John Ringo

Council Wars

Empire of Man

Into the Looking Glass

Legacy of Aldenata

Paladin of Shadows (Ghost)

Special Circumstances

Legacy of Aldenata F.A.Q.

Question
What the heck is a bun-bun??

Answer

Bun-Bun is a homicidal rabbit, the creation of Pete Abrams of Sluggy Freelance fame and legend. Bun-Bun's best introduction is arguably here: "Say the N word again..."
Warning: This comic is hilarious, twisted and addictive. I strongly recommend not eating or drinking while reading the section on "The Evil."

"I don't want to talk about it," Satan said. "Don't bring it up. Ever."

Seriously, go spend some time on Pete's comic. You won't be dissatisfied.

Question

From the Bar Ville asks: A very long question about Grav-guns

I got into a discussion of the Armored Combat Suits from Ringo's Legacy of Aldenata series a few days back, and the thing that came up as the biggest point of the discussion were the gravguns that the ACS use.

The first couple of problems we encountered were for one the actual kinetic energy of the rounds. Several passages in the books allow one to understand that multiple rounds are equal to 100 kilograms of TNT, however when Mike loads up all his spare ammo for the limpet mine in the end of A Hymn Before Battle, there is a passage that gives the impression that each round carries anti-matter equivalent to 200 pounds of TNT. So, I would like to clarify whether the 100kg of TNT means several rounds or a single round?

Second thing to come up was the mass and velocity of the rounds. In order to actually get the mass and velocity of the rounds to jive with the 100kg of TNT (assumed at this point to mean each individual round) the mass would have had to been around 0.1 grams and the velocity 3000km/sec (for it would stretch the definition a lot to use "relativistic" or "small percentage of lightspeed" for 100 km/sec for example). The problem is that the 0.1 grams sounds extremely low. Any clarifications here?

Third and finally, I noticed in A Hymn Before Battle that the mass of a single DU teardrop was mentioned to be 2 ounces, which would give one a huge mass of 58 grams. This lead to us figuring that this had to be a typo for the ACS are mentioned to carry over a hundred thousand rounds, and hundred thousand pieces of this ammo would mass 5.8 tons. As the suits are to my recollection mentioned to mass 0.5 tons, this is somewhat of a problem. Not to mention that it throws a spanner to any calculations trying derive logical figures for the mass, velocity and kinetic energy of the rounds.

Answer
John says: Unfortunately, the writer was not a physicist. And despite several people trying to help him get the physics right, some of it just ain't.

And I can't even _remember_ the discussions.

Unfortunately, Conrad Chu doesn't post here much anymore. He and I had a few discussions along the lines and he would be the most useful in answering the questions.

One thing I think I _can_ answer:

The impact was supposed to be the equivalent of x kg of antimatter. The "power charge" (a "droplet" of antimatter) attached to each round, however, was higher power (obviously). I think that might be where the discrepancy
lies. It still might be wrong ( take off my shoes to count from time to time), but that was the dichotomy you were specifically asking about.

Question
Grav-guns continued...

Answer
Conrad says: Ahh, the moral of this story is "A little antimatter goes a long, long way".

After A Hymn Before Battle was published, I wrote to John about how could O'Neal survive a nearly point blank range detonation of a 21.5 Megaton Device. John was chagrinned that I would claim such a large explosion when he deliberately omitted the size in the text. Unfortunately for John, he provided enough information for me to break down a lot of relative power / energy relationships that will become evident, John didn't delve to deeply into.

Chapter-39
He put it in the French backpack and started adding grenades from his suit, its cavernous ammunition storage disgorged two hundred and eighty-five. To this he added all of his magazines and all the ammo on the shuttle that was handy. He carefully duct taped his last grenade to the outside. In the end he had one hundred kilos total weight, at least .005 percent of which was pure antimatter.

100 kg x 0.005 mass fraction antimatter = 0.5 kg antimatter

I promptly thanked John mentally when I started doing this estimation. Since antimatter needs an equal amount of matter to react completely with, this resulted in a 1kg matter conversion. 1kg is very nice starting point for powers of 10 estimations (sometimes called Fermi numbers).

Gram of TNT 4.184 KJ
Megaton of TNT 4.184 x10^(15) J (or PetaJoules)
1kg matter + 1kg antimatter = 1.8 x10^(17) J

So 0.5 kg matter + 0.5kg antimatter is just 0.9 x10^(17) J
0.9 x10^(17) J divided by 4.184 x10^(15)J per megaton yields 21.51 megatons of TNT

Now for the fun part, how much antimatter per grav ammunition round?

Chapter-39 (con't)
When the conventional French grenade went off, it shattered a large number of the antimatter stabilization fields immediately around it. Each of these fields contained an antimatter charge equivalent to two hundred pounds of TNT. There were several hundred (???) in the backpack.

Discard the 200 pounds of TNT for now. Assume the editor deleted ??? which I will get back to.

Chapter-39 (con't)
The rupturing of the rifle ammunition in turn smashed the antimatter grenades. The grenades actually held a smaller charge than the rifle rounds, but the casing provided much more in the way of shrapnel and that proved providential.

The canister from the shuttle also contained antimatter. Quite a bit of it.

But by a few microseconds after the explosion of the conventional grenade thousands of forged particles were bombarding the outside of the canister. Under the assault, first the outer shielding, then the plasteel armor, and finally the inner shielding failed.

At which point nearly a quarter kilogram of antimatter detonated, with an explosion to rival the Big Bang.

Aha! More mass fraction info.
So, approximately half the antimatter was in the 'hefty' container. From textev the grenades had less antimatter than the rifle rounds did (grav rifle rounds). So for 285 some grenades the antimatter contribution compared to the remaining rifle rounds was negligible. If the follow is true, and I believe was the authors intent.

Chapter-26
Lieutenant O'Neal stripped the box magazine from his M-200 grav rifle and stared unseeing at the thousands of teardrop-shaped pellets within.

So if even one magazine of ammo made it into the pouch and grenades held a smaller charge of antimatter, then the grenade contribution of antimatter is negligible. (several thousand >> 285)

Proceed with the visual attack pattern! (vbg)
Mass fraction of remaining antimatter 0.25 kg for remaining mass capped at 100 kg of total bomb weight. For the sake of erring on the side of caution, the following estimates ignore the mass of the 'hefty' containers shielding and grenade mass. Realistically, the non-antimatter mass total should be less than 100kg.

Density of depleted Uranium is 19050 kg/m3
100kg of depleted uranium would displace 0.005249 m3.
1 liter is 0.001 m3 (cubic meters)
So the ammunition would displace roughly 5 liters volume. More than a gallon container, less than two. So far so good. A believable volume of matter, all of it grav ammunition. Still thanking John mentally, this is working out well. Having less mass for grav ammunition would lower the displaced volume, but, raise the mass fraction of antimatter with respect to the uranium projectile. Thus I'm still holding the mass at 100 kg of ammo.

Well, 100 kg of ammo has about 0.25 kg of antimatter, by mass fraction according to the author. The mass fraction is 400 to 1, uranium to antimatter.

If each teardrop had the equivalent of 200 pounds of TNT and we use pound (avoirdupois) of 7000 grains to 0.45359237 kg then 200 pounds is approximately 90.7184 kg. As one gram of TNT is 4.184 KJ, then the energy would be 3.79 x10^(8) J. (200 pounds of TNT)

If the mass fraction of antimatter remaining is 0.25kg, then the potential energy would be 4.5 x10^(16) J, there would be approximately 1.187 x10^(8) teardrops in the bomb if each teardrop was approximately 200 pounds worth of TNT. That is more than one hundred million rounds of ammunition. But, it is less than several hundred ??? that I mention above.

several hundred Billion
several hundred Million
several hundred Thousand

Okay, lets look at stepping up the TNT equivalence per teardrop.

Chapter-37
Despite the relatively small size of the teardrops, the explosive force on the first Posleen hit was equivalent to packing a hundred pounds of TNT into its body cavity and detonating it, splattering yellow finely distributed muck over the landscape. And then the teardrops, hardly degraded in form or velocity, would seek out the next Posleen in line, and the next and the next. Most of the fire drove six or seven layers into the mass, cleaving them like a nuclear weedeater.

Minimum 6 kills, so about 600 pounds of TNT energy spent on the posleen per teardrop. So lets cut the number of rounds by 3. 118 million rounds becomes 39.5 million rounds. Nope, no 'hundred' in that number. Anyway you start slicing it to get 'hundred' the equivalent amount of TNT keeps going up.

Okay, lets work in the other direction.
If there were 750 thousand rounds of grav rifle ammunition in the bomb, the equivalent amount of energy in each teardrop would be

4.5 x10^(17) J divided by 7.5 x10^(7) yields 6 x10^(9) J per teardrop
gram of TNT 4.184 kJ

each teardrop has an energy approximately 1.43 x10^(6) grams of TNT or 3161.5 pounds of TNT. Not too close to 200, but, only an order of magnitude off by this estimation.

Anyway you slice it the author's numbers are off a wee bit. This does depend on an arbitrary assumption the editor removed ??? from the text. I could have set the number of rounds higher or lower, but, if you do, the final estimate of velocity becomes problematic.

Lets conclude with an estimate of the grav round details. If each teardrop has 6 x10^(9) J of energy and there are 750 thousand rounds weighing approximately 100kg, then a typical teardrop weighs 0.133 grams. (Erring to a heavier than actual projectile.)

For E = 0.5 MV-squared
V-squared = 2 x 6 x10^(9) / 0.133 g
V = 9498713.8 m/s

Where the speed of light in a vacuum is only 299,792,458 m/s

Or approximately 0.03 of the speed of light.

Recap (this estimate of a grav round characteristic says)
0.133 g (max individual mass)
3 percent speed of light of projectile

Aside,
I don't know where the 100 kg of TNT comes from. I think you just got pretty close to a plausible mass when your estimate was around 0.1 g per grav round as the estimate above works out (close) to. Seems like you used a similar set of parameters. This estimate was redone, I lost the original several computers ago, so it took me while to work it all out again. If you stick too closely to any single facet of info, the estimate can swing widely out of control. This estimate is based on energy and less so on mass fraction that I started with. If you play with the mass and mass fraction of an individual teardrop, you can have speeds of 0.1 to 0.01 fraction of the speed of light very easily. Miss a decimal and you think the grav rounds are flying faster than the speed of light! It's easy to mess up and I wouldn't be surprised if I made a mistake above.

Hopefully this answers your question and provides a starting point if you really want to work out all the details you can deduce from the novel. Have fun!

(John says: I've tried to decide, over the years, if I love Conrad or hate his guts). :)