I wonder what the best orbits would be for an architecture like that.
I wonder if you really need to limit it to just one pass through the Van Allen belts though. From what I heard, the dosage for the inbound and outbound pass on Apollo was quite a bit less than the dose from the rest of the mission. I wish I had some hard numbers though. Cause not having to lug around a reentry capsule everywhere would be good, and being able to reuse all of your in-space hardware would also be very very good. The in space hardware was actually a very large fraction of the cost of an Apollo mission, and if you could reuse it even a few times without a huge amount of refurbishment, it might help the costs of missions go down.
I think it depends on how many braking passes are needed.
I think it depends on how many braking passes are needed. With NTRS down, the easily-accessible definitive numbers are hard to come by, but Wikipedia's page on the van Allen Belts indicates that Apollo astronauts got about 10 mSv from passing through the belts. The same page also says that a satellite in a 200-by-20,000-mile orbit, which sounds about like a braking orbit, will get 25 Sv/year, which works out to about 8 mSv per passage (perigee to apogee or apogee to perigee), which is roughly consistent with the Apollo figure.If you're off beyond the magnetosphere, you're probably going to be getting a sievert or so every few months from GCRs. So if the number of braking passes can be kept down to ten or so, my guess is that the van Allen Belts aren't going to be a major contributor to the overall dosage for the mission. The thing you really want to avoid is equatorial apogees around two Earth radii (15,000 km), because then you'll be going slowly through the most intense part of the belts, where the dose rate is something like 0.1 Sv per hour.Plus, the trapped van Allen particles are of relatively low energy -- more like the solar wind than GCRs. So shielding ought to be possible. If we're talking about a crew, they've probably got a solar storm shelter anyway, and they could just hang out there when passing through the worst of the van Allen Belts.
I think it depends on how many braking passes are needed. With NTRS down, the easily-accessible definitive numbers are hard to come by, but Wikipedia's page on the van Allen Belts indicates that Apollo astronauts got about 10 mSv from passing through the belts. The same page also says that a satellite in a 200-by-20,000-mile orbit, which sounds about like a braking orbit, will get 25 Sv/year, which works out to about 8 mSv per passage (perigee to apogee or apogee to perigee), which is roughly consistent with the Apollo figure.
The thing you really want to avoid is equatorial apogees around two Earth radii (15,000 km), because then you'll be going slowly through the most intense part of the belts, where the dose rate is something like 0.1 Sv per hour.
Plus, the trapped van Allen particles are of relatively low energy -- more like the solar wind than GCRs. So shielding ought to be possible. If we're talking about a crew, they've probably got a solar storm shelter anyway, and they could just hang out there when passing through the worst of the van Allen Belts.
(40,000 km)/(86,400 s) = 0.46 km/s, so I think the difference is under 1 km/s, depending on latitude, of course.
The more energy you want to dissipate, the deeper into the atmosphere you have to go. At some point you go so deep you don't come back out.
What is the fundamental reason why multiple passes through the atmosphere is necessary in the first place? Is it the heat load, limits of the actuators, limits on the control algorithms, something else?
Just saying it'll always be faster to do direct reentry.
Quote from: QuantumG on 10/18/2013 11:13 amJust saying it'll always be faster to do direct reentry.True, but the difference in time to return is a couple of days at most, if we can manage 1 km/s or so of deceleration on each pass. On an interplanetary mission, a "fast" abort is likely to take weeks anyway.
Quote from: Proponent on 10/18/2013 03:09 pmQuote from: QuantumG on 10/18/2013 11:13 amJust saying it'll always be faster to do direct reentry.True, but the difference in time to return is a couple of days at most, if we can manage 1 km/s or so of deceleration on each pass. On an interplanetary mission, a "fast" abort is likely to take weeks anyway.So... just the difference between life and death? Note, we're in the missions to the Moon section. If you're talking Mars missions, you'd probably be happy with aerocapture into EML2 and a stay at the Lagrange Hilton.
Quote from: Proponent on 10/12/2013 11:51 amI think it depends on how many braking passes are needed. With NTRS down, the easily-accessible definitive numbers are hard to come by, but Wikipedia's page on the van Allen Belts indicates that Apollo astronauts got about 10 mSv from passing through the belts. The same page also says that a satellite in a 200-by-20,000-mile orbit, which sounds about like a braking orbit, will get 25 Sv/year, which works out to about 8 mSv per passage (perigee to apogee or apogee to perigee), which is roughly consistent with the Apollo figure.I find the Wikipedia article confusing. They refer to a "safe zone" between 2 and 4 earth radii. But the article's graphics indicate high flux at 3 earth radii for protons more than 1 MeV and at about 1.4 earth radii for protons more than 400 MeV. It seems to me the 400 MeV or more protons are the biggest concern. The others are easier to shield against. If so, it seems the worst orbit would be one with a 1.4 earth radii apogee. Quote from: Proponent on 10/12/2013 11:51 amThe thing you really want to avoid is equatorial apogees around two Earth radii (15,000 km), because then you'll be going slowly through the most intense part of the belts, where the dose rate is something like 0.1 Sv per hour.A 6678x15000km orbit would have a period of about 3 hours. A perigee speed of about 9.2 km/s, about 1.3 km/s over a circular orbit.Goff's saying he wants to circularize over 3 to 4 passes which seems to indicate he's hoping to lose 1 km/s each perigee pass. If so, the astronauts wouldn't have to endure more than 1 or 2 orbits with apogees in bad spots. With two bad apogees, maybe 3 to 4 hours in high radiation zones. At .1 Sv per hour, that'd be .3 to .4 Sv. The Wikipedia article says 50 mSv is the annual dose set by the U.S. Atomic Energy Commission for people who work with radioactivity. Quote from: Proponent on 10/12/2013 11:51 amPlus, the trapped van Allen particles are of relatively low energy -- more like the solar wind than GCRs. So shielding ought to be possible. If we're talking about a crew, they've probably got a solar storm shelter anyway, and they could just hang out there when passing through the worst of the van Allen Belts.The 400 MeV protons are more like GCRs.
So what do we know about aerobraking the space-only part?