Apollo and the van Allen belts

In "The Space Environment" by N. H. Langton (1969) they say the
Apollo plan was to avoid most of the van Allen belt radiation by
going through near the edge. Cape Canaveral is 28.5 degrees North,
which is about right for going toward the Moon. The radiation
belts are inclined about 11 degrees (as is the Earth's magnetic
field) and are about +- 40 degrees wide. If you time your
departure for the Moon so you are at the Northern part of your
orbit while over the longitude where the radiation belts are South,
you can miss most of the radiation. From pages 134-136 I quote:

The problem of making a suitable exit through the trapped radiation
is not in fact particularly difficult. The lunar missions at
present proposed will leave from a parking orbit below the van Allen
region and the most opportune instant to leave this orbit will of
course be chosen. The radiation intensity is quite low at and above
magnetic latitude 40 degrees North or 40 degrees South and the
geomagnetic dipole is at an angle of 11 degrees to the Earth's
rotational axis. The rotational axis is at an angle of 66.5 degrees
to the plane of the ecliptic at the equinoxes and the plane of the
lunar orbit is inclined at 5 degrees to that of the ecliptic,
around which it rotates with a period of just over 18 years.
Accordingly, once every 18 years, at the equinox, there is an
instant each day when a straight line from Earth to the Moon is the
normal at magnetic latitude 39.5 degrees. The situation is shown
in Fig. 4.10. While this ideal path may not be followed (it imposes
considerable restrictions on dates and times of lunar missions, and
takes no account of solar flare incidence) the general principles
involved are clear, and a lunar mission is unlikely to incur
a high dose burden from the van Allen belts on its way from and
to the Earth.

-- Vince

PS This was in reply to something on sci.space.science but I think
it is worth posting here too.

In another book [1] they also indicate that Apollo should go through
the edge of the Van Allen belts. They studied a number of different
vehicle configurations and found that inherent shielding which exists
due to vehicle structure and equipment is sufficient to permit safe
passage through the edge of the belts.

They have a graph showing radiation intensity contour lines and the
normal path Apollo would take. The radiation level is much lower
(maybe 1/10th), and the distance through is much shorter (maybe
1/5th) at the edge where Apollo would go through than at the center.
So, in the nominal case, the radiation dose was small.

An abort path could end up going more through the center of the Van
Allen belts. They show abort cases with as much as 22 rem. They
note that adjusting the abort trajectory can reduce this.

From this it seems that avoiding the middle of the Van Allen belts
was an important part of the Apollo plan.

This work was done in 1961. So the ideas for avoiding most of the
radiation were known very early on. I suspect that NASA knew even
before Kennedy's speech earlier that year.

-- Vince

[1] "Advances in the Astronautical Sciences Vol 10, Manned Lunar Flight",
1963. This is the proceedings of the Symposium on Manned Lunar
Flight in Dec 1961. Pages 75 and 76.

In a 3rd book, "Spaceflight Dynamics", Wiesel, 1989, page 255, they
say that Apollo did in fact go near the edge (high latitudes) of
the Van Allen belts. I quote:

In a 300-km-altitude circular orbit, the daily radiation dose to an
unshielded human being would be only about 0.1 rad. However, at the
maximum intensity point within the Van Allen belts (at about 3000 km
over the equator), an astronaut would receive a dose of about 300 rads
per day. Apollo spacecraft are the only manned vehicles ever to
penetrate this region, and they crossed the belts at moderately high
latitudes and during the highest speed portion of their flight, so the
accumulated dose to their crews was within acceptable limits.

-- Vince