Seeder Ramjet Diagram

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Seeder Ramjet Don Juan from the novel A World Out of Time by Larry Niven.

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Larry Niven’s fictional Bussard Seeder Ramjet with some comparisons to the theoretical interstellar ramjet proposed by Robert W. Bussard in 1960, original paper here: Bussard, Robert W. (1960). “Galactic Matter and Interstellar Flight."

Larry Niven describes several types of Bussard ramjet in his fiction. One in particular proposed two nested magnetic fields acting in concert to protect the ship and gather hydrogen from the interstellar medium.  The fields interact in such a way that hydrogen is compressed for an external fusion burn rather than in a reactor, an unusual design feature — which distinguishes Niven’s conceptual ramjet from other theoretical Bussard ramjet concepts.   Per description this is the type of Bussard ramjet Niven’s State likely built for their Seeder Ramjet program, depicted in the short story “Rammer,” the novel’s “A World Out of Time,” “The Integral Trees,” and “The Smoke Ring.” The same concept is described in “The Ethics of Madness,” “The Warriors,” and later in the “The Man-Kzin Wars” collected short stories, most notably in “The Children's Hour,” where the vehicle is depicted as a one-off experimental prototype, a product of wartime research, used to attack the Kzin foothold on the Wunderland colony at α Centauri.

Bussard’s paper describes a vehicle which uses the interstellar gas as a source of energy by nuclear fusion and as a working fluid, turning to the concept of an interstellar vehicle which does not carry any of the nuclear fuel or propellant mass needed for propulsion, but makes use of the matter spread diffusely throughout our galaxy for these purposes.
 
From Winchell Chung’s Atomic Rockets site: “So, there is the obscenely-huge-mass-ratio problem, and the deadly-space-junk problem. SF authors were depressed. Then in 1960, a brilliant physicist named Robert W. Bussard proposed to use these two problems to solve each other.

If your starship is moving fast enough, the widely scattered hydrogen atoms will hit your hull like cosmic rays, and damage both the ship and the crew. One can theoretically use magnetic or electrostatic fields to sweep the hydrogen atoms out of the way so the ship doesn't hit them.

But wait a minute. Hydrogen is propellant, and could also be fusion fuel. Instead of sweeping it away, how about gathering it?

And if you are gathering your propellant instead of carrying it along with you, your mass ratio becomes infinity. This means you could theoretically accelerate forever.

This is the legendary Bussard Interstellar Ramjet. No mass ratio problems and no space junk problems. Accelerating at 1 g a Bussard ramjet could reach the center of the galaxy in a mere twenty years of proper time*and could theoretically circumnavigate the entire visible universe in less than a hundred years.
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* Keep in mind that twenty years to the galactic core is in terms of "proper time," that is, the time as experienced by the crew. The people who stay at home on Earth will still see the Bussard ramjet taking the better part of 25,000 years to make the trip.

Only there is a problem:

Things started to unravel in 1978. T. A. Heppenheimer wrote an article in Journal of the British Interplanetary Society entitled "On the Infeasibility of Interstellar Ramjets." Heppenheimer applies radiative gas dynamics to ramjet design and proves that radiative losses (via bremsstrahlung and other similar synchrotron radiation-type mechanisms) from attempting to compress the ram flow for a fusion burn would exceed the fusion energy generated by nine orders of magnitude, that is, one billion times. The energy losses will probably show up as drag. This was confirmed by Dana Andrews and Robert Zubrin in 1989.

The effect of drag? What it boiled down to was that the ramjet had a maximum speed, where the relative velocity of the incoming hydrogen equaled the drive's exhaust velocity. It has a "terminal velocity", in other words. It limits you to 12%c, which makes Bussard Ramjets pretty worthless.

However, in theory, the drag energy could be extracted and re-injected into the exhaust, to do an end-run around the terminal velocity problem.

It would be a Bussard Scramjet, in other words.

But only theoretically. It is incredibly difficult, as in "we might not manage to do it with five hundred years of research" level of difficult.

See Winchell Chung’s Atomic Rocket site, here for the thread Bussard Ramjet woes from rec.arts.sf.science (2001).

Niven’s fictional interstellar ramjets in context with theoretical work.

I’ve included Figure 4 from "Galactic Matter and Interstellar Flight," by Robert W. Bussard. This is Bussard’s schematic outline of one concept of an interstellar ramjet vehicle. I’ve also included the generalized depiction of a Bussard ramjet with its magnetic field arrangement (shown in red). In his fiction Niven described several types of Bussard ramjet design, the concept I’ve selected to focus on is nested field concept ramjet vehicle with an inner magnetic shield, labeled Deflection Field 1, and a electromagnetic ram scoop field, labeled Deflection Field 2, shown in blue and red respectively.       

Design Approach

To be clear, my purpose here is not to design a scientifically valid engineered Seeder Ramship, it is more to place Niven’s fictional concept in context with theoretical work, and to convey my impression of the Seeder Ramship from the text of the novel.

The design came about, in part, as a direct result of conversation with Winchell Chung on the subject of Bussard ramjet design. I was less than happy with my previous pass at visualizations based on Niven’s novel and was exploring ideas. Winchell reminded me of Niven’s concept of a Bussard ramjet starship riding inside an inverted conical magnetic field situated in a protective “pocket or bubble” within the convergence zone of the forward facing ram scoop field.

As Winchell related: “Around the starship is a cone-shaped magnetic field, but with the point at the front (opposite how a Bussard usually has it).” In my design the “point” is quite rounded and the body of the conical field is quite narrow, just enough to accommodate the hull. In the diagram above this field is blue and labeled “Deflection Field 1.” Around this, shown in red and labeled “Deflection Field 2,” is “a much larger cone-shaped magnetic field, with point at the rear in standard Bussard fashion—only the second field doesn’t close to a point, there is an opening at the rear, an opening just slightly larger than the base of the first magnetic field.”

“The idea is to make an arrangement much like a jet turbine: Hydrogen enters ram scoop collector field and forced by the first field into a ring shaped area to undergo hydrogen fusion. The starship would be safely behind the first field, and the reaction area would be spaced far enough away so as not to endanger the ship.”

—bold italic quotes above indicate Winchell Chung, speaking in conversation.

In Bussard’s paper no attempt is made to devise conceptual engineering approaches to the propulsion system design. This is left to future work. To the extent the propulsion system is described, Bussard states:

“For purposes of illustration we might sketch this hypothetical vehicle as in Figure 4 (see diagram above). Here we show the vehicle moving to the right so that in the vehicle frame ions appear to approach it from the right. As these cross the nominal frontal area plane A₀ they are deflected by an electric or magnetic field which causes them to arrive at a focal point some distance L back of the A₀ plane. At the focal point these ions are led into a fusion reactor of unspecified (indeed, unknown) type, made to react and generate power which is then fed back into the fusion products through a similarly unspecified conversion device, to the increase of their kinetic energy and momentum, with consequent reaction on and acceleration of the vehicle.”

The business end of my design consists of a large scale magnetic nozzle of design consistent with those envisioned for a theoretical fusion rocket. This is my own thought: Niven only describes a “ghostly ring of fusion flame” seemingly disconnected from the ship, and while this may be a rather dramatic description, I found it wanting in terms of illustrating how thrust is coupled to the vehicle. In most serious treatments of a pure fusion rocket the fusion thrust exits some form of magnetic confinement nozzle. In my modeling I applied the principles discussed here to the design of the superconducting rings and support cradle (the detail is hard to see in the final model). Each element is beveled with the narrowest edge facing inward so to present the most minimal profile to the fusion region, this cuts down on waste heat (and neutron bombardment) intercepted by the structure.

Since fusion generates a deadly neutron flux the center body base of my vehicle consists of a large neutron shield (see cutaway diagram above).

Going forward, each section is described.

Propulsion Bus: Electromagnetic generators for the magnetic nozzle and Bussard collector deflection field. In the novel Niven describes how the ram fields could be spread to catch the thinly spread hydrogen in the interstellar medium, and how Corbell could draw the ram fields in close as necessary to protect the ship: I made the collector support structure much wider than normally depicted to carry the nested superconducting rings (I imagine) might be required to alter the diameter and dynamically reshape the field.

Service Module: Power systems, thermal management, and a large LH₂ tank for in-system maneuvering: Per Niven’s text it would be necessary for the Seeder ramjet to slow down to some velocity near or below minimum ramjet speed and orient itself with the plane of the system in order to release the biological package probes and then accelerate back up to ramjet velocities to resume its journey). This would require an enormous propellant tank volume. The LH₂ tank would be slowly replenished from hydrogen collected by the ram scoop over a span of decades while in flight between systems. A large cylindrical radiator is mounted on the external surface of the hull.

Structural Spine: Niven’s text describes a nominal Seeder Ramjet mission of 300 years duration, proper time (time as measured by on-board chronometers). Since the crew compliment is one man total, who spends most of the voyage in cryogenic suspended animation, this would require autonomous AI expert systems coupled with advanced robotics to carry out maintenance and repairs (up to and including completely rebuilding critical ship systems in flight) along with a stock materials to do so. The structural spine seems a logical place to house (what I imagine to be) an array of hull-crawling robotic repair systems, raw materials, and fabrication equipment. Propellant for the reaction control system and tanked atmospheric gasses, and water, for the life support system, including the entire life support system habitat, which is described as three small rooms.

Biological Package Probes: Ten probes targeted for ten reducing-atmosphere (Venus type) worlds around ten different stars. This is the payload which gives the Seeder ramjet its purpose: to transform (over a span of thousands of years) the atmosphere of Venus-type planets into worlds with oxygen-nitrogen atmosphere, making them suitable for colonization. Pierce the Checker tells Corbell the biological package probes will “aim themselves” then says little more about them. The State allows Corbell only the information he needs to know. Per Niven’s text the probes are large and obscenely massive. I would think they would need to be: these would require all the capabilities of an advanced interplanetary spacecraft. Being self-aiming the probes would need to carry propellant and possess substantial propulsion systems capable of maneuvering to match orbit with the target planet. The probes would likely be guided by advanced AI systems (with a suitable instrument suite) to plot course and take advantage of any in-system planetary gravitational-assist and/or aero-breaking opportunities that present themselves.

Forward Module: In order for a Bussard ramjet to work, hydrogen ahead of the ship needs be ionized, otherwise it will pass right through your magnetic or electrostatic field, this can be accomplished with a powerful ultraviolet beam or a large bank of lasers, firing ahead of the ship. For Each laser element would need a service ready spare, the lasers would fire for years on end, they would need to be stood down in sequence for repair and rebuild. Sensors are housed in recessed compartments. An array of rather large chemical thrusters provide attitude control.  

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