Author Topic: Direct Fusion Drive  (Read 3608 times)

Offline sanman

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Direct Fusion Drive
« on: 09/12/2013 03:02 AM »
Princeton Satellite Systems is working on a Direct Fusion Drive, as featured in this TIME article:

http://science.time.com/2013/09/11/going-to-mars-via-fusion-power-could-be/?iid=sl-main-mostpop2

http://www.psatellite.com/research/fusion.php

Since the DFD runs on deuterium and Helium3, this then relates to the issue of being able to obtain Helium3 from places like the Moon. Would DFD itself be able to generate enough thrust to lift off from the lunar surface to orbit?

Offline R7

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Re: Direct Fusion Drive
« Reply #1 on: 09/12/2013 05:21 AM »
Would DFD itself be able to generate enough thrust to lift off from the lunar surface to orbit?

Not unless T/W is increased a lot from what they describe. 1MW 20N engine is estimated to weigh 1311kg. To hover that on the Moon with 55km/s exhaust speed and 0.72 conversion efficiency (all figures from their IAC paper) you need 8+ MW.

Needs to prove that even the prototype works first though. If it does who cares about reduced Mars trip times? Point the exhaust jet to a boiler and make steam to run power plant turbines, right here on Earth.
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Offline IRobot

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Re: Direct Fusion Drive
« Reply #2 on: 09/12/2013 06:14 AM »
There are already fusion reactors that provide (slightly)  more power than given. The 2 problems are: maintained steady power generation and to be commercial viable it needs to generate much more than the given energy, around 25x.

Offline QuantumG

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Re: Direct Fusion Drive
« Reply #3 on: 09/12/2013 06:55 AM »
There are already fusion reactors that provide (slightly)  more power than given.

Really?
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Offline R7

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Re: Direct Fusion Drive
« Reply #4 on: 09/12/2013 07:18 AM »
There are already fusion reactors that provide (slightly)  more power than given.

Really?

http://en.wikipedia.org/wiki/JT-60

Q = 1.25

But operation times less than half minute, and Q=5 needed to be self-sustaining.
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Offline IRobot

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Re: Direct Fusion Drive
« Reply #5 on: 09/12/2013 07:26 AM »
I think the MAST experiment also achieved a Q > 1, but can't find any reference.

In any case, the fusion engine does not have to be self sufficient, it could be complemented with solar power, what matters is the overall isp and higher thrust compared to ion engines.

Offline QuantumG

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Re: Direct Fusion Drive
« Reply #6 on: 09/12/2013 07:37 AM »
There are already fusion reactors that provide (slightly)  more power than given.

Really?

http://en.wikipedia.org/wiki/JT-60
Q = 1.25

But operation times less than half minute, and Q=5 needed to be self-sustaining.

If you read the page you linked to, you'll see that is purely theoretical.

Quote from: Wikipedia
During deuterium  (D–D fuel) plasma experiments in 1998 plasma conditions were achieved which would, if the D–D fuel were replaced with a 1:1 mix of deuterium and tritium  (D–T fuel), have exceeded break-even—the point where the power produced by the fusion reactions equals the power supplied to operate the machine. JT-60 does not have the facilities to handle tritium; currently only the JET tokamak in the United Kingdom has such facilities.

and on the JET page you'll find

Quote from: Wikipedia
During a full D-T experimental campaign in 1997 JET achieved a world record peak fusion power of 16 MW which equates to a measured gain Q, of approximately 0.7. (..) As of 1998, a higher Q of 1.25 is claimed for the JT-60 tokamak; however, this was not achieved under real D-T conditions but extrapolated from experiments performed with a pure deuterium (D-D) plasma.

So it seems no, there are not already fusion reactors that provide (slightly) more power than given.

I think the MAST experiment also achieved a Q > 1, but can't find any reference.

Okay..

Quote
In any case, the fusion engine does not have to be self sufficient, it could be complemented with solar power, what matters is the overall isp and higher thrust compared to ion engines.

Yeah, I can agree with that, Q>1 already being achieved just seemed like a strange claim.
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Offline 93143

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Re: Direct Fusion Drive
« Reply #7 on: 09/12/2013 08:37 AM »
If you read the page you linked to, you'll see that is purely theoretical.

"Purely theoretical" is too strong a statement, and could be misleading.  Rig JT-60 for tritium and you will get Q>1.  The reactor can do it.  It just doesn't.
« Last Edit: 09/12/2013 08:38 AM by 93143 »

Offline QuantumG

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Re: Direct Fusion Drive
« Reply #8 on: 09/12/2013 10:15 AM »
If you read the page you linked to, you'll see that is purely theoretical.

"Purely theoretical" is too strong a statement, and could be misleading.  Rig JT-60 for tritium and you will get Q>1.  The reactor can do it.  It just doesn't.

In theory.
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Offline Oli

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Re: Direct Fusion Drive
« Reply #9 on: 09/12/2013 01:31 PM »
They should put ITER in a rocket. The reactor only weights 23'000 tons.  ;D
« Last Edit: 09/12/2013 01:40 PM by Oli »

Offline sanman

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Re: Direct Fusion Drive
« Reply #10 on: 09/12/2013 02:51 PM »
Well, I was thinking that a burning plasma only needs to be sustained for <10 mins to get you to orbit. I'm talking about keeping it burning even in spite of bleeding off energy for exhaust thrust. But my point is that you don't need to strive for some perpetual indefinitely sustained fusion reaction - you just need one that lasts long enough to help you achieve escape velocity. Accordingly, you might be able to skimp on the parts you use and save launch mass, because you're not going to sustain the fusion reaction for more than 10 minutes.

Online RanulfC

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Re: Direct Fusion Drive
« Reply #11 on: 09/12/2013 04:03 PM »
Well, I was thinking that a burning plasma only needs to be sustained for <10 mins to get you to orbit. I'm talking about keeping it burning even in spite of bleeding off energy for exhaust thrust. But my point is that you don't need to strive for some perpetual indefinitely sustained fusion reaction - you just need one that lasts long enough to help you achieve escape velocity. Accordingly, you might be able to skimp on the parts you use and save launch mass, because you're not going to sustain the fusion reaction for more than 10 minutes.
"Burning plasma"? I'd have to think about that description for a while :)

Seriously, the plasma is going to be exhausted for the entire <10 flight so you need a source that constantly renews it so no matter how you cut it you're looking at a "plasma" generator of some type. If it's a "fusion" system then it has to produce and eject plasma the entire flight. And in order to get your T/W up there you're going to need to be injecting some kind of "reaction" mass into and mixing it with the plasma stream for thrust.

That's the main reason you want "break-even" or beyond fusion is because the amount of equipment and power generation required to GET that stream of plasma is usally prohibitive of the thrust you can get by itself.

VASMIR uses a nuclear power plant to generate the electricity needed to power the microwave plasma producers as well as the accelleration and direction systems. I've seen suggestions (can't call em actual "proposals" :) ) of running LH2 through a very high temp, NTR and using a secondary electrical generation system loop in the reactor to power microwave generators to produce a plasma stream, (which BTW would make it a "quad-modal" rocket at that point :) but the details have been very, very vauge on how you keep the generators intact that close to the exhaust.
(Interestingly, the MITEE series of reactor studies suggested that some would be able to reach temperatures high enough to cause disassociation of the hydrogen into mono-atomic hydrogen which would re-combines in the exhaust for added power. If so that might be a possible reactor design to look into for plasma production purposes)

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Online RanulfC

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Re: Direct Fusion Drive
« Reply #12 on: 09/12/2013 08:09 PM »
Seems I was mistaken in that someone HAS looked into "supercharging" the exhaust of an NTR. According to this paper:
http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-1107-LS-NTER.pdf

Both Aerojet and the ESA have looked into the concept.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Online karanfildavut

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Re: Direct Fusion Drive
« Reply #13 on: 04/08/2017 08:22 PM »
Just a quick update: NIAC phase II award. Perhaps a fusion engine will fly sometime in our lifetime.
https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Fusion_Enabled_Pluto_Orbiter_and_Lander

Offline Elmar Moelzer

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Re: Direct Fusion Drive
« Reply #14 on: 04/09/2017 05:34 PM »
The DFD is a very interesting concept and PPPL have made some good progress with it. I asked Sam Cohen about REBCO high temperature super conductors for their reactor last August. I am delighted to see that they are now working with the MIT on using them for their reactor/engine. The potential improvement over regular SCs is quite big. Joseph Minervini from MITs plasma science and fusion center will work with PPPL on this. This group around Dennis Whyte has had some fantastic results with REBCOs, which are the basis for their ARC and SPARC reactors.

Here is Princeton Satellite System's announcement with some more details:
http://www.psatellite.com/nasa-niac-phase-ii-selected/

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