The DARPA 100 Year Starship

[John O.]

sojourner wrote:

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DARPA isn't building anything. If you read the site you'll see that it's just a study in how to develop a path to develop the technologies needed to eventually build a ship.

Correct, it's a technology symposium - DARPA's just organizing it.

STR wrote:

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Are you allowed to tell the general public (schmucks like the rest of us on this board) a little bit about your proposal? I'm curious.

We take a helicon (which is a tube of plasma excited to extremely high ionization by a specifically tuned RF signal wound around a coil of electromagnets surrounding the tube), use it as a plasma source and inject that plasma into an IEC (which is an experimental fusion device). The motivation was that while fusion does not currently take place to any appreciable degree inside an IEC, it has a mode that's known to act as a plasma jet. In physics research they use what's called the 'star mode' of operation - and jet mode was always avoided because it destroys the internals of the IEC if you don't modify it to allow for the jet to exhaust. If you build a channel (as we have) to allow the jet to exhaust, then it can potentially produce thrust.

In the current evolution, it isn't a candidate for interstellar propulsion - but it is most definitely a candidate for satellite, trans-lunar injection and interplanetary propulsion. Isp is in the 4000-200,000 range depending on power levels and efficiencies we still have to work out. Nobody has modeled the electrostatic field in jet mode in an IEC before because it was of no interest, so that's a mystery right now as well. But at present the leading electric propulsion thruster is the Hall Effect Thruster and we think we can out-perform it - plus, the Hall thruster almost always has to use Xenon, which is becoming increasingly expensive and is absolutely not an option for interstellar travel because of the extremely large quantities it would require.

Actually just last week we got it to fire in jet mode briefly. There are still a lot of unknowns regarding the right conditions to fire in jet mode. Man it was a thing of beauty!

In the future, it may be possible to further develop the IEC concept such that you get fusion taking place and you get a much, much, MUCH hotter plasma coming out (i.e., higher velocity - higher thrust and ISP) in which case it is a candidate for interstellar propulsion.

Minus the fusion aspect, the helicon->IEC coupling is similar to VASIMIR but a lot mechanically simpler.

Here's a picture but this was just before we got it into jet mode. What you're seeing is a ball of argon plasma (the blue ball) inside a concentric electrostatic grid (the atom looking metal rings), with a hole to allow the plasma to escape the potential well in a specific direction. Unfortunately, it's not quite a jet there, it's diverging into a diffuse spray. As a jet it's a very narrow tight beam with what we believe may be shock patterns like you see in compressible flow.

[sojourner]

^Wow! those are some impressive isp numbers! Any idea at this stage on how small a unit this can be built at excluding the external power source? It sounds like it will be great for interplanetary use. Very cool pic to!

Does Bussard's Polywell research have any bearing on your work?

[John O.]

sojourner wrote:

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^Wow! those are some impressive isp numbers! Any idea at this stage on how small a unit this can be built at excluding the external power source? It sounds like it will be great for interplanetary use. Very cool pic to!

Does Bussard's Polywell research have any bearing on your work?

I wish I'd have gotten a pic in jet mode it's a lot more impressive looking!

I'm speaking a little outside my field of expertise because I'm an aerospace engineer and not a plasma physicist but my understanding of the relationship to the Bussard Polywell is that both the IEC and the Polywell devices trace their experimental lineage to the Farnsworth-Fusor device in terms of how the technology evolved via the search for viable magnetic/electrostatic confinement driven fusion devices.

In terms of scalability, I can't say much with any certainty - we haven't done force probe analysis yet so we don't even have an order of magnitude for thrust but we believe (hope!) that it's on the order of 1 N with the device scale and power scale we have now.

You hit on an important design point though - power production is, at present, a size limiting factor when you go to put this thing in space. In the lab, there's separate power supplies for the electromagnets, the IEC grid and the RF power - all of which are not small. The IEC grid is going to require 2-100 kV (I know that's a wide range, let's just say it'll ignite at 2 but a viable engine is probably going to require 50+); the RF power requires 1 kW or more; and I'm not entirely sure on the magnetic power.

One thing I'm actually in charge of investigating at present is the state of current research into permanent magnetic helicon coils - in order to reduce weight by doing away with magnetic power supply.

None of this is classified or restricted information of any sort... it's all in the academic public domain. But it's relatively 'cutting edge' because basically nobody has ever investigated coupling a helicon with an IEC before. My advisor got the idea because he's been in fusion research for nearly 50 years.

[sojourner]

John O. wrote:

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I'm speaking a little outside my field of expertise

Oh don't worry about that. My professional field is IT. I'm just a space technology enthusiast that never got past algebra in high school. I recognize the difference between thrust and ISP and understand the different ideas behind forms of rocket proplusion, but I couldn't tell ya actual physics in mathematical form.

Thanks for the info though! It's interesting and an idea I hadn't heard of. Is there a good website for source information?

Do you post on www.nasaspaceflight.com at all? I lurk over there to keep up to date on the space industry.

[John O.]

I've lurked over there for a few months now, a buddy of mine from undergrad works at JSC and he told me to go there one day when I needed some info on Juno or Phoenix or something - pretty cool site.

Yeah the neat thing about this design is that unlike VASIMIR, the Isp and thrust should be uncoupled - that is, you should be able to increase/decrease (design to spec) each of them separately based on the scale and power of the helicon vs the IEC. The design specs of the IEC should affect only the Isp and the design specs of the helicon should affect only the thrust (higher plasma density/ionization will produce more thrust).

When I first started working with the lab doing this research I did a lot of looking. There's a lot out there about IECs and a lot out there about helicons, just nothing that talks about the coupling - that is one of the aspects we're pioneering. You can look for a PhD thesis by a guy named Mike Reilly if you're really interested, he did the first 3-dimensional wave modeling of the helicon mode a few years ago, we're using a lot of his original equipment. It's titled, "Three Dimensional Imaging of Helicon Wave Fields Via Magnetic Induction Probes", from 2009. Another good paper is "Experiments on Helicon Plasma Sources" by Chen, 1991.

[MANT!]

I do like the *First one needs to have a destination* aspect of the project..
once the destination is found, the political and financial backing would be easier.

Also taking the long term approach is far better than any "Get There Now" as total costs (though high) would be spread out over a much longer period of time, making budgeting much easier.

the matter of interstellar flight can take several tacks.. a new propellant source or crude use of power..

http://en.wikipedia.org/wiki/Project_Daedalus
http://en.wikipedia.org/wiki/Project...ear_propulsion)

http://en.wikipedia.org/wiki/Project_Longshot

as designs already exist for crude interstellar spacecraft, it only makes sense that a destination needs to be found first...

[John O.]

I agree, another aspect this symposium will certainly have to tackle is that we can't just rely on ideas that may be technologically sound but pragmagically inviable. A pulsed nuclear propulsion system would require too many nuclear detonations to ever work, unfortunately, unless somehow the political climate about it were to change drastically or some powerful and influential politician were to get it into his head to make it possible..

[sojourner]

Pulse nuclear will never be politically viable near earth. Once we branch out into the solar system though, it may catch on.

[John O.]

sojourner wrote:

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Pulse nuclear will never be politically viable near earth. Once we branch out into the solar system though, it may catch on.

Yeah but we would have to be so branched out that even testing could be done at a safe distance, if the political environment about nuclear detonations remains the way it is.

Actually I think the idea of even building warheads again, for any purpose, would be difficult to sell, in part because it involves rebuilding a lot of government infrastructure associated with nuclear arms construction and there'd be a hoopla over that.

[sojourner]

^well, I could see a scenario where the warheads are built off earth. I don't think nuclear pulse will ever be allowed near earth. period.

[Yminale]

Uhm you do all know that this is a PR (and possible recruitment) even. DARPA is not serious about building a starship (America probably won't be around in a 100 years)

[sojourner]

^Nope, didn't know that at all..

sojourner wrote:

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DARPA isn't building anything. If you read the site you'll see that it's just a study in how to develop a path to develop the technologies needed to eventually build a ship.

[STR]

John O. wrote:

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Actually I think the idea of even building warheads again, for any purpose, would be difficult to sell, in part because it involves rebuilding a lot of government infrastructure associated with nuclear arms construction and there'd be a hoopla over that.

As far as I'm aware, they never really disassembled much of the warhead building infrastructure. You actually need a fair amount of infrastructure to maintain and refurbish existing warheads. In fact, it appears that they've recently begun replacing older structures with new ones, like the "Kansas City Plant" which makes electronic and mechanical components for warheads.

[STR]

John O. wrote:

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Correct, it's a technology symposium - DARPA's just organizing it.

We take a helicon (which is a tube of plasma excited to extremely high ionization by a specifically tuned RF signal wound around a coil of electromagnets surrounding the tube), use it as a plasma source and inject that plasma into an IEC (which is an experimental fusion device). The motivation was that while fusion does not currently take place to any appreciable degree inside an IEC, it has a mode that's known to act as a plasma jet. In physics research they use what's called the 'star mode' of operation - and jet mode was always avoided because it destroys the internals of the IEC if you don't modify it to allow for the jet to exhaust. If you build a channel (as we have) to allow the jet to exhaust, then it can potentially produce thrust.

In the current evolution, it isn't a candidate for interstellar propulsion - but it is most definitely a candidate for satellite, trans-lunar injection and interplanetary propulsion. Isp is in the 4000-200,000 range depending on power levels and efficiencies we still have to work out. Nobody has modeled the electrostatic field in jet mode in an IEC before because it was of no interest, so that's a mystery right now as well. But at present the leading electric propulsion thruster is the Hall Effect Thruster and we think we can out-perform it - plus, the Hall thruster almost always has to use Xenon, which is becoming increasingly expensive and is absolutely not an option for interstellar travel because of the extremely large quantities it would require.

Actually just last week we got it to fire in jet mode briefly. There are still a lot of unknowns regarding the right conditions to fire in jet mode. Man it was a thing of beauty!

In the future, it may be possible to further develop the IEC concept such that you get fusion taking place and you get a much, much, MUCH hotter plasma coming out (i.e., higher velocity - higher thrust and ISP) in which case it is a candidate for interstellar propulsion.

Minus the fusion aspect, the helicon->IEC coupling is similar to VASIMIR but a lot mechanically simpler.

Here's a picture but this was just before we got it into jet mode. What you're seeing is a ball of argon plasma (the blue ball) inside a concentric electrostatic grid (the atom looking metal rings), with a hole to allow the plasma to escape the potential well in a specific direction. Unfortunately, it's not quite a jet there, it's diverging into a diffuse spray. As a jet it's a very narrow tight beam with what we believe may be shock patterns like you see in compressible flow.

Thanks! I was a bit confused until I realized you were talking about a Polywell. Interesting stuff. Do you need a self-sustaining fusion reaction to make this a viable space motor?

[John O.]

STR wrote:

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As far as I'm aware, they never really disassembled much of the warhead building infrastructure. You actually need a fair amount of infrastructure to maintain and refurbish existing warheads. In fact, it appears that they've recently begun replacing older structures with new ones, like the "Kansas City Plant" which makes electronic and mechanical components for warheads.

Cool, I didn't know that. That's encouraging, actually.

I was actually thinking about also presenting on antimatter catalyzed thermonuclear pulsed propulsion on my own, I did some research on that a few years ago as well, but I doubted its political viability.

STR wrote:

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Thanks! I was a bit confused until I realized you were talking about a Polywell. Interesting stuff. Do you need a self-sustaining fusion reaction to make this a viable space motor?

We don't think so, no. Right now the math tells us that if the ions are coming out in a beam they will be highly energetic, enough to make it an extremely lucrative electric thruster to fill the gap between high-Isp Hall Thrusters and high thrust rocket motors. The "make or break" engineering problem right now is making sure it's in fact a neutral ion beam and not an electron beam. Electron beam = no appreciable thrust whatsoever (plus charge disparity issues). It may take some tweaking of the IEC to ensure it's a neutral ion beam.