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Liquid and solid chemical fuels in rockets are very expensive and inefficient. I have heard of solar sails but what are the most realistic space travel fuels that will be used in the future to get close to the speed of light?

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For what it's worth the fuels themselves are pretty cheap. – Daniel Chisholm Mar 5 '11 at 9:36
up vote 8 down vote accepted

In a lot of ways this is a technology---rather than physics---question, but lets look at some limits imposed by physics.

  • For rockets there are two numbers that matter: the velocity relative the spacecraft with which the fuel can be expelled (called the specific impulse) and the fraction of the original mass that is fuel. For very high mass fractions the spacecraft can have delta V equal to several times the specific impulse. That means that if you plan to stop at the other end your maximum velocity perhaps twice is the specific impulse. If you need to come home without refueling it is around the specific impulse.

That's the killer for fast burn rockets. We still use them for launch from the surface because they can develop the necessary thrust (our high efficiency engines (ion drives and the like) are all low thrust at the present time).

That suggests one of two strategies:

  1. Leave your engine behind. This is the mechanism used by laser sails, star wisps, etc.
  2. Pick up your fuel as you go. The strategy of ram-scoops.

Sail technology is--at least in principle--within reach of our current competence, but building and running one capable of efficient interstellar travel is beyond the current Gross Planetary Product. Star wisps are a subset of sails, but require considerable expertise in nanotechnology before they are useful even for exploration.

A second disadvantage of sails for traveling to other start in person, is that you're counting on politicians and policy makers you left behind to continue funding your trip...

Ram-scoops are a big unknown, as we don't know how to build them as yet. A BOTE calculation for a minimal, naive hydrogen fusion ram-scoop limits their top speed to about 14% of c (under the maximum drag assumption that the fuel must be brought to rest relative the craft).

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I found a nice article about ramscoops here: Thanks for the answer! – Cameron Feb 12 '11 at 21:02
I don't understand the 2x specific impulse limit. If your ship started out 99% fuel, shouldn't the max delta V then be -ln(1-.99). Of course your total mass id decreasing exponetially. But in any case, even for 99% fuel, you only get 4.6times. – Omega Centauri Feb 13 '11 at 0:21
@Omega I simply misremembered the form of the rocket equation. That's what I get for writing by the seat of my pants. – dmckee Feb 13 '11 at 1:43
dmckee: Its not that far off, restrict the mass ratio to e squared, and you would get 2! Unless you do something like store the fuel as an external block of ice getting a very high ratio of fuel mass to payload mass is probably unlikely. In any case the logarithmic scaling means you can't get very far. – Omega Centauri Feb 13 '11 at 4:49

I wrote a semi-popular book on this subject a couple of years ago:

A lot of this was worked out. I illustrate the relativistic rocket and the photon sail. The photon sail is clearly the most reasonable of these two. I also suggest using electromagnetically accelerated nano-probes, where millions of tiny probes are sent towards another star, with the prospect that a few might set up shop there and build up a robotic colony of sorts. The solar sail is capable of reaching a $\gamma~=~1.4$, or .$69c$, though a more realistic figure is $\gamma~=~1.15$ or about $v~=~.5c$. This analysis involve the redshifting of light on the frame of the sail craft and so forth. To accomplish this requires placing large Fresnel lenses in space which collimate light towards the sail.

There is not much point in sending probes towards extrasolar systems faster than $\gamma~=~2$, or $v~=~.86c$. The time saved becomes smaller and the energy costs enormous. A $\gamma~=~2$ probe has as much kinetic energy as its rest mass, which would be a considerable feat to achieve. At $.5c,~.7c$, and $.86c$ probes could be send within the interstellar neighborhood and signals returned in reasonable time.

The ramjet concept is useful IMO from one main reason. Suppose various instruments were packed onto a spacecraft with an acceleration of one gravity, or larger. These instruments look at the CMB and maybe particle detectors. The machine reaches a high $\gamma$ in order to probe the universe as it appeared earlier on. The CMB would be blue shifted into the IR, and maybe other detectors which can detect weakly interacting particles which pass through the early radiation dominated period of the universe. This is pretty speculative, but maybe not impossible in principle.

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The most realistic way is to get rid of humans, and let robots travel. They don't eat and can spend 1'000'000 years in flight without being bored too much. This way is achievable even with current tech. ;-)

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If the universe would be this much known that you wouldn't need or WANT a human around to discover and process the unknown ... then we shouldn't bother with exploring at all. – Leonidas Feb 13 '11 at 2:48
Such comments of the "down to reality" kind are disliked here. Some dream of manned interstellar flight! :=( – Georg Feb 13 '11 at 12:17

In order to make space travel truly affordable, the only realistic way is using thermonuclear explosions. The technology is relatively cheap and available, but it is dangerous, and must be strictly controlled by large stable governments. This puts it out of reach of commercial development, and there are well known issues with government controlled projects regarding efficiency. This makes it implausible that we will see such a thing realized soon or realized well. Since this is a problem of human political organization, not of technology, I won't discuss it further.

Ignoring politics and economics, the orion project proposed a ship propelled by thermonuclear explosion products, which produces ablation pressures from products at temperatures which are simply unreachable with any chemical fuel. This technology is not speculative in any way, and it certainly allows feasable economic travel to any point in the solar system in a time-scale of a few weeks. A large Orion can be the size of an office building, it can be shielded from cosmic ray radiation, and it could be built in a few years at a cost of a few billion dollars, with no technological innovation required.

The idea that further scientific discovery is going to do better than orion is not very realistic. It is actually remarkable that we know how to make Teller-Ulam devices. When people consider orion type things today, they are dreaming of an inertial confinement fusion, but even if this works, it will require an enormous laser in the ship, which is going to be heavy and require the ship to carry a ridiculously heavy power-source. Inertial confinement is also a dream. But plutonium triggered explosions are a reality, and it seems a waste to reject the peaceful applications of this technology because of the cold war nightmares everyone used to have.

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antimatter catalyzed fusion sounds pretty realistic to me, even if we have severe antimatter production shortages, and of course there is the big problem of containment, but nothing that can't be fixed with good diamagnetic confinement – user56771 Aug 31 '12 at 14:07
but then again, i'm afraid that any substantial amount of antimatter is likely going to be controlled by even worse nonproliferation politics – user56771 Aug 31 '12 at 14:10
@user56771: I can't imagine that antimatter, when storage equipment is taken into consideration is going to be any more efficient a propellant than thermonuclear weapons. A good nuclear weapon can be 1% of full efficiency, and I can't see a storage device for antimatter that doesn't weigh 100 times more than the antimatter it stores. There is no realistic alternative, I think antimatter and confinement fusion are just smoke-screens to get people to avoid talking about atmospheric nuclear explosions. – Ron Maimon Aug 31 '12 at 14:31
Ron, a Bussard ramjet using a small percentage of antimatter to catalyze hydrogen fusion could have a better efficiency than this. But i agree that banning nuclear weapons from space propulsion applications is stupid; any good propulsion fuel with be dangerously radioactive anyways – user56771 Sep 3 '12 at 14:44

I suppose that one could develop some sort of slingshot-like approach along side of some other approach using gravitaional pull of stars. My best guess is to look at technolgies that has yet to be discovered or developed. The current discussion seems limited to existing theory. Consider if similar discussion occurred 100 years ago about placing all of my phonograph records into a single device that I could carry with me and listen at whim to the different selection therein. We could very easily be discussing mini records, stacked in a tube with multiple needles to each disc. The techology of what would become an MP3 player would never occur to any in the discussion. This is why I enjoy - good - science fiction that tries to propose theory that is far beyond current and conventional understanding. I'm not sure but I would imagine that a better understanding of light itself may lead to a propulsion approach that far superior that what most consider.

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A fun question for science fiction fans. Ever since strings started to look like the Theory Of Everything I have been fascinated by the extra dimensions. It is the analogy of an ant in a two dimensional maze. If it sees the third dimension it can climb over and be million of ant miles away from his two dimensional path to that point.

So the TOE will be great fun once it is found and established. Even if all extra dimensions are curled, each particle in our four dimensions exists also in those curled dimensions. Who knows what the future may bring in technology. The best bet then is to go full steam ahead and find the TOE, before starting on these journeys worrying about fuel.

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So far strings do not predict that there is going to be practical uses of the extra fields of the microscopic structure. There is no reason to expect that the universe has to allow enginnering at all scales, it only has to admit engineering on those scales where life has evolved, and this is the chemical scale. There is no reason to suppose that we can harnass any other energy scale for limitless engineering--- nuclear engineering is notoriously limited to bombardment of natural nuclei, with no detailed microscopic control of particles of the type biology achieves with atoms in cells. – Ron Maimon Jul 17 '12 at 2:52
@RonMaimon You are too pessimistic. Are we not using lasers and transistors and nuclear energy, very far removed from chemical scales? so what if there are limitations? The optimistic view is that human ingenuity can find a way to get value out of anything that has been studied in some fashion. – anna v Jul 17 '12 at 3:54

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