Abstract
On May 25, 1961, before a joint session of the US Congress, President John F. Kennedy made what become known as the “Moon Shot Speech.” In it, the youthful president challenged the mightiness of the American aerospace industry to put a man on the Moon and return him safely before the decade was out.
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Notes
- 1.
This is analogous to an elevator motor where the elevator is respectively descending and ascending. In the first case, the motor acts as a brake to control the rate of descent and counteracts the downward pull of gravity. When ascending, the motor acts a generator that hauls the elevator up against gravity.
- 2.
These latches and wedges prevented the satellite from moving radially or along the vertical direction of the SSS. Their design allowed for the radial dimensional changes which the satellite would be subjected to while on-orbit, preventing unacceptable high stress when restrained in place.
- 3.
If a neutral atom loses a single electron, it becomes an ion with a single positive charge. If the ion still has electrons and is further heated, it may continue to shed electrons and become progressively more ionized until, in the extreme case, it is fully ionized. If hydrogen, the simplest of atoms, is ionized, the loss of the single electron leaves behind a proton.
- 4.
The solar wind is mostly diverted away from Earth by the planet’s magnetic field. However at the poles, the field allows some of the plasma to interact with the lower atmosphere.
- 5.
The wire and the magnetic field move with respect to one another. Regardless of whether the wire moves (usually by rotation) through a stationary magnetic field or the magnetic field moves (again usually by rotation) about a fixed wire, the result is the same with the generation of a current.
- 6.
The gravitational force follows the inverse square law, so the lower is the orbit the greater is the attraction felt by a spacecraft, but because the centrifugal force follows a linear proportionality, the farther away the orbit is from the Earth’s center the greater is the centrifugal force acting on the spacecraft.
- 7.
Note that despite being known as the gravity gradient , centrifugal force is an equally vital player in this tug of war between contrasting forces. In fact, up to one-third of the total force acting on the system is due to the centrifugal force.
- 8.
This is the same as the point where the gravitational and centrifugal force balance. However, it must be noted that for very long tethers, much longer that those used by the TSS mission, this does not hold true anymore because of the large mass of the tether.
- 9.
For this reason, the satellite was equipped with four in-line thrusters.
- 10.
Note that gravity, a downwards pulling force, is responsible for pulling the satellite outward. How is this possible? Remember that as the Orbiter and satellite change their positions, the system’s center of mass remains on the same orbit. As the Orbiter is pulled downwards, in order to keep the system’s center of mass on the same orbit the satellite has to go upwards. Hence, with the help of the centrifugal force, the satellite is pulled upwards as a consequence of gravity acting on the Orbiter.
- 11.
The gravity gradient is not limited to tethered systems. In fact, any sizable spacecraft will be subjected to it if an uneven distribution of masses of its internal components reproduces the same condition as two masses on two slightly different orbits. If a non-Earth oriented attitude is required, the spacecraft’s attitude control system must fight against the gravity gradient .
- 12.
As we have not applied any external force to the Shuttle-satellite system prior to deployment, the center of mass cannot change its orbital altitude. The satellite deployment brings about a shift in mass distribution but not in center of mass position. That is why the Shuttle has to move downwards. Obviously, given the different order of magnitude in mass between the Orbiter and the satellite, the displacement of the former is minimum compared to that of the latter. In fact, upon full deployment of the 12.4-mile tether, the Orbiter was expected to have fallen just 328 feet below the system’s center of mass.
- 13.
The angular momentum of a body is the product its moment of inertia (how the mass is distributed about specific axes) and its angular velocity. Unless a rotating system is perturbed, its angular momentum remains constant.
- 14.
This is similar to a skater pivoting on its axis with both arms stretched out. As soon as the arms are pulled in, the moment of inertia of the skater is reduced. In order to maintain a constant angular momentum, the angular velocity must increase. Thus the skater spins faster.
- 15.
It is worth noting that orbit eccentricity is also a source of libration . In fact, as the system’s center of mass travels along an elliptical orbit, the tethered spacecraft will be subjected to continuous changes in both its orbital velocity and altitude. Hence a continuous imbalance in the gravitational and centrifugal forces acts as a constant perturbation to the gravity gradient equilibrium, thereby generating libration .
- 16.
Do not confuse this mode with the libration rigid pendulous motion where the two masses rotate about the center of gravity of the system formed by the tether and the masses. In the pendulous mode, each mass rotates about its own center of mass with the tension in the tether being the restorative force. In contrast, in the libration rigid pendulous motion the restoring force is the balance between gravity and centrifugal force.
- 17.
If the current were pumped from the Orbiter to the satellite instead, the electromotive force would actually increase the velocity of the system. It is evident that this could be a good way for a spacecraft to change its orbit without using propellant. Tethers have been analyzed extensively as a means of de-orbiting satellites when their usefulness has expired.
- 18.
This is akin to playing with a jump rope and shaking it out of phase in order to damp out its vibrations.
- 19.
This is why the yaw maneuver of the Orbiter was designed to damp out the skipping rope to a maximum of 65.5 feet by the time the satellite had reached a length of 0.25 miles.
- 20.
A negative margin means that in a particular condition the structure will be subjected to a higher loading than that for which it was designed, thus increasing the likelihood of failure. Owing to conservatism in the design of any structure, a negative margin will not necessarily endanger safety, but certification rules out such negative margins.
- 21.
The Coriolis effect is a fictitious force that is used to simplify calculations involving rotating systems, in particular in relation to the Earth’s surface whose tangential velocity depends on latitude.
- 22.
It is common practice to manufacture, assemble, and store space-worthy hardware in clean rooms which maintain strict control of temperature, humidity, pressure, and particulates. Being akin to the operating theater of a hospital, it is not surprising that people who work in clean rooms must wear over garments similar to a scrub, inclusive of gloves, shoe covers, hat, and face mask.
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Sivolella, D. (2017). Something That Nobody Had Ever Done Before. In: The Space Shuttle Program. Springer Praxis Books(). Springer, Cham. https://doi.org/10.1007/978-3-319-54946-0_10
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