NASA backed fusion engine could fly to Mars in 30-90 days

[Algebraist]

Yes i'd think that for the fusion approach to be superior you would need to be getting a net energy gain from the fusion propulsion compared to the energy put in to generate the magnetic field (which would be from solar cells). Otherwise you might as well go down the VISMIR route but powered by solar cells (because the energy conversion is so efficient for that engine - 99% stated). So yes the fusion approach has a way to go.
Although its very possible that that a fuson engine useful for this purpose may not be particularly viable as a commercial power source (ie a power source would need to convert the power back into electricity and the cost of fuel/maintenance etc would need to be allowed for).

[Algebraist]

eg you wouldnt use a chemical rocket as a commercial power source!

[His Divine Shadow]

This fusion rocket does not require us to hit the breakeven point. It is reliant on external power sources (solar cells) like VASIMR; the main difference seems to be in how much thrust you get from a given amount of external power. If the figures in the article are accurate, this fusion rocket would get you a lot more acceleration for your constant 200 kW power input than a VASIMR thruster would.

Not sure if I understand this correctly, but how you could get a lot more acceleration from same 200 kW input than ion or VASIMR engines would produce if there aren't significant net power gain from fusion reaction. This of course assume specific impulse remain the same for both engines. You have to pass brekeven point to get more exhaust power than input of electricity.

I'm not sure it's entirely the same thing to compare a commercial fusion reactor generating a net-gain in terms of electricity and what we have here. The fusion reactor would need to generate heat which is then turned by turbines and generators into electricity, that output is what needs to be more than the power put into keeping the reactor going when we're talking about break-even.

The same stipulations don't apply for this engine, it does not need to be break even in that sense. The reaction will most likely generate more energy than is put into it because it'll be a fusion reaction, but it won't have to be harnessed and turned into electricity.

[Starglider]

Although its very possible that that a fuson engine useful for this purpose may not be particularly viable as a commercial power source (ie a power source would need to convert the power back into electricity and the cost of fuel/maintenance etc would need to be allowed for).

On the contrary any fusion rocket that managed enough Q (energy gain) to produce thrust augmentation worth the massive mass and cost penalty, would be very attractive for power purposes. For any given design Q would be much higher under earth conditions, where weight of the magentic coils, shielding and support machinery is not a factor.

eg you wouldnt use a chemical rocket as a commercial power source!

Actually we do; chemical combustion powers the vast majority of transportation and industry, increasingly using with turbo-compressors and high-pressure fuel injection equivalent to rocket turbopumps. We just don't need liquid oxygen on earth due to the ambundent atmospheric oxygen, and of course most engines are optimised for efficiency, durability and low cost not absolute maximum thrust-to-weight.

[Algebraist]

Although its very possible that that a fuson engine useful for this purpose may not be particularly viable as a commercial power source (ie a power source would need to convert the power back into electricity and the cost of fuel/maintenance etc would need to be allowed for).

On the contrary any fusion rocket that managed enough Q (energy gain) to produce thrust augmentation worth the massive mass and cost penalty, would be very attractive for power purposes. For any given design Q would be much higher under earth conditions, where weight of the magentic coils, shielding and support machinery is not a factor.

eg you wouldnt use a chemical rocket as a commercial power source!

Actually we do; chemical combustion powers the vast majority of transportation and industry, increasingly using with turbo-compressors and high-pressure fuel injection equivalent to rocket turbopumps. We just don't need liquid oxygen on earth due to the ambundent atmospheric oxygen, and of course most engines are optimised for efficiency, durability and low cost not absolute maximum thrust-to-weight.

You may very well be right about the commerical use for this type of fusion for power generation, but its interesting the way this particular proposal has been specifically pitched for this purpose rather than as a power station. Something used for one purpose can of course be adopted for another even if they end up looking rather diferent.

I could perhaps have more tightly worded the comment re the rocket - I was referring to a commercial reactor (ie electricity generation) rather than other transport and the pupose of what I said turbo-compressors and high-pressure fuel injections are not the same as a chemical rocket but rather an adaptation of technology! For example the high level of thrust for a rocket needs to utilize a very strong oxidiser through liquid oxygen or nitric acid in order to produce large volumes of hot gas for propulsion and this isnt really seen elsewhere (correct me if I'm wrong). However again the adaption of one technology for use in another is an important point.

[Algebraist]

To rephrase: A key point here is that each pulse from the fusion reaction will eject matter at very high velocity which is likely to be tricky to harness in an reactor for power. My (limited) analogy being trying to directly harness a chemical rocket for electric power (an actual rocket engine, rather than an adaptation of rocket tech!). Thus to make a reactor the approach is likely to be adapted or this may not be seen as very practical. Reactors may preferably use a sustained fusion reaction rather than a pulsed one but I gues they'll try to use whatever works. There is considerable speculation here though!

[Algebraist]

Ie anything that produces a lot more energy than put in is going to potentially adapted for a reactor.

[Simon_Jester]

Not sure if I understand this correctly, but how you could get a lot more acceleration from same 200 kW input than ion or VASIMR engines would produce if there aren't significant net power gain from fusion reaction. This of course assume specific impulse remain the same for both engines. You have to pass brekeven point to get more exhaust power than input of electricity.

I'm not sure this is true. The trick with confinement fusion is that you are, essentially, inducing a small nuclear (fusion) explosion by creating very temporary conditions that permit fusion, but which are by definition not sustainable. The definition of "breakeven" gets tricky.

Going by the article, which is rather imprecise, the exhaust velocity of the fusion rocket is around 30 km/s, quite a bit lower than the 50 km/s exhaust velocity typical of a well tuned VASIMR engine. It's quite possible that the specific impulse of a VASIMR of equal mass, hooked up to the same fuel tank, would be superior to that of the fusion rocket. But if the fusion rocket can sustain a higher acceleration for 10 days of engine burn, that may not matter so much. Sure, the VASIMR might take you up to a higher final velocity on the same initial fuel supply, but if that extends your engine burn by a factor of five because of the low thrust, you might prefer the fusion rocket for some applications like 'get there as fast as we can."

Another relevant point is that the specifications are for a 150 ton spacecraft, an unknown fraction of which is the drive. I don't know what a VASIMR built to the same scale might be capable of; we haven't seen the economies of scale that apply to VASIMR thrusters because they've only been tested in the lab.

[Starglider]

You may very well be right about the commerical use for this type of fusion for power generation, but its interesting the way this particular proposal has been specifically pitched for this purpose rather than as a power station. Something used for one purpose can of course be adopted for another even if they end up looking rather diferent.

While I'm as interested in blue-sky space applications as the next sci-fi fan, I suspect the space PR focus is a funding/politics thing. Between ITER, NIF and the gaggle of medium-sized tokamak/spheromak, stellerator and laser-intertial projects, fusion power funding is locked up pretty tight. Just look how difficult it was for the Polywell guys to get funding (for full-scale testing) even with a very promising design and a couple of decades of trying. Pitching it as a viable interplanetary thruster taps different sources of funding and avoids getting into an argument with the environmentalists and economists about fusion power cost/benefit.

For example the high level of thrust for a rocket needs to utilize a very strong oxidiser through liquid oxygen or nitric acid in order to produce large volumes of hot gas for propulsion and this isnt really seen elsewhere (correct me if I'm wrong). However again the adaption of one technology for use in another is an important point.

The specific requirements of liquid rockets were reached by further refinement of alloys, turbomachinery, cooling, valving, combustion and control techniques originally developed for conventional engines (steam, IC and jet). If you can build an F1 rocket engine, you can certainly build efficient coal/steam and gas turbine power plants; the reverse is not true. Proposing to build a spaceworthy fusion rocket before building a fusion power plant is actually quite analogous to trying to locally develop an F1 engine before your country has the ability to build its own gas turbine power station; yes you can skip developing some of the plant and handwave some of the costs, but the core technical challenges (metallurgy, tolerances, combusion modelling in this case) are an order of magnitude harder.

[Sky Captain]

I think I understood how they are trying to make it more efficient than pure electric thruster would be. Suppose they have 1 MW electrical power input. In pure electric thruster they would get around 0,7 MW of thrust power depending on drive efficiency. Now for the same 1 MW input they also get some fusion reaction going. The resulting thrust power is say 0,7 MW + whatever the fusion reaction is providing additionally. Let's say they have 0,7 MW thrust power from electrical input and anothe 0,7 MW from fusion. End result is drive that is seemingly 140 % efficient. Not enough for power plant but good enough for rocket.

[Algebraist]

In light of the discussion and a bit more research on this my overall understanding of this now is as follows:

This fusion engine has been proposed for space propulsion since it has the characteristics suitable for that. It ejects ionized metal at very high velocity out the back to act as propulsion. This is an efficient conversion of energy to thrust.

This method however is unlikely to be so ideal to adapt to electricity generation. For example conversion efficiency into electricity using a thermal system is only about 40% at best. In order to develop a commercial power station research is ongoing into developing direct energy conversion systems to potentially get electricity conversion of efficiencies over 70%! None of those techniques apear to fit though with this fusion engine (magnetic compression/expansion; controlled leakage, ion beam) although I'm speculating a bit.

There are a lot of parallel investigations going on into fusion. Given the dificulties of putting a fusion reactor into space compared to getting one to work on earth chances are we'll have working fusion power generation significantly before any fusion engine is seen on a space craft. However its reasonably clear those will be somewhat diferent approaches, although many technologicaly advances will be applied to both!

[Algebraist]

You may very well be right about the commerical use for this type of fusion for power generation, but its interesting the way this particular proposal has been specifically pitched for this purpose rather than as a power station. Something used for one purpose can of course be adopted for another even if they end up looking rather diferent.

While I'm as interested in blue-sky space applications as the next sci-fi fan, I suspect the space PR focus is a funding/politics thing. Between ITER, NIF and the gaggle of medium-sized tokamak/spheromak, stellerator and laser-intertial projects, fusion power funding is locked up pretty tight. Just look how difficult it was for the Polywell guys to get funding (for full-scale testing) even with a very promising design and a couple of decades of trying. Pitching it as a viable interplanetary thruster taps different sources of funding and avoids getting into an argument with the environmentalists and economists about fusion power cost/benefit.

Yes agree pitching this as an engine fo a Mars mission is a great way to get funding for fusion research from a source such as NASA when funding is difficult to come by. Any research that helps to develope fusion as an energy source is good and ultimately hopefully well see it on a Mars mission!