Fusion Rockets

[Comrade Jenkens]

Does anyone know what the predicted ISP and thrust of a fusion rocket will be? I've been searching on the internet but their is very little on the topic.

[Rune]

Try atomic rockets (the drive table). But the general answers is "it depends on your fusion engine".

[Comrade Jenkens]

Try atomic rockets. But the general answers is "it depends on your fusion engine".

Thanks for the link May have to check out that website.

I was kind of assuming an equivilent to the 1.25m NERVA in KSP as at some point i want to try to create it as a part. While researching though i've struggled to find much at all on it.

[Rune]

The performance for different fusion engine proposals is quite dissimilar. The ones that directly use the fusion products for the exhaust depend on the fusion reaction and the nozzle scheme (hell, one of those is thermonulcear Orion), and then you get to the concepts that use the fusion reaction to heat another propellant, with much lower isp (still thousands of seconds to tens of thousands of seconds). Also note in general fusion concepts in real life are pretty much built around the engine.

The best engine I can see to get into ksp reasonably would a huge magnetic mirror concept (forget about 1.25m), because those have the literature, or you could go in a really interesting direction and try to kerbalize the "Polywell QED ARC engine" (google it by those words and read the paper titled "from SSTO to Saturn's moons"). The very nature of a fusion engine would probably break KSP (much too high isp), but that concept goes from 1,500 to 10,000s with more or less compact engines (and the low isp ones don't need huge radiator systems).

Rune. Keep us posted!

Edited May 28, 2013 by Rune

[Comrade Jenkens]

Rune. Keep us posted!

haha will try but i'm struggleing to learn blender and even a simple fuel tank is taxing atm.

when i think of a fusion rocket I think of the type that heats the reaction mass similar to the NERVA which would probrably be slightly more balanced in KSP. When campaign and research trees are added though high tech parts would probrably be fine to add as they would have to be researched and bought and so would be late game. Would they produce more or less thrust than a NERVA though?

Thanks

[InfinityArch]

IMHO, if they used Helium-3 (or some kerbalized equivalent) in the reaction, fusion engines wouldn't break KSP since they'd require a resource not found in significant quantities on Kerbin, necessitating you to go mine the Mun for it, which requires a great deal of infrastructure to support even in sandbox mode, and depending on how scarce "Kelium-3" or whatever was on the Mun or other bodies, might not be enough for every flight to use it.

[Rune]

Oh, Thrust and Isp are related. The exact equation is Jet Power (watts) = 0.5 * T * Vex (Vex: exhaust speed, isp*9.81). IIRC. So for a given engine power, usually in megawatts, if you increase isp by a factor of two the thrust decreases by the same factor. So definitely less thrust by watt, but it depends on the engine power.

[Comrade Jenkens]

Hmmm, things don't look very hopeful for a futuristic, high thrust and ISP spaceship when high tech parts are added then.

[nyrath]

Comrade Jenkens, the sad fact of the matter is that among spacecraft propulsion, the rule seems to be High Isp, High Thrust, choose one.

Example: the ion drive.

The only drives that have both are the utterly outrageous ones, like Orion, Open-cycle nuclear thermal rockets, and Zubrin's infamous nuclear salt water rocket. All of which have a problem with exhaust plumes that are just sizzling with blue glowing radioactive death.

[Fractal_UK]

Comrade Jenkens, the sad fact of the matter is that among spacecraft propulsion, the rule seems to be High Isp, High Thrust, choose one.

Example: the ion drive.

The only drives that have both are the utterly outrageous ones, like Orion, Open-cycle nuclear thermal rockets, and Zubrin's infamous nuclear salt water rocket. All of which have a problem with exhaust plumes that are just sizzling with blue glowing radioactive death.

I definitely like the look of the magnetoplasmadynamic thruster, the thrust/KW performance looks good for an electric engine and may still offer specific impulse of over 11,000s. Working along very similar lines there is the associated quantum vacuum plasma thruster, which despite being decidely more theoretical, I understand is being looked at by an experimental propulsion team at the Johnson Space Centre. It may offer similar thrust/KW performance without requiring any propellant to be carried. The propellant is the the quantum vacuum itself, which acts like a virtual plasma.

You should definitely look into this engine for your website Nyrath, because despite it being an energy hungry little monster, it offers many characteristics that a budding interstellar civilisation might desire in a rocket, assuming said civilisation could find a great big reactor to power it with.

[Comrade Jenkens]

surely a fusion rocket which worked in a similar way to the LV-N (heating a reaction mass), would be able to provide at least as much thrust as the NERVA engine? They both use similar principle apart from the fact that fusion fuel has a much higher energy density. :S

[Rune]

surely a fusion rocket which worked in a similar way to the LV-N (heating a reaction mass), would be able to provide at least as much thrust as the NERVA engine? They both use similar principle apart from the fact that fusion fuel has a much higher energy density. :S

Ok, have a look here:

http://www.askmar.com/Fusion_files/From%20SSTO%20to%20Saturns%20Moons.pdf

The lowest end engine there, the one with lowest isp and only regenerative cooling, makes a variable isp of between 2,000 and 3,000 s with a thermal output of 5.3GW. But not much more numbers in this particular paper (you can hunt for the references, though). There is another version of the engine, basically the fastest you can do without additional radiators, for a Mars mission with much more detail. Isp is 5,500, engine mass 20mT, and thrust 220kN. As you can see, heavy. You can see a schematic of the different engine systems on page three. In fact, that paper has more information than you need to kerbalize any kind of engine at any scale, since there are fancy curves for system ratios in there (T/W as a function of isp at the system level, for example).

As you have probably guessed, this is my pet advanced fusionconcept. If only this panned out, the future would be awesomely sci-fi-ish.

Rune. Now go do some reading

[Fractal_UK]

surely a fusion rocket which worked in a similar way to the LV-N (heating a reaction mass), would be able to provide at least as much thrust as the NERVA engine? They both use similar principle apart from the fact that fusion fuel has a much higher energy density. :S

Yes but don't make the mistake of confusing fuel and propellant. The fusion fuel in an fusion engine could produce the heat necessary to expel variable quantities of fuel at a range of specific impulses, thus producing different thrust characteristics. You can thus make your fusion rocket higher thrust, lower specific impulse by pumping more mass of propellant through your engine but obviously this uses up your propellant increasingly rapidly making it less useful for long term use.

[Comrade Jenkens]

Wow! thanks Where do people find all this?

An engine with 5,500 ISP and 220 thrust could work as a late game research item which could be useful for reaching other stars if there isn't a warp drive. Way too op for the current sandbox though (then again NERVAs are prety OP atm).

I'm going to have to dig out my physics books to get half of this. Only been out of uni for 3 months and already all knowlage is leaking out :L

[Rune]

Well, I told you it would break it. And 5,500s is actually the low end for fusion proposals. It's 20mT, though, almost ten times KSP's NTR with T/W about one (without fuel or payload, that is). And the jet power is an astounding 6 Gigawatts. Talk about hot, that must be detectable from another system .

Rune. Of course that's a joke, the classic Nerva is already a few GWs.

[drakesdoom]

I read a report on some type of inertial confinement fusion pulse drive, though I cannot find the report now. Did anyone else see that?

[Rune]

I read a report on some type of inertial confinement fusion pulse drive, though I cannot find the report now. Did anyone else see that?

Atomic rockets to the rescue!

Rune. It's also interesting, but it doesn't really promise a SSTO.

[Poison Penguin]

That sounds like it would melt it.

[Mr Shifty]

I found this:

http://www.daviddarling.info/encyclopedia/M/microwavefusion.html

And this tokamak based one, which suggests 1.3Ms of Isp and 90MN of thrust, relying on some very unrealistic assumptions.

http://energyphysics.wikispaces.com/Tokamak+Thrust+Engine

(Disclaimer, I work at DIII-D, an experimental tokamak, which since I'm an engineer, not a physicist, gives me zero insight into the likelihood of this. I do know that working fusion reactors, even experimental ones, are still decades away.)