Clifford Singer: Propulsion by Pellet Stream

by Paul Gilster on July 15, 2014

Small payloads make sense if we can extract maximum value from them. But remember the problem posed by the rocket equation: It’s not just the size of the payload that counts. A chemical rocket has to carry more and more propellant to carry the propellant it needs to carry more propellant, and so on, up the dizzying sequence of the equation until the kind of mission we’re interested in — deep space in reasonable time frames — is ruled out. That’s why other forms of rocket using fission or fusion make a difference. As the saying goes, they get more bang for the buck.

But the idea of carrying little or no propellant at all has continued to intrigue the interstellar community, and numerous ways of doing so have been proposed. One early contender was a particle beam, which would be used to push a magnetic sail. Strip electrons from atomic nuclei and accelerate the positively charged particles close to the speed of light. There’s a benefit here over laser-beamed sail concepts, for the magnetic field creating the magsail has no heat limit. We’re less concerned about sail degradation under the beam and, unlike some of Robert Forward’s laser concepts, we don’t require a huge laser lens in the outer Solar System.

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At first glance, the idea stacks up favorably when compared to lasers. What it demands is a particle accelerator powered by solar energy sufficient to accelerate the charged particle beam, just as laser beaming to a sail would require large laser installations in an inner system orbit. But physicist Clifford Singer noticed early on that a stream of charged particles has an inherent problem — it will spread as it travels because particles with the same charge repel each other. Singer’s idea was to use a stream of pellets to replace the charged particles, each of them a few grams in size. The pellet stream does not ‘bloom’ as it travels. Singer believed that the pellets, accelerated to perhaps as much as a quarter of the speed of light, would be vaporized into a plasma when they reached the interstellar craft, turning into a hot plasma exhaust.

Image: Clifford Singer, whose work on pellet propulsion in the late 1970s has led to interesting hybrid concepts involving on-board intelligence and autonomy. Credit: University of Illinois.

When he came up with the proposal in 1979, Singer was at Princeton University’s Plasma Physics Laboratory, and it’s interesting to consider his work a kind of hybrid between beamed power and nuclear pulse propulsion, which is how Gregory Matloff and Eugene Mallove approach it in The Starflight Handbook. Singer envisioned pellets for acceleration, after which there would be a long coasting phase of the interstellar mission. Looking at laboratory work involving so-called ‘rail gun’ accelerators, he thought of scaling up the idea to an accelerator 105 kilometers long deployed somewhere in the outer Solar System.

Singer’s ideas, first broached in a paper called “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer” in The Journal of the British Interplanetary Society drew their share of criticism. Could the stream of pellets really be collimated so as to remain a single, coherent beam? One problem was that pellets might be dispersed due to interactions with dust grains in the interstellar medium. Singer would defend the concept in a second paper one year later in JBIS, acknowledging the dispersion problem for lighter particles but concluding that particles heavier than one gram should not be affected. Nor would interactions with the galactic magnetic field be a serious impediment.

Interstellar thinking of this era demanded thinking big, and while Singer’s pellets were tiny, they demanded not only that enormous accelerator but a series of deep space facilities spaced 340 AU apart — several dozen of them — to help keep the beam fully collimated. Such stations might be deployed from the departing starship itself, each of them measuring particle locations and correcting the particle flight path through the use of magnetic or electrostatic fields. Singer’s ideas have been enormously fruitful, leading to ideas the technology of the day would not render obvious, but as we’ll see tomorrow, they point to a fusion of digital tech and nanotechnology.

I like what Matloff and Mallove have to say about pellet propulsion in The Starflight Handbook:

Workable concept or not, the advent of the pellet-stream propulsion idea several decades after the beginning of serious starship speculation illustrates again how easy it is to overlook ‘obvious’ interstellar flight concepts. What other propulsion gems may be waiting to be found, buried in the armamentarium of twentieth-century technology!

Now a professor of nuclear, plasma, and radiological engineering at the University of Illinois, Singer is no longer active in interstellar work but keeps an interested eye on the propulsion method he created. The pellet concept is indeed a gem, and one whose facets keep changing as we hold it up to the light. For we’re seeing a metamorphosis away from the idea that the only kind of particles we can send are dumb objects. Gerald Nordley would enhance the particle stream with active intelligence that would allow collimation through course correction at the particle level.

No need for starship maneuvering or course correction stations along the way if we can deploy Nordley’s ‘snowflake’ pellets, which we’ll look at more closely tomorrow. I argue that an enhanced ‘smart pellet’ is one step away from becoming not just the propellant but the spacecraft itself. In any case, it’s into the interesting synergy between driving small objects — particles, pellets, micro-sails — to a spacecraft and the extremely rapid advance of digital tools and miniaturization that 21st Century interstellar thinking seems to be expanding.

Clifford Singer’s key paper is “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107–115. He followed this up with “Questions Concerning Pellet-Stream Propulsion,” JBIS 34 (1981), pp. 117-119.

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{ 12 comments… read them below or add one }

Brett July 15, 2014 at 13:15

No wonder there’s a Fermi Paradox. If this type of space infrastructure is what it takes to get interstellar probes out there . . .

Alex Tolley July 15, 2014 at 13:27

enhance the particle stream with active intelligence that would allow collimation through course correction at the particle level

So DARPA has already shown teh intelligence part is viable with
smart bullets.

We also need fine guidance using rocket propulsion in space. Already nearly there with electrospray ion propulsion.

So we have the basic parts for navigation and guidance of the particles. They just need to be put together in an integrated package.

That leaves the hard bit – a 100k km rail gun in the outer solar system, plus a beam power source in the inner solar system to power it. Onwards to the pellets being the “spacecraft”.

Michael July 15, 2014 at 14:46

‘But physicist Clifford Singer noticed early on that a stream of charged particles has an inherent problem — it will spread as it travels because particles with the same charge repel each other.’

Divergence of a particle beam is highly dependant on gamma, the temperature and the mass of the particles, see pg. 22

http://www-pnp.physics.ox.ac.uk/~delerue/accelerator_option/6_emittance.pdf

‘Singer’s idea was to use a stream of pellets to replace the charged particles, each of them a few grams in size.’

The problem I hit was that inducing a magnetic or electric field strong enough in the small pellet so that it could be accelerated by a linac made it prohibitively large. Getting smaller and smaller pellets made the ‘field’ problem much worse and they did not scale well at all.

Andrew Palfreyman July 15, 2014 at 16:43

This is beamer tech with real balls

Eniac July 15, 2014 at 20:33

It is good to see that Singer realizes that active course control is necessary. Tricky, but at least plausible.

The weak link here is that accelerator. No rail-gun, however long, can go much beyond orbital velocity. The reason is, as Michael alludes to, that macroscopic objects (more specifically, anything that is bigger than a particle or ionized atom) have vanishing charge/mass ratios and induced fields are subject to serious material limitations.

There is currently no plausible design of such an accelerator I know of. If Singer has come up with one, it should have been mentioned in this article.

Michael Spencer July 16, 2014 at 7:04

The pellets would arrive with a very high amount of kinetic energy, no? Isn’t this energy subtracted from the goal available during conversion? I wonder what fraction it would be.

Michael July 16, 2014 at 14:27

@Eniac July 15, 2014 at 20:33

‘There is currently no plausible design of such an accelerator I know of. If Singer has come up with one, it should have been mentioned in this article.’

There is the induction accelerator, but it requires a serious length to avoid the huge ‘g’ forces that would destroy a smart pellet. Pure particle accelerators are the way to go.

@Michael Spencer July 16, 2014 at 7:04

‘The pellets would arrive with a very high amount of kinetic energy, no? Isn’t this energy subtracted from the goal available during conversion? I wonder what fraction it would be.’

The energy available would be relative to the crafts velocity, an efficient magnetic catcher could absorb a large amount of momentum.

Eniac July 17, 2014 at 21:40

Michael:

Pure particle accelerators are the way to go.

With respect to acceleration, yes. With respect to collimation, on the other hand, no dice. You cannot have “smart particles”, and dumb cannot be focused onto a reasonably sized receptor.

Michael July 18, 2014 at 16:32

@Eniac July 17, 2014 at 21:40

‘With respect to acceleration, yes. With respect to collimation, on the other hand, no dice. You cannot have “smart particles”, and dumb cannot be focused onto a reasonably sized receptor.’

Don’t be so sure about the collimation of particle beams pg.22

http://www-pnp.physics.ox.ac.uk/~delerue/accelerator_option/6_emittance.pdf

‘You cannot have “smart particles”, and dumb cannot be focused onto a reasonably sized receptor.’

You may not need them, if the acceleration phase is high enough the divergence diameter at point of impact will be less.

Eniac July 19, 2014 at 12:46

When the beam is accelerated, its longitudinal mome
ntum is increased, … But the transverse momentum remains the same.

Using an ideal accelerator, yes. Not with a real accelerator, though. Even the tiniest deviations from straight ahead will lead to untolerable dispersion.

Otherwise, this document tells me that a beam in its moving frame of reference behaves like a gas, and like any puff of gas in vacuum, will dissipate quickly. And this is for a neutral beam. A charged beam is worse.

Michael July 20, 2014 at 6:25

@Eniac July 19, 2014 at 12:46

‘When the beam is accelerated, its longitudinal momentum is increased, … But the transverse momentum remains the same.’

‘Otherwise, this document tells me that a beam in its moving frame of reference behaves like a gas, and like any puff of gas in vacuum, will dissipate quickly. And this is for a neutral beam. A charged beam is worse.’

All processes in the moving frame of reference are slower by the gamma factor, including the expansion of the ‘ionic gas’ which we can make very cold and/or very heavy. So a 26 km/s expansion would be reduced to an effective ~8 m/s with 3500 gamma to the stationary observer, the spacecraft. At say mars distance the gas would have expanded by only ~6.5km.

‘Using an ideal accelerator, yes. Not with a real accelerator, though. Even the tiniest deviations from straight ahead will lead to untolerable dispersion.’

You will need an accurately designed linac that is for sure, we need to pay attention to the atoms of the kinetic fuel themselves just as much as we need to pay attention to the payload.

Eniac July 20, 2014 at 12:21

It is hard to make a beam that cold, a linac could not possibly reach those gamma factors, and 6 km is a very large diameter to be catching a relativistic beam in. Also, then, you are probably 6-8 orders of magnitude short in luminosity, and there is no way you can generate the field strength and extent needed to stop such high energy beams while keeping the craft light enough to benefit from the momentum.

This proposal is way out there. There are a lot of other ideas more worthy of spending time on, I think.

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