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Rocket technology predates space exploration by almost a thousand years, and although today’s computerized multi-ton launch vehicles are a lot more capable than 11th-century gunpowder-assisted arrows, some facets never change. A rocket is essentially a controlled explosion—when it works, all the energy comes out of one end and propels the vehicle forward. When it doesn’t, well, it is still a tube of explosive material. Risky? Sure, but rockets are ideal for transportation in conditions where other forms of locomotion would never function, like the sterile emptiness of outer space. Let’s break down how it works.
Gravity Doesn’t Compromise
Earth’s gravitational pull keeps us all safely rooted on our home planet, but what happens if you want to leave the planet? You need tremendous energy to accelerate even a small mass to escape velocity, which is probably faster than you think. To slip the surly bonds of Earth, an object needs to reach a speed of 6.9 miles per second (11.2 km/s), or 24,840 miles per hour.
The chemical reaction taking place in most rockets has the same constituent elements as a fire—there’s fuel, an oxidizer, and an ignition source. The problem with traveling high into the atmosphere (and into space) is that you won’t be able to use atmospheric oxygen as the oxidizer like, for example, an internal combustion engine does. That’s why rockets have to carry tanks of both fuel and oxidizer. The SpaceX Falcon 9’s Merlin engines use rocket-grade kerosene (RP-1) fuel with liquid oxygen as the oxidizer. NASA’s Space Launch System is based on liquid hydrogen fuel and liquid oxygen oxidizer. Some rockets use hypergolic propellants, which spontaneously burn when combined, but most require ignition.
A SpaceX Merlin engine being tested.
Credit: SpaceX
When a chemical rocket engine fires, it’s the purest expression of Newton’s Third Law of Motion: For every action, there is an equal and opposite reaction. So, the rapidly expanding fuel jetting from the nozzle pushes on the rocket, and the rocket pushes back. The result is enough thrust to overcome the force of gravity.
Naturally, this puts a great deal of stress on a vehicle. As it accelerates through the atmosphere, it will reach the most dangerous part of any launch, known as maximum dynamic pressure, or max q. At this point, acceleration and atmospheric pressure cause the highest mechanical stress. Engineers need to plan for this when designing rockets if they’re going to make it to space.
Up, Up and Away
We’ve all seen footage of rockets that fail to make it to space, often spinning out of control until they fly apart. More often than not, the first sign of impending doom is an escalating precession, the slight wobble that occurs in spinning objects. Rockets spin around their long axis to increase stability, but uncontrolled precession can cause the rotational axis to tilt until the vehicle is no longer going straight up. When a rocket is still in the atmosphere and accelerating, a slight tilt can cause tumbling, and tumbling causes high mechanical stress that will eventually tear the vehicle apart. Today, many rockets have remote abort systems that can destroy an out-of-control vehicle, something we saw in action during SpaceX’s first Starship orbital attempt (see below).
To keep a rocket pointed toward space, engineers use design elements like fins and gimbaled (movable) engines. There are two general types of engines, both of which have their uses to keep a rocket stable and moving in the right direction. Most of the interest today is in liquid-fueled rocket motors like the SpaceX Merlin and Blue Origin BE-4, but there’s still a place for solid rocket boosters, too.
Liquid-fueled systems keep the fuel and oxidizer in tanks, often cooled to super-low temperatures and stored at high pressure. With a powerful turbopump, the combustion chamber can be supplied with more or less fuel to change the thrust output. The fuel and oxidizer mix must be controlled perfectly. If too much fuel or oxidizer builds up in the chamber, a rocket can blow itself to bits.
Solid rocket boosters are sometimes used to give a rocket a little extra oomph at sea level. NASA’s SLS uses two SRBs, the most powerful ever constructed. SRBs have no fuel tanks—as the name implies, the fuel and oxidizer are combined in a solid combustible block. Unlike liquid-fueled designs, SRBs burn through all their fuel after igniting, with no way to alter the thrust on demand. However, it is possible to “program” a solid booster by shaping the fuel to provide more or less surface area for the reaction to occur as it burns.
Rocket launches are just a means to get a much smaller payload into space. Therefore, there’s no reason to keep all the rocket components attached to the payload after they’ve outlived their usefulness. Solid rocket boosters are usually the first parts to be jettisoned, but most rockets also have at least one main stage to drop. In the case of SpaceX, the first stage is flown down to land propulsively for reuse, but all other rockets active today just drop the expended stages in the ocean. A single-stage-to-orbit (SSTO) vehicle is the dream, but we don’t yet have the technology to make that a reality.
The Future of Propulsion
Scientists have proposed myriad alternatives to chemical rockets, but few are being seriously researched, and even fewer have been used in real life. While rocket launches will remain the realm of energetic chemical-based rockets, some spacecraft have swapped to more efficient, low-thrust forms of propulsion like the ion engine. For instance, NASA’s recent Dawn mission to study the dwarf planet Ceres relied on ion engines, which use an electric charge to accelerate ions out of the engine. This produces a small but continuous thrust that can get a probe moving quite fast, given enough time in the frictionless environment of space.
Credit: NASA / DARPA
For moving large payloads to distant locales, NASA and other space agencies are investigating various types of nuclear-powered rockets. The DRACO vehicle being developed in partnership with DARPA will use nuclear thermal propulsion (NTP), which could provide high thrust at several times the efficiency of chemical engines. We don’t know if NTP will be viable yet, but the idea is that the nuclear reactor will rapidly heat a propellant like liquid hydrogen, causing it to expand and push the vessel forward. A similar but more far-off idea is the fission fragment rocket. This type of vehicle would eject nuclear fission products from a reactor engine using magnetic fields. This could theoretically propel a craft to 3-5% of the speed of light.
While rudimentary rockets have existed for a millennium, the inexorable march of technological progress may deliver a new, as yet unexplored type of propulsion. There was some excitement several years ago when the wacky EM Drive appeared to conjure forward motion from thin air, but more analysis showed it was not working. Perhaps the next thing that sounds as far-fetched as the EM Drive will be the real deal. A rocket that can land on a floating platform sounded crazy not long ago, after all.