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."

Comments

Xyz

[Anonymous Coward]

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!

Re:Solar?

[tftp]

Well, didn't the voyager spacecrafts have solar power too?

There is very little sunlight where Voyager probes were supposed to go:

Voyager probes are powered by three radioisotope thermoelectric generators, which have far outlasted their originally intended lifespan, and are now expected to continue to generate enough power to keep communicating with Earth until at least around the year 2020.

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.

Re:Say what?

[tftp]

IANAAE either, but I read about them in a book; anyhow, none of your (1) and (2) applies. First of all, the proposed "new" engine is not one engine but two very well known and used engines. One is a chemical rocket that oxidizes (burns) the fuel, and the released chemical energy accelerates the molecules of gas and propels the vehicle. Another is an ion drive where an external energy source is used to accelerate molecules of gas. Since this external energy is solar batteries, you can accelerate a molecule to a very high speed, and so get a lot of bang from a given quantity of the reaction mass.

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.

Clarification of article

[Tx]

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.

Re:Uhm

[zippthorne]

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.