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Old June 30 2011, 01:58 AM   #34
John O.
Rear Admiral
Location: the bush
Re: The DARPA 100 Year Starship

sojourner wrote: View Post
^Which is the whole point of the project. It's not "how to build a ship", it's "how to build a sustainable program on a long term basis".
Exactly, I think it's a foregone conclusion that one of the goals of this symposium and future movements are to figure out how to make it self sustaining (think Men in Black funding itself through space technology patents, lol)

STR wrote: View Post
Thanks, just a couple more Q's. Really interesting stuff, though the engineering is beyond this MBA student.

Would you even be able to use the IEC as a fusion reactor while it's being used as a motor?

Also, regardless of power source, how many kW of power would you need to sustain the thing? I know enough that RF is a decently efficient way to heat things up, but I have no clue what it takes to get plasma temperatures.
1st Q - there's plenty of varying opinions on that. Some would argue quite ardently that in 30 years of trying, the IEC has not shown itself to be a promising potential environment for any significant amount of fusion to take place. Others disagree. I think the central point is that even if it never does, and the IEC functions as nothing more than an ion accelerator - in a sense, you could think of it as an electrostatic nozzle (versus the VASIMIR's magnetic nozzle), it still represents the potential for a very useful thruster compared to modern electric thrusters that are out there (Hall, Ion).

2nd Q - it's not so much an issue of needing a lot of power to sustain the engine - what you're talking about when ur talking about sustaining the engine is in fact just making sure the helicon ignites a plasma and the IEC fires too, which has more to do with striking the right pressure / gas flow rate balance. There are minimum values the voltage on the IEC grid to get ignition, and RF power to get a plasma, but they're low.

In other words, the engine doesn't have exceptionally high power requirements for missions like satellites - but if you wanted a big ole engine to get u to Mars, you are probably talking about needing anywhere from 2 kilowatts to... whatever you could generate. I mean as far as we can tell, it continues to scale upward without loss of efficiency in terms of power. I mean the point of increasing RF power is to get a higher plasma density (higher ionization fraction), but by the helicon's very nature it actually produces a very high ionization fraction (like .9) - hence recent (last 2 years) widespread interest in it as an ion source for propulsion systems, because that's much higher than previous ion production methods. In case it's not immediately clear, higher plasma density is desirable because in the IEC environment, only charged particles (ions) do you any good; neutral particles are useless. In academic discussions of modern electric thrusters this is not always made clear, but one of the major failings of electric propulsion systems to date has been a lack of an ability to generate a high ionization fraction, and the corresponding efficiency loss is termed "propellant utilization efficiency" which is often no higher than .6 or .7 in ion and hall thrusters, because the process of ionization is in fact, a somewhat difficult and high-energy process (conventionally). That's what's so cool about the helicon.

Now, power in space is potentially hard to come by, but solar arrays are really taking monumental steps forward. I did a really large photovoltaic report recently (some may remember I asked for PV specs on Juno and Phoenix) and Dawn produced about 2.5 kW for its ion engine; Juno's array will produce (launches in August 2011) about 15 kW in LEO and ~500 W at Jupiter. That is really pretty remarkable, and it's not simply a giant array - it's large but it also represents a couple of fundamental leaps forward for PV technology. It's kind of a rare island of prosperity in the space industry in that there are 2 high dollar sources of funding for PV use - both the growing international and domestic interest in clean energy technology (which is gradual but ever present), and secondly and probably a more significant short term boon for the tech, is the fact that every single satellite going up today - whether it's for telecom, military, academic, whatever - has to rely on solar for power, so there's a tremendous interest in the private sector to reduce costs and improve efficiencies. Lot of money going into it, which is going to help make some of these next-gen electric propulsion systems become more viable.
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