augeri@gwen.DEC (Mike Augeri) (09/10/85)
There is an interesting article in the September 1985 issue of Space World, the magazine published in cooperation with the National Space Institute. The article is "Propulsion Future" (pp. 17-19). It talks about past, present and future fuels for rocket propulsion. The article does not provide any equations to support the claims that it makes, and in some cases, it makes statements without any supporting data. In spite of these shortcomings, I thought the article was a good summary. I have some doubts about some of the statements in the article, but since I am not a qualified critic, I leave this to our readers. The article says that "[r]ocket performance is measured in many ways", but the best measure of the fuel efficiency is specific impulse, usually written as I(subscript sp) and has units of seconds. It says that thrust depends more on the design of the engine whereas specific impulse depends more on the energy content of the fuel and the pressure and temperature conditions under which it is used. Many fuel formulas have been tried over the years, but the best and safest performance to date is with a fuel first proposed by Konstantin Tsiolkovsky way back in 1903 -- namely, liquid oxygen and liquid hydrogen. This fuel combination produces a specific impulse of 400-450. The next best is hydrogen and fluorine at 480, but fluorine is a very difficult substance to handle. The problem with these fuels is that they are impractical if what you want to do is plan a manned mission to the outer solar system or to the stars. Many people are familiar with the NERVA (Nuclear Engine for Rocket Vehicle Application) program back in the 1960s. The NERVA program developed the "fission solid core" nuclear rocket. This rocket used hydrogen as a reaction mass and a uranium fueled reactor. The reactor was operated at a temperature as hot as possible without producing a meltdown. Liquid hydrogen was pumped into a jacket surrounding the reactor core where it was heated to a gas. The gas then flowed through holes in the reactor core to collect more heat, and emerged as a very hot gas that was expelled through an exhaust nozzle. The NERVA engine was first operated under full power in 1966 and produced a specific impulse of 850. Because of the massive weight of the engine and the amount of shielding it would have required, it was believed that the maximum specific impulse that could have been achieved in a practical engine would have been about 650. The NERVA program died around 1970 with the massive cutbacks in all space-related programs. Since then some theoretical work has been done on a "fission gas core" design. No information was given about this design other than saying that it used gaseous fuels. The article claims that such an engine "could make Mars in about 30 days with a five man crew." But a rocket of this type has never been developed and due to reasons that are said to be complex, some experts say it never will be developed. Others are pinning their hopes on fusion engines. It is claimed that a fusion reaction can liberate from 3 to 5 times the amount of energy liberated by a fission reaction per unit mass. Two major problems exist: first, we have not yet achieved a sustained, controlled fusion reaction, and second, we have not perfected a means to utilize fusion reactions in a rocket. However, in 1972 a couple of people at Lawrence Livermore Labs described a scheme that would burn small deuterium-tritium pellets by heating them with a laser beam. The pellets are injected into a thrust chamber at the rate of 500 per second where they are hit by laser pulse of one billionth of a second duration. The theoretical performance of this engine is an incredible specific impulse of 2,640,000. No one thinks that we could build such a perfect engine, but they do think that we could build an engine with a specific impulse of 1,000,000. Such an engine would revolutionize rocket travel. "A single stage rocket with a fuel to mass ratio of one to twenty ... could reach one tenth the speed of light. At full howl, Pluto would only be five days away, but allowing time for getting up to speed and slowing down at the destination, the real mission time might be more like three weeks." However, even this kind of performance is inadequate when you consider the requirements for an interstellar voyage. Assuming we could carry enough fuel (say 50,000 tons of helium-3 and deuterium), it would take about 50 years to reach the nearest star. Other propulsion systems we hear about are solar sails and ion engines. Both systems have been tested in space, at least in principle, and they do work. However, their acceleration is slow and they are not too practical for manned missions. Beyond the fusion engine, solar sails, and ion engines, we have a big question mark. To dream about interstellar travel is one thing -- to develop an engine to actually do it is another. "To launch a one-pound payload to one quarter the speed of light with an engine with [specific impulse] of 2000 would require a fuel load far greater than the mass of the universe. More efficient powerful engines is one answer; free fuel is another." One idea for a rocket engine is analogous to the atmospheric ramjet. The idea is that a rocket would travel through space and scoop up the interstellar hydrogen using a magnetic-field scoop generated aboard the rocket. Its a great idea, but many people doubt the feasibility of such an engine. "Energy losses involved in producing the fields seem likely to cancel out any net gain in velocity" and scoop sizes of "one million kilometers to half a light year in diameter" have aroused grave doubts. So what's left? "If one pound of fuel could be converted entirely into an exhaust beam, the result would be five billion times the energy released per unit mass in the best chemical rocket." But we all know that 100% efficiency is impossible to achieve. A close approximation is the ultimate in propulsion systems: the matter-antimatter engine. Such an engine would be about ten times more efficient than the deuterium-tritium fusion engine. For a quick trip to Mars "a 1000 ton vehicle using 4000 tons of water" for a reaction mass, heated by the matter-antimatter reaction, would require "about a gram of antimatter." However, there are some significant problems associated with using such an engine. First, about "half the [matter-antimatter] reaction is gamma ray radiation plus electrons and positrons, and we don't have any idea how to focus a gamma-ray exhaust beam." "Furthermore, the other half of the reaction is in neutrino form, and neutrinos can penetrate anything (including any shielding we can think of, and astronauts' bodies). Neutrinos also refuse for the most part to be directed by electric, magnetic, or any other sort of fields, so they are hard to get rid of." Aside from these problems we have the problem of producing "enough antimatter to power a spaceship. At present, the world's supply of antimatter is a few thousand antiprotons stored for a few days." The problem here is to make it cheap enough, make enough of it, and figure out how to store it. If we can figure out how to solve the problems of the matter-antimatter engine, maybe someday we can make it to the stars. Mike Augeri (DEC, Maynard Massachusetts)
rdp@teddy.UUCP (09/10/85)
In article <384@decwrl.UUCP> augeri@gwen.DEC (Mike Augeri) writes: > >Many fuel formulas have been tried over the years, but the best and safest >performance to date is with a fuel first proposed by Konstantin >Tsiolkovsky way back in 1903 -- namely, liquid oxygen and liquid hydrogen. >This fuel combination produces a specific impulse of 400-450. The next >best is hydrogen and fluorine at 480, but fluorine is a very difficult >substance to handle. > To say nothing of the fact that the resultant combustion product is good old hydroflouric acid, which is really grim stuff! There have been some reports of irritation resulting from the shuttle solid-fuel exhaust. This would preety much preclude in-atmosphere use of flourine.
king@kestrel.ARPA (09/10/85)
In article <384@decwrl.UUCP>, augeri@gwen.DEC (Mike Augeri) writes: > > ... > > "Furthermore, the other half of the reaction is in neutrino form, and > neutrinos can penetrate anything (including any shielding we can think of, > and astronauts' bodies). Neutrinos also refuse for the most part to be > directed by electric, magnetic, or any other sort of fields, so they are > hard to get rid of." > I certainly agree that neutrinos represent lost energy, but not a shielding problem. The particles will pass thru the astronauts' bodies, the Earth, and even the Sun without leaving behind any effects.
davidson@sdcsvax.UUCP (Greg Davidson) (09/12/85)
Let me add a couple of things to Mike Augeri's account of spacecraft propulsion methods (I won't say rocket, since not all the methods he talked about use rockets). First of all, he mentioned that light sails are too slow for interstellar propulsion, and aren't suitable for manned spacecraft. Not so! Although their acceleration might well be very low for massive manned spacecraft, they have the best performance for interstellar misions of all the systems he mentioned. Light sails have such excellent performance because they don't have to carry their fuel. With solar pumped lasers to keep them going, a manned light sail spacecraft can reach destinations 40 light years away, and reach a cruising speed of 1/3 light speed. Large manned spacecraft might have very low accelerations, but could reach cruising speed after several months or a few years. Unmanned untralight probes using light or microwave sails can have accelerations of 1000's of g's. Robert Forward has done much of the work on this and used the concept in his recent novel Rocheworld. Second topic. There is another method for obtaining total conversion of matter to energy, and not in the unusable forms of gamma rays and neutrinos. It does not need any dangerous and hard to make anti-matter. It needs a less dangerous, but MUCH harder to make mini-black hole. I don't know who first thought this up, but Clarke used this idea in Imperial Earth. You use a heavily charged black hole so you can hold onto it. You can dribble matter into it in such a way that most of the energy of the matter is turned to energy through friction. Only a tiny bit gets in past the event horizon. Assuming that there is nothing wrong with current black hole theory, I see no scientific barrier to this form of transportation. If we can't find any such black hole, we will eventually be able to make one (see next paragraph). And given one, I understand that you can make more fairly easily. Clarke envisaged using two very long (space borne) opposed mass drivers to create mini-black holes. Does anyone know how feasible this is? My background assumption is that in a few hundred years (maybe much sooner) we will be able to construct true von Neuman machines; that is, self reproducing automatic factories. Given space based von Neuman machines using solar energy and asteroidal matter I see no barrier to attacking really huge construction projects. So its really not a problem if you want a solar sail thousands of kilometers wide, with a bank of lasers big enough to drive it. Its also not a problem to construct mass drivers tens of thousands of kilometers long. Either of the above methods of propulsion should give us the stars! Ad astra, _Greg Davidson Virtual Infinity Systems, San Diego
henry@utzoo.UUCP (Henry Spencer) (09/12/85)
I haven't read the Space World article yet, but its author evidently
doesn't know much about antimatter propulsion. The following is mostly
from talks given by Robert Forward, who is (among other things) a
consultant on advanced space propulsion to the USAF. Papers (some his)
on antimatter propulsion in the Journal of the British Interplanetary
Society are also worth reading.
Matter/antimatter reactions do *not* immediately yield gamma rays and
neutrinos. A proton plus an antiproton yields a spray of particles,
all unstable, but mostly charged. They will eventually decay into
neutrinos and other particles, whose reaction will eventually yield
gamma rays. But the lifetime of the charged particles is amply long
enough to use a magnetic nozzle to collimate them into an exhaust jet.
Gamma-ray emission will still be substantial, but the nozzle problem
is manageable.
Production of antimatter appears to be a straightforward although very
expensive procedure. There are plenty of apparently-viable approaches
to handling it, although a lot of development work would be needed on
the details. Existing technology, suitably applied, appears adequate.
Costs are uncertain. It seems quite likely that it can be brought down
to a few tens of millions of dollars per milligram, given a large
dedicated production facility. If this sounds rather high to you, then
consider:
Antimatter at $50M/mg is cost-competitive for in-space
propulsion with H2/O2 lifted from the ground.
At $20M/mg, antimatter is cost-competitive with fission rockets.
At $10M/mg, antimatter is cost-competitive with fusion rockets.
Forward's work is being taken very seriously. I'm told (this is not
directly from him) that there is a symposium in the works on the subject
of building a "National Facility for Low-Energy Antimatter". That is,
a prototype antimatter factory and antimatter-handling-techniques lab.
This one's for real, folks. I very much doubt that anyone is ever going
to bother building fission rockets now, and I wouldn't spend a lot of money
on shares in a fusion-rocket company.
--
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henryfranka@mmintl.UUCP (Frank Adams) (09/14/85)
[Not food] While not disputing any of the specific conclusions in the original article (concerning spaceship propulsion systems), I think it is overly pessimistic about the possibility of interstellar flight. While none of the proposed systems is sure to work, I think collectively they produce a near certainty for this development. The anti-matter drive is probably the most likely to be possible. While we don't now have the technology to produce anti-matter at a reasonable price, it seems likely to me that this will be developed, in one way or another. I would rate this technology better than a 50/50 chance. The economics for ramscoops look much better if you assume there are areas of higher density than average, where you can "refuel". As a worst case, you can fly by other stars, so that the typical distance between stars is the limiting case. On the other hand, there may well be sub-stellar bodies scattered around the galaxy, which would presumably have large atmospheres. Light sails using ground-based lasers are another good prospect. You have to set up an installation at the other end if you want to get back, of course. Finally, there may be possibilities which haven't been thought of yet, perhaps because the basic science required isn't known yet. In short, I would conclude that we don't yet know how to build an effective interstellar spaceship, but that being able to do so is only a matter of time. Frank Adams ihpn4!philabs!pwa-b!mmintl!franka Multimate International 52 Oakland Ave North E. Hartford, CT 06108