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Harnessing Antimatter for Propulsion

Antimatter’s staggering energy potential always catches the eye, as I mentioned in yesterday’s post. The problem is how to harness it. Eugen Sänger’s ‘photon rocket’ was an attempt to do just that, but the concept was flawed because when he was developing it early in the 1950s, the only form of antimatter known was the positron, the antimatter equivalent of the electron. The antiproton would not be confirmed until 1955. A Sänger photon rocket would rely on the annihilation of positrons and electrons, and therein lies a problem.

Sänger wanted to jack up his rocket’s exhaust velocity to the speed of light, creating a specific impulse of a mind-boggling 3 X 107 seconds. Specific impulse is a broad measure of engine efficiency, so that the higher the specific impulse, the more thrust for a given amount of propellant. Antimatter annihilation could create the exhaust velocity he needed by producing gamma rays, but positron/electron annihilation was essentially a gamma ray bomb, pumping out gamma rays in random directions.

Image: Austrian rocket scientist Eugen Sänger, whose early work on antimatter rockets identified the problems with positron/electron annihilation for propulsion.

What Sänger needed was thrust. His idea of an ‘electron gas’ to channel the gamma rays his photon rocket would produce never bore fruit; in fact, Adam Crowl has pointed out in these pages that the 0.511 MeV gamma rays generated in the antimatter annihilation would demand an electron gas involving densities seen only in white dwarf stars (see Re-thinking the Antimatter Rocket). No wonder Sänger was forced to abandon the idea.

The discovery of the antiproton opened up a different range of possibilities. When protons and antiprotons annihilate each other, they produce gamma rays and, usefully, particles called pi-mesons, or pions. I’m drawing on Greg Matloff’s The Starflight Handbook (Wiley, 1989) in citing the breakdown: Each proton/antiproton annihilation produces an average of 1.5 positively charged pions, 1.5 negatively charged pions and 2 neutral pions.

Note the charge. We can use this to deflect some of these pions, because while the neutral ones decay quickly, the charged pions take a bit longer before they decay into gamma rays and neutrinos. In this interval, Robert Forward saw, we can use a magnetic nozzle created through superconducting coils to shape a charged pion exhaust stream. The charged pions will decay, but by the time they do, they will be far behind the rocket. We thus have useful momentum from this fleeting interaction or, as Matloff points out, we could also use the pions to heat an inert propellant — hydrogen, water, methane — to produce a channeled thrust.

But while we now have a theoretical way to produce thrust with an antimatter reaction, we still have nowhere near the specific impulse Sänger hoped for, because our ‘beamed core’ antimatter rocket can’t harness all the neutral pions produced by the matter/antimatter annihilation. My friend Giovanni Vulpetti analyzed the problem in the 1980s, concluding that we can expect a pion rocket to achieve a specific impulse equivalent to 0.58c. He summed the matter up in a paper in the Journal of the British Interplanetary Society in 1999:

In the case of proton-antiproton, annihilation generates photons, massive leptons and mesons that decay by chain; some of their final products are neutrinos. In addition, a considerable fraction of the high-energy photons cannot be utilised as jet energy. Both carry off about one third of the initial hadronic mass. Thus, it is not possible to control such amount of energy.

Image: Italian physicist Giovanni Vulpetti, a major figure in antimatter studies through papers in Acta Astronautica, JBIS and elsewhere.

We’re also plagued by inefficiencies in the magnetic nozzle, a further limitation on exhaust velocity. But we do have, in the pion rocket, a way to produce thrust if we can get around antimatter’s other problems.

In the comments to yesterday’s post, several readers asked about creating anti-hydrogen (a positron orbiting an antiproton), a feat that has already been accomplished at CERN. In fact, Gerald Jackson and Steve Howe (Hbar Technologies) created an unusual storage solution for anti-hydrogen in their ‘antimatter sail’ concept for NIAC, which you can see described in their final NIAC report. In more recent work, Jackson has suggested the possibility of using anti-lithium rather than anti-hydrogen.

The idea is to store the frozen anti-hydrogen in a chip much like the integrated circuit chips we use every day in our electronic devices. A series of tunnels on the chip (think of the etching techniques we already use with electronics) lead to periodic wells where the anti-hydrogen pellets are stored, with voltage changes moving them from one well to another. The anti-hydrogen storage bottle draws on methods Robert Millikan and Harvey Fletcher used in the early 20th Century to measure the charge of the electron to produce a portable storage device.

The paramagnetism of frozen anti-hydrogen makes this possible, paramagnetism being the weak attraction of certain materials to an externally applied magnetic field. Innovative approaches like these are changing the way we look at antimatter storage. Let me quote Adam Crowl, from the Centauri Dreams essay I cited earlier:

The old concept of storing [antimatter] as plasma is presently seen as too power intensive and too low in density. Newer understanding of the stability of frozen hydrogen and its paramagnetic properties has led to the concept of magnetically levitating snowballs of anti-hydrogen at the phenomenally low 0.01 K. This should mean a near-zero vapour pressure and minimal loses to annihilation of the frozen antimatter.

But out of this comes work like that of JPL’s Robert Frisbee, who has produced an antimatter rocket design that is thousands of kilometers long, the result of the need to store antimatter as well as to maximize the surface area of the radiators needed to keep the craft functional. In Frisbee’s craft, antimatter is stored within a fraction of a degree of absolute zero (-273 C) and then levitated in a magnetic field. Imagine the refrigeration demands on the spacecraft in sustaining antimatter storage while also incorporating radiators to channel off waste heat.

Image: An antimatter rocket as examined by Robert Frisbee. This is Figure 6 from the paper cited below. Caption: Conceptual Systems for an Antimatter Propulsion System.

Radiators? I’m running out of space this morning, so we’ll return to antimatter tomorrow, when I want to acknowledge Les Shepherd’s early contributions to the antimatter rocket concept.

The paper by Giovanni Vulpetti I quoted above is “Problems and Perspectives in Interstellar Exploration,” JBIS Vol. 52, No. 9/10, available on Vulpetti’s website. For Frisbee’s work, see for example “How to Build an Antimatter Rocket for Interstellar Missions,” 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 20-23 July 2003 (full text).


{ 23 comments… add one }
  • Alex Tolley March 13, 2018, 12:21

    produced an antimatter rocket design that is thousands of kilometers long

    Just looking at the dimensions, this would “difficult” to construct. The need for structural stabilization with external struts and tethers to keep it reasonably aligned is obvious.

    And people say constructing kilometer wide solar sails would be hard…

    • Robert March 13, 2018, 14:59

      One could envision kilometer scale solar sails being constructed with a process akin to that used in the Star Trek episode ‘The Tholian Web’.

    • J. Jason Wentworth March 17, 2018, 0:41

      There is a way around this “extreme length problem”:

      These two short videos (see: http://www.youtube.com/watch?v=pG-2Pvoq9Ck and http://www.youtube.com/watch?v=9KBSaam5Tkc ) show a “tractor” fusion-drive interstellar probe, which is a hybrid of the design concepts utilized in the familiar Project Daedalus starprobe and in another starprobe, Project Valkyrie (see: http://www.google.com/#q=Valkyrie+interstellar+probe&* ), and:

      The “tractor” propulsion takes advantage of the fact that most materials are stronger in tension than in compression; thus this probe’s front-mounted inertial confinement fusion engine *pulls* it along. Starprobes and starships using other types of propulsion (nuclear pulse, matter/antimatter, etc.) could also be of “tractor-rocket drive” configuration (like Goddard’s, the VfR’s, and the American Rocket Society’s early liquid-propellant rockets [the Russian Dnepr launch vehicle, a modified R36M ICBM, has a tractor third stage]).

  • Alex Tolley March 13, 2018, 12:36

    Photon sails get you the high exhaust velocity without all the attendant issues with anti-matter. Building massive beamers or solar concentrators to accelerate those sails seems far more feasible to me, at least for flight in and around the Sol system. Once we have established an industrial presence at the target stars, we can build similar beaming facilities to decelerate the sails. This seems far more tractable an approach for regular interstellar flight than anti-matter rockets that still suffer from the rocket equation and the [current] huge energy inefficiencies of anti-matter production that dwarf even laser inefficiencies.

    If anti-matter has a widespread role, I suspect that it will be in the nanodomain with microscopic amounts needed for processes.

    • Jens March 14, 2018, 18:12

      Is there any way to use anti matter in more efficient way than via the anti proton reaction?

  • Robert March 13, 2018, 14:11

    It seems to me we have had the concept of anti-mater for nearly a century and we’ve learned that annihilation occurs but in nearly a century, do we really understand why anti-matter and matter annihilate? And I don’t mean a simple statement about all its quantum numbers canceling out-that’s not understanding what’s really happening. A proton isn’t even thought to be a simple structure, it’s a complex arrangement of three quarks, gluons and such. If we really understood the process perhaps we could figure out how to replicate a similar process without having to make anti-matter directly. Of course, such knowledge may be too dangerous.

    • Enzo March 13, 2018, 22:21

      You mean quark fusion ?

      Much more powerful than normal fusion but, apparently,quarks do not exist for long enough. Maybe, in dense enough materials…….

      • Robert March 14, 2018, 13:08

        No, I’ve never heard of quark fusion. Just speculating on the unknown. Also, and probably everyone here already knows this, but according to the book Mirror Matter by Robert L. Forward, the best use of anti-matter for propulsion is not a 1:1 mix with normal matter but a small amount of anti-matter with a larger amount of normal matter for momentum generation.

    • J. Jason Wentworth March 21, 2018, 5:21

      I think I grok what you’re getting at:

      Protons, electrons, and their antimatter counterparts (anti-protons and positrons) are not solid, but are vibratory structures. If their vibrations could be brought “in-sync” with each other, maybe they could be coaxed into coexisting until their reaction is needed? (Perhaps matter/antimatter mutual annihilation is like two identical waves of sound or light [or in water] meeting in such a way that they cancel each other out?) I have no idea how matter and antimatter might be induced to “live together in perfect harmony” in this way, but if it could be achieved somehow, this type of starflight would suddenly be much more attractive and promising.

  • Spaceman March 13, 2018, 14:12

    Wow, a specific impulse of 0.58c!! That is huge. However, I thought rockets traveling at or above 0.5c are too dangerous because of the radioactivity induced by collisions with interstellar hydrogen atoms. Perhaps some new shielding technology could work around this constraint. Otherwise, who here thinks we will be limited to 0.1-0.5c for our fastest interstellar spacecraft?

    • Andrei March 13, 2018, 16:28

      I agree, even if we would find a method to travel very fast.
      The physics and conditions in the cosmos would have us go slower with the technology we could build or imagine to be possible.
      There has been some good ideas to create shielding that would work for slower speeds, such as a sheet of ice that fly formation ahead of the spacecraft or the beryllium plate plus tobacco smoke sized particles from Project Daedalus.
      The last alternative I can think of is to have a laser or some other kind of energy beam to vaporise any incoming object over a certain size. But it would both need to be able to identify the object at a safe range so that it not only have time to melt the danger but also so the resulting gas have time to spread out enough so it would not be any danger by itself.
      In the far future, physics research might lead to the development of new technologies or material science, ultra dense matter might make a more efficient shield for example.
      Some progress have been made, though in a study for possible fusion research.

    • Mephane March 19, 2018, 4:14

      An exhaust velocity of 0.58c does not mean that the rocket will ever move at such a speed. It is just a measure of efficiency, as thrust is a function of exhaust velocity and propellant mass per time. The higher the exhaust velocity, the less propellant mass needs to be used (and thus: carried aboard) for the same amount of net thrust.

  • Spaceman March 13, 2018, 14:19

    Ok, so assuming all currently known limitations, what is the best estimate for the maximum specific impulse of an antimatter engine in terms of a percentage of light speed?

    • Alex Tolley March 13, 2018, 17:24

      Use the rocket equation with your guess for M0/M1.

  • Geoffrey Hillend March 13, 2018, 16:46

    Anti-SH2 Tank 76 km, LH2 tank 76 km, 515 km. Kilometers in length? This thing is pretty long. I don’t see us building it anytime soon, but I don’t expect fast interstellar propulsion to be ready anytime soon.

  • Al Jackson March 13, 2018, 18:12

    As far as I can find out, from notes by his wife Dr. Irene Sänger-Bredt, Eugen started researching interstellar flight in the 1930s. He started research into photon propulsion in the late 1940s and then published first in 1953 [1]. The antiproton was not discovered until 1955 so he latched onto electron-positron annihilation. Sänger expended a lot of energy on trying to make photon rockets work. He was well aware of the ‘focusing’ problem and came up with pure electron ‘mirrors’ that only a Kardashev type III civilization would have built! (His last paper was in 1961[2] ) Why he didn’t latch onto antiprotons I can’t guess, have to admit the real hard engineering physics of using antimatter would take a number of more years. Amusing that Sänger noted in the Handbook of Astronautical Engineering that his photon ship could not be turned on in the solar system for fear of incinerating a planet!
    [1] E. Sänger, 4th International Astronautical Congress, Zürich, Switzerland 3-8 August 1953.
    [2] “Photon Propulsion”, Handbook of Astronautical Engineering, Koelle , 1961

  • Robin Datta March 13, 2018, 20:37

    For now it would seem that the most practical way to “harness” antimatter is with an anti-horse to an anti-cart on an anti-earth.

    Meanwhile here it would seem that there is a crying need to tease out the components of matter and antimatter involved in annihilation and the mechanisms that effect this phenomenon. It might even lead to ways to better control the process.

  • Andrew Palfreyman March 14, 2018, 13:41

    For interplanetary and interstellar travel, a network of beamers should be wholly adequate for the foreseeable future. Local fusion power for the beamers is the chief missing piece right now.

    It is unfortunate that the collision problem becomes acute before velocities are reached which result in significant time dilation.

    • Jens March 14, 2018, 18:07

      I agree with both.

  • Lepton March 15, 2018, 11:39

    The design is pure scientific exercise, addressing no engineering concern. Even Avatar ship knows to put the engine in front and PULL the ship.

    As many other people have said, it’s much better to focus on sails, develop space infrastructure and colonize the solar system. By then, we will have more knowledge, more resources to build other ships. This has added benefit of using beam stations to defend Earth from killer asteroid.

  • david lewis March 17, 2018, 13:19

    Why carry the antimatter? Couldn’t it be accelerated towards a vessel by a stationary source as it’s made? Perhaps as a particle beam, or as a series of small pellets that are vaporied by a laser on the vessel.

    Imagine a runway that’s 300 billion kilometers long. A vessel starts at one end while the antimatter is produced and projected at the vessel from the other end. (The antimatter doesn’t need to be moving very fast.) If the vessel’s acceleration is at 10G, then by the time it reaches the end of the runway the vessel would be moving at around 80% that of light.

    How to stop is another question entirely – crash into a planet and start an interstellar war maybe?

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