Best Antimatter Rocket Fuel

[SunJumper]

If asked what fuel an antimatter rocket would use, one might just say "Antimatter, duh". However, what anti-element/element combination would be best? Some ideas:

Anti-hydrogen - Cheapest (maybe?)

Anti-osmium - Densest but solid - or you could use the gaseous form, which should be at 5300K

Anti-Xenon - Densest gas (Unless SF₆ is denser)

Anti-Iron - Element that cannot be fused further

Anti-Argon - Lightest gaseous element that is lighter than Iron

Anti-Neon - Similar to above.

I think gaseous antimatter reacting with a solid matter core may be the best, but what are your ideas?

(The point of this drive is that the antimatter and matter react in uneven quantities, with excess matter used as hot exhaust)

Edited May 11, 2013 by SunJumper

[Kryten]

Anti-osmium would require huge amounts of energy to produce; as you noted, anything heavier than iron loses energy through fusion.

[SunJumper]

Anti-osmium would require huge amounts of energy to produce; as you noted, anything heavier than iron loses energy through fusion.

While you would reclaim some energy in making Anti-Argon.

[K^2]

Anti-osmium would require huge amounts of energy to produce; as you noted, anything heavier than iron loses energy through fusion.

This extra energy will be stored as extra mass energy, so it won't go to waste. And if you figured out how to make anti-matter efficiently, you can probably fuse stuff efficiently too.

[nyrath]

The best antimatter fuel is the fuel you can contain, that is, hold in some container so it does not touch any of the matter walls. Otherwise it is a worthless fuel.

This may put a severe restriction on what form the fuel can take. Dr. Robert Forward spoke of storing antimatter in the form of a frozen snowball of anti-hydrogen at temperatures below two Kelvin, levitated in a magnetic field to avoid contact with the chamber wall. In a vacuum, of course. The cold temperature is to keep the blasted stuff from sublimating any anti-atoms from the surface and starting an annihilation reaction with the chamber.

Anything heavier and non-ferrous is probably a non-starter.

http://www.projectrho.com/public_html/rocket/spacegunexotic.php#id--Antimatter

[K^2]

Dr. Robert Forward spoke of storing antimatter in the form of a frozen snowball of anti-hydrogen at temperatures below two Kelvin, levitated in a magnetic field to avoid contact with the chamber wall.

Frozen hydrogen would make a crappy material for magnetic suspension. It is diamagnetic, so it's possible, at least, but its susceptibility is less (by magnitude) than that of water. So against even 1g of acceleration the field strength required would be enormous. Considering the catastrophe that containment failure would result in, I would not pick H₂. Unless, of course, you manage to create it in the hypothetical MSMH state. That one should be superconductive, and then it would be fantastic for storage.

Of course, existence of MSMH would have so many other applications, it's not even funny. If it does, indeed, exist, I wouldn't be surprised if we one day end up mining comets for it.

Out of known materials, for magnetic confinement, the best options are either a superconductor, or at least, some ordinary conductor. For a superconductor, Aluminum is probably the best choice. For conductor, I'd go with Lithium.

[Tommygun]

Would having anti-mater make a Bussard Ramscoop a practical choice?

You wouldn't need to separate the hydrogen, just use whatever you collect as reactant.

You still have all those other issues with friction etc, but you start with a lighter ship or does it create more problems than it solves?

[K^2]

You still have all those other issues with friction etc, but you start with a lighter ship or does it create more problems than it solves?

Depends on how fast you want to go. Each atom/ion you capture is going to impart -γvm of momentum. If you then use matter-antimatter reaction to power an ideal photon drive, you can get at most 2mc of momentum out of it. In other words, you are going to reach terminal velocity when 2c = γv. That's v = 2c/Sqrt(5) = 0.894c. Realistically, of course, your photon drive is not going to be perfect, and your limiting velocity is going to be even lower.

In contrast, photon drive that carries its own matter and antimatter follows relativistic rocket formula. It can travel much, much faster.

Edit: It might not seem like a big deal. 90% is already almost as fast as you can go. True, from perspective of these remaining on Earth, travel time is not going to be much different if you go 90% or 99% of the speed of light. From perspective of ship's clock, however, this is more than 3x difference. Whether that matters or not depends on application. So again, it's all about how fast you want to go.

For an even slower ship, one might consider capturing interstellar medium and using it as fuel for an ion drive powered by matter-antimatter reactor. Then a very small amount of antimatter can go a very long way. Of course, if you do that, you might get away with hydrogen fusion as your power source and then you don't need to bring any fuel.

Edited May 12, 2013 by K^2

[Stochasty]

The problem with a photon drive (or any drive system with comparable ISP) is thrust. A 1N photon drive would require 300MW of power. This means you have two choices: consign yourself to really, really slow acceleration (which means that unless you plan on travelling between galaxies you'd be better of with a fusion powered ramscoop that could accelerate and decelerate more quickly) or somehow manage to build a photon drive that can put out ~3GW per kg of ship mass (if you want acceleration in the neighborhood of 1g).

Sure, if you are using matter-antimatter reactions obtaining that much power isn't an issue, but obtaining that much usable power without frying yourself from waste heat is.

[K^2]

Hm. That leads me down an interesting thought path. I've always considered "ideal" photon drive. One that converts energy into momentum. That assumes you have a "perfect" mirror handy, and none of the heat dissipation factors are a problem.

Instead, suppose that we burn matter-antimatter in the middle of a hemisphere. No reflection, pure absorption, so we don't need to worry about parabolic shape. We get À/4À = 1/4 of maximum thrust. That brings us up to 1.2GW / 1N of actual thrust.

Now, what are we going to make the "engine" shell out of? It's going to be hot. Really hot. I'm going with Tungsten. We'll need about 1cm thick shell to block all the radiation. Stuff will melt at 3695K, so lets set temperature at 3690K. We'll also be losing heat from 3ÀR² of the surface. (2À from outer surface, À from inner.) Mass will also scale as R², which tells us that TWR of this engine will be a constant regardless of the size.

Converting all of this to metric, and introducing density of Tngsten and Stefan-Boltzmann constant, we have the following.

Mass: 1.194x10³ R² kg.

Max power: 99.1x10⁶ R² W.

So that's 6.92x10^-5 N/kg, or TWR of 7.6x10^-6.

And that's actually not all that horrible. It is comparable to modern ion thrusters. And while ion thrusters will probably improve significantly in TWR department, the fact that scientists found uses for them even with that TWR suggests that matter-antimatter engines might actually be plausible propulsion method for future space probes. Because I_sp of gc/4 is really good if you want to cross interstellar voids.

[K^2]

[Accidental double-post. Sorry.]

[drakesdoom]

You know we already produce iron powder on a monumental scale for making carbide machine tools. I would imagine that anti matter iron dust could be held magnetically in a vacuum very easily.

[K^2]

Nah. Looking at accelerations we can get, magnetic confinement of anti-hydrogen will do just fine.

I did some estimates on mission times, by the way. 1,500 years to the nearest star. 2% of ship's mass will have to be fuel, with about 95% being the tungsten dome for the engine. That leaves 3% for payload, confinement, and the ion guns. I would propose using pellets of magnetically-confined anti-hydrogen placed in the center of the dome. Pellets will have to be microscopic, so that they don't overheat from their own radiation. But it shouldn't be a problem if we bombard these using an ion gun.

So now, the only unsolved challenge is getting enough anti-hydrogen. If we want to get 300kg of equipment to Alpha Centauri, we'll need 200kg of antimatter. Which is slightly more than 309 atoms of hydrogen that made up the largest batch ever made.

[ROFLCopter64bit]

If the confinement of anti-hydrogen is going to be a problem, then use ionized anti-hydrogen.

[Tommygun]

Are there any new ceramics that can take the stress and heat of an engine dome that would be lighter than tungsten?

[K^2]

"Lighter" won't do you any good. Thickness is determined by absorption, and absorption, at these energies, is pretty much determined by density. So TWR does not depend on how light-weight the material is. Melting point is the only factor. If you could increase that, you'd have more thrust. But alloys always have lower melting point than any constituent, and the only pure material with higher melting point is graphite. The melting point for graphite is only a little higher, and it tends to evaporate away quite easily at high T. So Tungsten is pretty much as good as it gets.

[Stochasty]

1,500 years to the nearest star. 2% of ship's mass will have to be fuel, with about 95% being the tungsten dome for the engine.

This is what I meant about "unless you're planning on travelling between galaxies." Consider a fusion powered rocket with an effective exhaust velocity of 0.119c and a fuel fraction of 97% (since you won't need the Tungsten shield) giving the same fraction for equipment and payload. Total delta-r is ~0.417c. Assume 1g acceleration up to half maximum, coasting, and then deceleration at the destination. Each burn lasts ~74 days (proper time), and the coasting velocity (in the rest frame of the origin) is ~0.205c. Under these conditions, subjective trip time to Alpha Centauri is about 21 years. You can do even better if you include a ram scoop, using the scoop to boost you up to ~0.1c first before burning onboard fuel (the scoop can also be used for deceleration, saving additional fuel).

In fact, in order to save time using the photon drive, the target destination must be at least 23925.1 ly away (assuming I've done my math right), with the subjective time to destination at the break even point a mere 113,844 years.

Edit: Actually, it's worse than that. If you need a fuel fraction of 2% to maintain acceleration for 1,500 years, then you'll need a fuel fraction of ~60% to accelerate for 114,000 years, increasing the mass of the ship by a factor of 2.5 - but that means correspondingly less acceleration. Upping the size of the shield to maintain the rate of acceleration won't work; you're beyond the maximum theoretical delta-r for that TWR.

Edit2: Granted, the fusion rocket is going to be dealing with the some of the same waste heat issues, since it needs to produce a minimum of ~300MW/kg to generate 1g of acceleration (although it might be possible to use the fuel itself as a heat sink). The means that you probably can't get anywhere near an effective exhaust velocity of 0.119c in practice, which would improve the break even numbers for the photon rocket considerably.

Edited May 12, 2013 by Stochasty

[Rune]

Wow, K^2, that analysis really puts antimatter rockets to shame. A simple thermonuclear blast can do better, or at least Orion could. About 2% of c cruise also, IIRC the paper by Dyson, and no confinement needed. Also a more than decent T/W ratio. Perhaps your numbers can be significantly improved with magnetic nozzles, for example? Pulsed operation may also help things.

Rune. Also, there is the fact all antimatter is is a really fancy battery.

[Flixxbeatz]

I'll go for Anti-Unununium.

[falofonos]

Anti - h2o may be something you can store magnetically.

Although if you have an antimatter storage solution it would probably be better to use a fusion reactor to heat some sort of propellant and throw that out of the back. Not as big a bang, admittedly but far less stress on the superstructure of your ship over the 1500 years of continuous operation.... Actually, the only structures humans have managed to keep continuously operating on those timescales are piles of rock whose main function is to be a pile of rock...

[Stochasty]

Hrmm, I thought of a potential way to improve the efficiency of the photon rocket. You're relying upon direct absorption of the gamma rays produced by the matter-antimatter reaction for thrust, and discarding the half that are headed in the wrong direction. However, a photon drive doesn't care about the wavelength of the emitted photons, just the total power, which means a blackbody radiator is theoretically just as efficient. So, just absorb the entirety of the emitted gamma rays in a reaction chamber and let it glow, then use normal mirrors to direct the radiation. You might even be able to get away with melting your Tungsten absorber if you can keep the molten Tungsten magnetically confined but allow it to radiate.

I'm imagining something like a spherical reaction "chamber" surrounded by a parabolic reflector. Just fire the anti-matter pellets directly into the Tungsten itself (using part of the Tungsten as reaction mass) and heat it up as hot as you can get it before significant quantities start boiling out of your magnetic trap.

Edited May 12, 2013 by Stochasty

[K^2]

Perhaps your numbers can be significantly improved with magnetic nozzles, for example? Pulsed operation may also help things.

Nope. We are dealing with hard gamma radiation. Can't manipulate that with magnetic field like you can with particle exhaust of any other type of rocket. The only field that can deflect hard gamma radiation significantly is the gravitational field. A gravitational nozzle would certainly help. Ball's back in your field, Stochasty.

But in terms of any sort of science we understand, that's basically the absolute limit for a matter-antimatter photon drive.

Ok, I can think of one possibility. There might be a way to utilize Mössbauer Effect. That would let you receive recoil from annihilation event by an entire lattice, minimizing heat production. If you can figure out how to emit photons exclusively with this zero-phonon recoil, you can make 100% (or near enough) efficient photon drive of legend. That would be the most efficient reaction drive physically possible. Unfortunately, our current understanding of relevant physics is akin to understanding of aerodynamics available to a caveman who is running in the field flapping two animal skins and wondering why he isn't flying like the birds.

Either way, we are missing some very important fundamental science background in order to even approach the engineering problem of practical matter-antimatter engine. But I am surprised to see that some sort of not-entire-useless device could be built with 1960's tech, have we had sufficient stock of antimatter.

[Stochasty]

Can the Mössbauer Effect produce collimated photons, even in principle? If so, then you're right that that would be the ideal photon drive.

[nyrath]

Would having anti-mater make a Bussard Ramscoop a practical choice?

You wouldn't need to separate the hydrogen, just use whatever you collect as reactant.

You still have all those other issues with friction etc, but you start with a lighter ship or does it create more problems than it solves?

Then it would not be a Bussard Ramjet, it would be a Ram-Augmented Interstellar Rocket (RAIR)

http://www.projectrho.com/public_html/rocket/slowerlight.php#id--Bussard_Ramjet_Derivatives

The difference being that while it may not be subject to the drag effect of a Ramjet, the drawback is that it no longer has infinite fuel, when the on-board supply of antimatter runs out, so does the thrust.

If the confinement of anti-hydrogen is going to be a problem, then use ionized anti-hydrogen.

Otherwise known as antiprotons. Since each atom of anti-hydrogen is composed of one antiproton and one positron, ionization removes the positron, leaving an antiproton.

Also, there is the fact all antimatter is is a really fancy battery.

Yes, got it in one. As long as there are no antimatter mines, antimatter is not a fuel, it is an energy transport mechanism.

http://www.projectrho.com/public_html/rocket/basicdesign.php#id--Power_Plant--Exotic_power_sources--Antimatter_Power

Edited May 12, 2013 by nyrath

[lyndonguitar]

I have an idea = anti-hydrogen + anti-oxidizer reacting with normal rocket propellant = Hyperdrive!!!

lol jk.

I would say anti-hydrogen, its the best fuel for me, and we are already very familiar in using hydrogen so it would be easier