Obegriplig varning av RostigHink

Hence for a returning Moon mission not only are the margins of error razor thin, the stakes are much higher. A bit of an undershoot, and the crew will be subjected to fatal G forces and the craft to unbearable thermal and mechanical stresses. A bit of an overshoot, and the craft could ‘miss’ the Earth and be sent on a wild elliptic orbit in deep space. And importantly, unlike a LEO return, there is only one chance to get it right.

The sheer difficulty in reliably performing ballistic re-entry from deep space especially with biological payload is why skip re-entry was developed. Skip re-entry is a technique whereby the landing module is designed and oriented to create aerodynamic lift such that it leaves the atmosphere and re-enters again for its final entry. The purpose is to reduce speed more gradually and thus reduce the peak G force on the crew and peak heat and stress on the spaceship. The tradeoff is a longer range from re-entry initiation to the landing point. This in turn can amplify errors through external variables like weather. Additionally, there are now two plasma blackout periods, one on the first dip and the other on the final re-entry.

The first attempt at skip re-entry for a biological payload rated return capsule was the Zond-4 (11 March 1968), which after a lunar flyby on re-entry was not able to skip back out into space causing it to go in ballistically. The ensuing G forces would have killed human occupants. The now shortened re-entry range necessitated a landing area over Africa, and this in turn activated a pre-armed explosive destruct system called APO. APO was activated over the Gulf of Guinea near the west coast of Africa.The APO served to prevent technology leakage as well as to conceal Soviet failures.

The next attempt was the famous Zond-5 (21 Sept 1968), the first instance of earthlings on a circumlunar mission. Here too the skip re-entry mechanisms had failed but were detected early enough for a more controlled ballistic re-entry with a landing in the Indian Ocean from where they would transport the vehicle to friendly Bombay and then back to Moscow. Human payload would have unlikely survived the 40G of deceleration.

The first correct demonstration of skip re-entry was the Zond-6 on 17 November 1968 which

“Entered its tiny entry corridor into Earth’s atmosphere at a velocity of 11.2 km/s. Passing through its 9000 km long entry corridor it skipped out of the atmosphere, having reduced its velocity to 7.6 km/s, and began a second re-entry that further lowered velocity to only 200 m/s. Throughout the re-entry, engines on the descent apparatus automatically fired to vary roll control so as to change lift force and reduce G-loads. Unlike its predecessor, the Zond-6 descent apparatus was subjected to a maximum of four to seven Gs. The complex re-entry was a remarkable demonstration of the precision of the L1 re-entry profile”.

Alas, a faulty gasket caused a leak which depressurized the capsule killing all the biological specimens.

Amongst the greatest feats of wizardry is how NASA hoodwinked the world into believing that on 21 December 1968, the agency successfully brought home a mammoth 3-man astronaut crew (Apollo 8) from the Moon; that too on their first such demonstration, and without testing it on guinea pigs before.

The literature surrounding Apollo’s re-entry methods are understandably ambiguous. When in doubt, mumble. There are three different versions of how Apollo re-entered, depending on who you ask. Apollo flight director Chris Kraft has stated that Apollo used a skip re-entry; the astronauts at Johnson Space Center in 2011 described it as a direct entry; and some experts qualify the Apollo direct entry as a ‘double dip’ (Kaya, 2008).

It is also a testimony of the unquestionable prestige and power that NASA wielded in the 1960s and 1970s. That edifice crumbles every day. At the time of writing NASA does not have the capability to bring back its own astronauts from the International Space Station, let alone from beyond LEO.