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!

Say what?

[ottffssent]

IANAAE (aerospace engineer), but obviously this is only applicable if either

1. high-thrust mode is hugely less efficient than low-thrust mode, or
2. there is a considerable fuel cost to starting and/or stopping the motor.

If neither applies, then you would simply run the motor at high thrust for shorter periods of time, without the added expense of a low-thrust mode.

The article wasn't what you might call detail-oriented, but this is some sort of electric ion propulsion scheme, which achieves high specific impulse (~3000sec, accd. wikipedia) and so optimising for efficiency makes sense. But it's still an ion drive, so there'll be no takeoffs in its future. At least not takeoffs from anything with a gravitational well deeper than an asteroid.

So we have an article about a thing. Only the article doesn't say what it is or what it's good for. I think I'll keep getting my space news from not-ZDnet, thanks.

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.

nope

[PopeOptimusPrime]

Last time I checked, the reason IC engines use transmissions is because they're only efficient in a dreadfully small window (1000-10000 rpm). Less than 1000rpm, you're not producing enough power to overcome friction. More than 10,000 and the whole thing goes boom. This range is even smaller for diesel engines and even greater for performance engines... but for rockets it's essentially infinite. (Relativity notwithstanding) I mean, there are no physical barriers to stop you from spraying hot gases from the business end of a projectile...

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.