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Rocket Aerodynamics: How Rockets Work
Even the Lunar Module—which Apollo astronauts used to get to the surface of the Moon and back—was a two-stage rocket. When we launched from the Moon to return home, the landing stage was left on the surface.
The first rockets that were built were single use, with no thought of reusing them again. The Space Shuttle was the first spacecraft that was designed to be reused, and it was capable of being flown to space one hundred times. Even its solid rocket boosters were partially reusable—they could be recovered after falling into the ocean, salvaged, cleaned and recertified, and refilled with fuel for later launches. Today, companies are building even more reusable rockets; SpaceX is able to launch and then land the first stage of its Falcon rocket, recovered intact and ready to be filled again with liquid fuel. Similar technology is also being used by Blue Origin for their New Shepard rocket.
There are two main types of fuel used to get rockets off Earth: solid and liquid. Solid rockets are simple and reliable, like a Roman candle, and once ignited there’s no stopping them: they burn until they run out, and can’t be throttled to control thrust. Liquid rockets provide less raw thrust, but can be controlled, allowing astronauts to regulate the speed of a rocketship, and even close and open the propellant valves to turn the rocket off and on.
The Space Shuttle used a combination of solid and liquid rockets for launch. The solid rocket boosters were used only to take the crew above the air; while the liquid fuel rockets burned the entire time.
Basic Physics of Rockets
The very basic driving force behind rocket construction is Newton’s Law that deals with variable physics. Since a rocket must be aerodynamic while shedding mass (the fuel that it burns through), Newton’s third law for actions and reactions comes into play. As a rocket ignites, fuel burns through and exits from the rear exhaust, causing the rocket to accelerate and propel forward with more and more velocity. This assumes that the rocket operates without drag force.
However, there’s a caveat: In order to fly in space, you need to get through Earth’s atmosphere, and then accelerate until you’re going fast enough so that you can successfully stay in orbit. The main impediment to achieving this is the drag caused by resistance from the atmosphere. Drag force is determined by the following equation:
D = 12 ρ v 2 C D S
D = drag. Drag is a force that slows you down. It’s important to remember that drag is a force. Drag force pushes against your spaceship and—if not thoughtfully allowed for in the spaceship’s design—can prevent the spaceship from going any faster, or even tear the ship apart.
ρ = rho, the density—or thickness—of the air around your ship.
As the spaceship moves away from Earth and higher in the atmosphere, air density decreases and so, per the equation, does drag. Note that the density of the atmosphere at any given altitude is variable since air expands when warmed by the sun—warmer air is less dense. And remember that out in the vacuum of space the density is essentially zero, so (by the equation) there is virtually no drag there.
v = velocity, or the speed of your spaceship. Notice that in the equation, drag is a function of velocity times velocity, or v squared. Thus as velocity increases, the drag increases rapidly—double the speed, four times the drag, etc. This is why famed astronaut Chris Hadfield says that “flying a rocket through the atmosphere is the hardest part”: at this stage the velocity of the rocket is continually increasing down where the air is still thick. Once you’re beyond the atmosphere, though, you can increase the speed without increasing the force of drag because there’s no atmospheric density.
CD = the drag coefficient, a characteristic of vehicle streamlining and surface roughness.
S = the cross-sectional area of your spaceship. A lower area (think: skinny versus fat rockets) helps lower drag. The implication is that atmospheric drag is a much bigger problem for spaceships that are still in the atmosphere and trying to leave than it is for a ship like the International Space Station, which is so high above the planet that there’s only a minute amount of air density acting against it. That’s why the ISS can be such an ungainly shape, and why rocketships have to be streamlined.
The drag equation creates a clear goal in rocket design and flight strategy. Not only do the most efficient rockets have lower areas, they also do as much of their accelerating (increase in velocity to orbital speed) as possible once they’ve gotten above the atmosphere into areas of lower air density.
Components of Rocket Construction
Rockets are specifically designed to withstand intense forces of weight and thrust, and to be as aerodynamic as possible. Thus, there are a few structural systems in place that have standardized the construction of most rockets. The nose cone, frame, and fin are part of the skeleton of the rocket’s shape, which is a large surface area often built from aluminum or titanium that is applied with a thermal protection layer. The pumps, fuel, and nozzle form part of the propulsion system, that enables the rocket to produce thrust.
In order to control flight path, there needs to be a level of adjustment over the flight direction of the rocket. Model rocketry, like bottle rockets, or other smaller rockets shoot straight up in the air and come back down where they please. A rocket destined for space requires much more control and flexibility: this is where gimbaled thrust comes in. As part of the guidance system, the gimbal angles allow the exhaust nozzle to swivel as needed, redirecting the center of gravity and repositioning the rocket to the right direction.
Improvements in Rockets
There have been few changes in the fundamental chemistry of rocket fuel since the beginning of spaceflight, but there are designs in the works for more fuel-efficient rockets. In order to improve their efficiency, rockets need to be less fuel-hungry, which means the fuel needs to come out the back as fast as possible to give the desired momentum, and achieve the same thrust. Ionized gas, propelled through a rocket nozzle using a magnetic accelerator, weighs substantially less than traditional rocket fuels. The ionized particles are pushed out the back of the rocket at an incredibly high velocity, which compensates for their small weight, or mass. Ion propulsion works well for long, sustained propulsion, but because
it creates a lower specific impulse, it so far only works on small satellites already in orbit and has not been scaled up for large spaceships. To do this will require a powerful energy source— perhaps nuclear, or something not yet invented.
Spaceships have improved since we began traveling to space in the 1960s, but a lot of our current technology originates from those first designs. Intuitively, it would seem to make sense that a spaceship should be pointy, like a high-speed aircraft. Research done in the 1950s, however, showed that, for orbital speeds, no material could be tough enough to take the tremendous heat on that pointed tip. A brilliant engineer named Max Faget realized that reentry spaceships need to be blunt, to spread the intense heat and pressure over a large area. He was key in designing Mercury, and thus the space capsule was born. Mercury and Gemini were essentially orbiting cockpits with mechanical systems to keep the crew alive: air pressure regulation, oxygen/CO2 processing, temperature control, and food and water storage. They proved that orbital spaceflight was possible for humans and opened the door to explore further, leading us to where we are in space exploration today.