Whether to Tether
Training for Work in a Free-Fall Environment
When astronauts are in orbit around the Earth, whether in the Shuttle Orbiter, on the International Space Station, or in any other spacecraft, they are in a free-fall environment. Free-fall creates the illusion that there is no gravity. Simulating this environment on the Earth is quite difficult.
One way in which this can be accomplished is to fly an aircraft along a parabolic trajectory. If the velocity of the aircraft is selected properly, so that the aircraft's flight path conforms to that of a projectile having a ballistic trajectory, then the occupants of the aircraft will experience free-fall. This creates a condition that is identical to that which astronauts experience in orbit.
Unfortunately the duration of this weightless condition is seldom more than about 20 seconds. This is clearly insufficient for training purposes.
For long duration training the best method of simulating a weightless environment is to practice space flight activities underwater where the buoyant force of the water can be adjusted to exactly balance the weight of the astronaut.
If the astronaut's weight is exactly equal to the buoyant force of the displaced water then the illusion of weightlessness will be created.
Figure 1 is a pool side view of NASAs neutral buoyancy facility in which astronauts train for EVA work in space. The effects that astronauts experience working underwater approximates the effects that they will experience while working in the free-fall environment of low earth orbit.
Figure 1: The large neutral buoyancy pool in which Canadian astronauts undergo their EVA training.
Figures 2 through 4 show typical activities that astronauts undergo in order to train for space missions. On Earth the EVA suits are extremely heavy and astronauts must have considerable assistance donning the suit.
The upper torso of the suit is suspended on an elevated rack and it is entered from below.
Figure 2: Dr. Dave Williams suits-up for EVA training in NASA's neutral buoyancy pool.
Figure 3: Dave Williams, suited-up and ready for immersion
Figure 4: Going ... going ... gone. Once totally immersed Dr. Williams is working in an environment that, to a large extent, simulates a free-fall environment.
Astronauts must be well trained to perform the tasks assigned to them. Once they are in orbit it is too late to work out procedures or to make mundane decisions such as which tools are needed or how much time a given task will require. All of these details must be worked out thoroughly before a mission can proceed.
Creating a detailed and reliable task schedule requires a great deal of practice. Various procedures must be tried, refined, and then practiced over and over again so that the possibility of delay or failure is completely eliminated.
To achieve this objective, underwater training provides many advantages.
Of course there are a few drawbacks to underwater training.
Working in Space: EVA (Extra-Vehicular Activity, Space-walk)
Unlike the swimming pool test facility, space provides neither safe boundaries nor an easy method of self-propulsion. In the pool astronauts can use the water itself as a medium against which they can push for slight maneuverability. To protect astronauts from drifting away into the emptiness of unbounded space a safety tether has usually been provided during the EVA phases of a space mission.
The first tethers that were used not only provided a strong mechanical link to the spacecraft, but they also contained wires and hoses for electrical, communication, and life support systems.
Figure 5: The earliest EVA missions were of short duration
and occured during the Gemini phase of the U.S. space program.
Once adrift in space, it is impossible to maneuver easily without mechanical assistance. To return to the spacecraft would require initiating the motion back towards the airlock by gently tugging on the tether. Even the ability to perform a simple task such as turning around provides some major challenges without the ability to exchange momentum with something else.
A little realized fact is that, on Earth, every time we initiate a motion we are exchanging momentum with the matter in which we are in contact. An astronaut "floating" in space is not in contact with anything ... changing one's motion under these conditions requires some ingenuity.
In figure 6 below, the astronaut is holding a small "rocket gun". It is designed to provide small amounts of thrust whereby astronauts can initiate changes in their motion.
Figure 6: Maneuvering in space is a tricky business. Small hand-held thrusters were tried with limited success.
One problem with a hand-held thruster is that the astronauts do not have both hands free to do other work. For EVA work on larger structures such as Skylab the astronauts can dispense with the hand-held thruster by clamoring carefully along the superstructure of the spacecraft. In the event that they should lose their grip and begin to float away the umbilical cable doubles as a safety tether.
However the development of very large spacecraft created a new problem. The umbilical tethers needed to work effectively during EVA tasks became much too long. The added mass and length of the tethers limited the astronauts' ability to perform many proposed EVA tasks such as construction of the International Space Station and servicing satellites such as the Hubble Telescope.
Much larger "maneuvering backpacks" were developed which gave the astronauts greater mobility and freed them from the limitation of long umbilical cables.
Early units were tested with umbilical cables to supply communications, electricity and life support services, and although mobility was increased, the astronauts' freedom of movement was still limited by the length of the umbilical cable.
Figure 8: The self contained back-pack with built in maneuvering capabilities was developed. The tether was still an essential requirement since it also supplied the astronaut with life support resources.
The EVA suits used today no longer require an umbilical cable. They are designed to provide fully self-contained communications and life support resources for as much as eight hours of intense EVA work.
These suits can be used with mobile EVA backpacks as shown in figure 9 or they can be used with a safety tether for EVA construction work on structures such as the International Space Station.
Tethers for Safety
Removing the need for system support via the umbilical cable did not, however, remove the need for safety and security when EVA tasks are undertaken.
For example during the construction of the International Space Station it would be impossible for an astronaut to implement any self-rescue procedure should he or she inadvertently fall away from the space station. Therefore each astronaut maintains a tethered link to specially designed fittings on the external structure of the Space Station.
The photo below shows Canadian Astronaut Steve MacLean at work on the International Space Station during mission STS-115 in September 2006. At all times Dr. Maclean was securely tethered to the ISS.
All persons and payloads that have the potential to drift free in space must be tethered.
The tether protocol applies to both astronauts and payloads. The protocol applies to payloads that are moved either robotically or by hand (glove).
"Make before break"
This is the safety catch-phrase used by astronauts working in space. "Make before break" means that whenever a tethered person or payload is moved from one location to another, one must always make a secure connection at the new location before one can break the connection at the original location. This insures that any item that has the potential to drift free is always tethered.
For example, a solar panel that is to be moved using Canadarm II must also be tethered. In the unlikely event that the Canadarm should prematurely release the solar panel before it can be bolted down, the tether would prevent the solar panel from drifting freely off into space.
Tether Construction and Designs
Tethers have two main components, quick-release and quick-attach buckles and a strong flexible cable between the buckles.
The buckles must conform to many design criteria.
They must be:
- light in weight.
- easily manipulated with thick gloves.
- able to withstand extreme temperature conditions.
Figure 11: Typical tether buckles. Note the quick release lever
is designed for easy manipulation with the gloves of the astronaut's space suit.
Similarly the cable between the buckles must satisfy many important criteria.
The cable must be:
- light in weight.
- tangle resistant.
- able to withstand extreme temperature conditions.
- resistant to damage from solar ultraviolet radiation.
Figure 12: Flat webbing designs are both strong and light in weight. The cross-section of the webbing tends to resist tangling better than cables and ropes which have a round cross-section.
The buckles on some tethers are designed to clip to specially designated attachment points but others are designed to clip easily onto any longitudinal structural member of a spacecraft.
Multipurpose tethers are also available whereby various items can be temporarily held in place at a convenient location until needed.
Figure 13: Various tether designs
As anyone who has every owned a boat will attest, ropes can create nightmares of entanglement. In space it is vitally important that all gear be well organized and safely stored to avoid creating hazardous situations. Long tethers pose a special problem that is solved by using spring-loaded reels. These allow the tethers to be uncoiled and recoiled as required.
Figure 14: Reels for deploying and storing tethers.
The design of tethers seems at first glance to be rather straight forward, but here is an activity that will challenge students.
Students are challenged to design and build a lightweight tether 1.5 metres long that will withstand the force of a 1 kilogram mass dropped from a height of 1.5 metres.
- A pair of hockey gloves.
- A cardboard box (or similar) with a cushion inside to place under the falling mass.
- A strong secure eyebolt supported about 2 metres above the floor.
- A stop watch.
- A 1 kilogram mass with an eye-bolt (a closed eye-bolt, not a hook) which can be used to pick it up.
The Set Up
- Students may prepare their tether in advance but they may not attach it to anything.
- When they are ready the instructor says "go" and starts the stop watch.
- Wearing the hockey gloves the students must attach their tether to the support and to the 1 kilogram mass, raise the mass to the support height and then let it free-fall to the end of their tether. (If the tether is strong enough it will stop abruptly before it lands in the cushioned box.)
- The stopwatch times the students until the mass has fallen to the end of the tether.
- Place the tether on an accurate balance and determine its mass (to the nearest gram if possible).
- The time from the beginning to the end of the drop should be timed to the nearest second.
- The score is determined by the mass (of the tether) x time (it takes the students to perform the experiment).
- Lowest score wins, however tethers which break are automatically disqualified.
Tethers & Gloves
For Primary/Junior Grades
This activity can also be done by using a (strong) pre-made tether and simply challenging the students to perform the experiment in the shortest possible time. The use of the hockey gloves makes this extremely challenging and highlights the difficulty of working in a space suit.
TIP: For P/J the height should be lowered and the tether shortened appropriately.
"Make Before Break"
For Primary/Junior Grades
This activity illustrates the "make before break" strategy used in all EVA/Payload operations in space.
- Students desks are arranged groups or "clusters".
- The students will have various tasks to perform, requiring them to move from group to group.
- A student cannot move to another group unless the student is tethered at all times.
- A tether must only be long enough to reach an adjacent group, but not long enough to reach other groups.
- Tethers made from lengths of light nylon cord (each about 2m long).
- 2 buckles for each tether, one at each end (with spring clips).
- 1 sturdy (immovable) anchor point for each student group. (TIP: Used 4L paint cans, filled with dry sand or gravel and with their lids hammered on make good objects to use as anchor points)
Prepare the following materials.
- Make up sheets of paper (enough for each group) with the words "Solar Panels" written on them.
- Repeat step one with pages for
- Food Supply.
- Life Support Equipment.
- Scientific Equipment.
- Rocket Fuel.
- Habitation Modules.
- Communications Equipment.
- Emergency Equipment.
- Give one set of each of the pages to each group. In other words, one group gets all the "Solar Panel" pages, another group gets all the "Food Suppy" pages and so on, so that each group has a monopoly on a specific resource.
- Give each group two tethers.
To assemble a space station.
- Decide, as a class, the basic items required to build the space station.
- Each group must collect (from the other groups) the items needed to complete their space station.
- [VERY IMPORTANT] Review the strategy and tether protocol outlined below. It is essential that everyone understand the rules.
- No more than three persons can be standing (call this "in EVA mode") at any one time.
- Resources must be collected directly from their source. i.e. They cannnot be passed from group to group.
- Tethers between groups can never cross.
- The "Make Before Break" rule must be adhered to at all times. No exceptions.
When all space stations have been completed discuss;
- any problems that may have arisen.
- possible solutions to such problems.
- how you might design a safety protocol that you could apply to make this construction simulation safer and more efficient.
TIP: The number of resources can be modified to suit the number of groups in the activity.
Also, this activity can be modified in many ways. For example, it might be combined with an art class whereby each group builds models of the resource package and they are "delivered" to a single group for assembly.
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