Quote from: Impaler on 10/16/2015 08:58 am...I don't think we will actually see 100 day transits mainly do to capture g-force limitations...What, exactly, are you talking about? Humans can withstand 6 gees for at least 10 minutes, which is 36km/s of delta-v, usually while still doing simple tasks (unnecessary in this case, since capture/entry/reentry would be automated). Such a long period of fairly constant deceleration is possible using a lifting (re)entry, actually including negative lift. Once captured, your next trip through the atmosphere can be for landing (possibly with a skip). So you just need to get rid of your hyperbolic velocity on the first pass because the next pass will take care of actual (re)entry.I don't think this is any more radical than anything else SpaceX is doing. And isn't more radical than Shuttle's reentry. Sure, we need a good estimate of Mars' atmospheric state, but that is also a solvable problem.36km/s of hyperbolic velocity is much more than enough for a 100 day transfer....lifting (re)entry is well within the state-of-the-art for both Mars and Earth. SpaceX already does this fairly regularly with Dragon. SpaceX also has a very good heatshield material (PICA-X) that they're very familiar with. I don't have any clue why they would shoot themselves in the foot by not leveraging these things to their full potential.
...I don't think we will actually see 100 day transits mainly do to capture g-force limitations...
Quote from: Lobo on 10/15/2015 07:12 pm[Dragon's trunk] It is 2/3 of the way to a lifeboat on a biconic MCT.
[Dragon's trunk] It is 2/3 of the way to a lifeboat on a biconic MCT.
and yet people here easily accept that MCT will be capable of landing dozens of people in the same vehicle that was used for the interplanetary transfer, or up to 100 tons of cargo?
The limitation I see with the magneto is that it doesn't produce lift. That makes the g-forces much higher because your doing a single short burst of drag.
Quote from: Impaler on 10/17/2015 05:37 amThe limitation I see with the magneto is that it doesn't produce lift. That makes the g-forces much higher because your doing a single short burst of drag.I recall from the relevant thread that the possibility was mentioned that producing lift by making the magnetic braking device unsymmetric should be possible. In that case it even might be able to do a lot of steering and adjusting to different conditions of the local atmosphere.
Quote from: Pipcard on 10/16/2015 04:59 pmand yet people here easily accept that MCT will be capable of landing dozens of people in the same vehicle that was used for the interplanetary transfer, or up to 100 tons of cargo?There are at least 4 threads on just this topic, in just this section, of people not merely accepting that.
Quote from: guckyfan on 10/17/2015 06:20 amQuote from: Impaler on 10/17/2015 05:37 amThe limitation I see with the magneto is that it doesn't produce lift. That makes the g-forces much higher because your doing a single short burst of drag.I recall from the relevant thread that the possibility was mentioned that producing lift by making the magnetic braking device unsymmetric should be possible. In that case it even might be able to do a lot of steering and adjusting to different conditions of the local atmosphere.The same thought had occurred to me, but I realized that the combined vehicle would simply pivot to bring the center of mass on the end of the tether into line with the center of drag. The effect is analogous to deploying two different sized round non-lift producing parachutes at the same time, they would create different amounts of drag but your line angles would simply be skewed such that the load is directly under the center of drag and you would descend vertically. Or at least this is my mental picture of what would happen when the plasma is not deflecting any air laterally which by definition this can't do because it's drag is entirely from ion collisions which scatter in all directions and end with the incoming gas just being swallowed into the existing plasma vortex, without deflection you can't have lift because you would be violating conservation of angular momentum.
Quote from: Robotbeat on 10/16/2015 11:27 pmQuote from: Impaler on 10/16/2015 08:58 am...I don't think we will actually see 100 day transits mainly do to capture g-force limitations...What, exactly, are you talking about? Humans can withstand 6 gees for at least 10 minutes, which is 36km/s of delta-v, usually while still doing simple tasks (unnecessary in this case, since capture/entry/reentry would be automated). Such a long period of fairly constant deceleration is possible using a lifting (re)entry, actually including negative lift. Once captured, your next trip through the atmosphere can be for landing (possibly with a skip). So you just need to get rid of your hyperbolic velocity on the first pass because the next pass will take care of actual (re)entry.I don't think this is any more radical than anything else SpaceX is doing. And isn't more radical than Shuttle's reentry. Sure, we need a good estimate of Mars' atmospheric state, but that is also a solvable problem.36km/s of hyperbolic velocity is much more than enough for a 100 day transfer....lifting (re)entry is well within the state-of-the-art for both Mars and Earth. SpaceX already does this fairly regularly with Dragon. SpaceX also has a very good heatshield material (PICA-X) that they're very familiar with. I don't have any clue why they would shoot themselves in the foot by not leveraging these things to their full potential.I never said crew was the limit. The limits are mostly on the vehicle and it's structure, remember that our aerocapture g's in a bi-conic vehicle are negative, aka nose bleed g's so they represent a 2nd direction that the vehicle needs to tolerate, and the more lift their is the more lateral g you get.Aerocapture of a large mass at mars from a hohmann trajectory (low incoming speed and low DeltaV needed) would be possible with a bi-conic shape, but 100 day transits result in MUCH larger deceleration needs which means you simply need more surface area then the vehicle itself can provide. So some kind of 'expander' is needed, either an inflatable device (which is disposable) or the magneto which looks to be fully reusable. The limitation I see with the magneto is that it doesn't produce lift. That makes the g-forces much higher because your doing a single short burst of drag.
If magnetic deceleration can be used it would help a lot with reusability. If a first pass can achieve capture then a second or third pass can achieve landing. Replacing the heatshield after every flight will cost time and money. I don't see magnetic as a requirement but i hope for it. A metallic heatshield will not do especially for EDL back on earth.
Quote from: guckyfan on 10/17/2015 03:21 pmIf magnetic deceleration can be used it would help a lot with reusability. If a first pass can achieve capture then a second or third pass can achieve landing. Replacing the heatshield after every flight will cost time and money. I don't see magnetic as a requirement but i hope for it. A metallic heatshield will not do especially for EDL back on earth.Even a PICA-X shield can take multiple entries (Musk said 100, which is an overstatement but is a lot more than MCT will need). And yes, a metallic shield would work, you just have to do it gently enough. But I suspect SpaceX is looking at PICA-X. A tanker going to LEO, most certainly, could use a metallic TPS without any hard limits on numbers of reuses (thousands are possible).
You're still speaking entirely in generalities. What sort of g-load do you believe is a limit? You can't make a claim like you're making (that g-loads are prohibitively high for 100 day transits) without giving some numbers.As a side note, a Falcon 9 first stage already has 8 deployed aerosurfaces that generate drag. You think SpaceX will not be able to have ANY deployed aerosurfaces for MCT?SpaceX, on the other hand, has never mentioned magnetohydrodynamic drag devices. You are basically handicapping them by disallowing any of their current ways of solving the problem so you can introduce your own pet solution.
I am already assuming separate aerocapture and entry at both ends and the DeltaV I stated were the minimum to capture, aka you go into a big elliptical orbit.Given the size of the MCT and the mass it has to work with I think 2 gs's is the best that can be reasonably hopped for and it will still in fact be hard to make the vehicle that strong. Your not taking into account the square-cube law that makes a large object proportionally weaker then a small one.
NASA did studies for Inspiration Mars. They came to the conclusion that direct reentry from Mars at earth return would be the way to go assuming PicaX as a heatshield material. A skip reentry would increase heatshield stress.I was considering using high pressure to stabilize MCT during reentry. Maybe 40 to 50 PSI or 3 times earth atomospheric pressure. This pressure during the minutes of reentry will not cause problems to crew given what we know from diving.
Is there any concept of a reverse gravity slingshot?- e.g. Use the gravity of the moon to slow down before re-entering earth's atmosphere.
NASA did studies for Inspiration Mars. They came to the conclusion that direct reentry from Mars at earth return would be the way to go assuming PicaX as a heatshield material. A skip reentry would increase heatshield stress.
...I was considering using high pressure to stabilize MCT during reentry. Maybe 40 to 50 PSI or 3 times earth atomospheric pressure. This pressure during the minutes of reentry will not cause problems to crew given what we know from diving.