Advanced Space Propulsion - DAEDALUS, ORION, and Others

by Louis E. Della Torre, Jr.

ORION was the name given to a proposal put forward during the 1960s for a very large vehicle using pulsed nuclear propulsion. In lay person's terms "pulsed nuclear propulsion" involves exploding nuclear warheads immediately astern of the vehicle. The force of the blast would expend itself against a pusher plate, driving the vehicle forward. At first blush this seems totally impractical. However, studies conducted at the time indicated that the concept was, from an engineering standpoint, feasible. (Although some very big numbers were involved.)

ORION was touted in some quarters as an interstellar vehicle. The consensus, however, seems to be that it was not practical for this purpose. On the other hand, what ORION *could* have done was to move very large payloads around the solar system relatively quickly.

First, as to the legal aspects: Glenn Reynolds was correct in stating in message #122072 that the only international legal obstacle to an is the Nuclear Test Ban Treaty. Just why that treaty doesn't exempt peaceful nuclear explosions is beyond me. It could be due to verification problems, i.e.-it's difficult to determine the purpose of a nuclear explosion without on-site inspection, which is not provided for in the Test Ban Treaty. However, I suspect that the main reason was that at the time the Treaty was negotiated (1963) no one thought of the possibility of peaceful nuclear explosions anywhere but underground. Since the Test Ban Treaty exempts *all* underground explosions, there was thought to be no need for such an exemption in the treaty. Then when ORION came along----

By way of a digression, this is not the only or even worst instance of the U.S. State Department blithely negotiating treaties which have come back to haunt us in space-related areas. Indeed, someone once commented that the problem with the State Department as an institution is that it has a tendency to forget its job is to represent the interests of the United States in its dealings with foreign governments rather than the other way round. To put it another way, the denizens of Foggy Bottom need to be constantly reminded that in their business enlightened selfishness rather than disinterested altruism should be the ruling principle.

The most serious example of this is, of course, the effect of the ABM Treaty on SDI development. Not to put too fine a point on it, sooner or later (and probably sooner rather than later) we're going to have to choose between abandoning SDI and renouncing the ABM Treaty. Another example is the Moon Treaty, with its potential for a Third World veto/ripoff of commercial exploitation of extraterrestrial materials.

Yet another example which might be cited is in the rather arcane area of radio frequency allocation. Third World nations probably have a legitimate interest in insisting that a certain portion of radio frequencies and communications satellite positions be reserved for them. Otherwise the developed countries may occupy all the usable portions of the electromagnetic spectrum. Then when less developed countries are finally in a position to, for example, put up their own communications satellites, it will be a case of "no room at the inn."

However, these international agreements have been structured in such a way as to give the Third World a veto over the microwave transmission of energy generated by a solar power satellite (SPS). I invite readers to contemplate as a "for instance" what the OPEC nations might do if development of the SPS coincided with another oil shortage.

As a final example, it seems abysmally stupid that the INF Treaty does not permit Pershing missiles to be "destroyed" by expending then in space launches. Of course, the Pershing probably couldn't put anything into orbit by itself. However, from my admittedly non-expert perspective it doesn't seem that there would be any insuperable problems in mating it with some other off-the-shelf components as an upper or lower stage. At a very minimum, it would seem possible to cluster Pershings around some other expendable launch vehicle (ELV) as solid rocket boosters (SRBs) thereby increasing that ELV's payload. If you want to get really exotic, the February/March 1989 issue of "Air & Space Smithsonian" mentions a scheme to replace the first stage of the Scout launch vehicle "with an electromagnetic catapult. This would shoot the rocket along six miles of rails built up the western slope of a Sierra-Nevada mountain such as Mount Hood or Mount Shasta." I suspect Pershings could have been launched in this fashion as well.

However it might have been done, since there are several hundred Pershings which have to be disposed of, this would seem to have been a cost-effective program. Also, since U.S. space launches are generally public, verification problems would seem to be minimal. One hopes that a similar blunder will not be made if and when a START Treaty is negotiated.

Treaty restraints aside, the major problem with ORION is that it would use up fissionable material at a prodigious rate. In fact one variant of the concept would have used the world's entire supply of nuclear weapons to power *one* ORION vehicle. Of course, there are ways of turning this drawback into an advantage. Apropos of what I mentioned previously, in the unlikely event that the U.S, the U.S.S.R. and what might be called the "minor nuclear powers" (if any nation possessing nuclear weapons can ever be called a minor power) ever *do* agree on complete nuclear disarmament, ORION might be a practical way of getting rid of no-longer-wanted warheads. One problem might be which nation would get to operate the vehicle. Perhaps, however, there could be one for each nation. "Rigel" for the United States and "Betelgeuse" for the Soviet Union. A more practical objection would be that an ORION vehicle would necessarily be at least semi-expendable. When you run out of nukes, you've run out of propellant. (Then too, Murphy's Law being what it is, we will just have finished using up all our nuclear weapons in this fashion when some unfriendly ETs wander into the solar system.)

Finally, contrary to suggestions made in the thread on Space Forum, I do not believe ORION was ever intended to launch payloads from Earth to LEO. It was supposed to be strictly an orbital transfer vehicle (OTV).

Also mentioned in the Space Forum thread was DAEDALUS. Unlike ORION, DAEDALUS was a concept for a true interstellar vehicle. Put forward by the British Interplanetary Society (BIS), it envisioned an unmanned 2-stage rocket which would be used for a flyby mission to Barnard's Star. (Barnard's Star was selected in preference to the Alpha Centauri star system because it was considered more likely to have planets. Each stage would burn for several years and the combined effect of both stages would be to accelerate the vehicle to a substantial fraction of the speed of light. The propulsion system would have used nuclear fusion. However, I'm not sure if it would have involved pulsed fusion (i.e.--small thermonuclear explosions) or some kind of continuous output system.

There are a couple of problems with this concept. There would be no deceleration at the target star, so the vehicle would obviously pass by it very quickly. In fact, about all DAEDALUS could have told us is whether Barnard's Star did or didn't have planets. Also, the propulsion concept involved would have used helium-3, which is rather rare in this neck of the solar system. It exists on Earth, but essentially only in trace amounts. There is some on the Moon, but nowhere near enough.

In fact, the nearest (and perhaps only known) place where it exists in sufficient quantity is the atmosphere of Jupiter. The scenario therefore called for DAEDALUS to be built in orbit around that planet. (Just why this was preferable to building the vehicle in Earth orbit and towing it out to Jovian orbit for fueling is something I've never seen explained.)

The biggest objection that I can see to DAEDALUS is that it would entail too great an investment for a single one-way space probe. However, the propulsion system concept might find other applications. For example, DAEDALUS like ORION, could represent an excellent means of moving very large payloads around the solar system relatively quickly. For example, since the fuel source is Jupiter's atmosphere, DAEDALUS might represent an excellent concept for moving mineral-rich asteroids for from the main belt to an orbit around Earth where they could be mined. (I believe it was G. Harry Stine who pointed out that a single nickel-iron asteroid, one mile in diameter would contain the equivalent of a couple of centuries of world nickel and iron production, at mid-1970s rates.)

Another advantage of DAEDALUS over ORION would be fuel supply. I don't know how much helium-3 is contained in the Jovian atmosphere. However, since just about everything about Jupiter is on a Brobdingnagian scale, I suspect there's quite a bit of it. In particular, there's probably enough to keep a fleet of very high capability (both in terms of speed and payload) orbital transfer vehicles (OTVs) running for as long as might be necessary or desirable. As a bonus, once the infrastructure to support such vehicles was in place, building one or more interstellar probes might become practical on a spinoff basis since the additional investment required would come down to acceptable levels.

Passing on to what Roy Pettis referred to in message #121995 as "reactor-based nuclear propulsion," there were, as I recall two concepts which, like ORION, were put forward during the 1960s. One was, of course, NERVA. To answer the question posed by Robert Mockan in message #122023, I believe the other concept was called DUMBO. DUMBO's performance would have been superior to that of NERVA. Supposedly, DUMBO could have powered a single-stage-to-orbit (SSTO) vehicle, which NERVA could not. One the other hand, the impression I got was that NERVA would have used more-or-less "off the shelf" technology, whereas DUMBO would have entailed pushing the technology to "the outside of the envelope."

However, Roy Pettis was incorrect when he stated in message #121995 that "reactor-based nuclear rockets put out nothing but clean hydrogen flames." At least DUMBO (and probably NERVA as well) would have released some radiation into the atmosphere. Admittedly, the amount would have been relatively minor. The estimate I remember seeing for DUMBO was that it could have orbited 1,000 SKYLAB sized payloads for a radiation release equal to that caused by one 20-kiloton atmospheric nuclear test. However, even 20 years ago the intentional release of *any* nuclear radiation into the atmosphere was politically intolerable. (It goes without saying that, in the wake of Three Mile Island and Chernobyl, the situation is even worse today.) I suspect that this "zero tolerance" for radiation release was as much if not more of a factor in the demise of research in this area as was Richard Nixon's perception that "we weren't going to Mars anytime soon" and that therefore "the nuclear rocket was a luxury research program."

As somewhat of another digression, I do not believe that "the SDI technology program is * * * recapturing some of the nuclear propulsion expertise," as stated by Roy Pettis in message #122109, at least not directly. As most Space Forum members know, I follow SDI pretty closely. As far as I know, nuclear propulsion does not figure in any aspect of the program. What SDI *is* doing is sponsoring some fairly intense research into large-output, long-duration, on-orbit power sources. As a practical matter that means nuclear reactors. One such project is designated as the SP-100 reactor. "SP" presumably stands for "space power." "100" refers to the output, but I'm not sure whether that is intended to indicate 100 kilowatts or 100 megawatts. (I'm also not sure whether the SP-100 is being developed solely for SDI or is considered to have wider potential.). In any event, of course, the reactor technology developed as part of the SP-100 or other SDI-related programs would probably be adaptable to propulsion systems, at least for OTVs. This may have been what Roy Pettis had in mind.

While I certainly agree that there is a desperate need for more efficient propulsion systems, both for Earth-to-LEO launch vehicles and for OTVs, I don't see nuclear rockets, whether pulsed or reactor based, fulfilling either of these roles. Nor do I see a nuclear fusion powered vehicle like DAEDALUS being used as an OTV.

In the case of OTVs, I think that fusion propulsion systems, pulsed nuclear propulsion systems and probably reactor based nuclear propulsion systems as well are going to be leapfrogged by antimatter propulsion systems. These are not as futuristic as most people think. The U.S. Air Force has identified antimatter propulsion as one of a number of critical long-range projects in which it is interested. Other reports suggest that antimatter propulsion may become practical as soon as the early years of the Twenty-first Century.

Furthermore, recent breakthroughs in the field of superconductivity may have made one technological problem related to an antimatter propulsion system easier to solve. (Very powerful magnets are required to confine or direct the particles resulting from matter-antimatter "annihilation" so that their energy can be harnessed for propulsive purposes. Shielding is of course also a problem, especially for a man-rated OTV. However, it is a problem which (as far as I know) differs only in degree from nuclear propulsion systems. Moreover, once we gain access to extraterrestrial materials, we have effectively gained access to unlimited amounts of mass. And as Jerry Pournelle has pointed out, mass is the best way to harden or shield anything.

The principal problem delaying the development of antimatter propulsion is our ability to produce and store sufficient quantities of antimatter. However, our capabilities in this regard have been increasing at a fairly steady rate, and only milligrams, or at most grams, are necessary to power a highly capable OTV. Hopefully too, the proposed super-conducting super-collider will, in addition to advancing knowledge in other areas, tell us something about how to manufacture and store antimatter.

Antimatter is obviously tricky to handle. It only has to come into contact with normal matter for an explosion to occur. Needless to say, this complicates storing it. The usual method is to suspend it magnetically in a vacuum. However, contrary to what might be expected, and for certain technical reasons which I do not profess to fully understand, the larger the amount of antimatter, the easier it is to store. Also relevant here is the fact that space gives you plenty of room to do nasty things. Manufacture and storage of antimatter can take place on-orbit, on the Lunar surface or at some other remote location. If something goes "bang," therefore "third party off-site damage" (to use the nuclear power industry's euphemism) will be minimal or non- existent.

Of course, as is frequently the case, the most likely potential "show-stopper" for antimatter propulsion is political. An antimatter warhead would make nuclear or even thermonuclear weapons look like firecrackers alongside a hand grenade. In actuality, this is not as serious as it sounds. Stability has been an essential requirement for military explosives all the way back to the days of black powder. For that reason nitroglycerin was never used in warfare and dynamite was used only more-or-less experimentally. (A "pneumatic dynamite gun" saw limited service in the Spanish-American War.) Combined with stability problems is the fact that the only advantage an antimatter warhead would have over a nuclear warhead would be greatly increased yield for its size. However, the recent trend has been towards lower yield, not higher yield warheads. Likewise, the physical size and weight of the warhead has long since ceased to be a significant problem in the design of nuclear weapons. For all of these reasons it is unlikely that antimatter warheads will ever be developed.

Lack of stability will also tend to make antimatter unattractive to terrorist organizations. Even the most fanatically suicidal terrorists, after all, want to blow up somebody *in addition to* themselves. The armed forces of a sovereign nation, especially a superpower, can, if they want to, afford to expend a great deal of time, effort and money to develop technology to keep antimatter stable under extreme conditions. (Although for the reasons previously stated, they are not likely to want to.) The resources to do so are not likely to be available to terrorist groups. Finally, if the manufacture and storage of antimatter takes place at an extraterrestrial location, then securing the site against terrorist incursions becomes relatively simple.

As always in the realm of politics, however, what seems to be is more important than what is. Perception in other words tend to override reality. If the numerous and vocal technophobes with whom our society is currently plagued can convince the public that antimatter presents unacceptable risks, then whether as a matter of objective fact such is or is not the case essentially becomes irrelevant.

On the question of Earth-to-LEO launch vehicles, as I previously mentioned, a necessary precondition for either a nuclear or fusion rocket is that it emit zero radiation into the atmosphere. A related problem is that the reactor has to be designed so as not to release radiation even in the event of a "worst case" launch or reentry accident. The latter problem might be soluble (although again, perception, and not just reality, has to be taken into account). However, I cannot see how the former could be done with a nuclear powered vehicle. A fusion powered launch vehicle might be practical in the very long term, depending on things like shielding requirements and how the energy of the fusion reactor is converted into thrust. Obviously, though, such a vehicle is a long way in the future.

For the foreseeable future, therefore, improvements in launch systems are apt to come from technologies other than nuclear or fusion propulsion. One possibility is the development of rockets using more energetic fuels. Metastable helium and tetrahydrogen (metallic hydrogen) have been mentioned as possibilities. Monatomic hydrogen, although a long time favorite with science fiction writers, does not appear to be given much of a chance of becoming a practical propellant. Be that as it may, any of these fuels would offer orders-of-magnitude improvements over present rocket propulsion.

However, while possible these advances do not appear likely in what might be called the "medium term." ("Medium term" in this case meaning between now and, say, the second or third decade of the next century.) During that period the most likely possibility of major improvements in launch systems involve techniques that will get around the necessity of having to carry all the energy needed to accelerate to orbital velocity aboard the vehicle itself. The system that is currently receiving the most attention in this regard is, of course, the National Aerospace Plane (NASP). Whereas the Space Shuttle must carry both liquid oxygen and liquid hydrogen in its external tank, the NASP's on-board tankage will accommodate only hydrogen. Oxygen will be drawn from the atmosphere. The NASP will also get a bonus from the fact that its engines will use gaseous rather than liquid hydrogen. Using air to burn liquid hydrogen produces several times as much thrust per pound of fuel burned as using liquid oxygen to burn liquid hydrogen.

Another possibility involves laser propulsion. A powerful ground based laser would be used to "illuminate" the launch vehicle. The propellant would probably consist of water, which the heat from the laser would turn to steam. As far as I know, this is not being actively pursued at the moment. However, it is an obvious potential spinoff from the SDI program.

A third type of launch vehicle propulsion system is the electromagnetic accelerator or "mass driver." While primarily discussed as a method launching payloads from the Moon, it is also adaptable to Earth to LEO transportation. The most extreme concept I've seen was presented as poster session at the Princeton Conference on Space Manufacturing either two or four years ago. This was dubbed the "Yokohama, Honolulu & Orbital Railroad." It involved another science-fiction favorite, the "vacuum tube." A airless tunnel would be constructed under the Pacific Ocean. The primary purpose would be to carry passengers and freight between Japan and the U.S. mainland, via Hawaii. One "branch line," however, would exit through the crater of a Hawaiian volcano. The launch vehicles would have no on-board power, but would be accelerated to such speed in the tunnel that they could "coast" into orbit.

At the other end of the scale, as proposals for mass-driver propulsion go, is something called "Electro-Scout," which I mentioned previously in connection with the disposal of Pershing missiles. Put forward in 1981 by six MIT scientists, it envisioned constructing a six mile long electromagnetic catapult on the Western slope of a Sierra-Nevada mountain such as Mount Hood or Mount Shasta. This would replace the first stage booster of the Scout launch vehicle. (Hence the name.)

However, the most promising application for a mass driver launch system would seem to be in combination with either a NASP derived vehicle or a laser propulsion system.

In the case of the NASP, its primary propulsion system will consist or supersonic-combustion-ramjet ("scramjet") engines. As the name implies, these engines can only operate when the airflow though them reaches supersonic velocities. The NASP will therefore need an auxiliary propulsion system to accelerate it to a speed at which a scramjet can operate. Current thinking seems inclined towards doing this by a combination of turbojet and conventional (subsonic combustion) ramjet engines. This may reflect the fact that NASP derived vehicles have potential for military applications (including launching military payloads), and as hypersonic transport (HST) civilian airliners, as well as civilian launch vehicles. For obvious reasons, both the Department of Defense (DoD) and the airlines want something that can simply take off from any airport runway of sufficient length.

However, such a capability would be of limited use for a civilian space launch vehicle. Moreover, the need to have two or three sets of engines instead of only one would obviously both increase the complexity of the system and reduce payload. Hence, the "elegant solution" for a civilian launch vehicle would seem to be a mass diver, which would accelerate the vehicle to scramjet speed before it leaves the ground.

In the case of laser propulsion, including a mass driver in the system goes from being highly desirable to being a virtual necessity. Some means has to be found of getting the payload up in the air so that the laser can be focused on it. Related to this is the fact that a laser works best if installed at the summit of a high mountain where most of the atmosphere, and especially most of the atmosphere's water vapor, are below it. This lends itself very readily to something similar to the Electro-Scout proposal. In effect, the first stage would be the mass driver and the second stage would be the laser.

Summing it all up, the proverbial "bottom line" is this:

There clearly exists a great need for improved forms of propulsion, both for launch vehicles (to reduce the cost-per- pound of Earth-to-LEO transportation) and for OTVs (to enable us to move large payloads around the solar system relatively quickly). However, it does not seem that either pulsed nuclear power vehicles like ORION or nuclear-reactor powered vehicles like NERVA or DUMBO or even fusion powered systems like DAEDALUS represent the likely answer.

In the case of launch vehicles, the release of radiation into the atmosphere makes nuclear powered systems, if nothing else, politically impossible. If a fusion powered propulsion system can be devised that does not entail releasing radiation into the atmosphere, it may become the basis of a practical launch system. However, such a vehicle is clearly a long way off.

As to OTVs, nuclear reactor powered vehicles such as DUMBO and NERVA are almost certainly practical. Assuming the Nuclear Test Ban Treaty can be amended and a sufficient supply a fissionable material is available, so is ORION. Once we have the ability to construct the necessary infrastructure in orbit around Jupiter, so is DAEDALUS. The problem with both nuclear and fusion powered OTVs, however, is that when we are in a position to build them they may represent an idea who's time has come--and passed. Antimatter propulsion systems may make them obsolete before the first one flys. The more advanced the system being considered the more the force of this point increases.

For example, it is probably hyperbole to say that DUMBO or even NERVA could be built using off-the-shelf technology. Nevertheless, a considerable body of knowledge obviously exists with respect to the design of nuclear reactors in general, and at least the preliminary work on nuclear rockets has already been done. A nuclear-reactor propelled OTV would therefore clearly be the system of choice if, for example, a manned Mars mission were to be mounted anytime soon. The same is probably true with respect to establishing and maintaining a permanently manned Lunar base. The problem is that neither or those missions *are* going to be undertaken anytime soon. For one thing, the political will (and hence the money) simply isn't there. In the case of a Mars mission, moreover, we need to learn a great deal more about such things as long-term, closed cycle life support systems and highly reliable fail-safe systems in general before such an expedition can be mounted.

Fusion powered OTVs are even more questionable. An major milestone in the development of fusion power is the demonstration of "zero net." That is, producing a fusion reaction that generates as much power as it consumes. Ten years ago it was being predicted that zero net would be demonstrated sometime in the early 1980s. Current predictions are that it will be demonstrated in the early 1990s. And, of course, zero net is a long way from a practical stationary fusion powerplant, much less a practical OTV propulsion system. The likelihood, therefore, is that by the time we really need a highly capable OTV antimatter propulsion system will be a practical option.