Revisiting the PGMP-12

[Carlobrand]

We've had discussions about the PGMP/FGMP family before. As presented by Book-4, it doesn't work: to keep recoil down where a person can hold it, and to keep to that 2 cm diameter jet, it comes across as a kind of rocket flamethrower.

I want to toss aside that 2 cm bit and look at it another way. I'm going to preface this by noting that my physics is both weak and rusty, so if anyone wants to chime in with a correction, it would be appreciated.

First, we look at our energy budget. The PGMP-12 is powered by a 3 kg battery giving it, at TL12, 12 megawatt-seconds in 40 shots. So, about 300 kilowatt-seconds per shot. In the description, it's described as taking the hydrogen close to fusion. It packs about a third the power of the FGMP, so that's about where we'll aim.

300 kilowatt-seconds, 300 kilojoules. Not a lot for that purpose. We start with liquid hydrogen, 70.85 milligrams per cubic centimeter. First thing is we need to increase the density by a factor of a couple thousand. Next thing is we need to raise the temperature to, say 15 million degrees? And we have to do that with only 300 thousand joules. My best guess says we can only do that for a few micrograms of hydrogen.

So now we have a few micrograms of hydrogen at roughly 15 million degrees and a density of around 150 grams per cubic centimeter - teeny tiny little pellet - and we release it out of the front of the gun. What happens next?

Well, it escapes, 'cause we let it out. It escapes at uber-scary velocities. Can't be faster than 2000 kps or, even with only a few micrograms, the recoil is more than the gunner can handle. I don't properly know how to calculate the velocity of the escaping plasma. Let's deal with the 2000 kps and see where that leads us, and maybe someone with a better knowledge of physics can tell us if that's even in the right ballpark.

So, I have a plasma starting at about 20 times the density of iron and 15 million degrees spitting out the barrel in a needle maybe 1/30 of a millimeter across. It will encounter air, but it's very dense. It will expand under its own pressure, but it's going 2000 kps and only needs to travel 75 meters to satisfy the book. Ideally it's supposed to be able to punch through 6 cm of steel inside of 25 meters. How large will it expand in 1/100 of a second? How large will it expand in 1/30 of a second? How will it interact with the solid matter it encounters along the way?

 

[mike wightman]

To make it work in an atmosphere you need something else added - a pre plasma bolt laser.

The laser can be used for aiming purposes/target designation, but just before the plasma needle is discharged the laser fires at full power to ionise the air, creating a vacuum for the plasma needle to travel through.

This tunnel will aid in keeping the plasm needle from expanding too quickly as its "walls" are a much lower energy plasma too.

The effect will produce quite a nice thunder crack and a fantastic light show depending on the gases in the atmosphere you are shooting through.

 

[Spinward Scout]

So, I have a plasma starting at about 20 times the density of iron...

Something about that doesn't sound right. A plasma is a less dense state than a gas, I believe. Hydrogen gas should be less dense than a solid like iron. If you're starting out with liquid or solid hydrogen, there would need to be a cooling device on the weapon.

 

[whulorigan]

300 kilowatt-seconds, 300 kilojoules. Not a lot for that purpose. We start with liquid hydrogen, 70.85 milligrams per cubic centimeter. First thing is we need to increase the density by a factor of a couple thousand. Next thing is we need to raise the temperature to, say 15 million degrees? And we have to do that with only 300 thousand joules. My best guess says we can only do that for a few micrograms of hydrogen.

So now we have a few micrograms of hydrogen at roughly 15 million degrees and a density of around 150 grams per cubic centimeter - teeny tiny little pellet - and we release it out of the front of the gun. What happens next?

Actually, 15 million degrees is about the temperature at which proton-proton (i.e. stellar) fusion occurs. Deuterium fusion can occur on the order of a few million degrees. That is for FGMPs.

Plasmas can form at a mere several thousand degrees. So the question really is: How hot do you need your plasma to be for a PGMP?

EDIT: Also, there is no reason why the "ammo" for the PGMP/FGMP could not be some form of fuel-pellet, as opposed to an L-Hyd (LH₂/LDH/LD₂) tank. It would probably be safer to carry on the battlefield as well.

 

[Carlobrand]

To make it work in an atmosphere you need something else added - a pre plasma bolt laser.
...
The effect will produce quite a nice thunder crack and a fantastic light show depending on the gases in the atmosphere you are shooting through.

A good idea.

The energy weapons are auto-spotted in Striker, most likely because they put on that show you describe.

Something about that doesn't sound right. A plasma is a less dense state than I gas, I believe. Hydrogen gas should be less dense than a solid like iron. If you're starting out with liquid or solid hydrogen, there would need to be a cooling device on the weapon.

As I understand it, a plasma is ionized matter. The solar core for example is an ionized plasma but nonetheless denser than what I described. If I start with a plasma and condense and heat it to extremes, I don't think it stops being a plasma. I'm not entirely certain it remains a plasma as it leaves the gun though. Given that the goal was to condense and heat the hydrogen to an extreme state, I'm not certain what benefit a cooling device would have - seems counterproductive.

Actually, 15 million degrees is about the temperature at which proton-proton (i.e. stellar) fusion occurs. Deuterium fusion can occur on the order of a few million degrees. That is for FGMPs.

Plasmas can form at a mere several thousand degrees. So the question really is: How hot do you need your plasma to be for a PGMP?

EDIT: Also, there is no reason why the "ammo" for the PGMP/FGMP could not be some form of fuel-pellet, as opposed to an L-Hyd (LH₂/LDH/LD₂) tank. It would probably be safer to carry on the battlefield as well.

Inasmuch as they're spitting hydrogen out as a weapon, I was thinking they were using ordinary hydrogen rather than deuterium. My criteria for "how hot" was based on the performance of the FGMP and the fact that the PGMP had about a third as much punch. Numbers were pretty much a ball park guesstimate, taken in part from what happens at the solar core, so I don't know how good those are for this situation. At the heart, I wanted something carrying about a third as much energy as the FGMP. The basics - the hydrogen being extremely dense, extremely hot, extremely tiny and so forth - don't change a lot if I'm off by a third or a half, so I didn't aim to be real accurate with the numbers.

I don't have information on fuel pellets. What are they made of?

 

[atpollard]

I sort of thought of it as a chemical laser where the chemicals reach plasma state and are expelled with the laser beam as a 'bullet' of plasma (later a bullet of fusing plasma for the FGMP) rather than being slowly and gently vented (like in a laser).

But that was just my WAG, I never really looked into the physics of it.

 

[whulorigan]

Inasmuch as they're spitting hydrogen out as a weapon, I was thinking they were using ordinary hydrogen rather than deuterium.

For starship reactors the use of light Hydrogen (Protium) is advantageous because of the desire to be able to skim fuel from Gas Giants or Water, which is mostly composed of Protium. But a Protium (Proton-Proton) reaction "ignites" at a much higher temperature. For a weapon system with the military logistics resources to resupply it, I would think such a weapon system would use a much lower-temperature ignition / easier to achieve deuterium reaction.

I don't have information on fuel pellets. What are they made of?

Fuel pellets are simply a chemical compound of some sort that is typically a hydride (often a metal hydride). This allows it to be stored at room-temperature safely (although it probably has a larger volume, depending on the compound). PGMPs and FGMPs in TNE used "Pulse Plasma Cartridges" and "Pulse Fusion Cartridges", IIRC, which were probably fuel-pellet based. They may be similar to atpollard's concept.

(BTW, it is also a reactor-fuel alternative option in MgT for Fusion Reactors. Metal Hydride takes up twice the volume of L-Hyd, but requires different storage tanks as compared to L-Hyd - i.e. they are not interchangeable. )

 

[Spinward Scout]

As I understand it, a plasma is ionized matter. The solar core for example is an ionized plasma but nonetheless denser than what I described. If I start with a plasma and condense and heat it to extremes, I don't think it stops being a plasma. I'm not entirely certain it remains a plasma as it leaves the gun though. Given that the goal was to condense and heat the hydrogen to an extreme state, I'm not certain what benefit a cooling device would have - seems counterproductive.

Hydrogen only becomes a stable liquid when cooled to -423 degrees. Storage of the liquid hydrogen before use is problematic.

http://en.wikipedia.org/wiki/Liquid_hydrogen

You can only create a plasma from a gas.

Heating a gas may ionize its molecules or atoms (reducing or increasing the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions. Ionization can be induced by other means, such as a strong electromagnetic field applied with a laser or microwave generator, and is accompanied by the dissociation of molecular bonds, if present. Plasma can also be created by the application of an electric field on a gas, where the underlying process is the Townsend avalanche.

http://en.wikipedia.org/wiki/Plasma_physics

 

[mike wightman]

A little thought from the outfield - gravitics.

The PGMP could incorporate a massive grav compression chamber to produce the stellar core plasma temperature and density you are after.

Higher TL versions would compress the plasma further (stronger gravitics) and possible use damper technology to initiate the fusion bolt.

 

[Carlobrand]

Hydrogen only becomes a stable liquid when cooled to -423 degrees. Storage of the liquid hydrogen before use is problematic. ...

And so it is today, and I haven't the foggiest how they might have solved that problem in the Traveller universe, but clearly they have. Hydrogen is routinely stored in that form in starships, spacecraft down to 10 dTons, tanks and armored vehicles, air/rafts, robots and even personal devices such as the Long Range Thruster Pack (Imperial Encylopedia, P. 68).

...You can only create a plasma from a gas. ...

Is there a physical limit to how dense the plasma can be? I keep hearing them talk about the plasma at the sun's core - is that a misnomer, or is it possible for a plasma to exist at densities a couple hundred times higher than water if you can get the temp up high enough? I understood the whole point of fusion research was to take that plasma and compressing it down to extreme densities and temperatures.

 

[flykiller]

Is there a physical limit to how dense the plasma can be?

http://en.wikipedia.org/wiki/Plasma_(physics)

nope. well, maybe the density that occurs during a nova ... but good luck carrying that in a "man-portable" fashion.

 

[Carlobrand]

http://en.wikipedia.org/wiki/Plasma_(physics)

nope. well, maybe the density that occurs during a nova ... but good luck carrying that in a "man-portable" fashion.

CT Book-4: "Fusion Gun, Man Portable - 14 (FGMP-14: Similar in design and function to the PGMP-13, the FGMP-14 differs only in that it contains the plasma slightly longer until a fusion reaction begins to take place."

Also, MT Imperial Encyclopedia: "FGMP (Fus/on Gun-Man Portable): The FGMP-14 is similar in design and function to the PGMP-13. The FGMP-14 differs only in that it contains the plasma slightly longer until a fusion reaction begins to take place."

They are arguably among the most magical of Traveller's magic-tech items, but there they are, compressing a plasma to fusion. Not quite nova-grade, but they're getting the job done and they're man-portable. Well, with battledress or a grav assist.

I've been directed to Wikipedia twice now, and I still don't understand why hydrogen at solar-core temperatures and pressures isn't a plasma while hydrogen in the sun's core is. I am not a physicist, and this is frankly confusing me. Is there something in the scale of the process that makes the hydrogen cling to its electrons in the gun while degenerating in the sun?

Ooh, that rhymes! I'm a poet!

And I'd be happier if we said the things ran on deuterium or D-T or little pellets so we could explain how those backpack fusion power sources work when both CT and MT are now saying that they're too small to exist. Frankly, I'd be glad if the thing didn't exist at all. I'm trying to figure this, and my best guess says my 20-times-as-dense superheated hydrogen bit is thinner than air long before it reaches 25 meters. I'm thinking maybe it can get 5 meters. I've got a plasma cutter! Nice idea for a melee weapon, but that's not what we're aiming for.

How does one calculate the velocity of a gas escaping under pressure? I need a lot more hydrogen to make this thing reach, which means my temps and pressure are lower by a couple orders of magnitude if I'm going to stay within my energy budget and keep the recoil within what a person can handle. (Also means the man-portable fusion weapons are just flatly impossible as portrayed, but that's an issue for another day.)

 

[Spinward Scout]

well, maybe the density that occurs during a nova ... but good luck carrying that in a "man-portable" fashion.

NGMP - I want one!

But, you know, then we'll need a Supernova Gun.

 

[Piper]

If you consider the fusion gun/plasma gun as a type of fusion rocket (you're just pointing the nozzle in unsafe directions) this might be of some help:

http://www.projectrho.com/public_ht...Inertial_Confinement--Magneto_Inertial_Fusion

 

[Spinward Scout]

I was thinking of it more as an advanced Gauss flamethrower.

 

[flykiller]

NGMP - I want one!

tech 16.

But, you know, then we'll need a Supernova Gun.

tech 17.