Fusion Propulsion

[NSEP]

The title explains it, using fusion energy to propell rockets, is it possible? Im extremely curious, and im sure some of you are too.

[kerbiloid]

Not yet.

KSPI-E tries to keep close to reality and has a description of different kinds of fusion/fission energy/propulsion plants in its OP.

Ideas are from RL. Just parts are scaled down to KSP.

Edited July 24, 2017 by kerbiloid

[KSK]

Yes it's possible but - so far as I know - only in the Project Orion sense. As in 'light off a bunch of hydrogen bombs behind your gigantic spacecraft and use the bombs as your combined power supply and reaction mass.'

Using a fusion reactor to power a spacecraft will remain science-fiction until we a) figure out how to build a working fusion reactor (this is a biggie) and b) figure out how to make that working reactor small enough to fit on a rocket (also non-trivial). I think there have been some proposals for fusion rockets that don't depend on maintaining the fusion reaction for any great length of time but I'm not sure of the technical details or whether they're at all plausible.

[sevenperforce]

The z-pinch fusion drive is promising, I think. Energy-negative but thrust-positive.

Or you can just use solar-electric propulsion; technically you're using fusion power.

[Green Baron]

5 minutes ago, sevenperforce said:

The z-pinch fusion drive is promising, I think. Energy-negative but thrust-positive.

Or you can just use solar-electric propulsion; technically you're using fusion power.

lol ... that's also true for a classical rocket launch with fossil fuel ;-)

[Bill Phil]

Fusion rockets may not come about for awhile, and they won't be torch drives, either, since those would basically evaporate your ship. This is why Orion is great, the heat is outside the ship.

But fusion drives are possible. In the sense that it's possible to use fusion as a heat source. I don't think they'll be able to beat nuclear thermal, though, as you hit temperature limits in the 1000 second isp range. Unless we can design it akin to a nuclear lightbulb, which could get our exhaust velocity pretty high. But I doubt the thrust will be huge. Luckily, in space, you don't need as much thrust to move around.

[Streetwind]

A fusion rocket is currently in development by MSNW LLC, supported by NASA. They've been proposing this since 2012 at least, and recently received funding as part of some of NASA's various new technologies programs (I can never remember which is which).

It gets around the need for an energy-positive fusion reactor, which nobody has built yet, by using a similar approach than an Orion drive: pulsed fusion events inside a special electromagnetic nozzle. Fuel is carried in the form of tiny pellets, which are ejected one by one out the back. A magnetic field then compresses a lithium liner around them in such a way that inertial confinement fusion is triggered. The power required for this trigger event is produced by solar cells that charge capacitors between pulses.

Specific impulse is projected to be in the neighborhood of 3000 seconds, and fixed. The amount of energy input only varies the rate at which pulses happen, which controls the amount of thrust produced. The system is expected to have significantly more thrust than a gridded ion thruster at the same input power and Isp, but not anywhere near what chemical rockets are capable of. (No statements more detailed than this were made, sorry.)

The last roadmap shown called for a ground based prototype that demonstrates the viability of the principle by 2020, with a test flight in space by 2025. But take this with a grain of salt, as that was like four years ago.

Google "magneto-inertial fusion driven rocket" and you should be able to find some videos, articles and powerpoint presentations.

Edited July 24, 2017 by Streetwind

[tomf]

In general pretty much every concept that anyone has ever had for rocket engines seems to be covered in teh atomic rockets list. http://www.projectrho.com/public_html/rocket/enginelist.php . There are plenty of fusion ideas in there.

[sevenperforce]

12 minutes ago, Streetwind said:

It gets around the need for an energy-positive fusion reactor, which nobody has built yet, by using a similar approach than an Orion drive: pulsed fusion events inside a special electromagnetic nozzle. Fuel is carried in the form of tiny pellets, which are ejected one by one out the back. A magnetic field then compresses a lithium liner around them in such a way that inertial confinement fusion is triggered. The power required for this trigger event is produced by solar cells that charge capacitors between pulses.

Specific impulse is projected to be in the neighborhood of 3000 seconds, and fixed. The amount of energy input only varies the rate at which pulses happen, which controls the amount of thrust produced. The system is expected to have significantly more thrust than a gridded ion thruster at the same input power and Isp, but not anywhere near what chemical rockets are capable of. (No statements more detailed than this were made, sorry.)

Yeah, that's the z-pinch.

IIRC the biggest problem is getting the magnetic nozzle to work.

You can also use an onboard fission nuclear reactor to charge the capacitors if you want higher thrust or want to operate out to Jupiter and beyond.

[Streetwind]

28 minutes ago, sevenperforce said:

Yeah, that's the z-pinch.

Nope, it actually isn't. It's incredibly similar though.

A z-pinch drive would compress a plasma, while this engine would compress solid metal. The difference is relevant because the z-pinch effect only works on plasma in the first place, so anything not using plasma cannot be z-pinch by definition. Otherwise, they both follow the same ideas.

...Though frankly, as long as it works, I don't care so much what it's called  

Edited July 24, 2017 by Streetwind

[sevenperforce]

1 hour ago, Streetwind said:

Nope, it actually isn't. It's incredibly similar though.

A z-pinch drive would compress a plasma, while this engine would compress solid metal. The difference is relevant because the z-pinch effect only works on plasma in the first place, so anything not using plasma cannot be z-pinch by definition. Otherwise, they both follow the same ideas.

...Though frankly, as long as it works, I don't care so much what it's called  

The term "z-pinch" refers to the z-axis of a cylindrical coordinate system. While the z-pinch term has a specific use in plasma fusion reactor research, it can also refer generally to any fusion concepts (metal or plasma) which uses a powerful magnetic field to compress the fuel along the z-axis of the reactor or engine.

[MatterBeam]

There's another, much simpler than Z-pinch, concept for initiating fusion. 

Gun Fusion.

Excerpt:

What is proposed here is a method to ignite fusion using well-understood technologies, requiring only minimal power inputs yet remaining well adapted for use in propulsion.

Gun-fusion configuration

The design is composed of two railguns, each accelerating a specially configured bullet, to be launched at each other with the intention of igniting fusion within a solid propellant cylinder. It involves a 5 step process:

 

-Launch. 

The bullets contain three elements: a thin (100nm) faceplate of gold, a chamber containing a gas mixture of deuterium, tritium and hydrogen, and a solid 'tail'. They are launched using a railgun to velocities of 20-100km/s depending on configuration.

The basic bullet configuration

-Impact.

Upon impact, the gold faceplates vaporize as kinetic energy is converted into thermal energy (10% efficiency). The heat radiated by the impact raises the temperature of the gas mixture (1% of bullet mass) to 3.125 million K. This lowers pressure requirements for ignition by a factor of 100000.

 

Diagram of impact: 2) Gold plate. 3) Gas mixture. 4) Optional backing plate (solid fusion fuel). 6) Point of impact 7) Radiated energy. 8) Vaporizing backing plate.

 

 

-Compression.

The momentum of the tails makes them act as pistons. Their movement compresses the gas mixture to over 80 million atmospheres. Temperatures rise to over 50 million K. A fusion reaction is ignited.

 

Diagram of compression: 3) Gas mixture being compressed. 4) Optional backing plate (solid fusion fuel). 5) Tail pieces.

-Propulsion.

The impact takes place under a half-sphere of polyethylene or other suitable propellant. The energy released by the fusion reaction vaporizes and ionizes the propellant. The momentum of the propellant is captured by electromagnetic or mechanical means. 

 

Configuration for gun-fusion drive

This 'gun' fusion has several clear advantages over the aforementioned methods of igniting fusion. 

 

Railguns are simpler than electromagnets cooled to superconducting temperatures or petawatt lasers using heavy, bulky supercapacitors. While the total energy involved is equal to or greater than that of inertial confinement fusion, the power levels are much lower and vastly more manageable. The fusion fuel does not have to be handled at cryogenic temperatures either.

 

With today's technologies, railguns are lighter than particle accelerators or Tokamaks. They are also more efficient than petawatt lasers, and the fusion mechanic is more robust. In the special designs discussed below, they can be replaced by even lighter methods of accelerating the bullets.

 

Another important characteristic for fusion propulsion is that the fusion equipment can be placed arbitrarily far away from where the fuel is ignited. This lowers shielding requirements. 

[wumpus]

One of the weird things about fusion rockets is that being power negative isn't a terrible thing.  The real issue is the exhaust temperature (all "fire breathing" rockets rely on exhaust temperature for Isp, although fission rockets "cheat" by heating H2).  If you confine your exhaust product in a magnetic bottle and then allow them to expand against a magnetic (instead of a physical nozzle) you suddenly can use exhaust temperatures higher than the melting point of any known material.  This lets you get away with some rather high ISPs.  The exhaust is also a mix of various monotomic hydrogen isotopes and helium, which should compare well to fission using H2 exhaust.

But it should be a lot easier just to build an Orion.  And all the fusion issues are solved (if not how to build the rest of the spacecraft).

[magnemoe]

23 hours ago, Bill Phil said:

Fusion rockets may not come about for awhile, and they won't be torch drives, either, since those would basically evaporate your ship. This is why Orion is great, the heat is outside the ship.

But fusion drives are possible. In the sense that it's possible to use fusion as a heat source. I don't think they'll be able to beat nuclear thermal, though, as you hit temperature limits in the 1000 second isp range. Unless we can design it akin to a nuclear lightbulb, which could get our exhaust velocity pretty high. But I doubt the thrust will be huge. Luckily, in space, you don't need as much thrust to move around.

Z-pinch and other modern fusion rocket ideas is fusion engines, downside is that they need external power from solar or an nuclear reactor. 
In short the fusion reaction generate very hot plasma who give very good isp. Better ISP and higher trust than vasimr because you get an huge bonus from the fusion reaction even if it don't go break even even in heat produced. Its far easier to get power from solar or nuclear than getting trust after all. 

Pulsed fusion like orion but gentle but you still need some shock absorbers. 
 

[kerbiloid]

Aneutronic fusion producing charged particles (mostly alpha-particles) allows to catch them by a magnetic field (powered with this reactor itself) and send into desired direction.
It's heat exchange differs from a simple heating device.

[MatterBeam]

3 hours ago, magnemoe said:

Z-pinch and other modern fusion rocket ideas is fusion engines, downside is that they need external power from solar or an nuclear reactor. 
In short the fusion reaction generate very hot plasma who give very good isp. Better ISP and higher trust than vasimr because you get an huge bonus from the fusion reaction even if it don't go break even even in heat produced. Its far easier to get power from solar or nuclear than getting trust after all. 

Pulsed fusion like orion but gentle but you still need some shock absorbers. 
 

If you don't break even, then you might as well directly heat an inert (non-fusion-fuel) pellet of hydrogen. You'd get the same thrust and Isp with better efficiency. 

59 minutes ago, kerbiloid said:

Aneutronic fusion producing charged particles (mostly alpha-particles) allows to catch them by a magnetic field (powered with this reactor itself) and send into desired direction.
It's heat exchange differs from a simple heating device.

These alpha particles can be very slightly slowed down by magnetic fields. If you slow them down by just 1%, you can power a magneto-hydrodynamic generator to create electricity out of the fusion explosion. If the MHD is 50% efficient, you can extract as much as 0.5% of the fusion yield.

0.5% of a terajoule blast is 500MJ. Current attempts at igniting fusion are using pulses in the 100kJ to 1MJ range, so powering the fusion ignition device should never be a problem.

[kerbiloid]

1 hour ago, MatterBeam said:

These alpha particles can be very slightly slowed down by magnetic fields.

You don;t need slow them to use in engines. It's enough to rotate them up to 180°.

Also who knows how much will they be slowed having a thermonuke power to do this.

[sevenperforce]

1 hour ago, MatterBeam said:

If you don't break even, then you might as well directly heat an inert (non-fusion-fuel) pellet of hydrogen. You'd get the same thrust and Isp with better efficiency. 

Not true. A net-negative fusion engine does not break even in terms of the power loop, but it more than breaks even in terms of kinetic energy. "Not breaking even" simply means that you cannot recover enough excess energy from the fusion to continue powering it, so it can't be used as a reactor, but it still makes an awesome engine.

[MatterBeam]

1 hour ago, sevenperforce said:

Not true. A net-negative fusion engine does not break even in terms of the power loop, but it more than breaks even in terms of kinetic energy. "Not breaking even" simply means that you cannot recover enough excess energy from the fusion to continue powering it, so it can't be used as a reactor, but it still makes an awesome engine.

Sorry. I though 'break even' referred to the commonly used 'Q-factor', which is the ratio of fusion energy to ignition energy. 

[sevenperforce]

6 minutes ago, MatterBeam said:

Sorry. I though 'break even' referred to the commonly used 'Q-factor', which is the ratio of fusion energy to ignition energy. 

Right; Q >>>> 1, but just like with thermonuclear bombs, one cannot actually recirculate enough of the heat to sustain the reaction.

[MatterBeam]

2 hours ago, sevenperforce said:

Right; Q >>>> 1, but just like with thermonuclear bombs, one cannot actually recirculate enough of the heat to sustain the reaction.

What do you mean by 'recirculating enough of the heat.?

[sevenperforce]

23 minutes ago, MatterBeam said:

What do you mean by 'recirculating enough of the heat.?

Depends on the type of nuclear reaction. One cannot use a chain of thermonuclear bombs as a fusion reactor; even though Q>1 fusion is taking place, the reaction is so energetic that it would be impossible to contain the heat and pressure enough to produce a sustained reaction. Same thing here, though on a much smaller scale.

Which prompts the question: would it be possible to use a ship-mounted railgun pair to fire a nuclear bullet at a target in outer space?

[MatterBeam]

@sevenperforce

You are right that it is practically impossible to contain a nuclear explosion. However, generating electricity does not need a sustained, contained nuclear explosion.

What all pulsed nuclear fusion designs use is a waste heat reclamation system. The simplest versions exploit the fact that much of the fusion reaction's energy becomes x-rays and neutrons. A magnetic bottle cannot deflect pure energy or neutral particles. So, these products slip through the magnetic fields into space. Some of these radiations are blocked and absorbed by the physical drive components, causing them to heat up. Electricity can be generated from this heat.

This process is incredibly inefficient, but even 0.01% of a fusion blast's energy is enough to ignite the next one.

More efficient methods use the magnetohydrodynamic effect of charged particles moving through a magnetic field. These generate electricity directly from the magnets by slowing down the particles slightly.

For your question:

Casaba Howitzers

Nuclear Shaped-Charge projectiles.

Edited July 25, 2017 by MatterBeam

[sevenperforce]

27 minutes ago, MatterBeam said:

For your question:

Casaba Howitzers

Nuclear Shaped-Charge projectiles.

Familiar with both, but I don't believe railgun-initiated nuclear fusion bullets have been proposed before.

[MatterBeam]

22 minutes ago, sevenperforce said:

Familiar with both, but I don't believe railgun-initiated nuclear fusion bullets have been proposed before.

Oh what a coincidence: Kinetic impact ignited fusion projectiles is third on my list of topics.