Energy density of annihilation

[Aarav Sangar]

<Moderator's note: Two threads on the same topic merged in order to have arguments and sources at one place.>

Can antimatter-matter be used as a fuel for a rocket?
There are various problems for anti-matter to be used as a fuel as it produces a lot of gamma rays. Gamma rays are not healthy to be around and can penetrate into the engine material.
I believe that positrons(which are also anti-matter) which are anti-electrons can be used as fuel as they produce less gamma rays which can be easily absorbed by lead.
Anti-matter like anti hydrogen should not be used in my opinion as they produce a lot of gamma rays.
But,
Positrons are lot more expensive than anti-hydrogen (like 250 million dollars for 10mg)...
So what in your opinion can be done too make spacecraft s powered by antimatter??

 

[mfb]

Can antimatter-matter be used as a fuel for a rocket?

Yes, as a quick google search would have told you.

The main problem is the lack of antimatter. We cannot produce amounts that would be useful for spaceflight. And even if we could, we could not store it in amounts useful for spaceflight.

Anti-matter like anti hydrogen should not be used in my opinion as they produce a lot of gamma rays.

Storing positrons alone makes it even harder.

Positrons are lot more expensive than anti-hydrogen (like 250 million dollars for 10mg)...

You cannot buy either.

So what in your opinion can be done too make spacecraft s powered by antimatter??

Going to the science fiction section.
It is probably not impossible, but way beyond our current capabilities.

 

[lekh2003]

And even if we could, we could not store it in amounts useful for spaceflight.

Is the method outlined in the book "Angels and Demons" in any way possible? I doubt so, but how do physicists store it?

 

[Dale]

You cannot buy either.

You can buy positrons, or at least you can buy positron emitters for PET Imaging.

 

[lekh2003]

You can buy positrons, or at least you can buy positron emitters for PET Imaging.

I don't think OP can launch a rocket with a positron emitter .

 

[Dale]

So what in your opinion can be done too make spacecraft s powered by antimatter??

I think that black hole power seems better

https://en.m.wikipedia.org/wiki/Black_hole_starship

 

[mfb]

Is the method outlined in the book "Angels and Demons" in any way possible? I doubt so, but how do physicists store it?

I didn’t read the book. The movie didn’t even try to make anything remotely reasonable.

We can produce and store antimatter. Something like 1000 atoms at a time. But these methods don’t scale well. Shoot protons at matter, filter everything that is not antiprotons, then decelerate and cool the antiprotons (most are lost in the process). Add positrons, hope that some of the produced antiatoms happen to be slow enough to stay trapped.

You can buy positrons, or at least you can buy positron emitters for PET Imaging.

You can buy the emitters but that doesn’t give you stored positrons, and the rate of their production is low. You could put the positron emitters on the spacecraft but that leads to a much lower energy density. You would get a better effect by putting some strong alpha emitter at the end of the spacecraft . While that has been proposed the activity needed make the approach impractical.

I think that black hole power seems better

That is probably even harder to make.

 

[lekh2003]

I didn’t read the book.

Well they store it in a vacuum and put in some kind of electromagnetic stasis. I haven't seen the movie, but I'm sure hollywood conjured some even more bizarre nonsense.

 

[mfb]

Well that is not really a useful description. Of course you have to keep it in vacuum, that also means electromagnetic forces are needed to balance it. But that doesn’t tell you anything else. It misses all the details.

 

[Dale]

That is probably even harder to make.

But straightforward. And once you have it the fuel is just any ordinary matter.

 

[lekh2003]

It misses all the details.

That was kind of the point. You can't disprove it, if you have no clue what it is.

 

[Aarav Sangar]

Is the method outlined in the book "Angels and Demons" in any way possible? I doubt so, but how do physicists store it?

They can be stored in magnetic fields..

 

[Aarav Sangar]

Yes, as a quick google search would have told you.

The main problem is the lack of antimatter. We cannot produce amounts that would be useful for spaceflight. And even if we could, we could not store it in amounts useful for spaceflight.Storing positrons alone makes it even harder.You cannot buy either.Going to the science fiction section.
It is probably not impossible, but way beyond our current capabilities.

They can be stored in magnetic fields...

 

[Aarav Sangar]

I think that black hole power seems better

https://en.m.wikipedia.org/wiki/Black_hole_starship

But in reality, it is impossible to go even near to a black hole, how can we take energy from them just like that. And if we can, then please reply how...

 

[Dale]

But in reality, it is impossible to go even near to a black hole, how can we take energy from them just like that. And if we can, then please reply how...

This is a fairly approachable paper on the topic.

https://arxiv.org/abs/0908.1803

Section V.B describes how you could conceivably handle a black hole in a starship.

 

[jerromyjon]

I assume all current means of antimatter production are terrestrial based, but what if it were created and stored in space? Wouldn't the microgravity, vacuum and low temperature make it much more productive and make storage less demanding?

 

[DrStupid]

Wouldn't the microgravity, vacuum and low temperature make it much more productive and make storage less demanding?

I don't think that the conditions in space are significiant better. At least antimatter fuel production in the spaceship makes no sense. It would be better to use the required energy directly for propulsion.

 

[jerromyjon]

At least antimatter fuel production in the spaceship makes no sense. It would be better to use the required energy directly for propulsion.

Yes, I understand that you would obviously be better off using the energy for propulsion, except that in the case of interstellar travel you lose the energy input from the star you are near. If you could produce more "fuel" close to a star and use it to propel your ship to another star to "refuel" you should see my point...

 

[jerromyjon]

While I have a qualitative understanding of energy density, I lack a quantitative ability to calculate required mass of fuel for propulsion versus mass of spacecraft it would propel. I am pretty sure matter/ antimatter annihilation has the highest energy density possible as all the mass is converted completely to energy, assuming the efficiency of conversion to propulsion is not lower than less energy dense fuels.

What I am curious about specifically is how much (equal parts of matter/antimatter, obviously) would be required to propel a ship of whatever realistic mass is necessary to contain such equipment to reach the nearest star. (4.3 lightyears) I realize this is very hypothetical and complicated, but I'm trying to think of how to simplify the problem. I know the acceleration could continue past the halfway point as there will be less fuel mass to decelerate for the remainder of the journey. Any help with understanding how this all adds up would be appreciated.

 

[jbriggs444]

How much propellant does it take to get a ship of unknown mass to go 4.3 light years at an unspecified speed? It depends on the unknown mass and the unspecified speed. To simplify the problem:

First get rid of the 4.3 light years. There is no interesting physics in the distance from Sol to its nearest neighbor. Instead, make delta V your figure of merit. Ask how much propellant is needed to get a specified delta V.

Second, factor the spacecraft mass out. Instead of asking how much propellant, ask for a propellant/mass ratio to get the delta V.

Third, Google for the relativistic rocket equation.

 

[DrStupid]

If you could produce more "fuel" close to a star and use it to propel your ship to another star to "refuel" you should see my point...

Even in this case it would be a good idea not to carry the manufacturing equipment with the ship whenever possible.

 

[DrStupid]

I am pretty sure matter/ antimatter annihilation has the highest energy density possible as all the mass is converted completely to energy

Only if the equipment required for the containment would converted too.

 

[anorlunda]

I bet you could do an approximate calculation yourself.

Propose a mission; how massive is the ship? how long to get to the star?

Assuming constant acceleration half the trip, and deceleration the other half, what is the maximum speed? What is the kinetic energy of the ship at the max speed? Multiply that by 2, and you have the energy for a one-way mission. As a first approximation, you could even neglect relativity.

Why don't you give that a try, and post your answer here?

My favorite for fanciful speculations was a solar sail trip to Alpha Centuri, but the proposal was for total mass of only 1 gram for the sail plus payload.

 

[DrStupid]

Propose a mission; how massive is the ship? how long to get to the star?

We would also need some basic assumptions about the propulsion system. Is it a pure photon drive or is there an additional reaction mass?

 

[jerromyjon]

According to this page: http://www.iflscience.com/space/antimatter-propulsion-could-sail-stars/
"Their proposal is for an antimatter-driven sail that could deliver a 10-kilogram (22-pound) probe to the next closest star system, Alpha Centauri, in just 40 years using 17 grams (0.6 ounces) of antihydrogen. The large 100-kilogram (220 pounds) sail would be five meters (16 feet) in diameter, made of carbon, and coated with depleted uranium."
This Forbes page about the same proposal: https://www.forbes.com/sites/bruced...-within-a-decade-say-physicists/#f292c2e58493
Says it's possible within a decade but doesn't go into how the 17 grams of antimatter fuel could be obtained.

As for all the calculations, the Delta v, the relativistic rocket equation, that's all out of my league. I got as far as model rocket solid propellant engines 30 some years ago... more Newtons of thrust for a smaller rocket goes further.

 

[DrStupid]

Says it's possible within a decade but doesn't go into how the 17 grams of antimatter fuel could be obtained.

Or how it is stored in a 10 kg probe.

 

[Vanadium 50]

using 17 grams (0.6 ounces) of antihydrogen

Which is something like a billion years worth of antiproton production.

 

[mfb]

But straightforward. And once you have it the fuel is just any ordinary matter.

"Build the required gamma ray lasers and focusing system" is a bit like "build the required antimatter production and storage system". With the difference that we can do the second one today on a much smaller scale, but the first one we cannot.

 

[jerromyjon]

build the required antimatter production and storage system

What would it take to scale it up, if that were the sole purpose and anything goes? Of course having a containment system that blends well with propulsion would be a plus...

 

[jerromyjon]

Which is something like a billion years worth of antiproton production.

I assume you mean net production, as in coincidental production, but what if you could filter out the matter and contain only antimatter? I feel like there has to be some way to separate them due to opposite charge... surely we could get it down to thousands or hundreds of years?

 

[Nugatory]

I assume you mean net production...

I'm not sure what you mean by "net" production. What other kind is there?

Don't assume when you can calculate. How many particles is 17 grams worth of antiprotons? How does that compare with the sort of production rates described by https://arxiv.org/pdf/1408.0759.pdf?

 

[mfb]

What would it take to scale it up, if that were the sole purpose and anything goes? Of course having a containment system that blends well with propulsion would be a plus...

If anything goes, build a trillion times the CERN accelerator complex (you can skip the SPS, the LHC and a couple of smaller components as they don't contribute). A trillion times 1000 antihydrogen atoms is 1.7 ng, you can use it to release 300 kJ. Well... not there yet. The bottleneck is the last part, the production of the neutral atoms and their storage. If you can build 10¹⁵ of them and keep the accelerators, you can produce 1.7 μg to release 300 MJ, roughly corresponding to 10 kg of rocket fuel. You cannot build 10¹⁵ antimatter traps, however. And even if you could, you would have the antimatter in 10¹⁵ different locations.

The first issue is the raw production rate of antiprotons. While that is not its target, MYRRHA should get a huge antiproton production once it is operational, something like 10²⁰ antiprotons per year, or 0.17 milligrams. That sounds nice, but you have to make a beam out of them, losing some of them, and they have to be fast to be kept in a beam - if you do that naively by just reversing an accelerator you lose basically all of them.
To get more slow antiprotons, you have to cool them. CERN's antiproton decelerator (AD) can do this, but it takes time, and it doesn't work well with a continuous antiproton source (such as MYRRHA). The AD uses a weaker but pulsed beam to produce a batch of 30 million relatively slow antiprotons (5.3 MeV) every 100 seconds, or 10¹³ antiprotons per year. Cooling them down to capture them in a trap loses something like 99.99% of these, so you get a few thousand every 100 seconds or a billion per year. These you have to mix with positrons and wait until some form antihydrogen, which takes some time, and you lose some more antiprotons.

Storing macroscopic amounts at the same place and time would probably need solid antihydrogen, levitated by electrostatic forces. That would lead to yet another lossy conversion process, and it is unclear how to start that process.

 

[Vanadium 50]

I'm with Nugatory - what other kind of production is there?

urely we could get it down to thousands or hundreds of years?

How do you calculate this? It sounds like your objection is that you don't like the answer.

 

[jerromyjon]

I'm not sure what you mean by "net" production. What other kind is there?

I meant that gross production of antiparticles, like how mfb broke it down, leads to a very low amount being containable, therefore net production would be the amount that could be contained. (perhaps as solid antihydrogen) Sorry for asking what you all probably think are silly questions but I can't find much of anything of value online, and the lack of detail of what I have found leaves more questions than understanding.
Thanks for the link, it looks very interesting, I'll read it tonight!

It sounds like your objection is that you don't like the answer.

Of course, that is my objection! It is just a slight step from "impossible"... and I really hate the word impossible. It's not that I doubt your estimate, based on existing methods, it's just that I'm dreaming that *someday* it might be possible, and wondering if there are any hypothetical ways of achieving it quicker.

 

[mfb]

We don't know any quicker way apart from increasing the beam currents, the cooling of things and so on, but these are all incremental steps.