Title: How Apollo will meet Soyuz, Salyut and Skylab
Author: Ted Wilding-White
Text: How Apollo will meet Soyuz, Salyut and Skylab
International cooperation in space, the subject of current US/USSR talks, may mean wider safety margins for astronauts and less wasteful duplicated effort. Ted Wilding-White reports on the design problems involved
Suggested coupling between Apollo command module and Salut space lab is a two man connector cone and airlock
At the end of this month, in Moscow, the third meeting should take place between Russian and American space technologists discussing a cooperative space rescue system. Since discussions began a year ago the two countries have been reviewing the technical requirements involved, largely concerned with the necessary ability of spacecraft of one nation to rendezvous and dock in space with those of the other. Previous meetings have gone well and it is hoped that technical requirements will be settled at this next meeting, to be followed by actual hardware development.
That this cooperation should have occurred at all is significant. Soviet space activity and, to a certain extent, that of the United States, has always been closely associated with military activity. The degree of cooperation is remarkable and no amount of speculation on motives can disguise its potential. Certainly no opportunity for additional safety can be ignored while the state-of-the-art is at its present relatively primitive level.
The accident which prevented man's third expedition to the moon, Apollo 13, from completing its mission, brought home more vividly than an actual disaster a few of the inherent dangers of space travel. Had the attached lunar module not been available, in effect acting as a lifeboat by contributing its oxygen and power supplies, the accident might well have proved fatal. The only alternative would have been to go into lunar orbit and await a rescue vehicle except that there was none. The time and cost required to prepare and launch a rescue vehicle prompted America's National Aeronautics and Space Administration to rely instead on multiple redundancy in spacecraft systems to ensure safety. In the foreseeable future, most of manned space activity will take place in earth orbit. Provided that the physical ability of a spacecraft to re-enter the earth's atmosphere is not affected, an emergency landing can be achieved within a couple of orbits. The problem arises if, for example, control is lost and the spacecraft is stranded in orbit.
Space exploration has now reached a point where a permanent manned space station in earth's orbit has become both feasible and desirable. Nasa plans to establish such a station by the mid-eighties, along with a quick turnaround, airline-type, re-usable shuttle vehicle to service it. Meanwhile, however, an experimental station/ laboratory called Skylab is to be placed in orbit in 1973 for an eight month operational period. During this time, teams of three men will visit it for one 28 day and two 56 day stays. They will reach it in standard Apollo capsules which will remain attached to the station throughout their stays and in which they will return. Any accident causing premature departure could also prevent the use of their ferry craft.
With the launch of its Salyut experimental space station, a modest equivalent to Skylab, the Soviet Union has demonstrated that it has plans along similar lines. Faced with the same kinds of problems, the Russians responded fully to Nasa's overtures. As a result, actual proposals for operational testing of docking systems emerged, earlier than anticipated, from the meeting last June at Nasa's
Manned Spacecraft Centre in Houston, Texas. An Apollo capsule is expected first to dock with a Soviet Salyuttype station, although probably not before 1974, followed by a link-up between a Soyuz, the Russian three-man Apollo-equivalent, and a postponed or second Skylab. The reason given for this order of operation is that Salyut is already proven while Skylab remains an unknown quantity. The last meeting concluded that "the technical feasibility of accomplishing an experimental test of this type exists in principle and will be studied further by both sides". If it comes off, the very action will symbolise its significance.
The reason that problems exist at all when orbital rendezvous and docking are virtually commonplace, is that both sides use radically different methods: consequently extensive redesign is required. Rendezvous consists of locating the target, and docking involves orientating on it and establishing a smooth and accurate approach to a firm connection. The Soviet Union first practised docking with unmanned satellites (Cosmos 186 & 188)
Docking is a critical manoeuvre carried out twice during American moon missions. First dock between the command module and lunar module was made on the Apollo 9 proving flight above, when astronaut Russell Schweikhart stuck his head out of the command module hatch to inspect the lunar module. Artist impressions, right and opposite, of Souz-Salut and Apollo-Skylab combinations show their essential similarity Current talks are aired at total docking compatibility between both Russian and American ferry craft and their respective laboratories
and initial manned tests were performed automatically. The major limitation was that no interconnecting tunnel was provided, so that the cosmonauts had to don space suits and manoeuvre their way along the outside to reach the other craft. The first evidence of internal transfer appeared with the occupation of Salyut by the crew of Soyuz 11 in June.
On this occasion a system fundamentally similar to Apollo's was used. Consequently, the Houston meeting was able to produce some approximate technical proposals. Both sides are now doing rapid design work, and their results will be compared and analysed in an attempt to finalise major technical requirements during this month's Moscow meeting. The problems that they face are best illustrated by a review of the considerations produced at Houston.
Communication was the first real problem. It was readily agreed that there must be radio contact between the two craft themselves and from each to astronauts carrying out any activity outside the spacecraft. It was also concluded that contact with the other country's control stations on earth was advisable, as was a link-up between ground stations for providing exact operational data.
This by itself indicates the progressive and uprecedented nature of cooperation. No comment has been made by Nasa, however, on its attitude to Soviet radio equipment which, in the past, has proved less than reliable by Nasa standards. None of the considerable technical material that must have already been exchanged has yet been released. However, Nasa is seriously considering, as a means of overcoming the language problem, a Soviet suggestion that specially devised codes could be transmitted by key-telegraph.
The language problem also entails terms of reference, particularly those for coordinating the two spacecraft during docking. It was agreed, however, that the Systeme International d'Unites (Sl) would be used for interspacecraft exchanges of data, "with exceptions for those particular units which both countries prefer because of common usage". Each crew would continue to denote attitude reference of its spacecraft (ie relative position and angle) in its own terms.
Other proposals are best described in the context of current hardware - though we know much more of Apollo than of Soyuz. The Apollo system is intended for dockings between the command module and the lunar module and is so designed that either may take the active role. Five docking lights, with colours identifiable at 1000yd are fitted to each - the standard red-port, green-starboard and with a white light forward and amber aft. Lastly, a flashing tracking light is fitted atop the spacecraft, producing a 60 degree beam spread and 60 flashes per minute. Such an arrangement would be fitted to the passive spacecraft (ie the Salyut, during Apollo/Salyut operations), except that the position lights would be flashing and the flashing beacon would be replaced by a steady floodlight capable of Providing 0.3 lumens/m2 to a distance of 150m and with a minimum 10 degree cone of illumination.
On Apollo, both command and lunar modules are equipped with a collimating docking sight - the crewman optical alignment sight (COAS). This provides gross range and closing rate information from 1 Soft out while giving a juxtaposed line-of-sight target image. The US-USSR system, being strictly one way, would employ a similar sight only in the active spacecraft and a target on the passive. Engineers on both sides considered that the active spacecraft must be capable of effecting all necessary manoeuvres while the passive vehicle would be responsible only for maintaining altitude control and be equipped with its own propulsion system, so that either spacecraft would be able to alter or correct the orbit. On Apollo, both spacecraft can perform all these tasks, so the docking tunnel has been aligned directly along the thrust centreline of the linked vessels.
For Apollo, relative motion between the two spacecraft is monitored from earth. At the June meeting, however, it was considered that the respective accuracy requirements of the two countries for ground-directed vectoring differed enough to merit an independent, onboard means of measuring the relative motion parameters. Also influencing this decision was the possibility of emergency situations arising from, perhaps, loss of communications with Earth. The proposed method was the installation of a transponder (an active radio-signal reflector) equipped with an omnidirectional antenna aboard the passive station so that the active spacecraft could measure signals on its own appropriate receiver. Other systems are also under consideration, notably use of lasers and of radio systems with passive reflectors.
The matter of the actual docking mechanism is purely one of mutual design. On Apollo, the command module is equipped with a probe mounted on a tripod and tipped with three latches. This is inserted into a conical drogue in the lunar module so that when the probe tip passes through a hole in the apex of the drogue the three latches are released to provide an initial capture. Around the base of the probe is the docking ring, in effect the edge of the command module's half of the docking tunnel. This contains 12 locking catches which are then connected with a similar ring around the brogue to complete a secure, airtight connection. Both the probe and the drogue are then
Apollo capsule interior, far left, and Soyuz interior are very much alike, compact and lacking room for rescued passengers. Below the present docking
systems explained (Nasa release full details of the sighting and docking mechanisms while the Russians have shown little of the mechanism beyond drawings of the two craft comma together). The same male-female joint is used, although the Soyaz device is larger and apparently less complex than that of Apollo.
In the past Russian astronauts have been reprimanded for cracking the obvious dirty jokes
unlocked and removed internally to leave a clear communication tunnel. a hatch at either end making it also an airlock. No technical details of a proposed connecting structure have yet emerged but it was proposed at the meeting that the interconnecting tunnel should measure at least 5ft square and 7ft long, or large enough to contain two spacesuited astronauts. No attempt was made to specify dimensions of the docking devices nor of the tunnel access latches.
One major problem has been that of spacecraft atmosphere. Until the cabin fire which killed astronauts Grissom, White and Chaffee during a countdown rehearsal in 1967, Nasa had used pure oxygen at 51b/sq in throughout missions. Following the fire, which was fed by the pure oxygen, US spacecraft have always used an oxygen/ nitrogen mixture at normal atmospheric pressure during launch, reducing to pure oxygen once in orbit. Soviet spacecraft have always used a near-standard, 14 71b/sq in mixture atmosphere throughout their mission. The proposed compromise was that an Apollo would carry out orbital operations at its normal pure oxygen low pressure atmosphere, reverting to an atmosphere compatible with the Soviet standard when docking with a Salyut.
Nasa has made some provision for a Skylab rescue eventuality. With three consecutive crews due to visit the station, the launch vehicle (a Saturn IB; small brother of the Saturn V moon rocket) for each next crew will be under preparation while the previous crew is still in residence. A special kit is being prepared which will enable a rapid conversion of the command capsule to enable two-man operation and accommodation for a further three astronauts. This technique is only applied to the Skylab programme but could well have subsequent applications. The system is limited to minor failures because the imperiled crew would have to wait up to four weeks for rescue.
Reviews of actions taken or being studied as a result of the Houston meeting should have been exchanged by the two countries during September. When they meet again in Moscow they certainly anticipate a very much busier session than before. "It could be a lot longer than when we met in Houston", observed one member of the Nasa team. "We've got a lot of work to do"