Aerobraking Reusable Deep-Space Vehicles

[QuantumG]

Just 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.

[KelvinZero]

Just 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.

I like the idea of a small direct reentry capsule.
* Because it does not have BEO requirements it could be very similar to an existing ISS crew vehicle, thus having a much higher flight rate than a BEO only design. Additionally, your BEO portion is much simpler now. (and recovery is merely a nice-to-have, like with the F9 first stage)
* Im guessing that there is a reasonable chance that a component that has had no problem during a hundred days in free fall could suddenly develop a flaw during a maneuver, even quite gradual acceleration. (for example I vaguely remember a story about large globules of water forming behind some panels on a russian station?)
* I suspect a small problem during/before/after braking could greatly affect the time it takes to rendezvous with something else in orbit.

So what do we know about aerobraking the space-only part?

[Proponent]

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.

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.

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.

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. 

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.

The 400 MeV protons are more like GCRs.

OK, you caught me, fair and square, shooting from the hip.   Now that NTRS is back, I've had a look around for a little more data.  It's harder to come by than I'd have expected, but the attached JPL report on manned radiation protection is helpful, if a little older than I'd like.  Look at figure A-13 on page 34 of the PDF.  It presents absorbed doses over five years in equatorial circular orbits at the altitudes indicated for two levels of Al shielding: 0.5 and 3 g/cm².  That's not much, by the way:  the Apollo CM had 7-8 g/cm², the Shuttle and ISS more.  This does indicate that van Allen radiation is actually pretty susceptible to shielding.  Equatorial orbits, by the way, are the worst case as far as van Allen radiation goes.  By "equatorial," I mean the geomagnetic equator, which is inclined about 10^o to the geographic equator.

For purposes of calculation, I've modeled the dose rate as the sum of two Gaussians in the log of the altitude.  My fit, done by Mark I eyeball, is attached.  I've also converted it to more modern units, as shown in the third attachment.  With just 3 g/cm² of Al shielding, the outer van Allen Belt turns into a pussy cat, but the inner Belt still looks a little scary.

The next figure shows apogee as a function of perigee speed for a perigee of 400 km.  The centers and approximate boundaries of the two Belts, respectively, are shown in blue by solid lines and by pairs of dotted lines.  What shows here is that if at least a kilometer per second of braking is possible at each perigee, then one can avoid apogees in the Belts.

Of course, even with apogees outside the Belts, it's still necessary to pass through them.  What matters is the does integrated over an orbit, which is shown in the final plot.  A third level of shielding, 7 g/cm², has been added in green, though this is just a guestimate (generated on the assumption of simple exponential absorption consistent with that seen between 0.5 and 3 g/cm[sup2).  If braking passes took place at 10.3, 9.3 and 8.3 km/s, the total does absorbed would be about 60 mGy with 3 g/cm².  Although the quality factor (the ratio of effective dose in sieverts or rems to absorbed does in grays or rads) probably isn't large (from what I've read; it seems to the hi-energy GCRs that have scary high quality factors), 60 mGy probably won't make the doctors happy.  But it looks like a little more shielding can probably solve the problem.  And, as remarked before, it's not a lot compared to what astronauts are going to get from GCRs in a few months outside the magnetosphere.  Doses could also cut by somewhat by avoiding equatorial braking orbits, especially if perigee (and, hence, apogee) can be moved away from the equator.

The largest perigee speeds shown correspond to an apogee of about 400,000 km, i.e., trans-lunar injection.  We can attempt to link the curves generated here to Apollo  experience by normalizing the to the trans-lunar values, as is done in the final plot (bear in mind, though, that the Apollo missions didn't in general fly in the plane of the geomagnetic equator).  The Apollo 8 crew, for example, averaged 1.6 mGy over the entire mission (see the  attached Apollo report).  Some of that must have come from source solar and galactic sources.  As a conservative figure, you could guess that the TLI point represents 1 mGy.

While it would be good to have some newer and more complete information for this calculation, I don't think van Allen radiation is going to be show-stopper.

Anybody who wants to look over my code is welcome to it (though it's in R).

EDIT:  Added green curve for Apollo-like shielding to dose-per-orbit plot and added Apollo-relative plot, Apollo experience report and related discussion.

[Proponent]

So what do we know about aerobraking the space-only part?

Well, you could do it very slowly like some recent Mars spacecraft, or you could put a big heat shield on, or you could use the magnetic braking scheme that jongoff suggests.

[redliox]

Aerocapture likely would be the key to a reusable lunar vehicle.  NASA politics aside, I think the real problem is getting a future LEM back to LEO where it can be refueled and refurbished, as LOX production or manufacturing on the Moon won't happen for a few decades even with ideal circumstances.

However note the concept for an inflatable heat shield:

http://www.igorstshirts.com/blog/conceptships/2013/manchu/manchu_14.jpg

After aerobraking into LEO, the LEM would jettison the shield.  Then when it's refueled and recrewed, a new shield-pod could be installed and sent moonward again.  Slightly more practical than dumping it like a booster rocket when anyone knows a lander is far more valuable.

[JasonAW3]

This might be a good idea for space probes that we want to use again.  Problem is, you'd likely want to send an aerobraking package with an adaptive cradle for the probe itself, to actually perform the maneuver.  (This is assuming that you don't want to carry the mass of the aerobraking recovery system with you to whatever target that the probe is investigating).

Such a package could be sent out on a high apogee orbit, use enough thrust to match the incoming probe's incoming velocity, attach itself to the probe, inflate the ballute, and use it's own guidance and thruster package to bring the probe home.

This could likelwise be done with large manned spacecraft, assuming that the crew would be utilizing a seperate high speed re-entry craft for Earth return.  Depending on the structure of the returning craft, it might need many passes to slow down enough to achieve a 300 KM orbit.  The deeper into the atmosphere a craft and ballute package can go, the more velocity can be bled off.  (Put another way, the weaker the structure, the more and higher altitude passes will have to be made.  The stronger the structure, (and how protective the ballute is) the deeper into the atmosphere the craft can go and the less passes it has to make).