17.3.3 - Interstellar Ramjet

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

The interstellar ramjet, first proposed by Dr. Robert W. Bussard at Los Alamos Scientific Laboratory in 1960, is a propulsion system which acquires energy and reaction mass from the surrounding medium.²⁷⁶⁶ Using some combination of electric and magnetic fields, the ramjet scoops up ionized interstellar gases to fuel its fusion rockets. This eliminates the need to carry large masses of fuel on board. Conventional chemical or nuclear rockets must be used to accelerate the starrammer up to about l-5%c, the threshold velocity at which the ramscoop mechanism becomes reasonably efficient.

The forward scoop would be immense (Figure 17.3). Bussard originally calculated that a 1000-ton vehicle would require a funnel diameter of 3600 kilometers to achieve a one-gee acceleration in normal interstellar space (~1 H-atom/cm³). In regions of dense hydrogen clouds, with perhaps 103 atoms/cm³, the ramscoop diameter could be as small as 120 kilometers. Theoretically, the acceleration could be maintained indefinitely, making possible the circumnavigation of the entire universe in less than a human lifetime.²⁷⁵⁵

Such monstrous scoops, of course, need not be constructed of physical materials. Most probably electromagnetic fields will suffice. To generate such fields, A.J. Fennelly of Yeshiva University and G.L. Matloff of the Polytechnic Institute of New York proposed in 1974 an annular copper cylinder coated with a layer of superconducting tin-niobium alloy (Nb₃Sn).¹⁴⁵⁴ The device would be rather modest in size (as starships go), measuring 400 meters in length and 800 meters in diameter. Energized with electrical current, an electromagnetic scoop with an effective diameter of 10⁴ kilometers would be generated. For braking at the destination, a drogue chute made of boron (noted for its high melting point) about 10 km in diameter is recommended. (An electrically charged wire mesh would give sufficient drag without being destroyed by erosion.^1066,1061)

As if to underscore the tremendous engineering difficulties involved in scoop design, Fennelly and Matloff announced in 1976 their original device was simplistic and probably would not work:

Nevertheless, assert the authors gamely, "we have hope that further analysis will lead to feasible scoop designs with some type of electromagnetic field to give a large scoop effective radius." It is now believed that a mixed electrostatic/electromagnetic field design will give the best results. (See Matloff,²⁷⁵⁹ Matloff and Fennelly,²⁷⁶⁶ and Powell.²⁷⁶⁰)

By adding "wings" to the starrammer, travel times may be cut in half.²⁷⁸² Explains one writer:

The Bussard ramjet is perhaps the most intensively scrutinized potential interstellar propulsion system. As a result, scientists are beginning to call attention to collateral problems involved in the design and operation of ramscoop vehicles.¹¹⁵⁵ One objection voiced by John Fishback in 1969 is fundamental.¹⁴⁶¹ He points out that the section of the starship which contains the sources of the magnetic scoop fields must be strong enough to withstand the forces generated by those fields. As the starcraft goes faster and faster, the required field strengths also increase. Since materials are limited by their maximum tensile strength, at some point the acceleration of the vehicle will have to be reduced to avoid the breakdown of its structure caused by the pressure of magnetic forces.¹⁴⁶²

For realistic building materials, this cut-off velocity at which further acceleration must be drastically curtailed occurs at about 99.999998%c. This is high enough to be of no practical significance for galactic travel at 1 gee, but may prove restrictive for higher acceleration rammers or for starships on intergalactic missions.

Another major difficulty, noted by Bussard and many others since, is that the proton-proton nuclear reaction is a poor candidate for fusion rockets. Most of the gas likely to be scooped up by the interstellar ramjet will be ordinary hydrogen, and hydrogen is very finicky when it comes to fusion. Deuterium reactions have a cross-section roughly twenty orders of magnitude greater, but this heavy isotope of hydrogen is relatively rare in the interstellar medium.

Recently, Daniel P. Whitmire has suggested the concept of a catalytic nuclear ramjet to overcome this problem.¹⁴⁷¹ In Whitmire’s scheme, the starship would carry on board a supply of "nuclear catalyst" consisting of carbon, nitrogen, and oxygen atoms. This fuel additive should catalyze a vastly in creased reaction rate among ordinary hydrogen atoms without itself being consumed. Calculations indicate that this technique will yield a rate of fusion more reasonable from the standpoint of interstellar missions. For this scheme to succeed, of course, a workable heavy ion fusion reactor must be developed, but, in Whitmire’s words, "the difficulty seems to be of a technological rather than fundamental nature." (He also proposes the use of a bank of forward lasers to ionize neutral atoms approaching the rammer’s maw, thus greatly increasing the reaction mass available for the starship’s engines.)

An interesting hybrid variation of the basic ramscoop technique involves a vessel that carries its own nuclear fuel supply and exhausts the reaction products for thrust, much like a conventional fusion rocket. However, this Ram Augmented Interstellar Ramjet, or RAIR as Alan Bond of the British Aircraft Corporation calls his device, enhances its performance by scooping up atoms from the interstellar medium and using them as reaction mass rather than for energy generation.¹⁴⁵⁵

In other words, fusion fuel is carried by the spaceship and additional reaction mass is collected from gas clouds through which the vessel passes. Preliminary calculations show that the RAIR design may save at least an order of magnitude of fuel savings at speeds up to 50%c, and as much as two orders of magnitude of fuel savings up to 70%c. Performance characteristics of RAIR starships have been worked out by Bond¹⁴⁵⁵ and Powell.^{1117,1115,2769}

Two other fascinating variations on the interstellar ramjet have been proposed by Whitmire and A.A. Jackson IV.²⁷³³ The first of these is called the Fusion Ramjet Runway. Micron-sized frozen deuterium pellets are accelerated electrostatically or electromagnetically out into space several years prior to the launching of a standard Bussard ramjet having a comparatively small scoop cross-section (perhaps it would be just a simple physical structure, such as a giant funnel). The starrammer could then collect a more concentrated fuel en route simply by staying on the "runway."

The other suggestion, rather bizarre and considerably less likely, is the Stellar Ramjet. This vehicle accelerates up to near-optic velocity across the photosphere of a star. Whitmire and Jackson propose that the envelope of a red giant or a large protostar would be ideal for this technique. Accelerative forces would be large but not prohibitive, and biological crews should survive if they are somehow immobilized or "frozen" during the starship’s relatively brief period of acceleration.