Low Exhaust Velocity Fusion Rocket

JoshNH4H · 2014-06-24 13:07:31

When we talk about fusion rockets, we're usually talking about ones with extremely high exhaust velocities, often discussed for their use in starships or interplanetary ships on brachistochrone (read: constant acceleration followed by constant deceleration) trajectories, for example making it possible to get from Earth to Mars in between 5 and 13 days accelerating at 1 g all the while depending on the orbital configuration. This would require between 4,200 and 11,000 km/s of delta-V (for reasonable mass ratios [Read: I used 5], exhaust velocities between 2,600 and 6,800 km/s-- .9% to 2.3% of c). I certainly agree that if you're going to fly brachistochrone trajectories like these you need ridiculously high exhaust velocities like those.

But what if you just want to build a better launcher from Earth? The thrust requirements are perhaps slightly higher but the exhaust velocity requirements are much lower. If you want to launch from Earth with a total trajectory delta-V of 14 km/s, which is enough not only to launch to orbit but also to slow down to 3-4 km/s entry speed, which is a much nicer re-entry, you need an exhaust velocity around 10 km/s, or 1000 s. I assumed a mass ratio of 4 so that you can have a good structural fraction.

This is a much less strenuous application than brachistochrone trajectories, because the engine power will be lower per unit thrust. Having said that, it will still be high. If you have a T/W at liftoff of 2 (because the mass ratio is lower than for a chemical rocket, you need to start off with a correspondingly higher thrust to weight to minimize gravity losses), and 1/3 of your structural mass fraction is the engines, the engines will need to have a thrust-to-weight ratio of 25. At an exhaust velocity of 10,000 m/s, this corresponds to a specific power of 2.5 MW/kg. This is roughly comparable to the highest engine chemical thrusters, which isn't really a surprise since taking into account the lower T/W of the engine and its higher exhaust velocity we're asking for something roughly comparable to a chemical engine in terms of performance.

Of course this is still an incredibly high power to weight ratio for an electrical device. Maybe if the polywell pans out, eh?

Terraformer · 2014-06-24 15:02:53

Or ultimate dialectric capacitors, with a capacity of 500MJ/kg... with 10% of your craft as capacitors, you'll be able to produce a jet with twice the Isp of Hydrolox...

JoshNH4H · 2014-06-24 15:15:46

What do you mean when you say ultimate dielectric capacitors? Are you referring to a real design or a feasible concept, or simply picking a number?

Terraformer · 2014-06-25 04:31:03

No, Adam Crowl posted the link somewhere in... Icarus Interstellar? That's the limit that they were talking about. Whether we can realistically make such capacitors remains to be seen, but at least it's possible. Kind of like a low mass, high power fusion reactor

But if we can, then cheap access to space becomes a much easier challenge.

JoshNH4H · 2014-06-25 08:31:58

Sure, fair enough. I wonder what that ship would look like.

I would imagine that these ultracapacitors would work best at low temperatures, so they would probably be surrounded by liquid hydrogen, which would also be used as the fuel for the rocket. Hopefully they would be dense but they might not be, who knows?

Depending what the q of the fusion reactor was, we might or might not want to power it separately. I assume that a high specific power, high net power reactor may have a lower q (ratio of energy output to energy input) than reactors designed to maximize q, so perhaps it would make sense to power it with a high-q, lower output reactor. Though then again larger reactors tend to have higher q's anyway.

:sigh: If only we had a propulsion system that was cheap, safe, high thrust, high Isp, reliable, and actually existed.

JoshNH4H · 2014-06-25 10:18:16

Do we have any guesses available as to what the thrust-to-weight of a mass driver might be?

Quaoar · 2014-10-16 15:20:16

Here there are some very interesting designs by Robert Bussard about polywell powered realistic fusion spaceships: a shuttle SSTO and a lunar lander, with a water propelled high thrust regenerative cooled electro-thermal fusion rocket (15-20 km/s of exaust velocity); an inner solar system ship with a radiator cooled moderate thrust electro-thermal fusion rocket (55 km/s of exaust velocity); an outer solar system ship with low thrust thermal fusion rocket (200-400 km/s exaust velocity).

http://www.askmar.com/Fusion_files/QED% … tation.pdf

Last edited by Quaoar (2014-10-16 15:20:56)

Tom Kalbfus · 2014-10-20 06:25:54

So basically fusion SSTOs require the fusion product be kept separate from the propellent, as contact with air would disrupt the fusion process by cooling the plasma.

Tom Kalbfus · 2014-10-20 10:14:43

It was a very interesting download by the way. So it says a closed fusion reactor could heat propellant and produce multiple g thrust to lift off of Earth, and using more efficient open fusion reactors in space, it could travel to Titan in 6 months, and those same fusion reactors would be the preferred power source on Titan, once the colony was established, as their is plenty of Deuterium on Titan in the form of various ices and in hydrocarbon lakes, streams, and rivers. In addition Lockheed says it could develop a compact fusion reactor that could fit in the back of a large truck in 10 years!

Lockheed says makes breakthrough on fusion energy project

By Andrea Shalal

WASHINGTON Wed Oct 15, 2014 1:09pm EDT
?m=02&d=20141015&t=2&i=983693043&w=580&fh=&fw=&ll=&pl=&r=LYNXNPEA9E0V6
The magnetic coils inside the compact fusion experiment pictured in an undated photo provided by Lockheed Martin.

Credit: Reuters/Lockheed Martin

(Reuters) - Lockheed Martin Corp said on Wednesday it had made a technological breakthrough in developing a power source based on nuclear fusion, and the first reactors, small enough to fit on the back of a truck, could be ready for use in a decade.

Tom McGuire, who heads the project, said he and a small team had been working on fusion energy at Lockheed's secretive Skunk Works for about four years, but were now going public to find potential partners in industry and government for their work.

Initial work demonstrated the feasibility of building a 100-megawatt reactor measuring seven feet by 10 feet, which could fit on the back of a large truck, and is about 10 times smaller than current reactors, McGuire told reporters.

In a statement, the company, the Pentagon's largest supplier, said it would build and test a compact fusion reactor in less than a year, and build a prototype in five years.

In recent years, Lockheed has gotten increasingly involved in a variety of alternate energy projects, including several ocean energy projects, as it looks to offset a decline in U.S. and European military spending.

Lockheed's work on fusion energy could help in developing new power sources amid increasing global conflicts over energy, and as projections show there will be a 40 percent to 50 percent increase in energy use over the next generation, McGuire said.

If it proves feasible, Lockheed's work would mark a key breakthrough in a field that scientists have long eyed as promising, but which has not yet yielded viable power systems. The effort seeks to harness the energy released during nuclear fusion, when atoms combine into more stable forms.

"We can make a big difference on the energy front," McGuire said, noting Lockheed's 60 years of research on nuclear fusion as a potential energy source that is safer and more efficient than current reactors based on nuclear fission.

Lockheed sees the project as part of a comprehensive approach to solving global energy and climate change problems.

Compact nuclear fusion would produce far less waste than coal-powered plants since it would use deuterium-tritium fuel, which can generate nearly 10 million times more energy than the same amount of fossil fuels, the company said.

Ultra-dense deuterium, an isotope of hydrogen, is found in the earth's oceans, and tritium is made from natural lithium deposits.

It said future reactors could use a different fuel and eliminate radioactive waste completely.

McGuire said the company had several patents pending for the work and was looking for partners in academia, industry and among government laboratories to advance the work.

Lockheed said it had shown it could complete a design, build and test it in as little as a year, which should produce an operational reactor in 10 years, McGuire said. A small reactor could power a U.S. Navy warship, and eliminate the need for other fuel sources that pose logistical challenges.

U.S. submarines and aircraft carriers run on nuclear power, but they have large fission reactors on board that have to be replaced on a regular cycle.

"What makes our project really interesting and feasible is that timeline as a potential solution," McGuire said.

Lockheed shares fell 0.6 percent to $175.02 amid a broad market selloff.

(Editing by Jeffrey Benkoe)

The key point being is that a small fusion reactor that can fit in the back of a large truck, can also go into space Titan Society anyone? With fusion as a power source, this would open up both Mars and Titan to colonization. In a fusion powered economy, which would have more resources to exploit? I think Titan definitely has more deuterium. Deuterium fusion powered heaters would be quite popular.

Last edited by Tom Kalbfus (2014-10-20 10:19:18)

Spaniard · 2014-10-21 01:03:58

Tom Kalbfus wrote:

So basically fusion SSTOs require the fusion product be kept separate from the propellent, as contact with air would disrupt the fusion process by cooling the plasma.

In Space, if you try to maximize impulse, you could use the byproduct of fusion (like Helium-4 isotope) as propellent.
But for a SSTO, a lot of mass is required as propellent to reach the high thrust needed to reach orbit, so yes, fusion plasma and propellent are separated.
I suppose that a aneutronic with high energy capture is needed to transfer into the propelent, because using pure thermal transfer, the core would be hotter than propellent, and normal exahust heat is too high for a nuclear wall to resist.
The solution is to capture the energy in usefull (electric) form instead of heat and use fast transfer into propellent. Like a Skylon with extra energy transfer from microwaves usign fusion energy source with high energy capture (to avoid overheat into the nuclear reactor). If the nuclear reactor is enough powerfull, you could even use water in liquid form because the energy of the reactor is enough to avoid the needed of hydrogen and oxygen as in Skylon, and use a more compact design of the spaceship. Or use methane because it has a better hydrogen ratio (you don't need oxygen as oxydizer with a fusion source).

In space, a thermal core is viable, because you can use low thrust and heat rejection.

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#1 2014-06-24 13:07:31

Low Exhaust Velocity Fusion Rocket

#2 2014-06-24 15:02:53

Re: Low Exhaust Velocity Fusion Rocket

#3 2014-06-24 15:15:46

Re: Low Exhaust Velocity Fusion Rocket

#4 2014-06-25 04:31:03

Re: Low Exhaust Velocity Fusion Rocket

#5 2014-06-25 08:31:58

Re: Low Exhaust Velocity Fusion Rocket

#6 2014-06-25 10:18:16

Re: Low Exhaust Velocity Fusion Rocket

#7 2014-10-16 15:20:16

Re: Low Exhaust Velocity Fusion Rocket

#8 2014-10-20 06:25:54

Re: Low Exhaust Velocity Fusion Rocket

#9 2014-10-20 10:14:43

Re: Low Exhaust Velocity Fusion Rocket

#10 2014-10-21 01:03:58

Re: Low Exhaust Velocity Fusion Rocket

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