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HoundsofDemos wrote:
Don't forget that the razor back transforms into a landraider lol

GrimDark Transformers: Raiders in disguise

w1zard wrote:
Actually, bolters shouldn't have bullet drop over distance. The gyros within each bolt should compensate for the gravity by angling ever so slightly downward. This is true for pretty much everything such as temperature, humidity, wind, Coriolis effect etc. The only thing a space marine should have to take into account is travel time.

Speaking as someone with actual experience designing and building firearms, no.

Fluff tells us that, excepting very rare examples, bolt shells are not guided. You say "missile" in your later post, but "rocket" is far more accurate.

As such, any talk of microthruster stabilisation is immediately ruled out. This means you want spin stabilisation (which immediately means no consistent "down")... and also, you actually want a very fast and brief burn. You want the kinetic energy and velocity immediately to maximise impact and minimise time to target. (This is how Gyrojets worked).
Another problem with long burn times can be seen with the RPG-7, which behaves really weirdly in cross winds. Normally you'd expect a projectile to drift with the wind, but the rockets actually turn into the wind, because the wind pushes the back of the rocket more than the front, like a weather vane, then the thrust pushes it into the wind.

Rapid thrust means maximum penetration, shortest time to target, and avoids weird rocketry ballistics.

Given that the bolter was designed by people who actually had a dang clue, this is and will always be my interpretation. It's a valid way to offset a large amount of the recoil that would result from trying to drive such large projectiles at rifle velocities, so you could perfectly reasonably believe that such a design would have existed in STCs - a generously lethal weapon that should hopefully be enough to kill many varieties of potentially unknown xenos, while providing a compromise between the pros/cons of traditional (high recoil) and rocket (low velocity at short range) propulsion for such a projectile.
(Admittedly, you'd probably still need power armour to be able to practically use one on rapid fire in combat, but I'd personally imagine that a boltgun has recoil maybe on the higher end of 12 gauge shotgun slugs).

Although it has no need to exist in our world, if you think about it, it is actually a surprisingly logical weapon to exist if you're sending people out to explore the universe. And with the Imperium largely having to scrounge from long forgotten technology, it does then make sense that you might give such a design to your super soldiers.

 MarcoSkoll wrote:
Speaking as someone with actual experience designing and building firearms, no.

Fluff tells us that, excepting very rare examples, bolt shells are not guided. You say "missile" in your later post, but "rocket" is far more accurate.

As such, any talk of microthruster stabilisation is immediately ruled out. This means you want spin stabilisation (which immediately means no consistent "down")... and also, you actually want a very fast and brief burn. You want the kinetic energy and velocity immediately to maximise impact and minimise time to target. (This is how Gyrojets worked).
Another problem with long burn times can be seen with the RPG-7, which behaves really weirdly in cross winds. Normally you'd expect a projectile to drift with the wind, but the rockets actually turn into the wind, because the wind pushes the back of the rocket more than the front, like a weather vane, then the thrust pushes it into the wind.

Rapid thrust means maximum penetration, shortest time to target, and avoids weird rocketry ballistics.

Given that the bolter was designed by people who actually had a dang clue, this is and will always be my interpretation. It's a valid way to offset a large amount of the recoil that would result from trying to drive such large projectiles at rifle velocities, so you could perfectly reasonably believe that such a design would have existed in STCs - a generously lethal weapon that should hopefully be enough to kill many varieties of potentially unknown xenos, while providing a compromise between the pros/cons of traditional (high recoil) and rocket (low velocity at short range) propulsion for such a projectile.
(Admittedly, you'd probably still need power armour to be able to practically use one on rapid fire in combat, but I'd personally imagine that a boltgun has recoil maybe on the higher end of 12 gauge shotgun slugs).

Although it has no need to exist in our world, if you think about it, it is actually a surprisingly logical weapon to exist if you're sending people out to explore the universe. And with the Imperium largely having to scrounge from long forgotten technology, it does then make sense that you might give such a design to your super soldiers.

If you truly had experience building firearms, you would know that the longer the impulse, the higher velocities and greater kinetic energy. This:

 MarcoSkoll wrote:
You want the kinetic energy and velocity immediately to maximise impact and minimise time to target.

Makes me really think you have no idea what you are talking about.

I envision bolts constantly accelerating until the moment of impact, or until they run out of fuel. Maximum kinetic energy, and the ability to correct in flight. I do not see them as advanced versions of gyrojet guns of the 70s, but rather as ~20mm smart-RPG machineguns. The old gyrojets started their rocket acceleration in the barrel, whereas a bolter has a conventional propellant charge (presumably supersonic) to get the projectile out of the barrel before the gyrojets activate mid-air.

 Deadshot wrote:

Northern85Star wrote:
I dont know how much more 40k lore i can take xD It gets worse the more i get to know!

Boltguns fire armour penetrating rockets?! xD

S4 0 AP

When is GW going to do a game with immersive lore? That is, integrity between the books and the battlefield. Never, is when!

They used to be AP5, ignoring anything less than Fire Warrior/Eldar Guardian armour, such as Imperial guard flak armour, most Orks and tyranid swarms, etc. To put in context, Imperial flak armour in pre-8th was a 5+ save, capable of saving vs Frag Grenades and Heavy stubbers (equivilent of a modern 50cal HMG). So they were accurately represented.

8th ed throws out lore representation in favour of balance and brownnosing up the new Bolt Rifle as the armour puncturing beast.

Nah, they used to be S4 with a -1 save modifer, before that silly AP system was introduced in 3rd.

I mean, who would design an armour system where the properties of a weapon wouldn't normally affect how much armour it penetrated...

w1zard wrote:
If you truly had experience building firearms

Custom and specialist airguns primarily. An example you might have heard of (although to be fair, my role on this one was only a very limited consultancy) is the "Eclipse" launcher, which was used a few times on Mythbusters.

In any case, while the US does not legally consider airguns to be firearms, the UK does, and the differences for these purposes are meaningless. Whether the projectile is driven by pre-compressed air or a combustible propellant has absolutely no effect on the external ballistics. Even if it did make a difference, the material I've studied on external ballistics is primarily researched using powder firearms.

you would know that the longer the impulse, the higher velocities and greater kinetic energy.

... if the force of the impulse is the same, in which case you have a larger impulse and a completely inaccurate comparison.
The bolt only has very limited volume for rocket fuel, and so a limited total impulse. Depending on the exact composition and grain of the rocket fuel, you can either burn that very quickly or very slowly.
Now, in reality, different propellants don't have exactly the same energy density, but the differences are relatively small, so we can generally assume that a fast burning propellant and a slow burning propellant will convey the same total impulse, even if they take a different time to do so.

Let's say that we have a 0.05kg bolt, with a muzzle velocity of 400 m/s from the "kicker" charge part of the hybrid propulsion, with our rocket propellant providing an impulse of 10 Ns in flight.
Rocket A provides that impulse over a tenth of a second. Rocket B provides its impulse over 1 second.

As such, Rocket A provides 100 N of thrust, and Rocket B 10 N of thrust, equating to accelerations of 2000 m/s^2 and 200 m/s^2. However, still exactly the same total impulse.
Ignoring aerodynamic drag for a moment, we can easily use the familiar equation v^2= u^2+ (2 * a * s) to calculate the velocity of the bolt at a distance of 50 metres. (Theoretically, we should be using the rocket equation, but we'll consider the mass of the propellant to be relatively negligible).

At 50 metres, Bolt A will have a velocity of 600 m/s, with its fuel *just* burning out (ergo, time to target is 0.1 seconds).
Bolt B will have a velocity of 424 m/s, massively lower. Time to target has been ~0.121 seconds.

Now, once you consider aerodynamics, which is naturally covered under f = (0.5 * Cd * A * D * V^2), drag forces are higher at increased velocities, so at long distances, a projectile that elects to maintain a lower velocity, using its rocket propellant more to offset drag than provide kinetic energy will see a higher velocity past a certain point, but its average velocity to that distance will have been lower, and therefore a higher time to target and more ballistic drop.

Looking into that, Bolts, in general, appear to have a ballistic shape not dissimilar to the G1 reference shape:

... so you know what, just for the giggles, let's actually use the G1 drag curves so that we can actually fully accurately model the projectile as it travels through the transonic region at longer distances.
(I would normally prefer to use the G7 shape for most modern projectiles, but bolts are always shown with flat bases, so G1 is going to be more accurate. I'll be honest, I'm not *entirely* sure how the rocket thrust from the base might affect the vortices around the base of the projectile, but to hell with the idea of setting up a full fluid dynamics calculation for an internet argument).

At this point, the maths becomes a bit complicated to post here, as it involves lookup tables to compare the projectile's current Mach # to look up its current drag coefficient. (I'm having to ignore normal precalculated ballistic tables,as they're not valid here because of the rocket thrust).

However, if you really feel the need to check my calculations, I'm assuming a projectile mass of 0.05kg with a diameter of 19mm, with an air density of 1.2 kg/m^3 with the speed of sound being 343 m/s.
This is the version of the G1 drag table I'm using, which is the most complete version of it I've managed to assemble. Although the chart's data points are very dense in the range we're considering, I'm interpolating to avoid possible errors in that respect:Once we consider this, Bolt A will hit a peak velocity of ~545 m/s at a distance of ~46 m
Bolt B will hit its peak velocity... well, actually at the muzzle, as the propellant's force is actually slightly outmatched by the drag.

At a distance of 245m, Bolt B will actually be moving faster, but it has still taken 18.6% longer to get there, meaning about 41% more ballistic drop.
Time to target (and thus ballistic drop) is only better past 565m (which is getting to be very long range for a small arm) by which point Bolt B has long since burnt out (it did so at ~363 metres).

If you're assuming that you're getting the same thrust force regardless of burn time, then yes, a longer burn time may seem beneficial, but this simply doesn't hold up. Faster burning fuels create a greater thrust force.

If you particularly object to my parameters, then feel free to re-simulate with other variables (or account for other factors like the consumption of propellant mass, which I've currently considered to be negligible), but I am going to insist that in all cases the total mass and impulse of the propellant is maintained, because otherwise it's not an equivalent comparison.

I do not see them as advanced versions of gyrojet guns of the 70s, but rather as ~20mm smart-RPG machineguns.

I'd reckon I'd be on a pretty safe bet if I were to say I could cough up a whole lot more fluff sources to support my interpretation of them than you can for yours.

Boltguns are only "crude" if you think a 19x165mm HEAP mini-autocannon firing supersonic to hypersonic rounds capable of penetrating several inches of plasteel is "crude". Boltguns are actually incredibly fiendish weapons well suited for dealing with what marines fight (Orks and Nids along with rebel humans) and even pose a serious threat to light vehicles and aircraft.

A lasgun, while an effective weapon for basic infantry due to its miraculous logistical properties, is a pretty poor weapon for shock troops considering as an energy weapon it will be losing power over range, performs worse in humid/gaseous environments and might not even function underwater, either punches a small hole through something or causes a very small explosion via boiling flesh, and it takes a lot of lasfire to put down something like a Tyranid Warrior, Genestealer, Ork Nob, or Black Legionaire.

Also the Imperium did originally outfit Astartes with las-esque weapons, Space Marines originally used Volkite. However Volkites were unfeasible logistically due to extremely difficult assembly slowing their production down, so they were scrapped, turned into a specialist weapon, and replaced with the boltgun.

And lasguns really aren't that long ranged. Siege of Castellax gives us a good example of this, with the range of a long-las maxing out at 2 kilometers. The Iron Warrior Champion however had a bolt pistol and was a crack shot, so his hypersonic bolt stretched out to 2.5 kilometers and downed the rebelling Janissaries.

If you're talking about Lascannons, those are very cumbersome and slow-firing anti-tank weapons, and not well suited for shooting out aircraft. Remember, a lot of aircraft in 40k are capable of near hypersonic speeds at high altitudes, and are pretty durable too. Using a lascannon to punch a hole through one might not even down it, and just hitting 40k aircraft is probably going to be a very hard feat for a marine to accomplish. You're better off leaving AA to people with homing missiles or proper AA batteries such as a Wyvern.

 MarcoSkoll wrote:

w1zard wrote:
If you truly had experience building firearms

Custom and specialist airguns primarily. An example you might have heard of (although to be fair, my role on this one was only a very limited consultancy) is the "Eclipse" launcher, which was used a few times on Mythbusters.

In any case, while the US does not legally consider airguns to be firearms, the UK does, and the differences for these purposes are meaningless. Whether the projectile is driven by pre-compressed air or a combustible propellant has absolutely no effect on the external ballistics. Even if it did make a difference, the material I've studied on external ballistics is primarily researched using powder firearms.

you would know that the longer the impulse, the higher velocities and greater kinetic energy.

... if the force of the impulse is the same, in which case you have a larger impulse and a completely inaccurate comparison.
The bolt only has very limited volume for rocket fuel, and so a limited total impulse. Depending on the exact composition and grain of the rocket fuel, you can either burn that very quickly or very slowly.
Now, in reality, different propellants don't have exactly the same energy density, but the differences are relatively small, so we can generally assume that a fast burning propellant and a slow burning propellant will convey the same total impulse, even if they take a different time to do so.

Let's say that we have a 0.05kg bolt, with a muzzle velocity of 400 m/s from the "kicker" charge part of the hybrid propulsion, with our rocket propellant providing an impulse of 10 Ns.
Rocket A provides that impulse over a tenth of a second. Rocket B provides its impulse over 1 second.

As such, Rocket A provides 100 N of thrust, and Rocket B 10 N of thrust, equating to accelerations of 2000 m/s^2 and 200 m/s^2. However, still exactly the same total impulse.
Ignoring aerodynamic drag for a moment, we can easily use the familiar equation v^2= u^2+ (2 * a * s) to calculate the velocity of the bolt at a distance of 50 metres. (Theoretically, we should be using the rocket equation, but we'll consider the mass of the propellant to be relatively negligible).

At 50 metres, Bolt A will have a velocity of 600 m/s, with its fuel *just* burning out (ergo, time to target is 0.1 seconds).
Bolt B will have a velocity of 424 m/s, massively lower. Time to target has been ~0.121 seconds.

Now, once you consider aerodynamics, which is naturally covered under f = (0.5 * Cd * A * D * V^2), drag forces are higher at increased velocities, so at long distances, a projectile that elects to maintain a lower velocity, using its rocket propellant more to offset drag than provide kinetic energy will see a higher velocity past a certain point, but its average velocity to that distance will have been lower, and therefore a higher time to target and more ballistic drop.

Looking into that, Bolts, in general, appear to have a ballistic shape not dissimilar to the G1 reference shape:

... so you know what, just for the giggles, let's actually use the G1 drag curves so that we can actually fully accurately model the projectile as it travels through the transonic region at longer distances.
(I would normally prefer to use the G7 shape for most modern projectiles, but bolts are always shown with flat bases, so G1 is going to be more accurate. I'll be honest, I'm not *entirely* sure how the rocket thrust from the base might affect the vortices around the base of the projectile, but to hell with the idea of setting up a full fluid dynamics calculation for an internet argument).

At this point, the maths becomes a bit complicated to post here, as it involves lookup tables to compare the projectile's current Mach # to look up its current drag coefficient. (I'm having to ignore normal precalculated ballistic tables,as they're not valid here because of the rocket thrust).

However, if you really feel the need to check my calculations, I'm assuming a projectile mass of 0.05kg with a diameter of 19mm, with an air density of 1.2 kg/m^3 with the speed of sound being 343 m/s.
This is the version of the G1 drag table I'm using, which was the most complete version of it I've managed to assemble. Although the chart's data points are very dense in the range we're considering, I'm interpolating to avoid possible errors in that respect:

Spoiler:

Mach# Cd
0 0.2629
0.05 0.2558
0.1 0.2487
0.15 0.2413
0.2 0.2344
0.25 0.2278
0.3 0.2214
0.35 0.2155
0.4 0.2104
0.45 0.2061
0.5 0.2032
0.55 0.202
0.6 0.2034
0.65 0.20995
0.7 0.2165
0.725 0.223
0.75 0.2313
0.775 0.2417
0.8 0.2546
0.825 0.2706
0.85 0.2901
0.875 0.3136
0.9 0.3415
0.925 0.3734
0.95 0.4084
0.975 0.4448
1 0.4805
1.025 0.5136
1.05 0.5427
1.075 0.5677
1.1 0.5883
1.125 0.6053
1.15 0.6191
1.175 0.6292
1.2 0.6393
1.225 0.64555
1.25 0.6518
1.275 0.65535
1.3 0.6589
1.325 0.6605
1.35 0.6621
1.375 0.6623
1.4 0.6625
1.425 0.6616
1.45 0.6607
1.475 0.659
1.5 0.6573
1.525 0.65505
1.55 0.6528
1.575 0.6501
1.6 0.6474
1.625 0.64435
1.65 0.6413
1.675 0.638
1.7 0.6347
1.725 0.63135
1.75 0.628
1.775 0.6245
1.8 0.621
1.825 0.61755
1.85 0.6141
1.875 0.61065
1.9 0.6072
1.925 0.60375
1.95 0.6003
2 0.5934
2.05 0.5867
2.1 0.5804
2.15 0.5743
2.2 0.5685
2.25 0.563
2.3 0.5577
2.35 0.5527
2.4 0.5481
2.45 0.5438
2.5 0.5397
2.55 0.5361
2.6 0.5325
2.65 0.52945
2.7 0.5264
2.75 0.52375
2.8 0.5211
2.85 0.51895
2.9 0.5168
2.95 0.51505
3 0.5133
3.05 0.5119
3.1 0.5105
3.15 0.50945
3.2 0.5084
3.25 0.50755
3.3 0.5067
3.35 0.50605
3.4 0.5054
3.45 0.5047
3.5 0.504
3.55 0.5035
3.6 0.503
3.65 0.5026
3.7 0.5022
3.75 0.5019
3.8 0.5016
3.85 0.5013
3.9 0.501
3.95 0.5008
4 0.5006
4.2 0.4998
4.4 0.4995
4.5 0.49935
4.6 0.4992
4.8 0.499
5 0.4988

Once we consider this, Bolt A will hit a peak velocity of ~545 m/s at a distance of ~46 m
Bolt B will hit its peak velocity... well, actually at the muzzle, as the propellant's force is actually slightly outmatched by the drag.

At a distance of 245m, Bolt B will actually be moving faster, but it has still taken 18.6% longer to get there, meaning about 41% more ballistic drop.
Time to target (and thus ballistic drop) is only better past 565m (which is getting to be very long range for a small arm) by which point Bolt B has long since burnt out (it did so at ~363 metres).

If you're assuming that you're getting the same thrust force regardless of burn time, then yes, a longer burn time may seem beneficial, but this simply doesn't hold up. Faster burning fuels create a greater thrust force.

If you particularly object to my parameters, then feel free to re-simulate with other variables (or account for other factors like the consumption of propellant mass, which I've currently considered to be negligible), but I am going to insist that in all cases the total mass and impulse of the propellant is maintained, because otherwise it's not an equivalent comparison.

I do not see them as advanced versions of gyrojet guns of the 70s, but rather as ~20mm smart-RPG machineguns.

I'd reckon I'd be on a pretty safe bet if I were to say I could cough up a whole lot more fluff sources to support my interpretation of them than you can for yours.

We already know how fast bolts are, the novels mention them to be supersonic/hypersonic. I also calculated the size of an "actual" bolt based on GW's numbers and Jes Goodwin's art, and the result is a 19.05x165mm or so round. It could be anywhere from around 155mm to 170mm, but unless bolt mags are bloody weird, it's probably 165mm.



Granted I'm terrible at drawing bullets and I only have MS Paint to work with, but I came up with this based on Goodwin's standardized artwork used in every edition and the bolts we see in stormbolters and the bolt models from the game Deathwing. Most of this is probably the kicker charge, but it's over 90 cubic centimeters of free space for the kicker charge. Considering how obscenely powerful Imperial fuel is (such as promethium), that's going to be a pretty big boom.

*snip* accidental double post

 Wyzilla wrote:
We already know how fast bolts are, the novels mention them to be supersonic/hypersonic.

Given the general consistency of the novels and the general firearms illiteracy of many BL authors, that's not very usable. (I reread Eisenhorn a while back because someone asked questions about his weapons, and the amount Abnett mangles the terminology is painful).

It's also a very vague description, and my ability to calculate these things would get much more ropey once into a truly hypersonic band, as there's relatively little study of ballistics at those velocities - even very fast APFSDS rounds fired by tanks (which have the advantage of very long barrels to accelerate their projectiles much faster than normal rifles) aren't even properly into the region.

Anyway, I'll admit that I picked relatively low values for my calculations, but if I pick higher values, it actually skews more in favour of a rapid burn.

If I pick 600 m/s as a muzzle velocity, and give the bolts 40 Ns of rocket propellant, then Bolt A is faster out to ~463 metres, and ahead out to 889 metres, and that's really getting a pretty long way for weapons where some of the design purpose must have been a weapon usable at close quarters.
Even if we calculate all the way out to 2 kilometres, Bolt B coasting rather than sprinting the distance still only makes it a fairly mild 13 m/s faster at 2000 metres.

(For reference, the peak velocity of Bolt A with these parameters would be ~1223 m/s at about 90 metres, which isn't technically hypersonic, but some people might describe it as so).

I also calculated the size of an "actual" bolt based on GW's numbers and Jes Goodwin's art, and the result is a 19.05x165mm or so round.

I can't actually recall having seen any art that assumes those proportions. Generally, bolt rounds I've seen in the art are proportioned more like a mix between pistol and shotgun rounds.

Space Marines are propoganda models made flesh, as operate as such. Blatant, loud, and photogenic.

w1zard wrote:
I envision bolts constantly accelerating until the moment of impact, or until they run out of fuel.

Any rocket propelled object will reach max speed, there are such weird things like wind resistance and max thrust, like it has always been since the days of rocket engines invention at WW2, thanks mr Hitler for the rocket power!

 MarcoSkoll wrote:
The bolt only has very limited volume for rocket fuel, and so a limited total impulse.

Why are you assuming that? What if in 40K land there is an extremely high-energy density fuel that is capable of burning very slowly and in an extremely controlled manner? Why not assume functionally unlimited fuel for negligible weight?

I am operating under the assumption that a bolt can reach maximum velocity in earth atmosphere whilst expanding only a fraction of its fuel.

 MarcoSkoll wrote:

Let's say that we have a 0.05kg bolt, with a muzzle velocity of 400 m/s from the "kicker" charge part of the hybrid propulsion, with our rocket propellant providing an impulse of 10 Ns in flight.
Rocket A provides that impulse over a tenth of a second. Rocket B provides its impulse over 1 second.

Again... why? Why not 1,000 NS total impulse over 100 seconds, or 10,000 NS total impulse over 100 seconds? You are making assumptions for real life materials that may not be applicable.

Titan armor is supposedly lighter than titanium despite titanium being only of the lowest atomic mass metals on the periodic table.

Your math seems to check out, but I am questioning the assumptions on which your numbers are based. I was following until the lookup tables, I understand that air resistance is a b**** to calculate but I don't mind admitting that is a little outside of my depth, I only took up to calculus based mechanics of solids and fluids in college.

 MarcoSkoll wrote:
I'd reckon I'd be on a pretty safe bet if I were to say I could cough up a whole lot more fluff sources to support my interpretation of them than you can for yours.

You are more than welcome to believe that. I however, have always envisioned bolts to be the equivalent of mini smart rocket-grenades that auto-correct slightly in flight. See my comparison of the gyro-rocket guns of the 70s to what we know about how boltguns work in 40k and you will see they are vastly different.

HexHammer wrote:

w1zard wrote:
I envision bolts constantly accelerating until the moment of impact, or until they run out of fuel.

Any rocket propelled object will reach max speed, there are such weird things like wind resistance and max thrust, like it has always been since the days of rocket engines invention at WW2, thanks mr Hitler for the rocket power!

True, it is still constant acceleration matched by an equal and opposite reaction though.

w1zard wrote:
What if in 40K land there is an extremely high-energy density fuel that is capable of burning very slowly and in an extremely controlled manner? Why not assume functionally unlimited fuel for negligible weight?

Because if they manage to push beyond what we currently know to be possible regarding the amount of energy tied up in chemical bonds, then those advances would almost certainly equally affect the development of rapid burning fuel. (And likely also high explosives and conventional firearm propellants).

If one of these things improved by a massive margin, it would almost certainly be through a development that would affect them all similarly, leaving things on level footing. With no canonical source to suggest that somehow they've managed to massively improve one area of chemistry but not several very closely related ones, the only reasonable assumption is that normal principles roughly hold.

You are making assumptions for real life materials that may not be applicable.

Not at all. Although any mathematical demonstration required choosing some actual values to work with, the general principles in question would apply even with wildly different values.
The example values were actually chosen pretty arbitrarily, me picking some easy-to-work with values that were vaguely in the right ball park

See my comparison of the gyro-rocket guns of the 70s to what we know about how boltguns work in 40k and you will see they are vastly different.

Not really. It's just a case of integrating two different forms of propulsion, which is not unknown in the firearms world.
While the world has never had a need to create a small arm capable of such a feat (although it'd be possible), the idea of hybrid propulsion is used in some light weapons.

 MarcoSkoll wrote:
*SNIP*

If we are assuming they are using modern technology you would be absolutely correct in your conclusions. However this is 40,000 years in the future and we have to assume they have something better than modern rocket propellant. Plug 1,000 NS total impulse at 10 seconds of burn time into your math and see what you get.

Humans in the 40k universe have made improvements on this front. I highly doubt space marines are still using semtex and Imperial Space ships use modern day rocket fuel to do maneuvers.

If we are assuming modern technology then titans would collapse under their own weight and lasguns couldn't even exist.

HexHammer wrote:
Never understood the reverse weaponry of IG and SM.

IG got ballistic based tanks, artillery and (some) air, then a few las gun thingies, but infantry with las rifles and pistols?
SM on the other hand has tanks with las guns, but infantry with old fashion ballistic bullets?

Imo SM should have the las guns so they can do all the awesome precision things, shooting targets from very far distance with extreme accuracy, easily shooting aircrafts etc.

IG should have the inaccurate old fashioned bullet based guns, that runs out of ammo, produces smoke and makes lots of noise.

Ballistic based weaponry are sensitive to wind and weather, if the target moves suddenly when you fire, the projectile will miss. Besides the amount of ammo you have to carry around would be insane just for a few weeks engagement.
On the contrary with a las based weapon a SM could in theory have unlimited supply of ammo with his powerpack.

Civilian or not is not even in the slightest a matter - space marines simply ignore pretty much all of it because they are so much trained and genetically upgraded they compensate beyond anything any human being could for any disadvantaging external interference. However, unlike IG infantrymen, they are able to fire in full auto little RPG rockets while mastering the recoil, able to carry trcuks of ammunitions if they wish. And you miss a huge point in current fluff: lasgun, as reliable as they might be, lack stopping power and dramatically lack armour penetration. By fluff standards, it is virtually impossible to punch your las round through a SMC power armour because it would require you to fire repeatedly at the very same spot a couple of time. Other issues in several environements and ranges were also dealt with in previous posts.

On the other hand, as far as IG is concerned, they don't care, because they litterally are supposed to fire in the tens or dozens soldiers at once. What they do care about however, is that laser technology is reliable to the point of being "idiotproof" (thanks for the expression), light, easy to (re)supply, and easy to manufacture.

And yes, the space marines do have other laser weapons, but those are AT weapons where standard solid rounds would not be as powerful. As far as basic flashlight/Bolter is concerned, the bolter is clearly the superior piece of armament in terms of firepower. So no, by current fluff standards there are no reasons the space marines SHOULD have las weaponry over IG. They COULD obviously, why not, but no further.

w1zard wrote:
If we are assuming they are using modern technology you would be absolutely correct in your conclusions.

But we're *not*. The figures are arbitrary.

If I plug your 1000 Ns total impulse for 10 seconds of burn into the equation, and continuing with the postulate that faster burning fuels are also better, an equivalent fuel with a 1 second total burn time... and I don't know, let's say a 1200 m/s initial muzzle velocity, while we're working with the idea that Imperial propellants are really energetic (which would create a really nasty recoil that would kind of defeat the point of two-stage propulsion)...

... well, now I'm having to extend the drag chart by looking up numbers for hypersonic drag (which my calculators don't normally cover, because bullets don't normally go this fast), because the faster bolt is getting *REALLY* fast now - we're talking so fast that it would actually explode on impact from just sheer velocity, so we're starting to see a ludicrous disconnect with the lethality that you'd get from stalker silenced shells (which are explicitly subsonic).

Anyway, now the fast-burn bolt has a higher velocity out to ~3870 metres, and a better time to ~5030 metres. The more we exaggerate the figures, the more it actually skews in favour of the fast burn bolts.

Just in case you think I'm deliberately pushing up the muzzle velocity at the same time because it favours my point, let's drop the muzzle velocity massively down to just 200 m/s (way lower than can be done with modern propellants, so we're now having to make some really contrived assumptions) and keep the super rocket fuels you're proposing.
The fast-burn bolt is still faster to ~3710 metres and sooner to ~4930 metres. Better and better rocket propellants actually favour fast-burn acceleration more and more in terms of velocity and time to target.

But okay, let's skew things more in your favour. Let's push the muzzle velocity back to a more plausible 600 m/s, but let's suppose that the fast-burn fuel is only half as efficient as the slow-burn, generating only 500 Ns of thrust over its 1 second burn.
It is still faster to ~2620 metres, and sooner to ~3515 m.

Okay, what about only 20% as efficient, only 200 Ns of thrust, leaving it with only twice the thrust of the slow-burn bolt, compared to just a tenth of the burn time?
Still faster to ~1518 metres, and sooner to ~2065 m.

I don't really know how much more I can do to demonstrate the point. I'm now supposing the super rocket propellants you seem to want, and even making the unrequested (and rather unreasonable) assumption of a very low initial velocity and that the fast-burn propellant has only a fraction of the energy of the slow-burn propellant.

Even if you want guided bolts, they'd get to targets at just about any common engagement range more energetically with a fast burn and then using fin guidance. (Hell, even if they used rocket guidance, even the 20% efficient fast burn bolts here would still be able to course correct out to somewhere beyond 1300 metres. The 100% efficient fast-burn bolts would be able to course correct out past 3 kilometres*)

*And yes, they're only burning for one second. I said they were going REALLY fast (but this is still within what rocket engines are capable of). Peak velocity is bouncing off Mach 10, and at those kinds of speeds, even a completely basic plastic projectile does things like this, leaving craters several inches deep in aluminium armour. Supposing fuels of this kind of energy density really starts to break the consistency with other lore - stalker silenced shells would be catastrophically outmatched, power armour would be useless**, etc.


** And in response to any cry of "But plasteel/ceramite/etc armour is stronger": At these velocities, it doesn't matter - when things go this fast, normal mechanical forces break down, and absolutely every material is like wet clay. Case in point: Aluminium wouldn't normally act like this under normal mechanical interaction, but it does versus anything at this kind of speed. As does copper, steel, tungsten, titanium, iridium, ceramic, concrete, wood, wet sand, whatever.

 MarcoSkoll wrote:
and I don't know, let's say a 1200 m/s initial muzzle velocity, while we're working with the idea that Imperial propellants are really energetic (which would create a really nasty recoil that would kind of defeat the point of two-stage propulsion)...

That seems awfully high for an initial velocity, why are you assuming it is that high? Just because we are assuming imperial propellants are energetic doesn't mean they put the same proportions in their ammunition that we do. Or that it burns as quickly. There are high energy density compounds that burn extremely slowly, C4 for instance. Yea, slow burning propellants are terrible for firearms but they could use a faster burn proellant for the initial kick out of the barrel and a slower burn propellant for the rocket on the back of the bolt.

BTW, a 1,200 m/s muzzle velocity with a 0.1 kg bolt, (assuming a boltgun weighs 10kg, which is not unreasonable considering their size and material) would generate a recoil of 12 m/s in the opposite direction (conservation of momentum, p=m*v). Assuming the space marine stops the recoil in .1 seconds, this would mean a space marine need only exert a force of 1,200 N for .1 seconds. This is not unreasonable considering that heavyweight boxers (unaugmented humans) regularly punch at 3,000 N, and as high as 5,000 N. A genetically modified space marine in power armor would most likely feel this like a normal human would feel the recoil of a .22 rifle.

 MarcoSkoll wrote:
Just in case you think I'm deliberately pushing up the muzzle velocity at the same time because it favours my point, let's drop the muzzle velocity massively down to just 200 m/s (way lower than can be done with modern propellants, so we're now having to make some really contrived assumptions) and keep the super rocket fuels you're proposing.

Again, what? I feel stupid for asking, but you are aware that subsonic ammunition exists with modern day propellants right? 200 m/s muzzle velocity is absolutely achievable with modern tech.

 MarcoSkoll wrote:
Anyway, now the fast-burn bolt has a higher velocity out to ~3870 metres, and a better time to ~5030 metres. The more we exaggerate the figures, the more it actually skews in favour of the fast burn bolts.

Just in case you think I'm deliberately pushing up the muzzle velocity at the same time because it favours my point, let's drop the muzzle velocity massively down to just 200 m/s (way lower than can be done with modern propellants, so we're now having to make some really contrived assumptions) and keep the super rocket fuels you're proposing.
The fast-burn bolt is still faster to ~3710 metres and sooner to ~4930 metres. Better and better rocket propellants actually favour fast-burn acceleration more and more in terms of velocity and time to target.

But okay, let's skew things more in your favour. Let's push the muzzle velocity back to a more plausible 600 m/s, but let's suppose that the fast-burn fuel is only half as efficient as the slow-burn, generating only 500 Ns of thrust over its 1 second burn.
It is still faster to ~2620 metres, and sooner to ~3515 m.

Okay, what about only 20% as efficient, only 200 Ns of thrust, leaving it with only twice the thrust of the slow-burn bolt, compared to just a tenth of the burn time?
Still faster to ~1518 metres, and sooner to ~2065 m.

I don't really know how much more I can do to demonstrate the point. I'm now supposing the super rocket propellants you seem to want, and even making the unrequested (and rather unreasonable) assumption of a very low initial velocity and that the fast-burn propellant has only a fraction of the energy of the slow-burn propellant.

Actually, I think there has been a misunderstanding. I was never claiming that a "slow burn bolt" would reach it's target faster than a "fast burn bolt". I was claiming that a slow burn bolt with greater fuel had the potential of reaching a higher total velocity over a long flight time, assuming the same energy density for fuel and neglible fuel weight. 100 NS total fuel at 1 second is 100 N of impulse total. 100,000 NS total fuel at 10 seconds of flight time is 10,000 N of impulse total, and while it is true that the bolt would be fighting air resistance and gravity for 10 seconds, I still think 10,000 N bolt is going to go faster than a 100 N bolt at some point on it's flight path. Guns with longer barrels have better muzzle velocities because they expose the bullet to longer impulses, in other words greater total impulse. I know that is a bit of a ridiculous example, but I hope it illustrates what I am trying to say.

 MarcoSkoll wrote:
Even if you want guided bolts, they'd get to targets at just about any common engagement range more energetically with a fast burn and then using fin guidance. (Hell, even if they used rocket guidance, even the 20% efficient fast burn bolts here would still be able to course correct out to somewhere beyond 1300 metres. The 100% efficient fast-burn bolts would be able to course correct out past 3 kilometres*)

I will reserve judgment on this. I don't know enough about the physics behind fin guidance to say whether rocket motor corrections or fin corrections would be more efficient in-flight. However, I always saw bolts as guided rocket propelled 20mm grenades. Almost like mini-cruise missiles without the fins.

Just out of curiosity, if the rocket motor on a bolt could correct for tumble due to air resistance, how feasible is angling the bolt ever so slightly downward to counter the effect of gravity over it's flight time? I know that is some really wonky ballistic physics, but since I seem to have an expert here I want to pick his brain.

w1zard wrote:
That seems awfully high for an initial velocity, why are you assuming it is that high?

I'm not. I deliberately used a wide range of possible initial muzzle velocities across the post in order to avoid any questions of cherry picking.

And yes, I am fully aware of subsonic ammunition. Although I'll accept that my phrasing was somewhat ambiguous, the intended meaning of "way lower than can be done" was not "physically impossible to achieve", but instead "nowhere near the limit".
Obviously it's always possible to make a projectile go slowly one way or another - Nerf guns aren't known for blowing people's heads off because they can't make them weak enough.

BTW, a 1,200 m/s muzzle velocity with a 0.1 kg bolt, (assuming a boltgun weighs 10kg, which is not unreasonable considering their size and material) would generate a recoil of 12 m/s in the opposite direction (conservation of momentum, p=m*v).

Well, it gets a lot more complicated than that, as you have to also consider the mass and velocity of escaping propellant gases, which, despite a couple of grams of gas seeming trivial against bullets weighing considerably more is actually a significant part of the equation. Bolters may have very simple muzzle brakes in the form of the gas ports, but like artists love rendering (with the massive ball of flame still emerging forwards), the design wouldn't actually be that effective.

Assuming the space marine stops the recoil in .1 seconds

Then he would be knocked back 60 cm, as per s = (v * t) + (0.5 * a * t^2). He needs to stop it a lot faster than that.

It's generally better to think of recoil energy rather than recoil momentum. Energy is Force times Distance, rather than Force times Time, so this gives a better idea of how hard and far the recoil pushes you.
For the figures you picked (and ignoring the question of propellant gases - we'll assume the muzzle brake exactly breaks even on that), we'd be looking at a recoil energy of 720 joules, which is more than the muzzle energy of many pistols. It's in the ball park of getting hit by a .357 magnum, and although that doesn't obviously doesn't mean that the bolter will punch through a Space Marine's shoulder (the energy is obviously applied differently with the different velocities and areas of impact), it's still going to be fairly brutal on even their shoulders and not going to be trivially held on target.
The recoil of the .22 rifle you mention, for comparison, is going to be in the vicinity of about 0.2 to 0.5 joules, dependent on the exact bullet and rifle weight. Space marines are strong, but not two thousand times stronger than a normal human for the recoil to be that trivial.

With the 600m/s, 50 gram bolt that I used for most of the calcuations and the same 10 kilo bolter, recoil would be a rather more palatable 45 joules, generally on the upper end of "12 gauge shotgun". (Which, if you watch videos of people trying to fire a Saiga-12 on full auto* gives you a sense of how effective most humans trying to fire a bolter on full auto would be).

* The recoil control technologies like in the AA-12 shotgun are a different matter. While technically the same recoil energy is generated here, but there's a massive spring damping system inside that "catches" the recoil over about a foot, massively reducing and flattening the peak forces on the user, so giving a lower "felt" recoil. However, bolter designs wouldn't allow such a technology, not having a stock to incorporate the mechanism.

Actually, I think there has been a misunderstanding. I was never claiming that a "slow burn bolt" would reach it's target faster than a "fast burn bolt". I was claiming that a slow burn bolt with greater fuel had the potential of reaching a higher total velocity over a long flight time, assuming the same energy density for fuel and neglible fuel weight.

I'm not seeing a meaningful difference. The mean velocity over a given distance is directly defined by the time it takes to cover that distance.
(And the time taken to cover the distance is what I covered in my "sooner" figures. "Faster" is the point that the slow-burn bolt starts having a higher velocity - always after the fast-burn bolt has burnt out - and therefore starting to catch up, "sooner" is the point it actually takes the lead).

Yes, at massive distances, you'd see slow-burn bolts having an advantage, but even the least favourable figures we're looking at above show fast-burn bolts to have the edge out to 2 kilometres, which is huge from the sense of actual combat distances. There's a couple of handfuls of people who've been shot at more than two kilometres, because generally people don't pick fights over salt flats with no buildings to break up the lines of sight.

Just out of curiosity, if the rocket motor on a bolt could correct for tumble due to air resistance, how feasible is angling the bolt ever so slightly downward to counter the effect of gravity over it's flight time?

Well, projectiles angling the tip upwards sort of naturally happens with rifles anyway. You have to have your bore axis pointing above what you want to hit so that as the bullet is pulled down by gravity it drops back down onto the target - with a rifle at fairly short distances, the angle is very shallow, but it does exist.
Because spin stabilisation gives the round gyroscopic stability, it keeps pointing in that same direction - it does not turn into the direction of travel as gravity starts arcing it down, so at longer distances, the nose of the bullet is actually angled above the flight path (which can complicate precise ballistic calculations at very long distances, as the airflow over the projectile changes).

The problem with rocket thrust here is that very small errors or inconsistencies in thrust magnitude or angle would heavily impact trajectory.

It's theoretically possible if we're supposing (very) smart ammunition, but this is a "if you had more money than sense" kind of "possible" - something like a smart sight that automatically measures range and compensates for the trajectory would be a much more practical and cost effective answer than having to massively increase the cost of every projectile by including guidance systems and micro-cogitators.
It would also be much easier to adapt to any given environment, just reprogramming the sights with the parameters for a given world's atmosphere, gravity and the like (or, potentially, even determining it automatically, which would be feasible for a fancy scope, but exorbitantly expensive to do on a per-round basis).

 MarcoSkoll wrote:
Then he would be knocked back 60 cm, as per s = (v * t) + (0.5 * a * t^2). He needs to stop it a lot faster than that.

It's generally better to think of recoil energy rather than recoil momentum. Energy is Force times Distance, rather than Force times Time, so this gives a better idea of how hard and far the recoil pushes you.
For the figures you picked (and ignoring the question of propellant gases - we'll assume the muzzle brake exactly breaks even on that), we'd be looking at a recoil energy of 720 joules, which is more than the muzzle energy of many pistols. It's in the ball park of getting hit by a .357 magnum, and although that doesn't obviously doesn't mean that the bolter will punch through a Space Marine's shoulder (the energy is obviously applied differently with the different velocities and areas of impact), it's still going to be fairly brutal on even their shoulders and not going to be trivially held on target.
The recoil of the .22 rifle you mention, for comparison, is going to be in the vicinity of about 0.2 to 0.5 joules, dependent on the exact bullet and rifle weight. Space marines are strong, but not two thousand times stronger than a normal human for the recoil to be that trivial.

With the 600m/s, 50 gram bolt that I used for most of the calcuations and the same 10 kilo bolter, recoil would be a rather more palatable 45 joules, generally on the upper end of "12 gauge shotgun". (Which, if you watch videos of people trying to fire a Saiga-12 on full auto* gives you a sense of how effective most humans trying to fire a bolter on full auto would be).

Erm, I'm no physics expert, but my math seems to indicate a 50 gram bolt traveling at 1,200 m/s would have 36,000 Joules of kinetic energy ((1/2)*(.05)*(1,200)^2=36,000) I may be dense, but I don't see how exactly this translate to 720 joules of "recoil energy"? Talking about recoil in terms of energy seems a bit wrong when factors like "impact area", weight of the weapon, and recoil time factor heavily into "felt recoil". It seems to me to be better to discuss recoil as a force instead. Don't get me wrong, I am not saying you are wrong, simply that from the perspective of a layman it just seems better that way.

Okay, redo my math on the recoil of the bolter. Using your example of a 50 gram bolt (my previous example was 100 grams as that felt more fair) moving at 1,200 m/s. The space marine would stop the recoil of his boltgun in .025 seconds, which is 4 times faster then my original time. Using the distance formula you provided and that I now remember from my physics class awhile ago and assuming constant deceleration ((6*0.025)+(0.5*-240*(0.025)^2)= 0.075 which is .75 centimeters and is roughly one quarter of an inch, which I assume to be acceptable to you in terms of stopping distance?

This translates to exerting a force of 2,400 N for .025 seconds which is pretty heavy recoil I admit, using my previous example it would be like getting slammed in the shoulder by a heavyweight boxer repeatedly and definitely be unsustainable for an unaugmented human. However, due to the nature of space marine armor and their genetically enhanced strength, I don't see a marine having a problem firing this kind of weapon indefinitely.

 MarcoSkoll wrote:
I'm not seeing a meaningful difference. The mean velocity over a given distance is directly defined by the time it takes to cover that distance.
(And the time taken to cover the distance is what I covered in my "sooner" figures. "Faster" is the point that the slow-burn bolt starts having a higher velocity - always after the fast-burn bolt has burnt out - and therefore starting to catch up, "sooner" is the point it actually takes the lead).

Yes, at massive distances, you'd see slow-burn bolts having an advantage, but even the least favourable figures we're looking at above show fast-burn bolts to have the edge out to 2 kilometres, which is huge from the sense of actual combat distances. There's a couple of handfuls of people who've been shot at more than two kilometres, because generally people don't pick fights over salt flats with no buildings to break up the lines of sight.

I wasn't talking about mean velocity over a distance I was talking about maximum velocity reached during flight.

I was simply pointing out that as long as both rocket engines have equal thrust, then a longer thrust (assuming negligible fuel weight and equal energy density) would produce a higher maximum velocity (also assuming maximum attainable velocity had not been reached yet).

A "fast burn" bolt dumping its 100 NS of thrust in 1 second has to have bigger engines then a "slow burn" bolt dumping its 100 NS of thrust in 10 seconds.

w1zard wrote:
Erm, I'm no physics expert, but my math seems to indicate a 50 gram bolt traveling at 1,200 m/s would have 36,000 Joules of kinetic energy ((1/2)*(.05)*(1,200)^2=36,000)

The 720 joules relates to a 10 kg bolter recoiling at 12 m/s.

Talking about recoil in terms of energy seems a bit wrong when factors like "impact area", weight of the weapon, and recoil time factor heavily into "felt recoil". It seems to me to be better to discuss recoil as a force instead.

The reason we don't use force is that resisting recoil is not a consistent even force - people are more like a spring, starting soft and getting harder the more you push back.
As such, energy is generally a better metric than trying to guess how much force is being applied to resist the recoil.

I've had to consider the logistics of firing half-pound projectiles from ten pound launchers (and let me tell you, a 1:20 ratio is a horrific mass ratio for recoil) where there simply wasn't the space/weight that could be spared for recoil control mechanisms, and the first thing I went to was a calculation of the recoil energy to work out whether it could be fired without a dislocated shoulder.
(The answer was yes, if you fully braced for it, but even when you did it right, the experience of firing it could only be adequately described by using a surplus of profanity. Suffice to say, that particular project had to go through a serious re-think).

Although yes, many other factors do come into play when considering felt recoil, such as the balance of the weapon, the bore axis relative to the stock*, the design of the stock, recoil control mechanisms, and the like, if you're boiling down recoil to one single figure, the kinetic energy of that recoil gives the best sense of the matter.

* Surprisingly though, on this front, even the notorious launcher above was surprisingly good at keeping the muzzle on target during recoil, because the design put the butt-stock and centre of gravity on the axis of the barrel, meaning the forces transferred directly back into the user's shoulder without any torque, so basically no muzzle rise.

I wasn't talking about mean velocity over a distance I was talking about maximum velocity reached during flight.

The fast-burn bolts hit the higher maximum velocities. The thing is that drag forces rapidly increase the faster you go, so eventually you hit an equilibrium where the bolt isn't accelerating. Higher thrust means a higher equilibrium speed.

EDIT: Pulling exact figures from my 600m/s calculations, slow-burn bolts hit an equilibrium at 1074.6 m/s. Even the 20% efficient fast-burn bolts hit 1527.6 m/s before they burn out.

 MarcoSkoll wrote:
The 720 joules relates to a 10 kg bolter recoiling at 12 m/s.

Sorry, I'm an idiot, I didn't connect the dots.

 MarcoSkoll wrote:
The reason we don't use force is that resisting recoil is not a consistent even force - people are more like a spring, starting soft and getting harder the more you push back.
As such, energy is generally a better metric than trying to guess how much force is being applied to resist the recoil.

Fair enough, that is a really good explanation actually.

 MarcoSkoll wrote:
The fast-burn bolts hit the higher maximum velocities. The thing is that drag forces rapidly increase the faster you go, so eventually you hit an equilibrium where the bolt isn't accelerating. Higher thrust means a higher equilibrium speed.

Which means you are making the assumption that the fast burn bolt has enough fuel to reach equilibrium speed, and also assuming that the fast burn bolt has larger engines than the slow burn one. I was doing this on the assumption that both bolts had the same thrust.

See my quote:

w1zard wrote:
I was simply pointing out that as long as both rocket engines have equal thrust, then a longer thrust (assuming negligible fuel weight and equal energy density) would produce a higher maximum velocity (also assuming maximum attainable velocity had not been reached yet).

A "fast burn" bolt dumping its 100 NS of thrust in 1 second has to have bigger engines then a "slow burn" bolt dumping its 100 NS of thrust in 10 seconds.

I like Bolt Weapons because even a glancing blow is likely to cause carnage?

Bolter would have mild recoil. It’s the equivalent of shooting a 12 gauge shotgun slug but even less recoil. Same sized projectile discharged at relatively the same velocity except the bolt gun is automatic so has some kind of recoil spring or system built in and the bolter itself is a heavy duty weapon which further absorbs the recoil.

A regular human with proper shooting technique could handle firing a bolter no problem. Weapons of war are made to be easily handled in adverse conditions. If it’s such a struggle to aim and fire a weapon they wouldn’t bother using the damn thing.

warpedpig wrote:
Bolter would have mild recoil. It’s the equivalent of shooting a 12 gauge shotgun slug but even less recoil. Same sized projectile discharged at relatively the same velocity except the bolt gun is automatic so has some kind of recoil spring or system built in and the bolter itself is a heavy duty weapon which further absorbs the recoil.

A regular human with proper shooting technique could handle firing a bolter no problem. Weapons of war are made to be easily handled in adverse conditions. If it’s such a struggle to aim and fire a weapon they wouldn’t bother using the damn thing.

I believe I saw somewhere it could break your arm if you're only a normal human. Could. And I just believe

In all due likelyhood Space marines aren't concerned with that, they are physically too powerful to not be able to master the recoil, after all they wield the strength of 10 humans and the armour further enhances it.

w1zard wrote:
Which means you are making the assumption that the fast burn bolt has enough fuel to reach equilibrium speed

No. The calculations all "assume" the bolt has the total impulse stated, over the duration stated. In a lot of the calculations I've done, the bolts have not reached their equilibrium speed.

also assuming that the fast burn bolt has larger engines than the slow burn one. I was doing this on the assumption that both bolts had the same thrust.

... how else did you imagine that two different engines would deliver the same total impulse over different durations other than by having different thrust levels? (Note, however, that "different thrust" does NOT automatically mean "larger engines". Rocket thrust is complicated). I've even specifically articulated the point that the fast-burn bolt is "assumed" to have more thrust:

 MarcoSkoll wrote:
Okay, what about only 20% as efficient, only 200 Ns of thrust, leaving it with only twice the thrust of the slow-burn bolt, compared to just a tenth of the burn time?

If we're making accusations of assumptions, what you're now doing is assuming that your slow-burn rocket automatically represents the absolute peak possible force that can be generated by any propellant that the Imperium has available, and this thus misses the entire point.
If you just pile on enough fuel reserves that your bolt can trivially generate maximum possible thrust for longer than it takes to get to even extreme engagement range, then it defeats the whole point of the argument, and represents a very wastefully designed bolt.

My point is that you want the rocket to generate the absolute peak thrust you can generate, even though this will be at the cost of the total burn time. With any fuel that's even broadly thinkable, even generously multiplying our current understanding of chemistry, then the idea of creating massive thrust for multiple seconds with the quantity of fuel contained within a tiny bolt is just not feasible.

If we stop with the idea that the fuel is an insignificant mass (which it actually physically cannot be - rocket engines are reaction engines, and work by expelling mass. The less we assume that mass is, the more ludicrous the exhaust velocity has to become*), then including enough fuel to get to such insane distances will unnecessarily increase recoil (as the fuel has to be fired, meaning more mass in that equation), reduce the acceleration of the bolt (actually reducing the impact of the bolt at likely engagement ranges) and simply be wasteful. Firing enough fuel to keep the bolt accelerating to 10 kilometres when almost everything you're firing at is within one kilometre, means more cost, and heavier, bulkier ammunition.

* Supposing, for a moment that our insignificant mass of fuel is 1 gram of the 50 gram bolt, generating 1000 Ns of thrust over 10 seconds from that amount of fuel would require an exhaust velocity of 1,000,000 m/s. Literally a million metres per second. That's utterly unthinkable with any chemical propellant - you're getting into energy densities that could only be achieved through mass-conversion (even ion engines don't get close to this) and I can't remember anyone mentioning bolter shells creating gamma ray bursts when fired...

(To be clear, the fact that your bolts are generating less total thrust does not make their exhaust velocities more plausible. The fast-burn bolts would have exactly the same exhaust velocities, because they'd generate the extra thrust by chucking their fuel mass out of the back ten times faster).

Even if we increase that to a significant 10 grams of the projectile, the figures you're proposing are still a good twenty times more energy dense than any existent chemical propellant, so the idea of maintaining these kinds of thrust impulses is basically out of the question in reality, so we're already getting into serious hypotheticals here. Keeping the same thrust, but slashing durations (and thus total impulse) to a tenth of these values would be way more plausible, but even then still stretching things.

 Maréchal des Logis Walter wrote:

warpedpig wrote:
Bolter would have mild recoil. It’s the equivalent of shooting a 12 gauge shotgun slug but even less recoil. Same sized projectile discharged at relatively the same velocity except the bolt gun is automatic so has some kind of recoil spring or system built in and the bolter itself is a heavy duty weapon which further absorbs the recoil.

A regular human with proper shooting technique could handle firing a bolter no problem. Weapons of war are made to be easily handled in adverse conditions. If it’s such a struggle to aim and fire a weapon they wouldn’t bother using the damn thing.

I believe I saw somewhere it could break your arm if you're only a normal human. Could. And I just believe

In all due likelyhood Space marines aren't concerned with that, they are physically too powerful to not be able to master the recoil, after all they wield the strength of 10 humans and the armour further enhances it.

SoB has downscaled bolters because the recoil is too powerful.

HexHammer wrote:

 Maréchal des Logis Walter wrote:

warpedpig wrote:
Bolter would have mild recoil. It’s the equivalent of shooting a 12 gauge shotgun slug but even less recoil. Same sized projectile discharged at relatively the same velocity except the bolt gun is automatic so has some kind of recoil spring or system built in and the bolter itself is a heavy duty weapon which further absorbs the recoil.

A regular human with proper shooting technique could handle firing a bolter no problem. Weapons of war are made to be easily handled in adverse conditions. If it’s such a struggle to aim and fire a weapon they wouldn’t bother using the damn thing.

I believe I saw somewhere it could break your arm if you're only a normal human. Could. And I just believe

In all due likelyhood Space marines aren't concerned with that, they are physically too powerful to not be able to master the recoil, after all they wield the strength of 10 humans and the armour further enhances it.

SoB has downscaled bolters because the recoil is too powerful.

The Departmento Munitorum can - or could - supply limited amounts of Boltguns to regular humans, as seen by Guard and Storm Trooper officers having access to them in their armouries (not sure if still true in new/current Codex/lore). What was the story there? I headcanon'd that they were "simplified", "stripped down", and lower caliber (like .50) than Marine versions in order for it to make a little more sense (obviously TT profiles keep things way more simplified).

 MarcoSkoll wrote:

w1zard wrote:
Which means you are making the assumption that the fast burn bolt has enough fuel to reach equilibrium speed

No. The calculations all "assume" the bolt has the total impulse stated, over the duration stated. In a lot of the calculations I've done, the bolts have not reached their equilibrium speed.

In which case the fast burn bolt that reaches maximum velocity (and a higher maximum velocity due to thrust differential) the fastest will only be overtaken by the slow burn bolt when the fast burn bolt runs out of fuel. I was never debating that.

 MarcoSkoll wrote:
... how else did you imagine that two different engines would deliver the same total impulse over different durations other than by having different thrust levels? (Note, however, that "different thrust" does NOT automatically mean "larger engines". Rocket thrust is complicated). I've even specifically articulated the point that the fast-burn bolt is "assumed" to have more thrust.

Then you have moved the goalposts of the arguing my original statement.

Look, this has been a very interesting and very enlightening conversation, but I originally said:

w1zard wrote:
If you truly had experience building firearms, you would know that the longer the impulse, the higher velocities and greater kinetic energy.

I was assuming constant force across both projectiles, which means the bolt with the longer impulse time gets more total impulse. I was never really arguing anything else, and am not sure how you came to that conclusion.

Give two bolts the same rocket engine. Give one bolt twice the fuel. Assume fuel weight is negligible. Assume the bolt with less fuel does not reach equilibrium speed before it runs out of fuel. Assume that we are out on salt flats or something so nothing gets on the way. My assertion: The bolt with twice the fuel will have a higher maximum velocity during it's flight time then the other. Unless I am understanding things very wrong, I think that is a correct statement.

I also apologize for being condescending at first. You obviously know what you are talking about. But, you wouldn't believe how many times I've heard "I work on guns for a living" or "I've been in the military" online...

w1zard wrote:
Then you have moved the goalposts of the arguing my original statement.
Look, this has been a very interesting and very enlightening conversation, but I originally said:

And my absolute first response to that was...

 MarcoSkoll wrote:

you would know that the longer the impulse, the higher velocities and greater kinetic energy.

... if the force of the impulse is the same, in which case you have a larger impulse and a completely inaccurate comparison. The bolt only has very limited volume for rocket fuel, and so a limited total impulse.

I set out exactly where I was putting the goalposts very early in the conversation, by contending that the fixed variable between fast and slow-burning fuels was not thrust but instead total impulse, and you never made any objection to that until very recently.

The entire point of my argument has always been on the principle that if you have two similarly chemically advanced rocket fuels (i.e. with similar exhaust velocities), then the one that burns/ejects its mass faster will create a greater reaction force, not an equal one.
As such, your entire argument relies on two bolts containing different masses of fuel, making for an entirely pointless comparison.

Yes, if two objects accelerate at the same rate but one accelerates for longer, then the idea that it's eventually going to go faster is a statement which would be commonly misattributed to Captain Obvious, but I believe it's actually in my book of quotations as first spoken by Sir Lord Grand Field Marshal Duke Obvious the Forty-Second of the Illustrious Obvious Dynasty of Planet Obvious in the Obvious sector.

I do not appreciate having my time wasted (and then being accused of disingenuously moving the goalposts) because you didn't read what I said right at the very start. If you'd objected right back then, then this could have been boiled down to "Well yes, if you're insisting on making that assumption, but I deeply disagree with that assumption".

 MarcoSkoll wrote:
I do not appreciate having my time wasted... because you didn't read what I said right at the very start.

I'm really not trying to be rude here buddy, but right back at you.

I'm not sure how you got from me simply stating "a longer impulse with the same force leads to a higher velocity" into a discussing larger, faster burning rocket engines on bolts vs smaller slower burning ones. Nor have I ever at any point claimed you were wrong... if you look back through our conversation I was simply trying to clarify your rather confusing assumptions the entire time.

You take a simple statement by me (delivered rudely, and I have apologized for that) and somehow turn it into a pagelong discussion of something completely unrelated to what I was originally talking about, and then blame me for wasting your time afterwards when I was trying to clarify what we were talking about the entire time.

Yes "a longer impulse with the same force leads to a higher velocity" is a rather dumb, captain obvious statement, which is why I was so confused about why you seemed to be trying to debate that.

 MarcoSkoll wrote:
I set out exactly where I was putting the goalposts very early in the conversation, by contending that the fixed variable between fast and slow-burning fuels was not thrust but instead total impulse, and you never made any objection to that until very recently.

No, I objected to it very early on in our conversation.

w1zard wrote:

 MarcoSkoll wrote:
The bolt only has very limited volume for rocket fuel, and so a limited total impulse.

Why are you assuming that?...

You will have to forgive me for not picking up immediately that you were implying the fast burning bolts had more thrust. It is obvious in retrospect, but at the time I didn't connect the dots. I am not an engineer or physicist. I am not used to making those kinds of connections and you never explicitly stated it, unless I missed something.

EDIT: If I wanted to get really snarky, I could point out that saying a bolt having a higher maximum velocity than another whose engine is "smaller" when they both have the same mass is just as "captain obvious" territory as my claim was.

Marines use bolters because when one shell hits a target, it not only takes it out, but the five nearby men surrounding it when it explodes. That's why there's like only 5 shells in a bolter magazine.

The lasgun just burns the target, but can shoot all day.