Electric Rocket

[Theory5]

Hey, I was reading a book called The Last Theorum (its pretty good, it is about a guy who solves the problem about A2+B2=C2.(its fiction)) and in the book there was talk about something called the Electric Rocket. Apperently it allows lots of accelleration so you can power it for 1/2 the ride then reverse and fire it to decellerate. Is this a real concept? Can anyone explain how it works? What kind of power sources would it need?

[CraigD]

Originally Posted by Theory5

... the Electric Rocket. Apperently it allows lots of accelleration so you can power it for 1/2 the ride then reverse and fire it to decellerate. Is this a real concept? Can anyone explain how it works? What kind of power sources would it need?

My guess (I've not read the book) is that what's being referenced is an ion thruster. The preceding wikipedia link and many other books and websites have good explanations of their working.

When I was in elementary and junior high school (1960-1975, grades 1-9 in my home town), working “ion rockets” were a popular science fair project (largely, I think, because how to build such a project was clearly described and illustrated in 1970s editions of the World Book Encyclopedia, a very popular reference book then). Suspended by string and connected to a 6 V battery, its possible to make the arrangement visibly swing after just a few minutes by switching it on and off with the right timing, either via an automatic or manual switch. Such rockets had an advantage over the kind actually used in space, in that they could use ionized air as their reaction mass, while spacecraft must carry a supply of some material to be ionized, such as xenon gas.

Another possibility is a rocket motor based on a “Gauss gun” or “rail gun”. Such ships were described in various SF of the 1950s and later, and typically used liquid mercury are reaction mass, and a nuclear generator for power.

All of the above are real. To the best of my knowledge, only ion thrusters have actually been used in spacecraft, the others only for projectile weapons.

[Theory5]

Originally Posted by CraigD

My guess (I've not read the book) is that what's being referenced is an ion thruster.


All of the above are real. To the best of my knowledge, only ion thrusters have actually been used in spacecraft, the others only for projectile weapons.

They actually made and used Ion thrusters? WHen was this? Do you have a link? :-)
And no I doubt they were taking about ion thrusters. Ion thrusters take a REALLY long time. This Electric rocket concept was explained as being quite fast, and if it was built it would allow for ships to reach places such as saturn or even the oort cloud in a reasonable amount of time. It would also be feasible to use this engine for going to mars. From what I understand Ion Thrusters would acellerate to slowly to get to mars.

[CraigD]

Originally Posted by Theory5

They actually made and used Ion thrusters? WHen was this? Do you have a link? :-)

Yes. Ion thrusters have been made and ground-tested since at least 1916. I’m not sure when the first ion thruster was flown in a satellite as a station keeping thruster (to make small correction to a satellite’s orbit), but it’s now a common feature in commercial satellites. The wikipedia link in my previous post has some good links on the history of ion thrusters.

According to This NASA FAQ page, the first spacecraft flown with ion thrusters as its primary motor was Deep Space 1, a comet flyby probe launched 10/24/1998. Since then, several additional spacecraft have been designed to use ion thrusters for main propulsion, but none have to my knowledge been flown. 7/12/2001, a EAS Artemis communication satellite made news when it used its station keeping ion thrusters to complete, over an 18 month period, an transfer orbit maneuver, after failure of its booster and having insufficient fuel for its chemical rocket motors.

Originally Posted by Theory5

And no I doubt they were taking about ion thrusters. Ion thrusters take a REALLY long time. This Electric rocket concept was explained as being quite fast, and if it was built it would allow for ships to reach places such as saturn or even the oort cloud in a reasonable amount of time.

Although ion thrusters have very low thrusts compared to chemical rocket motors, they have much higher specific impulse (around 29000 s, vs around 4400 for the best hydrogen liquid fuel motors), so in principle can produce much higher changes in speed (“delta v”) than chemical rockets by thrusting longer. In principle, an ion engine might be operated continuously, so even if its thrust was very small, it might reach Mars and beyond many times more quickly than previous spacecraft have.

Originally Posted by Theory5

It would also be feasible to use this engine for going to mars. From what I understand Ion Thrusters would acellerate to slowly to get to mars.

According to the numerous manned mars mission sites, including the preceeding NASA site, ion thrusters are among the serious candidates for the main engines for a manned Mars mission. However, what you saw may have been a mention of the VASIMIR engine. This engine has been under development since 1979, but has still not advanced to the stage that it can be flown. A VASIMIR was successfully ground tested in 2007, and there is discussion of a space testing at the ISS around 2011-2012. It’s a very exciting and promising technology, because it promise to have a specific impulse around 290000 s (around 10 times that of an ion thruster), while at the same time being able to produce high thrusts at lower – the “v for variable” in VASIMIR.

Any “electric engine” – ion thrusters, VARIMIRs, or exotic possibilities like rail guns – however, must contend with the problem of having enough electricity. Because one of the most proven and practical source of electricity for spacecraft are solar panels, this is especially a problem for spacecraft intended to be operated far from the Sun.

[Theory5]

Originally Posted by CraigD

Any “electric engine” – ion thrusters, VARIMIRs, or exotic possibilities like rail guns – however, must contend with the problem of having enough electricity. Because one of the most proven and practical source of electricity for spacecraft are solar panels, this is especially a problem for spacecraft intended to be operated far from the Sun.

I guess the Electric engine mentioned could have been an Ion engine, I wish they had elaborated more. It was said, however that this engine could be powered with some sort of power supply instead of relying on the sun's rays.

[CraigD]

Originally Posted by Theory5

It was said, however that this engine could be powered with some sort of power supply instead of relying on the sun's rays.

Radiothermal generators and other kinds of nuclear fission reactors can in principle be used to generate electricity for rocket motors.

The drawback with this approach is that, although they have fairly high power/fuel used mass ratios, their power/overall mass ratios are usually low, from 3 to 15 W/kg. Calculating the acceleration for some typical RTGs paired with typical ion thrusters gives maximum acceleration, not including any other components, of around 0.0005 m/s/s, Even such a low acceleration, however, has useful spaceflight potential. For example, a Earth/Mars transfer orbit, which requires delta Vs of about 2945 and 2649 m/s, could be accomplished by such a powerplant-motor system operating for a total of about 68 and 61 days at each end of its trip.

A political barriers to the use of nukes in space is due to the danger of a high-speed collision with the Earth of a spacecraft containing the 1 to 8 kg of Pu238 in a typical RTG or other power plant. IMHO, this objection is unwarranted.

[Theory5]

To me nuclear power supply's are not worth it, especially in an enclosed metal tube flying through space. There are risks with just going into space, but adding a nuclear power plant as your power-source, that is kind of scary. Especially if there are no materials to use to repair it should something start leaking radiation.

RTGs are usually the most desirable power source for unmanned or unmaintained situations needing a few hundred watts or less of power for durations too long for fuel cells, batteries and generators to provide economically, and in places where solar cells are not viable.

Those engines sound good except for only powering something that uses a couple hundred watts or less of power. And every so often (years) they lose a bit of their power. What i think would work better would be a power source that generates more power, and is not a passive system. Then there is the need of a backup system, which can probably be a battery of some sort.

A political barriers to the use of nukes in space is due to the danger of a high-speed collision with the Earth of a spacecraft containing the 1 to 8 kg of Pu238

Why would there be controversy over a small mass of pu238?