What Keeps a Satellite Up?

What Keeps a Satellite Up?

Before we answer this question, we should first make sure we know which way is up.  Since the planet Earth is round, up means any direction that is away from the center of the Earth. And if we are talking about the planets, which are satellites of the Sun, up means any direction that is away from the Sun. Whatever keeps a man-made satellite up is the same thing that keeps the Moon from falling on the Earth and the Earth from falling into the Sun.

Now, if you had a ladder about 200 miles high, and if you climbed up to the top carrying a satellite under your arm (some satellites are small enough to carry under your arm), and let go of it, it would fall down exactly as you had dropped it off the top of a building. It wouldn’t stay up at all. So it isn’t just a matter of getting a satellite up there.

What does keep a satellite up is its velocity – the right amount, and in the right direction. And that leads us to Newton’s law stating that every action has an equal and opposite reaction. Newton has another law that states that anything that is moving will keep on moving at the same speed in a straight line forever, unless some force makes it do something else. This means that if we used a rocket to launch a satellite from the earth’s surface, would like to keep going in the same direction it had when the rocket burned out, and at the same speed. And in fact, if the earth were not there, that is just what it would do. But the Earth is there, and the Earth’s gravity is the outside force that makes it do something else. We have a tug-of-war, where the satellite is trying to sail off in a straight line into space and the Earth is trying to pull it back down.

Photo: Elena

Now if the satellite is going fast enough it will break loose from the pull of the Earth and sail off into space (but not in a straight line, because then the Sun’s gravity will start trying to pull it in toward the Sun). But if it is going just fast enough to balance the pull of the Earth it will keep going around the planet as though it were on a leash; it keeps trying to go straight and the leash (gravity) keeps pulling it in. That is what the Moon – our biggest and oldest satellite – has been doing for millions of years.

The force of gravity gets smaller and smaller as you get farther and farther from the Earth (that’s another of Newton’s laws). This means that for any distance from the Earth there is one particular speed that just balances the pull of gravity, and the higher a satellite goes (that is, the farther from the Earth), the less speed it needs to stay up. On the other hand, you need a more powerful rocket to get it there, because the rocket has to push it farther against the pull of gravity. And if a satellite is at a lower orbit, it has to be going faster just to hold its own against the pull of gravity. That is why lower satellite orbits have shorter periods of revolution – time it takes to go around the Earth – than higher ones.

Jupiter and its satellites. Photo in public domain

The first astronauts in their Mercury capsules went around the Earth in about an hour and a half. They were only a hundred miles up. And they were going over 17,000 miles an hour. The Moon, though, is 240,000 miles up and takes 28 days to go around the Earth. That means it goes only about 2200 miles an hour. For any altitude in between, there is a particular speed that you must go to stay in orbit at that altitude.

Of course, if the satellite or even the Moon were to stop in their orbits they would fall straight down to Earth. And if anything slows a satellite down it will fall a little bit because now the force of gravity is stronger than the force trying to keep it going in a straight line. Like anything else that falls, it will speed up as it falls until once again it is going fast enough to maintain a new orbit at the lower altitude. That is how the Gemini astronauts changed from one orbit to another. If the wanted to go to a lower orbit they fired their retro motors to slow them a little bit.

The process is just the reverse if you want to change from a low orbit to a high orbit.  You give a spacecraft a short push with your rocket motors, and that starts it moving up to the higher altitude. As soon as the motors stop thrusting, however, the spacecraft just coasts the rest of the way until it gets as high as it can go with the amount of thrust. While it is coasting, of course, it slows down, like a car that is coasting uphill after you shut off the engine. And if you’ve given it the right amount of thrust, when it gets to the higher orbit it will be going at the slower speed that corresponds to that orbit.

So although you speed a satellite up to get it to a higher orbit, by the time it gets there it is actually going more slowly than before. And although you slow a satellite down to get it to a lower orbit, by the time it gets there, it is actually going faster than before.

You already know that a satellite has to be lifted above the Earth’s atmosphere to stay in orbit at all, because it had to push its way through the air while it was in orbit, it would slow down so soon that it could stay up for only a very short time. Illustration: Megan Jorgensen.

Propellants

Propellants

Today`s rocket engines use chemical propellants, which may be either liquid or solid. Notice that we say “propellants” and not “fuel”. This is because a rocket engine usually has to have two propellants. One is called fuel and the other is called the oxidizer. An automobile engine or an airplane engine needs to carry only fuel, because it uses the oxygen from the air to burn with the fuel. But most of the time a rocket has to operate where there is no air, so it must carry its own source of oxygen, and that is called the oxidizer.

Most liquid rocket engines have separate tanks for the fuel and the oxidizer, which are combined and burned in the combustion chamber of the engine pretty much the way gas and air are combined and burned in the burner of an ordinary gas stove. The difference is that in the rocket engine both fuel and oxidizer are stored in liquid form in the tanks and pumped or gravity-fed to the combustion chamber, where they are sprayed in and burned.

Different rocket engines use different combinations of fuel and oxidizer, but the most common oxidizer is just plain liquid oxygen. In the Atlas and in the Saturn V first stage, the fuel used is kerosene, very much like jet airplane fuel. For upper stages like the Centaur or the third stage of the Saturn V, liquid hydrogen is used as the fuel. Both oxygen and hydrogen have to be very cold to become liquid (-297 degrees F for oxygen and -423 degrees F for hydrogen), and are hard to store and handle. These propellants are called cryogenic, from the Greek word kryos, meaning icy cold.

And remember that even the most efficient rocket engine works on the same principle as the toy balloon, that is, it must expel something from the nozzle in order to create thrust. Photo: © Elena

The Titan II rocket and some others used propellants that did not have to be cold, such as hydrazine and nitrogen tetroxide or nitric acid. Some liquid propellant combinations are called hypergolic, which means that they ignite just by coming together and therefore do not need any ignition system. The TRW-built Descent Engine for the Apollo Lunar Module used hypergolic propellants.

A solid propellant engine also has a fuel and an oxidizer, but they are mixed together before-hand and poured into the rocket casing. The rocket case is placed nose down and a core is placed in the middle of it. The mixture is then poured in around the core and cured by heating until it becomes a solid rubbery mass. The core is then taken out, leaving a hole in the middle of the propellant. When it is time to fire the engine, an igniter shoots a tongue of flame down this hole and the propellant burns all along this inner surface, shooting the hot gases out the nozzle.

Solid-propellant rockets have no tanks or pumps or valves and are very simple and reliable. The Minuteman missile, for example, has three solid propellant stages and a ready to take off ant time at less than a minute`s notice. One trouble with a solid rocket, however, is that once you have started it you can`t shut it off as easily and safely as you can a liquid engine. Also, we haven’t perfected a way yet to fire it, shut it off, and then start it again later as we sometimes need to do for space missions.

Another type of rocket that will be used to power the deep-space missions of the future is the nuclear rocket. The first such engine was designed by NASA and the Atomic Energy Commission. It was called NERVA (Nuclear Engine for Rocket Vehicle Application). It was supposed to use the heat from a nuclear reactor to heat liquid hydrogen to a high-temperature gas, which was then expelled out the nozzle like the gases from a chemical rocket engine. The Isp from such an engine is much higher than that of chemical propulsion; it would be about 1000 seconds, compared to 300-500 seconds for chemical propellant rockets.

Still more advanced is the electrical propulsion engine, which has much higher Isp`s. This engine uses electricity to heat hydrogen to a high-temperature gas that is expelled from the nozzle. So far, only relatively small electrical propulsion systems have been built, because it is hard to generate in space the enormous amounts of electrical power that would be needed to produce a thrust comparable to that of a chemical or nuclear rocket engine. The ion engine and the plasma engine are other types of electrical propulsion system that are also very efficient, but so far they too are limited to very small thrusts of a pound or less.

Notice that electricity by itself could not propel a rocket at all. It can make the rocket move only by accelerating some kind of particle and expelling it.

What Makes a Rocket Go

What Makes a Rocket Go

The force which propels rockets, called thrust, has often been demonstrated with an ordinary toy balloon. If you suddenly release a blown-up balloon, the air inside will rush out its open neck. Obeying Sir Isaac Newton`s law which states that every action sets up an equal and opposite reaction, the rushing air creates a reaction force – thrust – which drives the balloon in the opposite direction.

Just as a balloon is thrust forward by expelling air, a rocket is thrust forward by expelling particles – usually in the form of a gas – from its nozzle. The greater the flow rate through the nozzle, the greater the forward thrust.

Notice that the forward motion of the balloon is not caused by the air expelled pushing against the atmosphere, the way an airplane propeller does. Even if there were no atmosphere to push against, the balloon would still zip around the room. In fact, it would even go a little farther and faster, since it wouldn`t have to push its way through the air.

Thrust is measured in pounds. In order to launch a rocket from the ground, the number of pounds of its thrust must be greater than the number of pounds it weighs. It`s like a tug-of-war between Earth`s gravity and the rocket engine`s thrust: if the engine can push up (thrust) more than gravity is pulling down (weight), the rocket will move up.

An ordinary balloon can be used to demonstrate thrust. Photo: © Elena

How rapidly the rocket moves up depends on how much greater its thrust is, compared with its weight. Rocket engineers call this the thrust-to-weight ratio.

The Saturn V moon rocket, for example, together with the Apollo spacecraft, weighted about 6 million pounds. The thrust of the first stage engines was 7, 5 million pounds, so the engines could win the tug-of-war and the whole vehicle lifted off. The thrust-t-weight ratio in this case was 7, 5 (million pounds of thrust) to 6 (million pounds of weight). This is the same as 5 to 4, or we could say 1.25. That means that the thrust is 1.25 times the weight.

Now you can see that if the thrust were 2 or 3 times the weight it would move even faster. Well, that is what happens as the rocket starts to move. The thrust remains the same, but since the engines are burning up propellants very fast (in the case of the first stage of the Saturn V, 15 tons every second), the weight is getting smaller all the time. That is why the rocket moves slowly at first but accelerates very fast and is usually out of sight in two or three minutes.

Another important factor is specific impulse, which we usually writ with the symbol Isp. To the rocket engineer, this means the same kind of thing as miles per gallon; it tells him how much thrust he can get from a pound of propellants. For example, if he says that a certain type of engine will give him an Isp of 300 seconds, he means that I pound of propellants will generate 300 pounds of thrust for 1 second, or 1 pound for 300 seconds, or some other combination of thrust and time that can be multiplied together to make 300. Naturally he wants to get the most thrust he can from each pound of propellant weight, so he likes the Isp to be as high as possible.

One more very important thing that is considered in rocket design as the mass ration. That simply means the total weight of the rocket loaded with propellants compared with the weight left after the propellants have all burned up. We like to have the mass ration as high as possible, because that means we get more payload placed in orbit, or we can place the same payload in a higher orbit. In order to improve the mass ration, rocket engineers often design rockets with more than one stage, because in this way they can throw away part of the weight when they don`t need it any more. When the first stage has burned up all of its propellants, it is separated and allowed to fall back to the ground. The the casing, tanks, engines, and so on for this stage don`t have to be carried up any higher. Most rocket vehicles have two or three stages.

Rocket Technology

Rocket Technology

In the beginning, the growth of rocket technology in the U.S. received an enormous impetus from the development of the Air Force`s long-range missiles. These programs – Atlas, Thor, Titan, and Minuteman – representing one of the most urgent, complex, and costly tasks ever undertaken by American industry, involved over 200,000 firms as well as many hundreds of thousands of scientists, engineers, and technicians. Selected by the Air Force as systems engineer and technical director, TRW Systems Group coordinated the technical, cost, and schedule aspects of each of these vast efforts.

Atlas

Atlas, the first U.S. ICBM, was a 1-1/2 stage cryogenic liquid-propellant missile featuring a sustainer engine and two booster engines. The two boosters, visible on each side of the rocket, were jettisoned in flight, leaving the sustainer engine to continue firing until it burned out and the correct ballistic trajectory was achieved. This is why Atlas was called a 1-1/2 stage rocket, it did not truly have a second stage. The reason for this design was that rocket engineers had not completely solved the problem os starting a second stage engine at high altitudes.

The initial flight was made at Cape Canaveral in June of 1957, only a little over 2-1/2 years after development had begun. Athough deactivated in 1965 as a weapon system, Atlas continued to play an active role in the space program for a few years, having served as a versatile and reliable booster for lunar and deep space probes as well as for Project Mercury and Project Gemini, as engine improvements increased its thrust to over 400,000 pounds. Mated with an Agena D upper stage, this vehicle was able to insert in earth orbit a payload of 11,500 pounds.

Minuteman I. This image or file is a work of a U.S. Air Force Airman or employee, taken or made as part of that person’s official duties. As a work of the U.S. federal government, the image or file is in the public domain

Thor

The intermediate-range Thor, a one-stage cryogenic liquid-propellant missile, took just 13 months to go from drawing board to first flight. It was ready for military operation as an IRBM in December of 1958, just 3 years after starting development – a record for such a vast undertaking. Mated with second and third stages derived from the Navy`s Vanguard booster and dubbed Thor-Able 1, the vehicle saw its first service in the nation`s space effort in August of 1958. In October of the same year, it launched TRW`s Pioneer 1, the world`s first deep space probe. Deactivated as a weapon system in 1963, the Thor became the nation`s most widely used space booster, earning the title of “space-age workhorse”.

It was used with several different upper stage combinations, and was upgraded almost continuously. The early Thor-Delta configuration, based on the original Thor-Able combination, was able to orbit 610 pounds. One of the last improvement in payload capability was seen in Long Tank Thor, which with its three strap-on solid propellant boosters and an Agena D upper stage was able to orbit 2640 pounds.

Titan

The Titan program resulted in the development of two weapon systems: Titan I and Titan II. Titan I, a two stage cryogenic liquid propellant missile with first stage thrust of 300,000 pounds and second stage thrust of 80,000 pounds, was first successfully flight tested in February of 1959. Like Atlas, Titan I was deactivated in 1965. The later generation Titan II employed non-cryogenic storable propellants and inertial guidance. With first stage thrust increased to 430,000 pounds and second stage thrust to 100,000 pounds, it offered greater payload weight and increased range, as well as better accuracy.

It could also be launched from underground silos, providing greater security against attack. Still deployed and fully operational as ICBM, Titan II also saw duty in the US space program, notable as the booster for the two-man Gemini capsule. Utilizing various improvements to increase thrust, Titan II evolved into an even more powerful vehicle Titan III. One member of this family, the Titan IIIC, featured two 1,2 million-pound-thrust solid propellant boosters, strapped on each side, an improved first stage with thrust increased to 474,000 pounds, and a 16,000-pound-thrust hydrogolic fueled third stage known as a Transtage. Used extensively as by the Air Force as a booster for military communication satellites, TRW`s Vela nuclear detection satellites, and other spacecraft, the Titan IIIC had sufficient thrust to orbit 25,100 pounds.

Minuteman

Studies begun in 1957 to develop a second-generation ICBM which would have less vulnerability to enemy action, be capable of launching in a shorter time, be more mobile and reliable, cost less, and require fewer operating personnel. The ensuing program resulted in the development of the three-stage Minuteman, the first solid-propellant IVBM to become operational in the US. The first version of the missile, Minuteman I, was test-flown from Cape Canaveral in 1961, and later the same year was first launched from an underground silo. The first two flight units (20 missiles) were declared operational at Malstrom AFB, Montana, in 1962.

An improved version, Minuteman II, was first launched in 1964. It incorporated a new second stage engine and an improved guidance system, providing greater range and payload and greater ability to withstand the effects of enemy attack. In 1967, the goal of 1000 operational Minutman was reached.

Development of the next version of the missile, Minuteman III, began in 1967. Minuteman III had an improved third-stage engine and a new re-entry system which allowed even heavier payloads and greater accuracy. A key feature of the new design improvements was their ability to be grafted onto Minuteman II missiles. This resulted in a new weapon system at a fraction of the cost of developing an entirely new one. Dispersed in hardened underground silos throughout seven western states, that prime deterrent force was steadily being updated with the eventual goal of replacing all older models with Minuteman III.

Revealing Triton

Revealing Triton

Triton, Neptune’s largest moon, was discovered on October 10, 1846. William Lassell, an English amateur astronomer, found the satellite just seventeen days after Neptune itself had been discovered.

Until Voyager 2 streaked past the intriguing moon in August of 1989, Triton remained little more than a tiny point of light to earthbound astronomers. Scientists knew from spectroscopic studies that Triton possesses an atmosphere and that it orbits about 220,000 miles from the center of Neptune, but they remained uncertain of its size and many other physical properties.

Triton’s most unusual characteristic, at least from Earth-based observations, is its orbit, which is “backwards”. Unlike any other major satellite in the solar system, Triton orbits in the direction opposite to its parent planet’s rotation. Scientists theorize that Triton formed separately from Neptune and was captured later when it drifted too close to the giant blue planet.

Neptune’s Extraordinary Moon Triton. Photo: NASA, public domain

The television cameras of Voyager 2 revealed an amazing frozen world of mountains, fault lines and ice volcanos. Triton is one of only three worlds in the solar system, along with Earth and Jupiter’s moon Io, known to have active volcanos.

Several Voyager photographs showed dark irregular blemishes on Triton’s surface. The patches may be slushy ponds of frozen nitrogen spewed from volcanic vents. Although the surface is so cold that nitrogen freezes solid, tidal forces below the surface could turn the nitrogen ice into slush. As pressure builds, the slush forces its way to the surface in an explosive eruption. Voyager’s instruments detected thin clouds above the dark spots, further evidence of volcanic eruptions.