Talk:Fusion rocket

IEC

Look, I know the Farnsworth-Hirsch Fusor is cool, but it's not going to yield a useful fusion drive (Analog article notwithstanding). To build a useful engine you need two things: energy and reaction mass. Ideally you'd combine the two. The goal of a fusion drive is essentially to extract energy from the fusion reaction; this is impossible with the fusor. Now, when I say extract above, what I actually mean is "use it to accelerate reaction mass". So if the problem were that various high-energy ions went flying out, we could direct them backward and get acceleration. But what comes out are omnidirectional X-rays. There's no way to usefully direct X-rays, and they're not much good at propelling you. The problem is that if you were to try to use a fusor as an engine, you'd have to take along some other energy source (possibly a fission reactor) to power it: the vast majority of the energy you put into the fusor comes right back out as X-rays, regardless of whether fusion occurs or not. --Andrew 06:26, Dec 31, 2004 (UTC)

I'm taking out the old text and putting in an explanation. --Andrew 06:47, Dec 31, 2004 (UTC)

The most practical approach might eventually be a Farnsworth-Hirsch Fusor. A fusor uses inertial electrostatic confinement. Since an electron volt equals 11,604 degrees, electrostatic confinement can and has achieved fusion in large vacuum tubes. The reactors still have not broken even, but the problems may be solvable.

Fusors have three important advantages:

1. They can react fuels that no other design could. For example, they might be able to fuse protons and Boron 11. This reaction produces neither gamma rays nor neutrons.
2. They are mostly vacuum, and therefore very light weight, suitable for vehicles.
3. They might be able to generate electric power directly. The ionized reaction products would be permitted to fly from the reaction site through a high voltage field of several million volts, and then hit a grid. This would create a small current at several million volts. This power could either be used directly, or pulsed, and used to operate a transformer to get more normal voltages.

Unfortunately, Dr. Todd Rider has shown [1] that any non-equilibrium fusion system, such as the Farnsworth-Hirsch fusor, must produce X-rays due to bremsstrahlung which will carry away many times the energy that is released by fusion; with currently imaginable technology it is infeasible to capture the energy from these X-rays with sufficient energy to obtain a net release of energy from such a reactor.

Of course, in a propulsion system the goal is not to 'capture' the fusion products but rather to direct the 'flow' of these products out of the 'exhaust' with a velocity that is orders of magnitude greater that that achieved by chemical rockets. In such a system, the issue of 'break-even' is largely irrelevant, as propulsion is proportional to 'Mass times Velocity squared' (see Spacecraft propulsion ) - which means more is to be gained by inceasing exhaust Velocity than be increasing the exhaust Mass (hence ION Engines / ION Drives, see Ion thruster )

Thrust with neutrons

The article currently states that it would be "very difficult" to use the D-T reaction directly for thrust. Is it really that hard? Take the worst case that you have to fly a D-T tokamak, can't you just leave off the bottom half of the blanket? Then half the neutrons would stream out the bottom, the other half would be absorbed by the remaining part of the blanket, providing thrust. The energy of the absorbed neutrons could also be converted to electricity, but it might be better to just directly heat a gas to use for additional thrust. (I am assuming that you are not trying to breed tritium from lithium on board!) --Art Carlson 09:52, 22 September 2006 (UTC)

You'd probably build up a lot of heat that way. Also, some elements become radioactive when they absorb neutrons.

In at least one design for the Project Daedalus rocket, the D-He3 pellets would contain a deuterium-tritium "trigger" at the pellet's center. Presumably the total amount of neutrons emitted would still be manageable--Robert Treat (talk) 03:05, 24 September 2008 (UTC).

Fusion "Afterburner" for Plasma Engine

I had the experience while in Huntsville AL of meeting a aerospace engineer who is working on a type of plasma/electric engine for spacecraft. While in a meeting to discuss research, the topic led to fusion engines, of which he briefly discussed an engine he had designed and was contemplating at the time sending to NASA for proposal (this was late in year of 2008). His design included the plasma engine that they were perfecting at the time, along with a lengthy confinement section which narrowed to the degree where fusion would be accomplished by means of compression of exhaust plasma. He mentioned it as a "fusion afterburner" though it seemed similar to a ramjet in design.

The engine, as he mentioned it, would weigh 10 - 15 kilotons, and would fit aboard the Ares cargo launcher, allowing for a test run of the vehicle within the next decade. There was also mentioned of another two feasible fusion propulsion devices being contemplated elsewhere in the US, one of which would require multiple accelerators aimed at a central target, and both having approximately the same weight.

1stcontact2035 (talk) 17:32, 6 December 2009 (UTC)

Fusion Rockets don't Require a Net Energy Gain

Unless I'm very much mistaken, practical fusion rockets, as opposed to practical fusion power reactors, don't require a net energy gain. I.e. it is perfectly OK if, for each kWh of electricity (or whatever other energy source) you supply into the engine, you only get e.g. 0.4 kWh of kinetic energy of the exhaust plasma coming out of the nozzle. It's just important that the engine imparts those 0.4 kWh of kinetic energy onto a very tiny amount of plasma, so that the plasma is accelerated to a correspondingly high velocity (which is the efficiency (specific impulse) of the engine). I think this is a very important difference between fusion reactors and fusion rockets, one that's not understood by a lot of interested laymen, and it might make fusion rockets easier to build. After all, achieving hot fusion (outside H bombs) isn't all that hard and has been done for decades, it's the "ignition" (net energy gain) part that's really difficult. Shouldn't we mention this fact briefly? Olaf Klischat (talk) 02:23, 6 April 2013 (UTC)

No. You just described a 40% efficient electric propulsion thruster. High specific impulse minimizes propellant mass, but maximizes power system mass (thrust is mv but power is mv^2). Photon rockets (flashlights) have very high ISP and are very efficient at generating energy, but don't add significant momentum. For space travel you need both. The point of a fusion rocket is that it has gain. A nuclear electric powered electric propulsion thruster has a maximum efficiency 100%, each kWh electric is 1 kWh jet energy. For a gain of 200, each kWh electric (and power system mass) is 200 kWh jet energy! User:guest 11 April 2013. — Preceding unsigned comment added by 24.18.138.177 (talk) 05:15, 12 April 2013 (UTC)

I didn't describe an ion engine: Whether an engine is an ion engine or not depends on the mechanism used to accelerate the plasma, which I didn't specify. An ion engine uses electric fields for that, a fusion engine uses nuclear fusion neutron energy and associated heat and pressure. Why would a fusion engine necessarily have to have a net energy gain? If so, why does the technology for fusion rockets seem to be relatively close to practical application (see e.g. http://www.washington.edu/news/2013/04/04/rocket-powered-by-nuclear-fusion-could-send-humans-to-mars/), when at the same time practical fusion power reactors, which definitely have to have a positive net energy gain to be of practical use, are decades away from it? I would say that even without an energy gain, fusion rockets may offer advances over ion engines in that they may achieve a much higher mass flow rate (and thus, higher thrust) at the same specific impulse. No? Olaf Klischat (talk) 15:10, 14 May 2013 (UTC)

Edited out 'ion engine' to be more general. Fundamentally, rockets that use electricity to create thrust are limited by the power. So, a 1 MW fusion rocket has the same thrust and flow rate as a 1 MW Hall thruster or ion engine with the same specific impulse (discounting efficiency losses). The reason to use fission or fusion is that you get more jet power than input electrical power. A 1 MW fusion rocket with a Gain of 100 generates 100 MW of jet power compared to a 1 MW EP system jet power. It doesn't appear MSNW thinks they can use this technique for to generate fusion electricity. Fusion bombs generate thermal energy with very large Gains.

Isp

"This design would thus be considerably smaller and more fuel efficient due to its higher exhaust velocity (Isp=700 km/s)"

While exhaust velocity is measured in km/s, specific impulse is not. I'm removing "Isp=" as it just confuses the article. (For those who are curious, specific impulse is measured in seconds. It's a measure of how long one kg of fuel produces one kg-force (9.8 Newtons) of thrust. So for a fusion rocket with 700km/s exhaust velocity, it's about 71000 seconds.) -- PaulxSA (talk) 03:40, 13 April 2013 (UTC)

A high potential Isp is one of the main benefits, so it would be nice to mention the Isp or exhaust velocities that the sources say might be possible for the different fusion reaction/fuels that might be chosen. Rod57 (talk) 11:23, 16 April 2024 (UTC)

What thrust to weight ratio predicted, and what sustained acceleration on a long voyage

Most of the confinement schemes use additional propellant (hydrogen or argon) to increase thrust, but the article doesn't say how that would be accelerated and what the thrust to weight ration might be overall (fusion reactor + propellant thruster). Since article mentions journey times, perhaps the sources include the extra detail on mass & thrust. Rod57 (talk) 11:29, 16 April 2024 (UTC)