cooling than the MCT which will only need to keep its propellant from boiling off near Mars and between Mars and Earth but will not need to keep it cool for long in the 10 radii range of the Earth.Not sure this is a problem. Other depot studies have noted this as an issue for hydrogen but methane is quite a mild cryogen in comparison. With solar power a methane prop depot should be able to be zero boiloff anywhere.There was a paper I read in the last month (and I know it is linked to here on NSF and I will look for it later) that suggested LOX and Methane would be fine more than 10 radii from Earth at Earth's distance from the sun with simply passive cooling, but that near Earth and potentially Mars more cooling would be required. And remember a LEO depot will spend roughly half its time above a sunlit Earth that is radiating significantly more than its black body night time amount and that it will cover a significant fraction of the visible area around the depot.