Rockets whiz around in space with no air to kick off. What’s her secret?
It turns out that the engines that power rockets are different than the kind of engines that power airplanes or other ground-based devices. Rocket engines carry everything they need into space instead of relying on air.
Like terrestrial engines, rocket engines work with combustion. Since all forms of combustion require oxygen, rockets carry an oxidant, such as liquid oxygen, into space. This means that they are not dependent on ambient air like a car engine.
“Then the rocket still has fuel, whether it’s kerosene or methane or liquid hydrogen, to create a reaction,” Cassandra Marion, science advisor at the Canada Aviation and Space Museum in Ottawa, told Live Science.
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The rocket’s design includes a combustion chamber, where the oxidizer and fuel react, and then a nozzle, where the products of combustion exit, she explained.
“The explosion caused by this combustion will produce very hot gases that will be ejected from the bottom of a rocket,” Marion said. “If you drive enough force to the bottom of the rocket, the reaction is the rocket moving in the opposite direction.”
This is a reference to Isaac Newton’s third law of motion. We often put it this way that every action produces an equal and opposite reaction, although Newton didn’t put it that way.
An older English translation of his 1766 Latin “The Mathematical Principles of Natural Philosophy (Volume 1) (opens in new tab)‘ describes this law: ‘Every action is always met by the same reaction: or the interactions of two bodies with one another are always the same and directed towards opposite parts.’
In other words, rockets work in a universe of forces. Sometimes the forces are unbalanced, which is what we see when a rocket’s acceleration pushes its inert body up into space. Sometimes, however, forces are balanced, such as a book resting on a table (or a rocket awaiting launch on the launch pad).
“According to the third law, the table exerts an equal and opposite force on the book. This force is created because the weight of the book causes the table to deform slightly so that it presses on the book like a spiral spring,” Britannica wrote (opens in new tab).
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The rules of movement must also take into account orbital mechanics. Put simply, around large planets like Earth, every possible altitude is associated with a certain speed.
The highest point of an orbit is a periapsis and the lowest point is an apoapsis. As NASA explained (opens in new tab)Rockets can only increase their periapsis by turning on their engines (or otherwise increasing their energy) while in apoapsis. Or if rockets want to decrease their altitude, they have to remove energy (turn on thrusters) at the periapsis.
Earth’s atmosphere acts as a constant stress on spacecraft and the International Space Station, forcing them to periodically fire rocket engines to prevent a fallback to Earth. Therefore, missions in all but the highest Earth orbits must carry enough fuel to prevent this “fallback” from occurring.
“There are very accurate measurements of how much fuel to put in the rocket, depending on the size of the rocket, the type of fuel, and anything that adds mass to the rocket,” Marion said. Designers must also consider Newton’s second law. One way to put it another way is that forces acting on an object give it acceleration, where the amount of acceleration depends on the mass of the object.
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Therefore, before sending a vehicle into orbit, designers must consider the specific impulse of a rocket. That’s a measure of how efficient the rocket fuel is in terms of amount of thrust per amount of fuel burned, NASA said. “The higher the specific impulse, the more ‘push off the pad’ you get per pound of fuel,” the agency added (opens in new tab).
Adding more fuel to a rocket isn’t always the solution to orbital problems. That’s because more fuel means more mass, which increases the cost of a mission since more energy is needed to push the spacecraft and rocket off the launch pad.
NASA often uses liquid hydrogen and liquid oxygen because, according to the agency, this combination provides the highest specific impulse of any commonly used rocket propellant. However, hydrogen has such a low density that using the propellant alone is impractical: the tank would be “too large, too heavy and with too much insulation to protect the cryogenic propellant to be practical,” the agency said .
Because of this, many launch rocket missions require boosters. An example today is NASA’s Space Launch System (opens in new tab), a space rocket for lunar missions designed to use two boosters. Together, the boosters provide 75% of the total take-off thrust (opens in new tab)required to lift the SLS off the ground.
For more distant targets, space agencies get creative. To save money when shooting at distant planets like Jupiter, some spaceships whip around a planet (e.g. Venus) and use its gravity to give a speed boost. This shortens the time it takes to get to other targets and requires a missile to carry less fuel than is required for such a long distance.
Follow Elizabeth Howell on Twitter @howellspace.
Originally published on Live Science.
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