We’ve come a long way but there are still vast propulsion hurdles to overcome if mankind expects to conquer space.
One of the most fundamental hurdles to space exploration is the problem of propulsion, efficiency and thrust. There are many types of drives in operation and in testing but so far the limitations of each have not resulted in a viable solution for human space travel. One drive may be very efficient but does not generate enough thrust for interplanetary journeys in a reasonable time. Other drives have the exact opposite problem in that they can get us there and back again quickly but consume an enormous amount of fuel or are simply downright dangerous. Here are some of the propulsion options we have today; with their strengths and weaknesses.
Chemical propulsion is the most commonly used method of propulsion but one of most limited as well. It’s what we use to launch objects into space like satellites and supplies for the International Space Station (ISS). It is the one means most commonly thought of; the rocket, and it achieves thrust through combustion of a fuel and oxidizer. Chemical rockets can produce massive amounts of thrust, enough to exceed the escape velocity of Earth itself. Each main engine of the space shuttles was able to produce a thrust of 1.8 MN (1.8 million N) by reacting 1340 liters of propellant each second and ejecting gaseous water at a speed of 3560 m/s (12 800 km/h). We can easily move heavy objects with chemical propulsion-it just depends on how much fuel you want to burn.
Limitations of Chemical Propulsion
- Chemical engines are mass-limited. How long you can burn depends on how much fuel you can carry and the fuel isn’t easily replenished in space.
- Fuel is heavy and pricey. It takes a lot of fuel to get where you’re going. More fuel means larger fuel tanks and more structure to hold everything together.
- It’s just slow. While chemical propulsion is great near Earth for moving massive objects, the problem of interplanetary travel requires more speed than it can provide. A spacecraft cannot easily exceed the exhaust velocity of its propellant and chemical combustion just isn’t efficient enough to shorten long range interplanetary missions. A delta V of 2.3 to 3 times the exhaust velocity is doable. That corresponds to a vehicle whose mass is initially 90% to 95% fuel. Beyond that, there’s not much hope for a single stage rocket.
The Ion Drive
NASA’s answer to the efficiency problem is the ion drive. The ion drive accelerates gas (usually xenon) to high speeds in order to release electrons and produce positively charged ions which are released in an ionizing event to produce thrust. The fuel is stable and uses only a fraction of chemical propulsion engines. Over 100 communication satellites currently use this drive to remain in geosynchronous orbit and the Dawn spacecraft uses it in its deep solar system exploration. The ion drive with its glowing blue ion beam looks like something right out of science fiction and although it isn’t suitable for atmospheric use, it has been invaluable for long term space missions.
Limitations of the Ion Drive
- • Ion drives are limited by their power supply. Ion engines are limited by how much energy (electricity) a vessel can carry or how much energy solar panels can collect. In the inner solar system, solar panels can provide enough energy to power the drive, especially if the drive is used intermittently like satellites do to maintain orbit, but the drive requires an alternative power source like nuclear energy when traveling deeper into the solar system where solar energy is less abundant.
- • Thrust is relatively low. Ion drives don’t have enough thrust to launch spacecraft from Earth and thus are used only in space. Because of the low thrust it takes an ion drive a lot longer to increase the speed of large objects; a manned ship to Mars for example. The journey would take much longer for a large spacecraft using a current ion drive than other means of propulsion and the length of time astronauts would be exposed to radiation in deep space is not within safe limits by a long shot. For now, the ion drive is a great engine for unmanned spacecraft on long missions due to its fuel efficiency, but it is not a good option for manned missions.
Even though ion drives are not a preferred option for manned space travel yet, recent advances are showing promise. Each new generation of ion drives produces a higher thrust-to-power ratio than the last. This means of propulsion is definitely one to keep your eye on.
Nuclear Propulsion Options
Understandably, nuclear powered spacecraft have taken a long time to develop. The regulatory hurdles alone to develop such spacecraft are astronomical in and of themselves but the potential for space exploration is enormous. To confound the problem, fuel is in short supply. Plutonium-238 hasn’t been refined in the U.S. sine 1988 and NASA is almost out of its current stockpile although the Department of Energy plans to restart production in 2019. The experimentation and development of nuclear drives very costly and time consuming with its own set of risks due to security and safety. Nevertheless, there are a few candidates that are under serious consideration for development.
The Fusion Drive
In layman’s terms, the fusion drive uses nuclear power to excite a propellant for thrust. It is far more efficient than chemical propulsion, using up a much smaller amount of propellant and produces around the same magnitude of thrust. Fuel for the fusion drive would also be easier to come by in space. A possible source for fusion fuel is Helium-3, which occurs in abundance on the moon. Imagine the moon as a refueling station; it opens up amazing possibilities. Because of its fuel efficiency it can burn longer providing greater periods of acceleration and deceleration and hence, a faster trip. The amount of time it takes for a round trip to Mars from Earth is about 500 days with chemical propulsion, but a fusion powered spacecraft can do it optimistically in an estimated 130 days. That’s a big chunk of solar radiation exposure time subtracted from our astronauts. Despite the difficulties of building a nuclear powered ship, we are already in development, Russia plans to test a fusion drive in 2018.
Limitations of the Fusion Drive
- It’s radioactive. Spacecraft must be constructed in a manner which protects astronauts from the reactor.
- It’s expensive. Nuclear powered vessels aren’t cheap, just look at the price tag on some of our submarines.
- It’s weapons grade. The fusion drive uses the same reaction that powers a hydrogen bomb. Security is a logistical nightmare for the development and operation of any fusion drive powered ship.
The Fission Drive (FFR)
The fission drive, also called the fission-fragment rocket, takes fusion a step further and directly harnesses power of the atom directly generating its own plasma from the nuclear reactor to provide thrust. Fragments of the nuclear reaction are expelled in the exhaust to propel the ship. This drive is extremely efficient (up to 90%) and seems plausible to develop. The mass expelled is very small but the speed at which it is ejected is huge in comparison to other types of propulsion. We’re talking scales of magnitude. To put it into perspective, the fission drive ejects plasma at 5% of light speed, with a theoretical specific impulse of 1.5 million seconds. Compare that to the Space Shuttle Main Engines with a specific impulse of a mere 360-450 seconds. With an exhaust velocity that high, the FFR generates a tremendous amount of thrust with very little mass ejected. It is estimated that the fission drive could generate an amazing 150 newtons of force at a fuel consumption rate of only 1/100th of 1 gram of fuel per second. The most amazing part is that we are within our technological capabilities to build it today; we could soon build the alcohol burning, pro-drag car equivalent of spaceships.
Fission Drive Limitations
- • It’s hot! The reaction used to produce thrust (over 10,000°C) can melt the engine if we don’t regulate it safely. A rotating reactor is needed to stay cool.
- It’s highly radioactive. The radiation levels for the fission drive are much higher than the fusion drive.
- Again, the volatility of the fuel and security risks inherent in the use of such fuel.
We won’t delve too deep into Project Orion as the project was given up in the early 1960’s but the concept is jaw-dropping if not bordering on insanity. The concept is to use a series of controlled external nuclear detonations impacting a protective plate to accelerate a spacecraft. The idea was nixed due to safety concerns and potential fallout from its propulsion.
The Impossible Drive
The most controversial drive today is the EM Drive or radio frequency (RF) resonant cavity thruster but more popularly known as “The Impossible Drive” or “Reactionless Drive” because it defies the law of conservation of momentum. It essentially produces thrust without any exhaust, there is no equal and opposite reaction, it just moves forward. The “thrust” is generated by an electromagnetic field applied within a cone shaped metal cavity. There are many skeptics to the claim that it actually produces thrust at all. Measurements so far have been very small, but in fact, measurable as illustrated in a test by NASA scientists. The Chinese even say they have it figured out and testing in space will soon follow. If the drive does work, it will have huge implications in physics and space travel, but for now, all we know is that it might work and if it does, it’s very power hungry. Let’s hope this one doesn’t go the way of the Dean Drive.
Limitations of the EM Drive
- Does it even work?
- It’s a power hungry beast
- If it does work, it doesn’t produce enough thrust so far to be useful
Spacecraft drives continue to become more efficient over time, but here’s to hoping it’s not too little too late. We can always hope we figure out the way around it and learn to fold space. But, warp drives are a different topic altogether.
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