July 13, 2024

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Science It Works

Conversation-Stopper – Why Don’t the Space Shuttle and Space Station Fall to Earth?

Conversation-Stopper – Why Don’t the Space Shuttle and Space Station Fall to Earth?

What happened to gravity? Doesn’t it operate in space?

The answer to the above questions is the same as for, “Why doesn’t the moon fall onto Earth”, or “.. the Earth into the Sun”. Gravity – the attractive force between two masses (or bodies – the product of their masses divided by the square of the distance between the two centers) – does act continually and everywhere; the answer, therefore, is that the Shuttle, Space Station and Moon are each traveling at high speeds and (corresponding) heights, circling Earth, as does the Earth in its orbit around the Sun – and that all are always “falling around” the curvature, e.g. of the Earth. Although the flight path of the orbiting vehicle (or moon, or planet) attempts to be a “straight” line – it is continually being pulled down by gravity – so that as “satellites” they continually travel in stable orbits, circular or elliptical. The same is true throughout the universe, and while attractive forces exist between all bodies in the Universe, each to each other, the factor of distance-squared in the denominator effectively eliminates the significance of all other bodies in comparison to the two involved in satellite orbiting.

To put a numerical perspective upon what has become a casual acceptance of space activities, to achieve a stable orbit, the Shuttle Orbiter vehicle – which does not have propulsive power while orbiting – must rise to a sufficient height above the Earth’s air layer, where the vacuum of space produces no “drag” resistance (generally about 125 miles altitude) – to achieve this, the lift-off propulsion system must propel the orbiting vehicle to approximately 18,000 miles per hour (note: traveling in the easterly direction gains the Earth-surface rotational speed of about 1000 mph).

At the completion of the mission, to return to Earth, the Orbiter is slowed slightly – dropping closer to Earth – smashing into individual air molecules, which are “vaporized” by the impact – a tiny pulse of both “drag” (causing further slowing and lowering of the Shuttle) and also of “heat”. As the Shuttle is slowed and lowered for the reentry mode, the heat build-up develops tremendous temperatures of up to 3000 degrees Fahrenheit – requiring the insulating “tiles”, which cover the lower wing and body surfaces.

The concept of the Space Shuttle is remarkably and functionally (and beautifully) simple and reliable – as a result of reliance upon this function of insulation – in an absolutely hostile, unforgiving space environment of cryogenic iciness plus vacuum. The rentry insulation tiles, invented and developed by NASA and Lockheed Aircraft, shield the Orbiter Spacecraft (fabricated of conventional light, aluminum-alloy thin skin-stringer construction, similar to most sub-sonic aircraft of that era) from re-entry heat – temperatures which would melt the strongest alloy steel.

  • The tiles are individually designed for the anticipated reentry temperatures, 6×6 inches in size and average about 1 inch in depth;
  • Inside, they are comprised of extremely long, fine filaments of quartz, compressed into the vacuumized tile volume, with a covering of thin glass. The fabrication process – double vacuum, pressurized, is extremely complex. Tile surfaces are relatively easily damaged, the inside appearance likened to white styrofoam – however, when the outside temperature is 3000 degrees F, the backside (attached to the aluminum structure of the Orbiter by ordinary RTV Room Temperature Vulcanizer) – is only 80 degrees F.
  • The highest reentry temperatures are on the bottom of wing and body, and at the trailing edges of the control surfaces; there are 30,000 black tiles; NASA has reported the cost at $2500 each.

However, because of the tremendous insulating capability of the tiles, a greatly simplified and reliable Space Shuttle concept has been achievable:

  • The Orbiter vehicle itself was essentially designed and constructed much as a conventional aircraft – its only flight function, landing after reentry, uses conventional flight controls, tires and brakes (the landing speed is similar to a commercial jet aircraft, about 160 mph).
  • There is no propulsive capability for orbiting or landing, the speed and momentum of the vehicle after reentry, permitting the astronaut-pilot to maneuver the craft into the pre-selected NASA airport and runway for landing.
  • The most powerful and efficient rocketry arrangement is therefore designed for the lift-off sequence: two solid rockets (approximately twelve feet in diameter), plus the three Orbiter engines (fueled by the large center fuel-oxidizer tank); all five are fired simultaneously for maximum thrust at lift-off (approximately seven million pounds thrust required) – along with the giant ground-retention explosive bolts at the base of the solid rockets (only attachment of the entire assemblage to the launch platform).
  • After twelve minutes of flight, the exhausted solid rockets are depleted and dropped (parachuted and recovered in the ocean); the three on-board engines continue until the center tank is empty, when it is separated; afterwards, the Orbiter, traveling at orbiting altitude and speed, has no more propulsive capability.

A human-interest worry about mankind’s 21st century “Space adventures” is space debris – the remains of space-hardware rocketry that have not, as yet, returned to Earth. Varying in size from complete rocket stages to tiny particles, they are true hazards because of their thousands of mph travel speeds. The larger ones are monitored – just recently, March 5, 2009, warnings about a possible strike of the International Space Station forced the US astronauts to take shelter in the parked Russian Soyuz capsule. Two months ago, two satellites collided in orbit, adding several hundred new pieces of “junk” to the space debris litter-belt circling Earth. The NASA Orbital Debris Program Office is at Johnson Space Center, reporting that about 13,000 such threats are constantly tracked, of about 600,000 total debris items.