Section 3.6 - Alternative Rocket Propulsion

Although today's rocket thrusters rely almost exclusively on chemical rockets, since the dawn of the Space Age engineers have been aware of and considered alternative rocket propulsion systems that may be able to dramatically improve performance. There are a few that are worth exploring here

Aerospike Engine

We learned about bell shaped converging-diverging nozzles in Section 3.2, and that they lose efficiency at different altitudes as atmospheric pressure changes in Section 3.3. There's an alternative design called the aerospike engine that would eliminate this inefficiency. 

Think of an aerospike engine as an inverted bell nozzle. At low pressures, it's the ambient atmospheric pressure that pushes the exhaust against the spike and forces it straight down. As the rocket rises and the pressure decreases, the shape of the aerospike allows it to "adapt" to the changing air pressure, ensuring the exhaust is always directed straight down. See the below video for how the nozzle "adapts." 

So if it's so great, why don't we use it? There are real prototypes that have been test fired (see video here), but none that are operational. The main problem is that it's extremely difficult to cool - the gas remains at high temperature all along the surface area of the spike, especially the thin tip, which makes it prone to melting and other structural failures. If this could be solved, we may have a real breakthrough in rocket technology

Ion Propulsion

Ion thrusters are a relatively new concept that uses an electric charge to ionize a gas (commonly Xenon) and a large voltage (difference in electric potential) to zoom these ions out the end of the spacecraft
For more detail, watch this video, particularly from 2:30 - 4:00

While conventional rockets produce a massive thrust to quickly accelerate a spacecraft to high speeds in a short period of time (and admittedly, relatively inefficiently), ion thrusters are the opposite. They accelerate the spacecraft incredibly slowly (it would take 4 days to accelerate a spacecraft from 0 to 60 mph), but when fired over a long time, they can accelerate a spacecraft to enormous speeds very efficiently. Ion thrusters can achieve specific impulses in the thousands of seconds, while the best chemical rockets can't get past 400 seconds.

As such, they aren't meant for getting rockets off the ground, they're meant for constantly accelerating a spacecraft in deep space towards its target after having already been launched into space by a conventional chemical rocket. Ion drives have seen operational use on a number of missions like Japan's Hayabusa, NASA's Dawn, and the ESA's BepiColombo

Nuclear Thermal Propulsion

Wouldn't it be great to combine the high thrust of chemical rockets with the high efficiency of ion thrusters? Nuclear thermal propulsion holds the promise of the ultimate solution - it uses a nuclear reactor in the rocket to heat liquid hydrogen to super high temperatures, then expel the hot gas out the nozzle. Nuclear rockets should be able to launch from Earth at twice the efficiency of the best chemical rockets
For more detail, watch this video, particularly from 4:15 - 6:45 and 11:30 - 15:00. The video may be retro, but it's great!

So if nuclear rockets are so great, why don’t we use them? Actually, we tried – in the 1960s at the height of the Apollo program, NASA successfully tested several experimental nuclear prototypes through their Rover and NERVA projects. Engineers hoped to use this technology to take humans to Mars by 1980. But after the Moon landings when NASA’s budget got slashed, the entire project was mothballed, and the technology has been lost and ignored for the past 5 decades.
This engine from the 1960's could've been 2x better than anything we have today

Solar Sails

This propulsion system would allow a spacecraft already launched into space by a conventional rocket to use the radiation pressure exerted by sunlight on large mirrors on the spacecraft. Just as a sailboat uses the wind, light exerts a force on the solar sail that can gradually accelerate a spacecraft. But how, you may ask, does light exert a force when it has no mass? If you recall from the quantum mechanics section, light has properties of both a wave and a particle, so although we don't typically think of light as having mass, it still possesses momentum that's proportional to its frequency (read more here

Regardless of how counter-intuitive it seems, this force has been demonstrated to exist. The force exerted on an 800m x 800m solar sail is about 5 Newtons at Earth's distance from the Sun. For added boost, a super-powerful Earth-based laser could hypothetically add additional thrust. The advantage of solar sail propulsion is the spacecraft wouldn't need any propellant at all! Japan's IKAROS was the first successful solar sail spacecraft

Illustration of Japan's IKAROS

Space Elevator

Even if a spacecraft is traveling to a distance location in the solar system, most of the rocket's energy is expended just achieving Earth orbit. A space elevator is a hypothetical structure that would leverage's Earth's rotational force to send objects to space without the use of rockets at all!

The combination of the gravity on the Earth-tethered end and the centripetal force at the top would keep the cable up, allowing astronauts and cargo to ride up to space. 

Unfortunately, we don't yet have any material with sufficient tensile strength to build a space elevator - the excessive centripetal force would case the cable to snap. Even if we did, this would unquestionably be the most expensive structure ever built, and it would be subject to terrorist attacks, meteor strikes, and weather, and general wear and tear.