Antimatter.. how good is it for propulsion?

[Spacescifi]

 

Say you had a device to neutralize gravity's pull on a spaceship with the mass of an aircraft carrier (97,000 tons fully loaded).

 

You are using liquid hydrogen as propellant and mixing it with antimatter. You have 3 tons of antimatter on board.

Would your thrust to weight ratio be decent enough to make several take offs and landings? Or just one or two before running out of propellant?

What about fuel to cargo ratio? Would the aircraft cartier mass spaceship be more than 10% fuel tank? 50%? 80%?

 

Granted this works better for take off with grav-neutralization, but I still wanna know.

Edited June 14, 2019 by Spacescifi
Texts

[Nightside]

Weight is a function of mass and acceleration (mainly gravity). If you have a “gravity neutralizer” then your weight is zero, so any amount of thrust would give you an infinite TWR.

Spoiler

What I want to know is: how does the gravity get neutralized??? I think the device should work by completely annihilating all mass on the planet, removing its gravitational pull. Useful, but with some functional limitations.

 

[Spacescifi]

1 hour ago, Nightside said:

Weight is a function of mass and acceleration (mainly gravity). If you have a “gravity neutralizer” then your weight is zero, so any amount of thrust would give you an infinite TWR.

  Hide contents

What I want to know is: how does the gravity get neutralized??? I think the device should work by completely annihilating all mass on the planet, removing its gravitational pull. Useful, but with some functional limitations.

 

 

Just accept the effect as a plot device, shielding the vessel from gravity. It can be turned on or off. Which makes it useful for reaching space, and then the ship can reach orbital velocity and shut it off.

[WinkAllKerb'']

t r i c k y ask your grand grand grand grand grand son ...

edit: aka pkm set gps network first, send something inside then ...

Edited June 14, 2019 by WinkAllKerb''

[WinkAllKerb'']

and while you can fold space in your mind like in "dune" or with a paper sheet with two drawed point so the points meet each others, i hardly this happen for biped and space travl aside when two (more) black hole encounter each otehrs ...

or when two universe encoutner each others in the primitive soup flow ... time scale, distance scale <=> flow <=> void, gazeous, liquid, solid

(starmade game "lore" in oracle java sun mycrosystem as exemple)

Edited June 14, 2019 by WinkAllKerb''
typoZzzzzzzz some not all + scale lifespan

[RCgothic]

1 hour ago, Spacescifi said:

 

Just accept the effect as a plot device, shielding the vessel from gravity. It can be turned on or off. Which makes it useful for reaching space, and then the ship can reach orbital velocity and shut it off.

Or don't turn it off and just hover. If the device uses less energy to run than it would take to accelerate to orbital velocities, you just wouldn't.

 

And if it does use more energy than it takes to get to orbital speed, might not be worth having it at all. The ascent is a relatively small part of the energy requirement for accelerating to orbital speed.

[kerbiloid]

If no gravity, then what are "take off" and "landing"?

[WinkAllKerb'']

1 hour ago, WinkAllKerb'' said:

 void, gazeous, liquid, solid

 

 ? >> void, gazeous, liquid, solid << ?
flow jsut flow @DistScale and time@scale (+biped lifespan), memories and idea lifespan (dna, mirco frag inter species, environnemetnal(s) etc etc etc ... spad coheris ... noaa noah not all may be if lucky a few @energy required) to "bring" (some) and "reach" (what)& (eventually)

life is about flow @scale(s)(Zzzzzzzzz) and encoutner on the road aka herbert "fanny mae" (others writers similar metaphor that speak to one or another of bipeds) or USa mortgage comp.

 

 

Edited June 14, 2019 by WinkAllKerb''
@kerbi

[KSK]

Depends how much handwaving you want to use. Your gravity neutraliser makes things much easier since you can ignore gravity and orbital velocities entirely. Your take off speed can be 1mm/s and you’ll still get to space - eventually. You won’t be in a stable orbit when you get there of course but that’s okay because you’re not going to fall down again under gravity.

So to answer your question.

First level approximation - figure out how fast you want your ship to go, calculate it’s kinetic energy at that speed, and divide the that kinetic energy by two multiplied by c-squared. That will tell you what mass of antimatter you need to annihilate to accelerate the ship to that speed.

That assumes that all the energy from antimatter annihilation is converted to kinetic energy of your ship - which won’t be anywhere near the case in practice. However it’ll give you a lower limit on the amount of antimatter required (or, conversely, an upper limit on how many times you can accelerate your ship to that chosen speed and back, using that 3 tons of antimatter).

Then you can scale that based on assumed engine efficiency.

Figuring out how much propellant you need is a lot harder. At a bare minimum you’ll need to know the exhaust velocity of your antimatter rocket and I have no idea how to even approximate that. One problem (if I recall rightly) is that most of the energy from antimatter annihilation is released as gamma radiation - which won’t be terribly efficient at heating your propellant.

Edit. Google is probably your friend here. I can almost guarantee that there will be detailed numbers on antimatter rocket designs somewhere on the internet. Atomic Rockets site is probably a good start.

 

Edited June 14, 2019 by KSK

[wumpus]

9 hours ago, Spacescifi said:

 

Say you had a device to neutralize gravity's pull on a spaceship with the mass of an aircraft carrier (97,000 tons fully loaded).

 

You are using liquid hydrogen as propellant and mixing it with antimatter. You have 3 tons of antimatter on board.

Would your thrust to weight ratio be decent enough to make several take offs and landings? Or just one or two before running out of propellant?

What about fuel to cargo ratio? Would the aircraft cartier mass spaceship be more than 10% fuel tank? 50%? 80%?

 

Granted this works better for take off with grav-neutralization, but I still wanna know.

This is two questions:

1. What effects would a device to neutralize gravity's pull have?

The amount of fuel required to launch/land/move between planets approaches zero.  Simply burn enough fuel to reach a speed that will get you there in a reasonable time, then burn the same amount of fuel to stop.  Note that according to general relativity, the internal effects on a spaceship due to acceleration is identical to gravity: your gravity neutralizer should remove all acceleration issues from your craft (possibly also providing a steady 1g for the crew), so you can accelerate as fast as the engines can without squishing your crew.

2. How much power does antimatter have?

Good question.  The energy levels are extremely straightforward: E=(2)mc² tells you how much energy you can get out of the antimatter (the 2 is because regular matter is "free", and we only care about the antimatter).  v=mv²/2, so a 100% antimatter engine would net a velocity of 2*sqrt(m_antimatterc²/m_{total_rocket_mass}).  Switch to relativistic equations (not much harder) if you notice your spacecraft is going more than 10% of the speed of light.

Don't expect 100% conversion, and a better method would be figure out how the Orion worked (it was designed to efficiently use the momentum generated by atomic explosions, should be similar to your antimatter), and work out the math from there.

Also liquid hydrogen is a terrible material to use for reaction mass: it is one of the least dense things in the universe and has to be kept terribly cold to avoid being even less dense.  Water, CO₂, air (N₂), argon, iron, regolith would all make better forms of mass to through at the antimatter (you might care to have something that can work with a "fuel injector" of some sort, just don't be too picky about it.  The means of constraining the antimatter will be really weird and high-tech, so melting and squirting molten iron would be a no brainer compared to injecting antimatter.

[Spacescifi]

7 minutes ago, wumpus said:

This is two questions:

1. What effects would a device to neutralize gravity's pull have?

The amount of fuel required to launch/land/move between planets approaches zero.  Simply burn enough fuel to reach a speed that will get you there in a reasonable time, then burn the same amount of fuel to stop.  Note that according to general relativity, the internal effects on a spaceship due to acceleration is identical to gravity: your gravity neutralizer should remove all acceleration issues from your craft (possibly also providing a steady 1g for the crew), so you can accelerate as fast as the engines can without squishing your crew.

2. How much power does antimatter have?

Good question.  The energy levels are extremely straightforward: E=(2)mc² tells you how much energy you can get out of the antimatter (the 2 is because regular matter is "free", and we only care about the antimatter).  v=mv²/2, so a 100% antimatter engine would net a velocity of 2*sqrt(m_antimatterc²/m_{total_rocket_mass}).  Switch to relativistic equations (not much harder) if you notice your spacecraft is going more than 10% of the speed of light.

Don't expect 100% conversion, and a better method would be figure out how the Orion worked (it was designed to efficiently use the momentum generated by atomic explosions, should be similar to your antimatter), and work out the math from there.

Also liquid hydrogen is a terrible material to use for reaction mass: it is one of the least dense things in the universe and has to be kept terribly cold to avoid being even less dense.  Water, CO₂, air (N₂), argon, iron, regolith would all make better forms of mass to through at the antimatter (you might care to have something that can work with a "fuel injector" of some sort, just don't be too picky about it.  The means of constraining the antimatter will be really weird and high-tech, so melting and squirting molten iron would be a no brainer compared to injecting antimatter.

 

I had envisioned the grav-neutralizer as a field that merely deflected the planetary gravity field around it. The field would have a 2 kilometer radius. Anything falling from above into the field would be deflected by falling around and away from the ship at 1g (deflection is 1g because it is set according to planet gravity).

Also if you walked into the grav deflector field you would fall forward at 1g, and fly out beneath the vessel and be very dead once you reached the edge of the 2 kilometer field on tbe opposite side.

 

In other words, my grav-neutralizer does not shield the spaceship from it's own acceleration. Only the acceleration by gravity from a planet.

Also molten iron as propellant? Wow. I guess heavy propellant and thermo-jet engines,would be popular with these spaceships then

As using the air for propellsnt would be as simple as using antimatter by mixing it with air.

And a molten iron plume would not look like your ordinary rocket plume LOL!

 

It would be more dangerous.

[wumpus]

1 minute ago, Spacescifi said:

And a molten iron plume would not look like your ordinary rocket plume LOL!

It would be more dangerous.

Once it hits antimatter it isn't [molten] iron anymore.  It is pure energy.  And that plume will have many of the nasty effects of neutron bomb radiation flying around so you don't even notice the trickle of unburnt iron.

Antimatter is something that exists and researchers have managed to store.  Anti-gravity is pure fantasy, although for all I know we might manage anti-gravity before finding a means to mass produce antimatter.  But I'd still recommend leaving the anti-gravity off, and simply using the anti-matter drives to leave each planet (it should barely take any antimatter).

If you want an even less exotic fuel (although it might seriously date your science - it could be quickly proven impossible/wrong): try metallic hydrogen https://www.youtube.com/watch?v=nMfPNUZzG_Q

 

[Flibble]

3 hours ago, wumpus said:

2*sqrt(m_antimatterc²/m_{total_rocket_mass})

Using this the original shop as proposed has a delta v of 3337 km/s. Pretty sure it can make orbit.

I reckon you need about 27 grams of antimatter for orbital velocity.

[WinkAllKerb'']

magnetic/electrostatic accel gating ... hum ... that a step "metaphorical"concept, then their an energetic cost and where to feed to from in void when far from light

insert nano collide on the way and star trek this thing taht field around earth ... searching img

Edited June 14, 2019 by WinkAllKerb''
metaphor

[wumpus]

7 hours ago, Flibble said:

Using this the original shop as proposed has a delta v of 3337 km/s. Pretty sure it can make orbit.

I reckon you need about 27 grams of antimatter for orbital velocity.

Note that my equations are a tad ... optimistic.  A better assumption would be to convert the entire energy to gamma rays, and then compute the momentum they contain.  Of course, the reason I mention gamma rays is from electron/positron annihilation while antiprotons (and anti-neutrons) would provide most of the energy.  But the point is that you need very, very little antimatter to get off a planet.  Just don't ask how much it costs to make even that amount of antimatter using 21st century methods.

[Spacescifi]

9 minutes ago, wumpus said:

Note that my equations are a tad ... optimistic.  A better assumption would be to convert the entire energy to gamma rays, and then compute the momentum they contain.  Of course, the reason I mention gamma rays is from electron/positron annihilation while antiprotons (and anti-neutrons) would provide most of the energy.  But the point is that you need very, very little antimatter to get off a planet.  Just don't ask how much it costs to make even that amount of antimatter using 21st century methods.

 

What I really want to know is how much propellant the 97,000 ton spaceship would need to reach orbital velocity.

Since unless one is willing to totally photon rocket nuke the launch site, using ONLY antimatter seems less than optimal for safety of those living on the planet below.

Thus somr type of propellant is needed for getting off the planet in a more safe manner that won't obliterate everything within a 3 kilometer radius just to reach orbit.

I know, that is the worse case scenario for lift off, but still a concern.

At any rate, perhaps using the air for propellant would be enough. But I am not sure.

 

Lifting 97,000 tons to orbital velocity is... no joke. That takes some serious lifting, and I am not sure air mixed with antimatter is enough to get 97,000 tons to orbital velocity.

 

[kerbiloid]

1 hour ago, Spacescifi said:

What I really want to know is how much propellant the 97,000 ton spaceship would need to reach orbital velocity.

0.
Because the gravity is neutralized, so ship is already in orbit..

[Guest]

 

If the pull of the planet is neutralised you are basically in orbit around the sun and the planet is only sharing your relative velocity. You could change your speed relative to the planet using any insignificant amount of thrust. The amount of deltaV required for landing and taking off becomes only a matter of how much time you want it to take for you to move away an appreciable distance then re-encounter the planet and equalise your orbit around the sun with its orbit. (Not taking into account atmospheric drag)

If you could neutralise all gravity effecting you you wouldn’t need any deltaV to move away, all you’d need need to do is turn the device on.

Edited June 15, 2019 by Guest

[Shpaget]

4 hours ago, Spacescifi said:

What I really want to know is how much propellant the 97,000 ton spaceship would need to reach orbital velocity.

I'm trying to wrap my head around it, but just can't figure out why you specify 97 000 tons?

False precision is a thing.

[KG3]

19 hours ago, wumpus said:

Once it hits antimatter it isn't [molten] iron anymore.  It is pure energy.  And that plume will have many of the nasty effects of neutron bomb radiation flying around so you don't even notice the trickle of unburnt iron.

What happens when a single antiproton hits an atom?  I'm assuming it only takes out a single proton's worth of mass from that atom.  Does it simply change the atom's atomic number and create a different element or does it cause some sort of fission reaction, blowing the atom to pieces?  Is it possible to get a bigger bang for your antimatter buck by using single antiprotons with a fissionable element?   

[Shpaget]

16 minutes ago, KG3 said:

  Is it possible to get a bigger bang for your antimatter buck by using single antiprotons with a fissionable element?   

Nope. Fission transforms only tiny fraction of matter into energy. Since normal matter is free, anihilation (100% matter to energy transformation) will always be more bangy.

[kerbiloid]

9 hours ago, Shpaget said:

I'm trying to wrap my head around it, but just can't figure out why you specify 97 000 tons?

96 997 t of dry mass + 3 t of antimatter.

[Shpaget]

Math seems to check out.

[Flibble]

15 hours ago, wumpus said:

Just don't ask how much it costs to make even that amount of antimatter using 21st century methods. 

Bigger issue might be time, with current facilities it's estimated to take 100 billion years per gram.

[Spacescifi]

14 hours ago, Dale Christopher said:

 

If the pull of the planet is neutralised you are basically in orbit around the sun and the planet is only sharing your relative velocity. You could change your speed relative to the planet using any insignificant amount of thrust. The amount of deltaV required for landing and taking off becomes only a matter of how much time you want it to take for you to move away an appreciable distance then re-encounter the planet and equalise your orbit around the sun with its orbit. (Not taking into account atmospheric drag)

If you could neutralise all gravity effecting you you wouldn’t need any deltaV to move away, all you’d need need to do is turn the device on.

 

Unless one is making scifi this whole discussion is a complete waste of time. As it would not produce anything.

Yet I am.

What you said is true, but such a method is too slow for a setting that has masdive SSTO spaceships casually launching and landing from planets.

I finally decided to just use a fictional drive that looks cool and also has some connection to reality, only for fun interactions. Since this new fictional drive emits extremely bright and repulsive diamagnetic rays as exhaust.

 

It also produces sufficient momentum transfer on the ship itself to reach orbital velocity and fly around in space.

 

The dianagnetic part is interesting since it would allow the spaceship to 'tractor beam' tow ferromagnetic objects in space while flying in front of them. Which would not work in atmospheres because the air wind blast would prevent that.

Edited June 15, 2019 by Spacescifi
Wind blast