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Fuel Efficient Five-Gear Rocket Engine Designed
Roland Piquepaille writes "Georgia Tech researchers have had a brilliant idea. Rocket engines used today to launch satellites run at maximum exhaust velocity until they reach orbit. For a car, this would be analog to stay all the time in first gear. So they have designed a new space rocket which works as it has a five-gear transmission system. This rocket engine uses 40 percent less fuel than current ones by running on solar power while in space and by fine-tuning exhaust velocity. But as it was designed with funds from the U.S. Air Force, military applications will be ready before civilian ones. Here is how this new rocket engine works."
For those who haven't read TFA: This is different from VASIMR. They use solar power and ion drives.
For all the rest: Beware beware! This is a Roland post! Last link goes to his page!
This is an ion drive. It works by using an electric field to accelerate small amounts of propellant to very high speeds. It is not useful for launching a rocket from Earth at all. But it is a very efficient way to travel in space. Acceleration is very slow, but adds up over time when there is no friction. The article is about tweaks to improve the efficiency of this type of engine.
There is very little sunlight where Voyager probes were supposed to go:
Sounds like a no-brainer to me : use conventional rocket engines to escape earth, and then switch to solar/ion drives.
Indeed there is little novelty in this.
Modern satellites are inserted into orbit using kerosene or hydrogen or hydrazine, with some oxidizer (liquid oxygen for example.) These fuels contain both the energy and the reaction mass, and require nothing but a nozzle and a few pumps to work, and they are very powerful.
Once in the orbit, many satellites can be moved a little using a very limited amount of fuel that is stored in the satellite, and once used up the fuel can't be replenished. Sometimes it may be just compressed gas. It would be very useful to replace these with ion engines; the only trouble with them is that it will take a year to move a satellite from one orbit to another, since the thrust of an ion engine is measured in grams, and typically satellites weigh a ton. Try pushing your car with a feather, gently :-)
Ion engines are efficient because of throwing your precious reaction mass away at low speed it accelerates individual molecules to a very high speed, and saves the reaction mass for future use. The energy for this is provided by an external source; typically, only nuclear sources are sufficiently powerful to provide a reasonable thrust, but of course a solar panel will do as well, if you have plenty of time.
It seems to have a similar principle to VASIMR engines. Basically, they can adjust the specific impulse in flight, higher specific impulse giving lower thrust but better efficiency.
Of course, the thrust that an engine like the one described would output is miniscule. It would not be useful for launch vehicles, but only for keeping satellites in orbit. That said, if you can reduce the amount of fuel a satellite uses by 40%, you can keep it up there for almost twice as long, which is never a bad thing.
Late reply, but maybe it will help somebody. The article is crap, and it's not surprising that all the other replies have misunderstood what it's about. The engine is not used for launch, it is only used for maneuvering the satellite once in orbit. It's not that it takes 40% less fuel to launch the satellite, rather that 40% less fuel needs to be carried for subsequent orbital maneuvering/adjustment due to the efficiency of this engine.
The Georgia Tech press release is slightly less misleading than the various summaries derived from it.
Oh no... it's the future.
TFA is pretty badly written. What they mean is that the satellite can now carry 40% less fuel because its motor is more efficient. Since you are carrying less fuel, you now have more room for other stuff, or you might be able to use a smaller launch vehicle to carry the smaller satellite. Obviously this would reduce launch costs.
W..w..W - Willy Waterloo washes Warren Wiggins who is washing Waldo Woo.
Not only is the cars analogy poor, it's also wrong. At least as per the summary: It is the maximum exhaust velocity that is fifth gear. Specific Impulse is directly related to the exhaust velocity.
But, KE = mv^2 (or P=rho*v^2 in this case) The power available (photovoltaic, chemical, nuclear-electric, etc.) is relatively constant, so the density of the exhaust gasses (and therefore the thrust since it goes like momentum T={mass flow rate}*v) goes way down at higher exhaust velocities.
High-thrust, low impulse would be "first gear" and only needed for launch. It is the maximum flow rate, and obviously uses up propellant like nobody's business. Orbital transfers would use the "fifth gear" mode as once in orbit, there is no atmosphere to rise out of before turning tangentially to the ground, the energy source (solar power) is relatively unlimited over time, and continuous thrust is even more efficient than even the ever useful hohmann transfer.
There are maximally energy efficient thrust modes between max flow and max velocity depending on orbital velocity, but with a continuous source of energy, the life limiting factor is actually propellant, and since it's not easy to resupply to a spacecraft, it's usually better to be miserly with the thing you can't replace and run all the time in high-Isp (specific impulse) mode.
AND the engine's not even new. It's not even a completed version of a recent project like VASIMR. It's a Hall thruster, which impressive though they are, are mature enough to have actually flown already. Not to mention that it will probably never be capable of the kind of thrust needed for launch.
Can you be Even More Awesome?!
The concept is already in use with the parallel staging of the Space Shuttle. The Space Shuttle Solid Rocket Boosters are effectively a high thrust, low velocity exhaust first gear. The first gear is most effective when losses due to drag in the atmosphere are high. At sufficient height and speed, high thrust low speed exhaust is traded for lower thrust high speed exhaust when the SRB's are jettisoned and the main engines take over. This higher gear is analogous to the one described in the link.
an ill wind that blows no good
DS1 could turn its thrust up and down, but the specific impulse rose with increasing power. From the sketchy descriptions in TFAs, this unit can increase the specific impulse while running at constant power or perhaps even lower power. This results in much lower thrust, but stationkeeping operations require little.
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I really hate the front-page article, because it makes no distinction between the payload boost from a lighter stationkeeping system and the payload increase which would result from a more efficient booster rocket (burning chemical fuels). It also confuses the ultimate energy source for the ion system (the Sun) with the direct power source (electricity); the unit could easily run nuclear-electric, it doesn't care. This sort of nonsense makes it difficult or impossible for the naïve reader to understand the matter at hand. What's the point of putting an article on Slashdot if you're only going to make a logical mess of it? If the stuff here is going to leave people knowing less than before they read it, people shouldn't read it.
If the editors here are idiots, the readership here will slip to that level by both selection and mis- and dis-information.
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Sustainability and energy independence essay
The problem you were pondering is not solved by a transmission. It's solved this way...
Aerospike Engines (how they work)
Aerospike Engine (history)
Linear Aerospike Engine (see the "efficient at all altitudes" section)
Rocket engines are more efficient (see: specific impulse)when the exhaust velocity of the escaping gas is higher. The shape of the bell of the "traditional" rocket nozle is static and thus operates at maximium efficiency at a particular altitude. The linear aerospike engine makes one side of it's bell continuously variable -- by using the air as one side of the nozle and taking advantage of the changing atmospheric pressure as the rocket ascends. The rocket engine will then have a continuously variable, uh, "transmission", to borrow the terminology of this discussion which beats a five-speed hands down. : )
The article summary, the RP/ZDNet press release rehash, and indeed the original press release itself are all very poorly written.
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