Pulsed Fusion Rocket
Project Daedalus - Pulsed Fusion Rocket. (Credits: Nick Stevens and GrahamTG)

Pulsed Fusion Rocket

A pulsed fusion rocket burns fusion fuel in a pulsed manner as opposed to a steady-state burn (Check out the previous edition for the Axisymmetric Tandem Mirror). This mode of pulsed fusion is also known as Inertial Confinement Fusion (ICF) and it has been studied for decades at the National Ignition Facility (INF). Although INF has made great strides in the past several years, useful energy (in the form of electricity) has yet to be extracted from such reactors.

The application of ICF to propulsion, was originally proposed by the British Interplanetary Society who called their interstellar spacecraft Project Daedalus and later Project Icarus. The proposed spacecraft was a two-stage interstellar vehicle, with the first stage used on the outbound journey (leaving the solar system) and the second stage used to slow-down inbound to Proxima Centauri (our closest stellar neighbor). However, it was never clear as to how one would build such a device, or how long would such a trip take.

The two stages of Daedalus spacecraft shown separating (credit: Adrian Mann)

At the core of any ICF rocket is the fuel-capsule, a BB-ball sized sphere (referred to as a "target") that is a technological marvel in its own right. The sphere is hollow, the outside of the shell consists of a thick layer of carbon composite or similar material. The inside layer consists of a super-cooled solid ice-layer of a mixture of Hydrogen isotopes (Deuterium, Tritium or Helium-3). With every engine pulse, a target is shot by a "pellet injector" into the innards of the reactor. When the target reaches the center of the reactor, a set of high-power lasers irradiate the outer layer of the target causing the carbon to evaporate in all directions. By the law of conservation of momentum, the target begins to collapse. Much like a supernova, the core of the target undergoes a rapid fusion reaction and explodes releasing high-speed particles in all directions. Since the reactor is in the shape of a magnetic nozzle, the charged particles (plasma) are deflected right out of the back of the engine. In theory, the ICF engine converts fusion energy directly into thrust - an optimal design goal. High-velocity particles mean high specific impulse - an important efficiency metric for rocket engines. Moreover, the pulsed mode of operation for this engine means instant start-up, shut-down, and re-start capabilities as well as throttling through the changing of the firing-rate.

A modern pellet at NIF uses a "Hohlraum" to spread the laser heating onto the spherical fuel (credit: NIF)

Unfortunately, the advantages end right there. The device itself is plagued with technological challenges (I would refrain from using the word impossibilities). Firstly, during an explosion event, a large amount of energy is released in the form of a plasma radiation (Bremsstrahlung) which cannot be deflected by a magnetic field. This radiation impacts the lasers and magnetic coils of the reactor which greatly diminishes their effectiveness. Furthermore, a small portion of the output must be converted to electricity to power the spacecraft and engine. This can be achieved by means of a pick-up coil (called an induction-loop) located about the exterior of the reactor. During each pulse, the plasma "shot" would transfer some of its energy via magnetic induction, but this method has never been tested before, not even in the lab. The production of targets is also an incredibly time-consuming process, and each target could cost hundreds of thousands of dollars to produce. Lastly, the lasers require special amplifiers which are massive to say the least - packing it all up on an interstellar vehicle seems unlikely without some serious technological breakthroughs.

The amplifiers used to drive the lasers of the NIF occupy an entire underground floor. (credit: NIF)

We certainly can't expect an ICF power-plant here on earth any time soon. However, when considering a vacuum-based space engine, the design requirements change drastically. Only a small portion of released energy must be converted to electricity - just enough to power the spacecraft and engine. The rest of the energy is released via high-velocity particles ejected right out of the engine to produce thrust. It just maybe that this is the sort of technology that is great in the lab but simply cannot be scaled up for any practical applications - I certainly hope that the guys at NIF prove me wrong!

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Michael Diettrich-Chastain

Leadership Consultant | Empowering Mid-Sized Business Leaders to Master Communication, Emotional Intelligence & Build Engaged, High-Performing Teams | Passionate about conscious leadership & positive change.

6 个月

Fascinating concept!

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Sam Brewer

Former USAF Pilot, Energy Sector Manager, and Author.

6 个月

Alex, if you have any information on a spacecraft system using a fission reactor powering lasers to initiate micro-fusion propulsion, please share some concept specs and perhaps an energy balance. Your probably right that it is feasible and a better practical solution than the atomic pusher.

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Jason O. Harris

Keynote Speaker ??| US Air Force Pilot| Girl Dad| Building Trust Like Your Business & Life Depends On It ????| I help CEOs, C-suite execs, & HR leaders build top-tier teams & foster trust & accountability for excellence.

6 个月

The idea of harnessing controlled fusion explosions for propulsion is mind-bending. While the technological challenges are immense, the potential for revolutionary space travel capabilities makes this an exciting area of research to watch.

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Jason Greer - Employee and Labor Relations Expert

I solve the problems that keep CEOs, Executives, Human Resources professionals, and Attorneys up at night. I also help improve productivity in your company for a more profitable outcome!

6 个月

It's exciting to consider how this could potentially revolutionize interplanetary travel.

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Jim Britt

World's top 50 most influential Key Note speaker. Over 5,000 audiences worldwide. World's Top 20 Life and Business Strategist. Produces Powerful, Lasting, Results!

6 个月

Fascinating concept!

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