Antimatter Engine

[Dr. Kerbal]

Hey. But I have a questions how would a Antimatter engine work? Would there be a Antimatter tanks and a matter tank. Then realizing those types of matters creating Gamm ray explosions to create trust? Or will it just be the end of so one who uses it.

[RealKerbal3x]

@Dr. Kerbal The matter-antimatter reaction would probably be used to heat water. The resulting superheated steam would then be expelled out of the back of the spacecraft as reaction mass to produce thrust. You couldn't use the gamma rays as reaction mass as photons have zero mass. Some serious radiation shielding would be required - you might need to coat your spacecraft in lead, which is extremely heavy.

[mikegarrison]

29 minutes ago, RealKerbal3x said:

You couldn't use the gamma rays as reaction mass as photons have zero mass.

Not zero momentum, though. Photons produce thrust.

[RealKerbal3x]

1 minute ago, mikegarrison said:

Not zero momentum, though. Photons produce thrust.

Would they produce as much thrust as vaporising something else (like water, as I said above)? Using the radiation itself as propulsion would undoubtedly be more efficient though. 

[sevenperforce]

@Dr. Kerbal

For any rocket engine, you need two things: an energy source and a working mass (also known as reaction mass or remass).

The simplest rocket engine is a cold-gas thruster. You've got a compressed tank full of nitrogen gas. The energy was put into the nitrogen gas when it was pressurized; open the valve and the pressure pushes the gas out. The momentum of the escaping reaction mass confers equal and opposite momentum on the vehicle.

A monopropellant thruster (like HTP or hydrazine) comes next. Here the energy is not contained in pressurization, but in the chemical energy of the propellant. When hydrazine decomposes into ammonia and nitrogen gas, it releases heat which in turn accelerates that ammonia and nitrogen gas to produce thrust. It's important to note that even though the exhaust and the reaction mass are the same thing, here, that doesn't necessarily need to be the case. You could have a hydrazine thruster that mixed the exhaust with water in order to produce a cooler exhaust with more thrust, even though the water would make the entire affair less efficient. You could even have a hydrazine thruster for a jet aircraft which used a turbofan to compress air and mix it into the exhaust, which would actually be more efficient. The energy source and the remass can be separate.

One example where remass and energy source are partially separate is actually none other than the Space Shuttle Main Engine. When burning liquid hydrogen and liquid oxygen together, you achieve the maximum amount of energy with two pounds of hydrogen for every one pound of oxygen. However, the SSME actually mixed in four extra pounds of hydrogen to help cool the engine and provide additional remass.

For a nuclear thermal rocket like NERVA, the energy source and the reaction mass are completely separate. The remass is liquid hydrogen and the energy source is the nuclear reactor. You could use any remass -- liquid hydrogen, liquid methane, even water -- and get a different performance level, but the energy source remains the same.

A hypothetical antimatter engine would work the same way. Antimatter is a way of storing energy. So you would inject a small amount of antimatter into a large propellant flow (like liquid hydrogen or liquid methane or even water). The antimatter would annihilate with a small amount of the propellant, which would heat up the rest of the propellant and push it out as reaction mass.

[mikegarrison]

I will point out again that while we say "reaction mass", we actually mean "reaction momentum".

There is nothing physically impossible about accelerating a spacecraft by either emitting photons from it or reflecting beamed photons off of it. But thrust would be very small.

[Gargamel]

18 minutes ago, sevenperforce said:

The simplest rocket engine is a cold-gas thruster. You've got a compressed tank full of nitrogen gas. The energy was put into the nitrogen gas when it was pressurized; open the valve and the pressure pushes the gas out. The momentum of the escaping reaction mass confers equal and opposite momentum on the vehicle.

[sevenperforce]

27 minutes ago, mikegarrison said:

I will point out again that while we say "reaction mass", we actually mean "reaction momentum".

There is nothing physically impossible about accelerating a spacecraft by either emitting photons from it or reflecting beamed photons off of it. But thrust would be very small.

Photons do have mass, just not rest mass.

[magnemoe]

22 minutes ago, mikegarrison said:

I will point out again that while we say "reaction mass", we actually mean "reaction momentum".

There is nothing physically impossible about accelerating a spacecraft by either emitting photons from it or reflecting beamed photons off of it. But thrust would be very small.

True, however this require that the photons are IR or weaker, 
Create an good gamma ray mirror as in one who works like standard mirrors but with gamma rays, yes then it might work. 
But antimatter engines would not be on the table of that you could do with it. 

[mikegarrison]

17 minutes ago, sevenperforce said:

Photons do have mass, just not rest mass.

Well, that kind of gets into a discussion of what "mass" actually is.... Not something I know the answer to.

[sevenperforce]

8 minutes ago, mikegarrison said:

Well, that kind of gets into a discussion of what "mass" actually is.... Not something I know the answer to.

Well, no one really knows the answer to that.

But we do know that most of the mass we deal with on a daily basis is relativistic mass, not rest mass. One kilogram of water has the "rest mass" of approximately 0.015 milligrams...roughly 1/6 the mass of a human eyelash.

[Elthy]

30 minutes ago, sevenperforce said:

But we do know that most of the mass we deal with on a daily basis is relativistic mass, not rest mass. One kilogram of water has the "rest mass" of approximately 0.015 milligrams...roughly 1/6 the mass of a human eyelash.

How exactly? As far as i remember the mass of ordinary matter is very close to the mass of the protons/neutrons its made of, isnt it?

[sevenperforce]

Just now, Elthy said:

How exactly? As far as i remember the mass of ordinary matter is very close to the mass of the protons/neutrons its made of, isnt it?

Nope -- the vast majority of mass is relativistic mass from the energetic motion of the particles it contains.

The nucleus of an atom has more mass than the constituent neutrons and protons it contains.

A proton has a mass of 1.6726e-24 grams and it is made of two up quarks and one down quark. The rest mass of an up quark is 4.02e-27 grams and the rest mass of a down quark is 8.59e-27 grams. Add that up and you find that the constituent quarks in a single photon mass just 1.663e-26 grams, just under one percent of the mass of the proton. 99% of the mass of the proton comes from the kinetic energy of those quarks and the gluons that hold them together. When you thrown in the mass of the neutron and the associated binding energy of the gluons which hold the nucleus together, the total "rest mass" in an atom is less than 0.2% of the total mass.

You are quite literally made of energy.

[Bill Phil]

33 minutes ago, sevenperforce said:

Nope -- the vast majority of mass is relativistic mass from the energetic motion of the particles it contains.

The nucleus of an atom has more mass than the constituent neutrons and protons it contains.

A proton has a mass of 1.6726e-24 grams and it is made of two up quarks and one down quark. The rest mass of an up quark is 4.02e-27 grams and the rest mass of a down quark is 8.59e-27 grams. Add that up and you find that the constituent quarks in a single photon mass just 1.663e-26 grams, just under one percent of the mass of the proton. 99% of the mass of the proton comes from the kinetic energy of those quarks and the gluons that hold them together. When you thrown in the mass of the neutron and the associated binding energy of the gluons which hold the nucleus together, the total "rest mass" in an atom is less than 0.2% of the total mass.

You are quite literally made of energy.

From what I recall it's the binding energy between the quarks, but that can be construed to be kinetic energy as well.

[Elthy]

35 minutes ago, sevenperforce said:

Nope -- the vast majority of mass is relativistic mass from the energetic motion of the particles it contains.

The nucleus of an atom has more mass than the constituent neutrons and protons it contains.

A proton has a mass of 1.6726e-24 grams and it is made of two up quarks and one down quark. The rest mass of an up quark is 4.02e-27 grams and the rest mass of a down quark is 8.59e-27 grams. Add that up and you find that the constituent quarks in a single photon mass just 1.663e-26 grams, just under one percent of the mass of the proton. 99% of the mass of the proton comes from the kinetic energy of those quarks and the gluons that hold them together. When you thrown in the mass of the neutron and the associated binding energy of the gluons which hold the nucleus together, the total "rest mass" in an atom is less than 0.2% of the total mass.

You are quite literally made of energy.

Thanks, it never crossed my mind that the quarks could be at fault, i usualy forget they even exist.

[sevenperforce]

10 minutes ago, Bill Phil said:

From what I recall it's the binding energy between the quarks, but that can be construed to be kinetic energy as well.

The binding energy between the quarks is the relativistic kinetic energy of the gluons which mediate the strong force.

[Dr. Kerbal]

Wouldn’t the explosion form the two matters trying to eliminate each other be enough or is it more complicated?

[K^2]

6 hours ago, RealKerbal3x said:

Would they produce as much thrust as vaporising something else (like water, as I said above)? Using the radiation itself as propulsion would undoubtedly be more efficient though. 

No. Photons produce less thrust for the same energy input, but they produce far better I_SP. The I_SP of a perfect photon drive can be as high as c/g. I_SP of a steam rocket would be comparable to LH₂/LOX chemical rocket, so it's not a huge improvement at a big increase in cost. There are other intermediate technologies that can use antimatter as power source and a dedicated propellant to produce much higher I_SP than a steam rocket, but it's a bit more complex than just heating water with gamma radiation from a matter-antimatter reaction. Simplest one is firing pellets or plasma beams of antimatter into larger propellant pellets and have the explosion push the damper similar to nuclear pulse propulsion, but there are more creative ideas as well, down to using antimatter to drive a plasma rocket of some sort.

Nonetheless, the absolute best I_SP you can get is if 100% of fuel mass is converted into energy, and that's a photon drive. That, too, comes with huge technological challenges. We don't have mirrors that work at these photon energies, which means the shielding behind a photon rocket will be absorbing the radiation. That wastes a major fraction of your theoretical I_SP. Indeed, the best you can do is c/(2*pi*g) with a kind of photon rocket we know how to build. But worse, it produces enormous quantities of heat you have to get rid of at anything like reasonable thrust. If you want to get full impulse you'll have to get rid of over 1GW of thermal energy for every 1N of thrust. That's a small nuclear power plant's output worth of energy for a few ounces of thrust. If you sacrifice some I_SP you can bring that down to about 300MW per 1N, but that's still an enormous amount of energy to get rid of for any practically useful thrust.

So antimatter propulsion is a bit complicated. It's not just a problem of making antimatter and storing it, but even turning that energy into useful thrust is very far from being a solved problem. I would go as far as to say that for practical interstellar torch ship, black holes are probably more practical than antimatter. Outside of the problem of making one, the only real difficulty with these is confining that black hole, and that's basically equivalent to balancing a skyscraper on top of a needle point. While clearly not an easy task, I can at least picture how to reduce that one to an engineering problem rather than requiring conceptually new understanding of matter and energy.

[sevenperforce]

15 hours ago, Dr. Kerbal said:

Wouldn’t the explosion form the two matters trying to eliminate each other be enough or is it more complicated?

Well, it wouldn’t be an explosion in the conventional sense. The initial fireball from a nuclear weapon, for example, is primarily surrounding air that was boiled into plasma by the extreme x-ray flux from the nuclear chain reaction. In space, however, there’s no air, so there’s nothing to “explode” in that familiar sense. An antimatter annihilation reaction would produce an extremely bright burst of x-rays and gamma rays, enough to fry the ship, but the only momentum imparted to the ship would be from the photons themselves. As @K^2 noted, we are talking about gigawatt levels of heat for just a single Newton of thrust.

If you want more thrust then you’ll need to surround the annihilation reaction with water or liquid methane or liquid hydrogen or something to both absorb the heat and have something with enough mass to actually explode outward and produce meaningful thrust.

[Gargamel]

15 hours ago, Dr. Kerbal said:

Wouldn’t the explosion form the two matters trying to eliminate each other be enough or is it more complicated?

It depends on the amount of reaction material you provide.   I believe this interaction strictly follows E=mc², if I'm wrong the more learned members will correct me, but for a simplistic description, it should be good enough. 

If you have a single Electron and Positron (Anti-electron) interact and explode into pure energy on the palm of your hand, you might not even notice, as the masses of the two particles are so tiny.  I don't know the mass required before you start losing fingers, but it's a not insignificant amount.   A few mg perhaps?    (For those wanting to churn these numbers, what would be the equivalent reaction mass of an M80 Firework?)  But for the sake of argument, lets call it something the size of a small pill.  

So we have to step up the mass and volume to get a bigger explosion. 

And now we have to somehow directly harness that explosion.   Pusher plate drives are a well researched, if theoretical, technology.  Do some reading on Orion Drives, the idea would be identical, instead of a nuclear explosion, you'd use a matter/antimatter reaction.   Therefore you could use far less total mass as fuel to get the same amount of dV, as the reaction is more complete, if not total conversion to energy. 

But the rub is getting the the antimatter outside of your ship into the proper position.   Once you've launched it outside it's magnetic containment field, it's going to react with anything it comes in contact with, maybe prematurely, sending a chunk of it spinning of in the wrong direction, which includes your ship.    Remember you can't mix it before hand and light a fuse. 

So yes, you could use a pusher plate for this, but due to the containment question, it would be far easier to use it as a controlled power supply to power some other sort of thruster, which would be far more reliable than an antimatter pusher plate drive. 

[kerbiloid]

A funny fact is that gamma-rays are so much penetrating that you have to have several decimeters of matter around the annihilation zone just to absorb significant part of the gamma-rays energy.

And a snowball of several meters in diameter to absorb it mostly, to prevent the ship irradiation and to spend its energy on purpose.

This in turn means that to effectively use AM you should take a homeopathic dose of AM surrounded by fusion fuel as both propellant and absorber.
And in this case you have not a pure annihilation engine, but just a AM-catalyzed impulse fusion engine with electromagnetic reflector.

In turn, this means that as the AM is used in amounts producing small fraction of total energy itself, it's better not to store AM onboard, but to generate positrons and inject them into the reaction zone.
So, finally you get an impulse fusion engine with electromagnetic reflector, catalyzed by self-produced positron beams.

[sevenperforce]

37 minutes ago, Gargamel said:

I don't know the mass required before you start losing fingers, but it's a not insignificant amount.   A few mg perhaps?    (For those wanting to churn these numbers, what would be the equivalent reaction mass of an M80 Firework?)  But for the sake of argument, lets call it something the size of a small pill.  

Any time you think "this is how explosive antimatter is" without doing the math first, multiply it by a thousand. And then you'll still be too conservative. 

A classic M-80 holds three grams of pyrotechnic flash powder which I believe has a TNT equivalence of 0.45 or thereabouts. Do the math on TNT tonne equivalence and that gives you 5,648 joules for an M-80 firework. Using e=mc² (and keeping in mind that you only need half as much antimatter because it will react with anything), we learn that you need 3.1e-11 grams of antimatter to equal the blast of an M-80. For reference, that's slightly more than the mass of a single human sperm cell.

(small squibble -- it's not "reaction mass" in this context, but "blast yield")

What about something the size of a small pill? Well, I have a bottle of extra strength 500 mg Tylenol in my desk. Each pill is half a gram. A half-gram of antimatter would annihilate a half-gram of normal matter, so that's one gram of mass being converted into pure energy. Dial up e=mc² again, and we get the answer: 9e13 Joules, or 21 kilotons, the same as the blast yield of the Fat Man nuclear warhead that destroyed Nagasaki. The Fat Man was a 6.4-kg critical mass inside of a 4.7-tonne implosion warhead and it converted almost exactly one gram of its critical mass into pure energy -- an efficiency of 0.016%. Antimatter, in contrast, is 100% efficient.

51 minutes ago, Gargamel said:

And now we have to somehow directly harness that explosion.   Pusher plate drives are a well researched, if theoretical, technology.  Do some reading on Orion Drives, the idea would be identical, instead of a nuclear explosion, you'd use a matter/antimatter reaction.   Therefore you could use far less total mass as fuel to get the same amount of dV, as the reaction is more complete, if not total conversion to energy. 

In the Orion Drive, the actual nuclear blast does practically nothing to accelerate the ship. The propulsion comes from the propellant -- a dense tungsten tamper which is placed in the casing. The tungsten propellant is about 30-40% of the total mass of the warhead. When the nuke is triggered, the x-rays vaporize the tungsten and turn it into a jet of plasma that is blasted back toward the ship, delivering the desired impulse.

The only reason you need the pusher plate, though, is because there is a minimum size that a nuke can be. If you have antimatter containment, you can simply feed microgram-quantities of antimatter into a stream of propellant continuously. The Raptor engine, for example, has an energy output of 3.52 GW (yes, that's three times the infamous Back To The Future value). To match that with an antimatter engine, you'd only need a flow of 19.6 micrograms of antimatter per second...that's just under five gnats per minute. Of course, to match the thrust of the Raptor engine, you're also going to need to feed about 700 kilograms of cryogenic propellant into the chamber per second. The more liquid propellant you use, the more thrust you get, but the lower your efficiency is.

 

[Guest]

23 hours ago, Gargamel said:

snip

You made me almost lose my breath I laughed so hard.

I do have a question about this. I got the question about this after having a chemistry course on Quantum stuff and alternate universes so I am still wrapping the head around it but.

wouldn't the Matter and Antimatter attract to each other and just explode? Causing no real way of containment? 

[magnemoe]

2 hours ago, kerbiloid said:

A funny fact is that gamma-rays are so much penetrating that you have to have several decimeters of matter around the annihilation zone just to absorb significant part of the gamma-rays energy.

And a snowball of several meters in diameter to absorb it mostly, to prevent the ship irradiation and to spend its energy on purpose.

This in turn means that to effectively use AM you should take a homeopathic dose of AM surrounded by fusion fuel as both propellant and absorber.
And in this case you have not a pure annihilation engine, but just a AM-catalyzed impulse fusion engine with electromagnetic reflector.

In turn, this means that as the AM is used in amounts producing small fraction of total energy itself, it's better not to store AM onboard, but to generate positrons and inject them into the reaction zone.
So, finally you get an impulse fusion engine with electromagnetic reflector, catalyzed by self-produced positron beams.

In this case you could just as well go for nuclear bombs who have the benefit of don't exploding if power to the storage systems get minor interruptions. 
If you could get the antimatter to hit something who mostly generated charged particles its an option, if not its no better than fusion, and fusion should be easier than storing antimatter safely for months. 

[kerbiloid]

6 minutes ago, magnemoe said:

If you could get the antimatter to hit something who mostly generated charged particles its an option, if not its no better than fusion, and fusion should be easier than storing antimatter safely for months. 

To stay safe, teh ship should better use aneutronic fusion (for example, ¹¹B+H or ¹⁵N+H), and these reaction have greater ignition temperatures than D+T.

So, a positron beam could quickly heat such pellet up to the ignition temperature. Also, the positrons are accelerated in same direction like protons of the ionized pellet.

Edited October 20, 2020 by kerbiloid