But it was much harder to produce weightlessness here on the earth. Scientists and engineers did devise a system of a large plane flying a hyperbolic loop across the sky that could produce weightlessness for about half a minute. The effect was very similar to what a person feels for about half a second on the top of the arc when swinging on a playground swing. This allowed some experimentation and training of personnel, but duplicating the hours and days of weightlessness that would occur in space was not possible without actually entering orbit.
Even the fact that spacecraft would experience weightlessness in an orbit around the earth was poorly understood by the general public. I’ve read several old science fiction novels that didn’t understand the basic physics of weightlessness and assumed the gravity of the earth or the moon or mars would affect the astronauts. So why are people and things weightless when in orbit around the earth? Obviously they are not beyond the reach of the earth’s gravity. After all, that force holds the moon in orbit around our globe.
It is not about being out of the reach of the force of gravity, but rather being in “free fall.” Free fall means falling without a force acting upon your body. If you sky dive from an airplane, you are falling, but the force of the air against the body slows you down and you feel that “acceleration” as weight. If you were falling in open space, with no air to slow you down, then you would not feel gravity. That is what an object in orbit does. It is falling around the earth and there is nothing to give weight.
Reentry into the atmosphere, however, does cause weight because the friction of the air is slowing down the space craft and that is what the centrifuge training was for … that and the launch when gravity is magnified by acceleration instead of canceled out by freely falling. So that is why voyagers in space have no weight except when undergoing changes in velocity, called acceleration, as rocket motors or air friction apply forces to the spacecraft and the occupants.
Since most of a space mission and even a voyage to the moon are done with the engines off and just floating, weightlessness is the normal condition of most of space flight. Not only did weightlessness have an effect on the people, their physiology and movement, but it affected the equipment taken into space too. Besides the often portrayed floating of things in the space craft or the need for squeeze bottles to drink liquids, the lack of gravity had a profound effect on the design of some equipment, and the physics of operation. Weightlessness didn’t have an effect on the flow of electrical currents and electronic circuit components such as transistors or switches, but it had a profound effect on the certain devices.
For example, engineers know that the standard meltable-link fuse is a simple, passive, reliable, and very effective way to protect against damage due to short circuits and overloads; it's normal and wise practice to use these on power and signal lines. Their operating principle is simple: when excess current flows through the fuse, the link heats and melts, and then the molten blob falls away, breaking the circuit and current path.
Whoops … the word "falls" is the key to why a fuse won’t work in the weightless world: there is no force (such as gravity) to cause the blob to go anywhere. It will melt and then stay in place, making and breaking the circuit intermittently. To operate reliably, “spring loaded” fuses were required. That way, when the link melted, the spring would pull the connection apart stopping the current flow.
Another counterintuitive situation has to do with cooling, an omnipresent concern for electronic and other systems. In the vacuum of space, of course, there is no option of conduction or convection cooling; only radiation cooling is possible. This complicates the design of satellites and must be carefully factored into the thermal planning and system design.
But what about the Space Shuttle, Skylab, or the International Space Station, all of which have a "normal" air atmosphere? That should allow convection cooling of the electronics as heating air rises, you might assume.
Whoops, wrong again … another mistaken assumption: the word "rises" has no meaning in this weightless environment. What happens is that the heated air just stays where it is, accumulating around the heat source and acting as a warm — and therefore destructive — blanket. So forced-air or active fluid-based cooling is needed, since passive convection-only cooling does not happen.
Let me explain these last paragraphs and the physics of heat transfer for you non engineers. There are three ways that heat moves from one point to another. The first is “radiation.” That’s how heat gets to us from the sun. The energy that we describe as heat moves across free space via photons. In other words, the heat is in the light (or more accurately, the radiation) from the sun. That’s why shade feels so good on a hot and sunny summer day.
Heat also moves by “conduction.” That’s why a metal handle on a pot on the stove gets hot. Heat is actually the vibrational movement of molecules. The fast moving (or “hot”) molecules bang into nearby molecules making them “hot” too. Metals are very good at conducting heat, while other materials resist this motion more. That’s why some pot handles are made of plastic or wood which doesn’t conduct heat as well. A hot pad is also a poor conductor of heat as is styrofoam. Now you can understand everything form home insulation to coffee cup design is premised on this physical phenomenon called conduction.
The third method of heat transfer is “convection.” It is based on the fact that “hot air rises.” You know … what makes a hot air balloon lift off. The reason is that hot air (or any hot fluid from air to water to oil to …) is less dense than the material when cool. If it is less dense, then gravity forces the more dense fluid “down,” thereby forcing the hot fluid “up.” That’s what is working in your old fashioned steam or hot water radiators. That’s also why it is hotter on the second floor of a two story house and quite cool in the basement. (Convection plus the insulation effects of the ground and solar radiation … it is a combination of physics.)
Convection is what makes the wind blow and thunderstorms and tornadoes and hurricanes and most of the weather effects. Convection is also how a “heat sink” works. Heat sinks are large metal devices that conduct the heat away from the transistor or I.C. or other electronic device. But then the metal fins common on a heat sink heat up the nearby air which floats away by convection, thereby cooling the device. So heat sinks don’t work in weightless air, since the heat is not carried away.
The lack of convection in a weightless condition had some advantages. The lunar spacecraft was a rather cold environment with the hundreds of degrees below zero space just outside the metal walls, the interior was a bit chilly. Yet the astronauts slept very comfortably because a thin layer of warm air from the heat of their body would wrap around a sleeping astronaut like a transparent sleeping bag and keep them warm all night. Normally, convection would disperse this heated air, but in the still air of the weightless spacecraft, the layer clung to the body keeping the sleepers quite comfortable.
Almost every small action, perspective, and operating mode we know on Earth has the presence of gravity effects as an unspoken, "given" assumption — and how its absence in space make these standard operating concepts almost meaningless. It's tough on equipment, and even worse on humans, with bizarre micro- and macro-consequences due to the absence of gravity impact as a pervasive force.
Another example was the simple “fuel gauge” designs on the lunar lander. Just like an automobile gas tank design, the “gas gauge” works with a float in the tank and gravity to force the liquid fuel to settle in the bottom of the tank. As the lunar lander approached the moon’s surface, only when the engines were firing did the fuel gauge work. This led to very stressful moments as the first lunar lander approached the moon’s surface and nearly exhausted the fuel in last minute maneuvering to avoid large boulders. NASA was counting down the fuel supply based on telemetry from the space craft, but they weren’t sure of the amount remaining. They were down to 15 seconds of fuel left when the “Eagle Landed.” At least that’s what their instruments showed, but they were very worried the readouts weren’t accurate due to problems making these devices work in space and during a weightless free fall toward the moon.
To the early NASA scientists and engineers, they had the responsibility to design reliable space craft that would sail into unknown waters. Truly dragons were there. Through a process of “baby steps,” and incremental advances, the staff and crew learned how to build craft and train astronauts that could operate in space. (Even though, as the picture above shows, space is not good for hair styles.) Now we have the perspective of not only the flights to the moon, but the months and years of survival in orbit in the International Space Station. Now we know from experience how to build devices to work in the weightlessness of space. At first we had to figure it out in our heads before our actual experience. The reason that all early astronauts were “test pilots” is that was exactly the job they were taking on.