[FPSPACE] Zond 8

Geert Sassen geert at navtools.nl
Sun Aug 14 06:50:06 EDT 2005


At 00:12 14-8-2005, you wrote:

>I'm surprised that no one caught me - I had one of those "Uh-oh!" moments
>while out shopping. I made an error re: Zond 8. Once upon a time it was 
>assumed
>that its Indian Ocean splashdown was evidence of a failure - especially since
>it did a ballistic trajectory. When I wrote my MIR HARDWARE HERITAGE in 1994,
>there was still some question about that, though Mishin had claimed that 
>it was
>intentional. The consensus these days is that it was in fact intentional -
>that it was meant to pioneer an L3 return trajectory. The ballistic 
>reentry was,
>however, unintended. That occurred because the guidance system failed.

All other Zond missions landed on an ascending track (south to north) while 
Zond 8 landed on a descending track (north to south). My understanding is 
that reentry over the northern hemisphere had the advantage of good 
radio-contact during the important last hour prior re-entry (when the craft 
had to be positioned for re-entry), however it leads to a landing in the 
Indian Ocean (unless you make an extremely steep ballistic re-entry like 
for instance the YE-8 returncraft did, however 100+ G is a bit too much for 
the crew..).

The 'normal' Zond double-skip re-entry profile has the advantage (apart 
from reducing G-forces, which any gliding re-entry would do) of a very long 
groundtrack (first re-entry over the Indian Ocean and landing in 
Kazachstan), which they needed to prevent the craft from landing somewhere 
on 'hostile' territory. In order to recover the craft in case of a 
ballistic re-entry (if the guidance system failed) first re-entry had to be 
either above USSR territory or above the ocean. Furthermore the craft 
probably needed to be within range of a tracking station (ship) at a 
certain moment prior re-entry in order to monitor correct orientation of 
the craft etc. Launch constraints and the position of the sun at certain 
moments during the mission probably also played a part in this (see Sven 
Grahns very good article at 
http://www.svengrahn.pp.se/histind/Zondmiss/Zondmiss.htm ). In other words, 
the first re-entry over the Indian Ocean was more or less dictated by 
mission-constraints, and in order to avoid landing somewhere in 
India/Pakistan/China they had to fly a very long re-entry profile, and the 
only way to do so is to fly this skipping re-entry technique.

On the other hand, if you decide on an Indian Ocean landing on a south 
bound heading the craft will be within range of several land-stations 
shortly prior re-entry and there is no direct need for such a long re-entry 
groundtrack, any (Apollo-style) gliding re-entry which sufficiently reduces 
G-forces would do. In other words, if L3 was destined to land in the Indian 
Ocean i doubt whether it would use the complicated Zond skipping-re-entry 
profile. This leaves open the question on whether or not the ballistic 
trajectory of Zond-8 was intended or not, but personally i do not believe 
Zond-8 was intended for a skipping re-entry á la Zond 6/7, its first 
re-entry was already above the Indian Ocean and a 'normal' gliding re-entry 
to reduce G-forces would have been sufficient, there was no need to fly an 
extremely long re-entry profile (no way it could have reached Kazachstan, 
as it was heading south, all it could have done was land somewhere in 
Africa..). Maybe its guidance system failed, resulting in the ballistic 
re-entry, of maybe its guidance system was simply not capable for any other 
re-entry profile and the ballistic re-entry was intentional.

Remains open the question on why Zond and L3/LOK used different landing 
sides (and re-entry techniques). Maybe there were just two different teams 
at work which independently came to different solutions to the re-entry 
scenario, or it might be that the method of orientation of the craft was 
different or there were other launch/mission constraints (Zond relied 
heavily on its sun and star sensors to correctly orientate the craft prior 
re-entry).

In the past there have been questions in this group by others on how 
exactly the Zond guidance system worked during re-entry, but I never saw 
any answer to this. Failure to fly the skipping re-entry on Zond 4/5 seemed 
to have been caused by failure of the 100K Star sensor prior re-entry, 
resulting in incorrect orientation of the landing craft ("Challenge to 
Apollo", page 618). However, this sensor is on the instrument module, which 
is discarded prior actual re-entry, thus an other system in the landing 
module must take over once the craft is correctly orientated and the 
instrument module is discarded. It sounds a bit like the landing craft 
itself did not actually do that much 'steering' during the actual re-entry: 
once orientated properly it just kept this orientation, and together with a 
very shallow re-entry angle this resulted in the skip back into space, 
similar to how you can let a stone skip over a water-surface without 
requiring the stone to actually change its orientation during flight. 
Disadvantage of this profile, however, is that it actually seems to 
increase G-forces compared to an gliding re-entry (staying within the 
admosphere). The first entry into the admosphere (up to about  45 km 
altitude) will be quite shallow with not that much G-forces, however after 
jumping back out of the admosphere (up to about 145 km) the craft will have 
lost a lot of 'horizontal' speed and the second re-entry will be with less 
speed but on a much steeper angle, resulting in higher G-forces (although 
not as high as on a direct ballistic re-entry). Maybe the whole technique 
'sounds' complicated but the actual complications are in flying a correct 
(very shallow) re-entry angle and correctly positioning the craft prior 
re-entry, all done by groundcontrol and systems in the instrument-module 
prior first re-entry, thus saving weight and complexity on the guidance 
system inside the landingcraft...  If this is true, then Zond-8 simply did 
not have any other option but a ballistic re-entry...

Regards,

Geert Sassen. 


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