{"id":4765,"date":"2012-12-04T06:00:47","date_gmt":"2012-12-04T11:00:47","guid":{"rendered":"http:\/\/www.allandouglas.com\/blog\/?p=4765"},"modified":"2025-04-21T01:56:41","modified_gmt":"2025-04-21T05:56:41","slug":"scifi-space-propulsion-engines","status":"publish","type":"post","link":"https:\/\/pineymountainfoster.org\/thoughts\/scifi-space-propulsion-engines\/","title":{"rendered":"Science Fiction Fact & Fancy: Space Propulsion-Engines"},"content":{"rendered":"
\"rocket<\/a>
Via Wikipedia commons<\/figcaption><\/figure>\n

In my last post on this topic (Space Travel<\/a>) I mentioned that many Sci-Fi authors borrow heavily from naval vessels and aircraft in depicting the behavior of space craft in their writing.\u00a0 And while this sometimes irritates me too, I encouraged authors to write what they dream; don\u2019t pay too much attention to us hard-SciFi\u2019ers, because something is impossible only until someone does it.\u00a0 And humankind has done the impossible many times already.<\/p>\n

However, for those who would like to put a more factual edge into their Sci-Fi writings, I will continue to look at some of the specific areas that draw fire by being more science fantasy<\/em> than science<\/em> fiction and ways to lessen this by employing currently viable technologies as a starting point at least.\u00a0 This week I want to look at propulsion systems: engines.\u00a0 Because this is a blog post, not a treatise, I\u2019ll need to keep it brief and select the most suitable topics \u2013 or serve espresso and cookies with each reading to keep you from wandering off.<\/p>\n

Solid Fuel Propulsion<\/h1>\n

Invented about a thousand years ago in China, solid-fuel rocket engines were the first engines created by mankind and have been used widely ever since. The line about “the rocket’s red glare” in the USA national anthem (written in the early 1800’s) is talking about small military solid-fuel rockets used to deliver incendiary devices. As a kid I (a few years after the national anthem was written) built model rockets using solid fuel \u201cengines\u201d and sent them soaring into the sky.\u00a0 I could even make multi stage rockets with cartridges that would blow out the top as a final act and ignite the next engine in the stack. C130 Hercules aircraft use solid fuel rocket boosters to get heavy loads off the ground on a short runway.\u00a0 And, of course, the Space Shuttle program used solid fueled SRBs<\/a> to help lift the shuttle into space.<\/p>\n

Solid fuel rockets have the advantage of being cheap to make (relatively speaking), simple (no moving parts) and safe (as long as the seals hold). Their disadvantages include the fact that once they\u2019re lit they cannot be shut down, if they go out on their own they cannot be re-lit, and their thrust cannot be controlled in flight (throttled up or down).\u00a0 The fuel charge can be \u201cshaped\u201d, as in the Shuttle\u2019s SRBs, to provide more thrust early in the burn then taper off, but a pilot cannot \u201chit the gas\u201d to increase thrust.<\/p>\n

Still, solid fuel boosters could be used to give a large, heavy ship a boost out of an orbital position toward its destination.\u00a0 The boosters would then be discarded (lightening the ship) and recovered by the orbital facility to be reloaded and used again \u2013 or not; if you\u2019re happy with scattering space junk all over the place.<\/p>\n

Solid fuel rockets could also be used in weapons systems; missiles and torpedoes.<\/p>\n

Liquid Fuel Propulsion<\/h1>\n

Liquid fueled rockets are the mainstay of our modern space program. Robert Goddard tested the first liquid-propellant rocket engine in 1926. He worked on and solved a number of fundamental problems in rocket engine design, including pumping mechanisms, cooling strategies and steering arrangements. These problems are what make liquid-propellant rockets so complicated.<\/p>\n

Aside from being complicated, the biggest problem with liquid fueled rocket engines is the amount of fuel they consume \u2013 and the associated weight.\u00a0 Goddard used gasoline and liquid oxygen (LOX) to fuel his experiments, the Germans used alcohol and LOX in their V2 rockets, kerosene and LOX were used in the Apollo Saturn 5 rockets, liquid hydrogen and LOX by the space shuttle and nitrogen tetroxide\/monomethyl hydrazine by the Cassini engine.\u00a0 Each is an improvement over its predecessor, but all still suffer the problem of having to lift vast quantities of itself to blast into space.\u00a0 NASA states<\/a> that using a hydrogen\/oxygen fuel to lift a rocket from the surface of the Earth to stable Earth orbit requires a rocket that is 83% fuel and 17% payload\/vehicle (by mass).<\/p>\n

Once out of Earth\u2019s gravity well, far less fuel is consumed.\u00a0 It takes as much fuel to get off Earth as it does to fly to and land on the surface of Mars.\u00a0 And of course, once a space ship is away from a planet the engines can be turned off and the ship will \u201ccoast\u201d until it needs to flip over and decelerate.\u00a0 Traveling around our solar system with chemical rockets is possible \u2013 especially if we set up mining bases on asteroids as refueling stations for them \u2013 but traveling outside our system, or getting anywhere very fast, will require something more sophisticated.<\/p>\n

Nuclear Propulsion<\/h1>\n

In the 1970s tests were done using a small nuclear reactor to heat hydrogen as a propellant.\u00a0 The results were promising.\u00a0 Thrust is measured as Payment Energy in kilometers per second (km\/s).\u00a0 A solid fuel rocket delivers a PE of 3.0 km\/s, liquid fuels around 4.5 km\/s, but the nuclear rocket delivered a PE of 8.3 km\/s! Nearly double the thrust of the best liquid fuel combustion engines.<\/p>\n

The biggest problem with this engine is containing the radiation, which required heavy shielding that increases a ship\u2019s mass so much the thrust gain is lost again.\u00a0 If light weight but effective shielding could be developed, this would become an excellent engine for long range space craft.\u00a0 Not only is it powerful, but hydrogen is the most abundant element in the universe.\u00a0 It exists as a free element in outer space, in nebulae and is a component of many asteroids and planetary atmospheres.<\/p>\n

Ion Engine<\/h1>\n

\"ion<\/a>The Ion engine works in a similar fashion to the ion fans you see around that \u201cblow\u201d air via electrical fields instead of whirling blades.\u00a0 An ion engine produces thrust by tearing apart a large, stable molecule such as xenon, then using magnetic fields to separate positive from negative and accelerate them using electrically charged (magnetic) grids to produce thrust. Once the particles have passed through the acceleration grid, the positive and negative elements find one another and recombine to produce the inert substance again.\u00a0 Check out this NASA\/JPL tutorial<\/a> if you want to dig into the guts of this engine.<\/p>\n

Ion engines do not produce great amounts of thrust; they could not be used to escape from a planet\u2019s surface, but once in space their extremely low fuel usage would mean that the engines could run continuously for years on the fuel they carry.\u00a0 In space, this ongoing thrust produces cumulative speed: the longer the spaceship burns the engines, the faster it goes, which could be handy for long trips.\u00a0 It would have to flip over at the half way point to begin decelerating or it would splat into its destination like a bug on the windshield of a supersonic aircraft, but it could achieve some very impressive top speeds.<\/p>\n

Ion thrusters come with a fundamental price: the power imparted to the exhaust increases with the square<\/em> of its velocity while the thrust increases only linearly<\/em>. [1<\/a>]<\/sup> This is a fancy-shamcy way of saying that stepping on the gas gets really, really expensive in terms of energy consumption.\u00a0 Ion engines will have a set sweet spot for peak performance; on or off, no throttle.<\/p>\n

Bussard Ramjet<\/h1>\n

\"BussardThe physicist Dr. Robert W. Bussard theorized an engine in the 1960s that used a huge scoop to collect hydrogen from space and funnel it into a magnetic constriction chamber that would compress the hydrogen until it ignited in a fusion reaction.\u00a0 This reaction would then exhaust out a nozzle at the rear of the ship to provide thrust.\u00a0 In practice, it was found that although hydrogen is most abundant, it is not as readily available in open space as he had predicted and this model turned out to be unworkable.\u00a0 However, the Bussard collector idea could be used to collect hydrogen so it could be compressed to a liquid state and stored as fuel for a nuclear rocket.<\/p>\n

Antimatter Drive<\/h1>\n

\"particleThe darling propulsion system of sci-fi writers everywhere. There are several thoughts as to how an antimatter reaction could be used to propel a space ship.\u00a0 In efficient antimatter reactions (positron annihilation), matter and antimatter are combined in balanced amounts and the result is nothing but gamma rays; lots and lots of gamma rays.<\/p>\n

Gamma rays are like X-rays on steroids. They penetrate matter and break apart molecules in cells, so they are not healthy to be around. High-energy gamma rays can also make the engines radioactive by fragmenting atoms of the engine material. However, it is difficult to get gamma rays to directly propel a space ship; they have a lot of velocity but no mass, so they offer no thrust.<\/p>\n

Gamma rays can be used to heat a fluid propellant that would then provide exhaust momentum and thrust, but then you\u2019re back to carrying fuel (propellant). The antimatter reaction could be used to heat fluid to steam and used to power generators to run an electrical drive like the ion engine.[2]<\/sup>\u00a0 Or, a small antimatter reaction could be used in an ablative reaction engine<\/a>, like what NASA is working on as propulsion for future Mars missions, which would actually capitalize on the normally destructive aspect of gamma rays.<\/p>\n

Antimatter does have a few small drawbacks aside from the gamma rays.<\/p>\n