I found this diagram while looking up diamagnetic machines. Could anyone explain what engine this is?
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3$\begingroup$ Can you add the link to where you actually found this? Right now that's just a link to a google search. $\endgroup$– uhohAug 12, 2018 at 22:52
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1$\begingroup$ @uhoh that is where I found it. $\endgroup$– MuzeAug 12, 2018 at 22:52
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$\begingroup$ quora.com/Are-antimatter-engines-possible $\endgroup$– uhohAug 12, 2018 at 22:58
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$\begingroup$ Your link does not produce the results you think it does for everyone other than yourself. The link uhoh provided and this blog link to it, but there's no source except a name. Robert Frisbee however is a JPL (NASA) engineer, so he might be able to explain it and tell where it came from (perhaps he's drawn it himself). $\endgroup$– MastAug 13, 2018 at 5:15
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2 Answers
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It's a "beam core" engine, described in some further detail on Project Rho's Big List O' Engines:
Microscopic amounts of antimatter are reacted with equal amounts of matter. Remember: unless you are using only electron-positron antimatter annihilation, mixing matter and antimatter does NOT turn them into pure energy. Instead you get some energy, some charged particles, and some uncharged particles.
The charged pions from the reaction are used directly as thrust, instead of being used to heat a propellant. A magnetic nozzle channels them. Without a technological break-through, this is a very low thrust propulsion system.
answered Aug 12, 2018 at 22:58
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$\begingroup$ Low thrust, yes, but high ISP. We can do better if we can extend the field enough to also push the muons the pions decay to. $\endgroup$– JoshuaAug 13, 2018 at 4:42
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9$\begingroup$ @Joshua: Want low-thrust, high ISp? I can give you a photon drive of 30 million seconds and 1 piconewton of thrust. For like a dollar. And it weighs under a gram. I'll just have to dismantle my flashlight. The big problem with all these extreme-ISp drives is as soon as you're past nanonewton thrust level, losses (inefficiencies) of even a fraction of percent necessitate cooling of such mass that the whole TWR goes down the drain, and you're getting accelerations of order of 30m/s per month, $\endgroup$– SF.Aug 13, 2018 at 8:47
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$\begingroup$ SF: Only if you can make an efficient antimatter power plant. On the other hand, I've seen claims that this thing puts out enough thrust to lift off the moon. Rho lists 10,000,000 N $\endgroup$– JoshuaAug 13, 2018 at 13:36
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$\begingroup$ @Joshua: Apply that 10 MN to 100 ton spacecraft, for 1 second. That's 500 megajoules per second, or 500 megawatt. Have 0.2% of that transfer as heat to the engine and you have a megawatt of heating power to dissipate. Ouch. $\endgroup$– SF.Aug 14, 2018 at 0:31
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$\begingroup$ @SF well if you wanna pull 10 gees and be in lunar orbit in 18 seconds. 1000 tons yields better numbers. Now we can afford the cooling water that's vaporized on the outside of the engine bell. $\endgroup$– JoshuaAug 14, 2018 at 0:49
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It's difficult to say exactly what type of engine this is as Anti-matter engines are solely conceptual. Mainly due to the highest production available for Anti-Hydrogen seems to be about 1 gram per 100 billions years (see comments). That aside we can still analyse the engine in question.
You have a storage tank containing Anti-Matter Hydrogen atoms (A-H2) which are contained and out of contact with the walls of the container using high powered magnets. Most likely superconducting magnets. From there you have a feed system which is able to get a specified amount of A-H2 at a time. The A-H2 is then channelled towards the nozzle, once again using magnets where it meets a stream of H2, which is relatively inexpensive to store so let's blast the A-H2 with it. Eventually, one A-H2 molecule will meet a H2 molecule and annihilate converting into photons. The energy of the photons will be pretty high and most likely be gamma radiation (so would essentially pass through the engine, making the engine kind of useless however, let's assume you can retain the energy within the nozzle).
Looking at energy produced by the engine. We have 0.5kg of A-H2 and H2 so we have 1kg of fuel. Using a relation to calculate the theoretical maximum isp using the specific J/Kg and $E = mc^2$.
$$ I_{sp} \leq \frac{\sqrt{2 \times 9\times10^{16}\frac{J}{Kg}}}{g} = 212100000s $$
where g is gravitational acceleration. Comparable to Space Shuttle Main Engine (SSME) Isp which is $453$s. So you can see why conceptually anti-matter engines are theoretically great.
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$\begingroup$ Project Rho suggests a 10 million second Isp, rather than 200 million, as proton-antiproton interaction doesn’t get pure $E = m c ^2$ conversion rates. $\endgroup$ Aug 13, 2018 at 1:15
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$\begingroup$ Welcome to Stack Exchange! If you have a chance, could you add a link to the source for "1 gram per 100 billions years"? I know getting to $10^{23}$ is hard, but there is a lot of pressure to increase production of low energy antiprotons and to decelerate and trap them (see here and here) and rates in the near future may be much larger than past predictions. $\endgroup$– uhohAug 13, 2018 at 2:26
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$\begingroup$ @RussellBorogove granted achieving 200 million isp is unfathomable but as stated this is a theoretical maximum isp if all energy was converted to kinetic energy. There are always more mechanisms to think about, here I assumed it would all be photonic energy, but this reaction could produce particles too. In reality this is incredibly difficult to achieve no matter the engine. Just assumed max conversion rate because it's easier to explain. LH2-LOX has max theoretical isp of 530 ish, off the top of my head, but SSME only gets about 450 due to heat losses and entropy hiding behind every corner. $\endgroup$– AdrianoAug 13, 2018 at 7:26
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1$\begingroup$ @uhoh Thanks for the welcome! Production rate atm as far as I'm aware is in the realm of 10 million antiprotons per minute at the ALPHA experiment at CERN, this works out to be 100 billion years ish for a gram, this is up from the thousands per minute stated here rsta.royalsocietypublishing.org/content/368/1924/3671.full . Granted the value for ALPHA is from Physics profs at Uni of Manchester I know, and they heard of that value from their colleagues but the value increases by the day and you can find several different like 1B yrs for 1g - 100B for 1g. Hope this helps. $\endgroup$– AdrianoAug 13, 2018 at 7:46
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1$\begingroup$ @uhoh Given current production rates only for for anti-matter is experiments but this is much needed as there is much to learn about anti-matter. $\endgroup$– AdrianoAug 13, 2018 at 7:47