Bussard Revisited

Sir Harold Spencer Jones, British Astronomer Royal, famously remarked “Space travel is bunk”.   This observation attracted much ridicule over the years although it is clear that Jones was talking about inter stellar travel which may well be impossible.

The search for exo-planets has made great progress recently so that potentially habitable planets such as Kepler 62c are being found almost routinely. Given mankind’s innate curiosity there will be attempts to visit these planets.  “Sci Fi” fantasies like Star Trek imagine faster than light travel based on “Warp Drives” and “Worm Holes”.  Are there any more practical approaches backed by physicists and engineers? Probably the best known is Freeman Dyson with his Orion project and space habitats.

Dyson suggests that large structures can be built in space enabling communities to live there indefinitely. Problems such as providing artificial gravity can be overcome by spinning the structure or attaching a rocket motor that can operate continuously. Chemical rockets can provide acceleration in excess of one “g” for only a matter of minutes. Even the nuclear impulse rockets in Dyson’s Orion proposal are demonstrably inadequate for inter stellar travel over reasonable time scales. So what else is there?

In 1960 Robert Bussard proposed a fusion powered ramjet that collects its fuel by scooping up the hydrogen making up almost 90% of the gas in inter stellar space. Bussard made calculations based on hydrogen and CNO fusion reactions which showed how difficult it would be to achieve the velocity needed to collect sufficient fuel to run the fusion rocket. One needs something even more powerful than hydrogen fusion!

Proton annihilation rocket

The most efficient nuclear reactions demonstrated to date involve reactions between matter and anti-matter. In the laboratory, electron/antielectron and proton/antiproton reactions are taking place every day in several laboratories around the world. Probably the best known is the LHC (Large Hadron Collider) in Geneva. When a proton encounters an anti-proton both particles are annihilated while two 938 MeV photons are emitted. The photons emerge in opposite directions. In order for the process to be useful in a rocket, a mirror would be required to “beam” the photons. A gamma ray mirror seems implausible but…………you can’t be sure.

Another problem is that while matter/antimatter reactions are 100% efficient, the process for producing antimatter is staggeringly inefficient. To produce the fraction of a gram of antimatter in Dan Brown’s “Angels and Demons” would take more energy than mankind uses in an entire year.

As our knowledge of physics improves it may be possible to annihilate protons without mating them with antiprotons, while ensuring that the emitted photons emerge as a beam. I call this a “Proton Annihilation Rocket” which has the advantage that the reaction mass is ejected at the speed of light. Let’s assume that the space ship includes a Freeman Dyson “habitat” and enough fuel to reach a speed at which the gas collecting scoop could offset the reaction mass. A ship with a mass of 100,000 tonnes might be big enough. So how much reaction mass would be required to provide a steady one “g” acceleration? It turns out to be roughly 3 kg per second or 103,000 tonnes per year. Thus the gas scoop needs to be made large enough to collect hydrogen at a rate exceeding 3 kg per second within months rather than years.

Scoop assumptions

Our galaxy has gas densities that cover an enormous range. Near hot stars, gas density can be as low as 100 atoms per cubic meter while “Stellar Nurseries” have densities billions of times higher. To find enough gas, a Bussard ramjet would need to spend as much time as possible in the galaxy’s spiral arms which are relatively rich in gas. Even so the scoop would need to be at least 10 km in diameter.

Scoop area



Gas density



Trip statistics

The chart below shows the velocity and distance plotted against “Ship Time”. While an Earth based observer would agree on those measurements he would disagree on time measurements. A crew member would time the trip to the galactic core as 10.6 years while an Earth based observer would record 27,002 years. This strange situation follows from the characteristics of a relativistic rocket.


At a constant acceleration of one “g” it takes only days to get around the solar system. Reaching the stars takes only a few years.

Time (years)

Time (days)

Alpha Centauri




Kepler 62




Galactic core




So how did that scoop thing work out? It might work although building a scoop over 10 km in diameter will be tricky. It might look like a flimsy butterfly net dragging behind the space ship.



There are some huge problems with this concept. For example the fuel consumption is deceptive. Annihilating 3 kg of protons each second sounds reasonable unless you realize that it is 17,000 times the total energy consumed by mankind.  Harmless photons in the visible spectrum will be “Blue Shifted” into lethal gamma rays.

While this presentation contains no “Deliberate Mistakes”, I may have dropped a decimal point or two given that this is not a peer reviewed publication.   Corrections and comments are welcomed!

About gallopingcamel

Physicist & engineer
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8 Responses to Bussard Revisited

  1. Verity Jones says:

    Thanks for posting this. The numbers certainly make you think don’t they?

    I reckon we’ll make some sort of ‘breakthough’ like how to fold space-time. Go back 100 years and telephony was still young; the ‘beaming’ of images through the air (television, pictures by email, wifi etc) would seem inconceivable.

  2. While proton annihilation is an improbable fiction, fusion rockets (hydrogen or CNO) are within the bounds of existing physics. Fusion rockets will be enough to make the solar system accessible with the result that the mining of metals such as Nickel on asteroids will become economic. Once you have the profit motive Freeman Dyson’s “Habitats” will happen.

    Here is a movie that I consider quite excellent in its portrayal of the gritty life style that is likely to result except that there won’t be any huge “Picture Windows”. Everyone will be living in caves with a few small port holes!

  3. Bloke down the pub says:

    One ‘flaw’ in the Orion project was that entailed detonating hundreds of very small nuclear warheads starting at ground level. Dyson worked out that the number of excess deaths from radiation induced cancers was not high (at least when compared to things like deaths in cars) but it would have been a hard sell to get that one past the tax-payers. Space travel is not difficult once outside the pull of Earth’s gravity. The short term solution is to use a cheaper method of getting large quantities into orbit, which is where Skylon might be an indication of where we should be investing. http://www.reactionengines.co.uk/

  4. Another Ian says:


    Jerry Pournelle (not quite on this thread) has some items on education that might be worth keeping an eye on at


  5. Wayne Job says:

    Main stream science took a wrong turn about 100 years ago and their view of the universe as in the standard model is a total crock. When a discrepancy turns up rather than say they got it wrong they invent imaginary stuff to plug the hole. 95% of the matter is missing for their model
    to work, no problem it is dark matter, that did not work out so now it is dark energy.

    The relatavistic equations used will be found to be wrong and the missing matter is a charge field separate from the EM field thus they see it not. The universe is teeming with this field 95% of the energy in the universe. This is the ether that even Einstein said was needed to make sense of the universe, it is a part of the light spectrum which is a separate entity from the EM spectrum. Our ether is photonic and it is photons that are quantised it is they that cause the quanta in quantum mechanics. When and if our scientists admit they are wrong instead of inventing imaginary friends to make their set in concrete models work, then they will start to make progress.

  6. Wayne Job,
    Every theory in physics will eventually be proved incorrect. Even so, as long as a theory is close enough to make useful predictions we will keep on using it long after we have a better theory. For example, we seldom use relativistic mechanics because Newtonian mechanics is accurate enough for most purposes.

  7. Chic Bowdrie says:

    Peter, It’s probably a good thing you don’t get this peer-reviewed. Since our current planet will soon be inhabit-ably hot, a consensus may form around the need to relocate to another solar system. This will compel an intergovernmental panel to leverage spending other people’s money on it.

    • Chic,
      Inter stellar travel using Bussard ram jets or anything else seems highly improbable but it is always fun to spend other people’s money on something. That may be why we have so many useless wind turbines and solar power plants.

      On a more serious note I expect nuclear rocket propulsion to be developed within 100 years capable of getting us to Jupitre within a week. With that kind of technology, mining asteroids will be profitable and another Chixalub asteroid will be a manageable problem.

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