Splashdown point

[Kiwi]

One of the pleasures of studying Apollo is that there is always something new to learn. In all of the reading I'd done between 1969 and 2004 I never learnt the following, which I discovered a year ago on page 64 of the Apollo 11 Press Kit, so I post it here in case anyone else has missed it:

Apollo 11 Press Kit, page 64
The latitude of splashdown depends upon the time of the transearth injection burn and the declination of the Moon at the time of the burn. A spacecraft returning from a lunar mission will enter the Earth's atmosphere and splash down at a point on the Earth's farside directly opposite the Moon. This point, called the antipode, is a projection of a line from the center of the Moon through the center of the Earth to the surface opposite the Moon. The mid-Pacific recovery line rotates through the antipode once each 24 hours, and the transearth injection burn will be targeted for splashdown along the primary recovery line.

This makes me wonder if achieving the right angle of entry into the Earth's atmosphere wasn't quite the difficult thing I'd always thought it was. It seems that celestial mechanics were on their side when it came to getting the astronauts back safely.

If anyone would like a typescript of this press kit, PM me with your email address and I'll send a 404 kb formatted rich text file. It doesn't include any of the diagrams, but is far superior for searching for information than the 8651 kb pdf document because there are many errors in the scanned electronic text of the pdf.

[scooter]

I just was reading a technical description of the reentry profile. In it was discussed the lifting capabilities of the capsule as well as programs used to control crossrange and downrange errors throughout the stages after EI...really made such a dynamic reentry seem sorta "common sense"...can find you the link.
Dave

[Kiwi]

Nobody has commented on what I'm particularly interested in: Whether this is something that everyone here knew all along, or, like me, didn't know at all.

It's such an important point that I'm sure I wouldn't have read it long ago and forgotten it. I was rather startled to read it for the first time last year, and I'm pretty certain that it's not mentioned in the many books I have. Is it mentioned in yours?

[gwiz]

Not sure how difficult you thought it was. The procedure is to use the radio tracking to calculate the orbit, compute the manoeuvre required to correct to an orbit with the right entry angle, tell the crew to perform the manoeuvre, repeat as necessary. It needed a computer on the ground that was capable for the times, but the basic maths of orbital mechanics needed to program it goes back centuries. Once away from the moon, the vehicle is essentially in an elliptical orbit and the aim is to adjust the perigee altitude.

[Kiwi]

Thanks for that, but achieving the correct re-entry angle isn't my main point, it is the following that I never knew until last year:

A spacecraft returning from a lunar mission will enter the Earth's atmosphere and splash down at a point on the Earth's farside directly opposite the Moon. This point, called the antipode, is a projection of a line from the center of the Moon through the center of the Earth to the surface opposite the Moon.

Obviously, entering the atmosphere and splashing down don't occur at the same latitude and longitude, as Moon Man seems to think, so the above isn't entirely accurate, but I had no idea that either point was near the antipode.

I also wonder how this would have affected the landing of a Soviet lunar mission, as they seem to have landed most of their spacecraft at a fairly high latitude.

[gwiz]

Well, yes. If you have an elliptical orbit with the apogee at the moon, that's where the perigee will be.

Some of the Zonds had problems with the attitude control which meant they had to do an unguided re-entry, and these came down at low latitudes. The ones without this problem did a skip out of the atmosphere to reach the USSR.

[Bob B.]

kiwi said:

Thanks for that, but achieving the correct re-entry angle isn't my main point, it is the following that I never knew until last year:

A spacecraft returning from a lunar mission will enter the Earth's atmosphere and splash down at a point on the Earth's farside directly opposite the Moon. This point, called the antipode, is a projection of a line from the center of the Moon through the center of the Earth to the surface opposite the Moon.

The reentry point is always at or near perigee. For a lunar return mission the perigee is at the antipode. Similarly, reentry from Earth orbit occurs 180 degrees from where the deorbit burn takes place. A deorbit burn does nothing but lower the perigee to an altitude inside the atmosphere. One-half orbit after the burn as the spacecraft reaches perigee and enters the atmosphere.

kiwi said:

I also wonder how this would have affected the landing of a Soviet lunar mission, as they seem to have landed most of their spacecraft at a fairly high latitude.

I believe the Zond missions landed in the Indian Ocean rather than on land as the Soviet orbital missions had.

[Kiwi]

gwiz said:

If you have an elliptical orbit with the apogee at the moon, that's where the perigee will be.

Aha, thanks -- never thought of it in those terms. This brings up one of my old regrets about Apollo: I have never seen a decent diagram of the trajectories of the Apollo craft to and from the moon -- they are always grossly oversimplified. I'm particulalry interested in seeing the positions of the spacecraft and moon on the outward journey between the craft coming under the influence of the moon's gravity and lunar orbit insertion.

My first contact with the moonlanding hoax was William L. Brian's book "Moongate". Regarding his arguments about the neutral point and gravity on the moon, I drew myself a rough scale drawing of a spacecraft's trajectory from the Earth to the moon, plotted positions of the moon at different stages of the journey, and immediately saw that the craft would never linger near an imaginary straight line between the two bodies. The further the craft was from that line, the further from the moon the neutral point would be. Brian's major arguments didn't stand up to the simplest scrutiny and Bennett and Percy have repeated his error in "Dark Moon".

I think that many laypeople think of the path between the two bodies as being a straight line, and some probably don't even think of that line on the outward journey pointing to an empty point in space where the moon and the spacecraft will meet in two or three days' time. They instead think of it travelling along a straight line that points directly at the moon at translunar injection, in which case it would miss the moon altogether by a great distance.

It's only recently that I realised the line would actually be curved, though I don't have a clue about how much curve, or how flying southward to miss the thickest parts of the Van Allen belts affects the path.