Table of Contents

STS-74 Mission Overview

Docking Module Installation

As Atlantis caught up to Mir, the shuttle crew used the Canadarm to put the Russian-built Docking Module (DM) into position on top of the Orbiter Docking System (ODS). The DM was designed to permit the shuttle to dock to the side of Mir without hitting the Mir solar panels.

The DM was essentially a tube 5.1 metres long and 2.23 metres in diameter with identical Mir docking mechanisms, called Androgynous Peripheral Docking Systems, at each end. Two large solar array boxes, which werel later moved onto Mir, are stored on the upper sides of the DM.  Docking aids such as special targets, television cameras and lights were mounted at strategic places about the module.

A grapple fixture protruded from a short post on one end of the DM to permit the Canadarm to grab it. Eighteen black-painted inconel ASVS dots had been bonded to either white panels or directly onto the orange-coloured thermal blankets that enclosed the DM. These dots, ten or fifteen centimetres in diameter, were arranged in a useful pattern for targetting by the Advanced Space Vision System and were surveyed to within a millimetre of their exact positions before launch. The Docking Module was latched near the back of the Atlantis payload bay by four support fittings. A rotating umbilical arm carried power and data between the DM and the orbiter.

The Orbiter Docking System (ODS) was made of a Mir docking mechanism attached to an airlock that connected to the shuttle cabin by a short tunnel. A strong truss held the ODS securely in the payload bay. One television camera and two lights were bolted to the truss and six black ASVS dots were attached in three two-dot patterns near the top of the ODS tube.

The operation to mate the DM to the ODS consisted of two different phases: unberthing and moving the DM into position over the ODS, and joining the two pieces together. The job took about an hour and a half and began when Chris Hadfield and Bill MacArthur powered up the Advanced Space Vision System (ASVS). The major steps that followed include:

ASVS Checkout

The ASVS processed video images of the target dots attached to the DM and ODS and determined their exact location in space. There were two identical systems on board and both were made ready to run. The second of the two units was configured as a "hot spare," immediately ready to take over if the primary failed. Each unit was a notebook computer with special video processing hardware and links to the onboard video system. For the DM installation, the system was  mounted to the right of Hadfield as he looked aft from his window into the payload bay. Two TV monitors, also on his right, showed views from cameras in the bay and on the Canadarm that helped him see what he was doing. The second monitor was configured to display one of two outputs from the ASVS system. He could switch between either an enhanced video display, which is a normal camera view with target tracking boxes superimposed on it, or a synthetic display that graphically indicates the precise location and orientation of the DM as it is moved around by the arm. Before using the ASVS, the camera positions and target dot locations had to be calibrated so that the system could accurately converge on a solution of their relative geometry.

DM Grapple

Hadfield uncradled the arm and moved it to a pre-grapple position by flying the arm manually so that its end effector was in line with the Docking Module grapple fixture. When ready, Hadfield moved the arm forward about a half metre, until the grapple shaft of the DM was within the "can" of the end effector. Triggering a switch snared the shaft and a rigidize command pulled the arm securely to the DM.

DM Unberth

Just before unberthing, MacArthur and Hadfield configured the ASVS to track the DM from camera B (port-aft). Then the rotating umbilical that connected the orbiter to the DM were demated and the four latches holding the Docking Module to the orbiter were released.

Move to Low Hover Position

Using a light pen on the ASVS enhanced video on monitor 2, MacArthur moved designator boxes over the target dot images, permitting the ASVS to track the dots. Once the ASVS had acquired them, the monitor was switched to a synthetic view which Hadfield could consult to help him carefully guide the DM straight up to a hover position, four and a half metres above the payload bay.

Move to Intermediate Position

Next, Hadfield flew the arm through a complex sequence that lifted the DM two metres up and moved it eight metres forward, near the Orbiter Docking System. At the same time he pitched the DM down 90 degrees and rolled it right 155 degrees. This sequence was entered into the arm computer as an automatic manoeuvre which Hadfield could enable. The ASVS tracked the move.

Move to Pre-install Position

Camera A, the port-forward camera, was then selected for the ASVS, and targets on both the ODS and the DM were acquired and tracked. The ODS docking mechanism was powered up and its capture ring extended in preparation for docking. Hadfield flew the arm manually to position the DM about one metre up and directly over the ODS. He cleared this path by looking out the window, looking at a number of other camera views, and consulting the ASVS synthetic display.

MacLean Manoeuvre

A test of the ASVS tracking accuracy preceded the final act. Hadfield put the arm into single drive mode and selected the shoulder yaw joint for movement. As the ASVS tracked the DM target dots, he swung the arm back four degrees and then returned it to position. The move was named for Canadian astronaut Steve MacLean, who, after tests with the arm in orbit in 1992, found that single joint rotations of the arm were extremely accurate and could be used as a position "ruler" for the ASVS.

Move to Install Position

The final manoeuvre put the DM’s docking mechanism within 15 centimetres of the extended ODS capture ring. This was the most difficult part of moving the DM into position because the alignment between the two docking mechanisms had to be nearly perfect for the installation to succeed. The ASVS tracked this movement from Camera A.

Install

The actual installation was dramatic. Each capture ring of each docking mechanism had three guide petals with latches that protruded and, when joined, engage a lip on the opposite ring. These latches were very stiff and the Canadarm could barely push the DM into the ODS with enough force to trip them. So, instead, Hadfield took the brakes off the arm joints allowing the arm to go limp, and Ken Cameron fired six downward jets of the Primary Reaction Control System for 1.52 seconds.

This imparted enough acceleration to the orbiter to slam the ODS up into the stationary DM and effect a capture. If it didn't work (the alignment between capture rings might not be good, causing the DM to bounce off), then Hadfield would have had to try again to position the two rings, but this time he would have, very gingerly, crept the DM down to within three centimetres of contact. Again Cameron would have commanded the jet sequence. If this "quasi-static" method had also failed, then they would have modified their original attempt for one last try. By this time Mission Control would have analysed exactly the position of the arm on the first try and could have given Hadfield new, slightly biased arm angles that would have aligned the DM properly. Then one final jet sequence would have been commanded.

If this last attempt had failed, Jerry Ross and Bill McArthur would have been ready to quickly don their space suits and perform an EVA in the payload bay. They would have cinched straps between the DM and ODS, keeping the relative positions stable while the ODS capture ring was extended into the DM, pushing the latches over-centre. Once the latches sprung home, the ODS capture ring would have retracted, pulling the docking system seals together. Twelve hooks would have been driven to mate the two structures securely.

At this point, Hadfield left his station and floated down to the middeck, into the airlock and joined Ross as they cracked the hatches and checked for leaks in the ODS and DM. Hadfield also installed a centerline camera that pointed straight up through a window in the forward DM hatch in preparation for the Mir docking. Meanwhile, MacArthur shut down the ASVS systems, and stored video tape recordings of both the enhanced and synthetic displays from the operation for later analysis. He also took over control of the arm, ungrapple from the DM, and put the arm in a poise for docking position.

Rendezvous and Docking with MIR

The STS-74 mission would not have been successful unless Atlantis met up with Mir. This most crucial phase of the flight was also the most difficult and required careful attention from Mission Control and the flight crew. The rendezvous began at launch when Mir’s orbit passed almost over the launch site so that the Atlantis orbit was on the same plane. When the shuttle made orbit, it started behind Mir and spent nearly the next three days slowly catching up. Because of the nature of orbital mechanics, as explained in the next section, Atlantis had to fly in a lower orbit to move faster than its target. Nearly five hours before docking, the final rendezvous procedure began when the orbiter was about 76 km behind Mir.

A big burn with several corrective burns, and a final smaller burn with its corrections, put Atlantis in an orbit that was almost two kilometres lower than the space station. Throughout this time, the shuttle crew tracked Mir, first optically with its star trackers and then, when in range, with its KU-band antenna. These measurements were fed into the onboard computer which was navigating and guiding the flight. The shuttle soon caught up and, when it passed directly beneath Mir, Ken Cameron, the commander, fired its jets to push it into a station-keeping position 52 metres away. Atlantis was now on the Rbar, an imaginary line from Mir to the centre of the Earth. When the go-ahead was given, Cameron pushed his translation controller in and begin to close with the station.

This phase is called the approach and was very difficult. He had to control the trajectory precisely within a narrow corridor, but, fortunately, he had many aids to help him. The most important cue that Cameron received was from a camera mounted in the hatch of the Docking Module looking straight up at a target within the docking ring of the Kristall module of the Mir. If this target was perfectly in line, then he knew he was okay. In all, there were eight cameras to help him with his position. As well, the Canadarm was in a poise for docking position, hanging out over the left-hand sill into space so that its wrist camera could point up at Mir and its elbow camera could point exactly in line with the plane of contact between Mir and Atlantis. This latter view gave the crew visual confirmation of exactly how far apart the two giant craft were in the last stages of the join-up. Two laser range-finders in the payload bay and one hand-held laser, called Lidar (pointed by Chris Hadfield out an overhead window), gave him his range and speed of closure.

When Atlantis was at nine metres, Cameron stoped the rate of closure to correct any last misalignments between the two ships. When Mir was ready and communications with Moscow and Houston were okay, he started a three centimetre per second approach. Five minutes of slow, careful flight ended with a nudge when the two docking rings engaged. Cameron immediately punched a button to command a jet firing sequence that pushed Atlantis into Mir to guarantee capture. The final mating happened when the seals of the docking rings were drawn together and twelve hooks were driven to secure the connection.

Orbital Mechanics - "Brake to Catch up"

All objects in space move through gravitational force fields that bend their paths. As Isaac Newton discovered, gravity is a force that all objects possess and the attraction between two objects is related by their masses and their distance apart. In the case of the shuttle moving in orbit around the Earth, we can neglect the mass of the shuttle and say that it moves under the influence of a central force field.

Movement is what is important. We are all pulled towards the centre of the Earth, but, if you move fast enough and in the right direction, the horizon will always curve away beneath you and you'll be in orbit. In a circular orbit, the speed and direction of your movement (your velocity) stays constant relative to the Earth's surface. The height of the orbit is proportional to the amount of energy you put into it. With more energy, you climb outward from the "gravitational well" of the Earth, making a bigger orbit. The climb costs velocity, however, so you will now move more slowly relative to the Earth and your orbit will take longer.

This is the strange effect of orbital dynamics: to go slower, you ACCELERATE; to go faster, you BRAKE! To drop behind an object, you go higher and slow down. To catch up, you go lower and speed up. In effect you trade between potential energy (height) and kinetic energy (velocity). All orbits are a balance of these forms of energy.

The shape of an orbit is another of its distinguishing features. Two orbits of the same size will take the same time to complete one revolution. They could, however, have different shapes. Practically all orbits around the Earth follow elliptical paths, descending to a low point, called the perigee, and climbing to a high point, called the apogee. By picking when and in which direction you fire your engine, your burn will change either the size and shape of your orbit, or just its shape.

Firing your engine to push away from the Earth (outward) puts you in a climb which causes your orbit to shift to a new apogee - but the orbit doesn't get any bigger! The converse is true of a burn towards the centre of the Earth (inward). You dive to a new perigee in your shifted orbit.

To rendezvous, approach and dock with a target object requires a careful playing of the size and shape of your orbit: losing energy (braking) when you want to catch up, adding energy (accelerating) when you are near, and then shifting your shape and adding energy in ever smaller amounts until you meet your target exactly.