Making Gyrocs Great

[Varyon]

Originally Posted by Fred Brackin

Ve2 wouldn't let you build a rocket motor with a duration less than half a second and that might be a problem for your goals but even that would let you build 100mm missiles that accelerated 10x faster.

I may give remaking gyrocs in Ve2 a shot. Fortunately, my gyroc rocket motors last around 4 seconds (longer for the Flight variants), so that shouldn't be a problem. Even the most short-lived hot gyroc lasts around a second.

Originally Posted by Anthony

Rockets actually have a max chamber pressure, not a max acceleration, so small rockets can go faster. Gyroc ammo is 0.1 lb per shot, unclear how much of that is projectile, but if it's all projectile that's a total energy of 0.5 x 45g x 450m/s^2 = 4,556J. Accelerating to full speed in 10 yards is about 500J/m or a force of 500N, 15mm is a cross-section of 174mm^2, so that's 2.9 MPa or 420 psi, which is a believable figure.

That's not so bad, then.

[Black Leviathan]

Since there's some control over velocity how about a Slow Gyroc, One that caps out just under the speed of sound with a very massy projectile? chances are you wouldn't get much range out of it due to the speed but with a round that accelerates rather than decelerating you could pack a whallup without breaking the sound barrier, maybe the round would be big enough to contain a chemical payload or a super-heating core or some other fun gimmick that would be useful in a silent weapon.

[Anthony]

Originally Posted by Black Leviathan

Since there's some control over velocity how about a Slow Gyroc, One that caps out just under the speed of sound with a very massy projectile?

Is there some reason to continue calling that a gyroc? In the end, it's just a small missile.

Originally Posted by Black Leviathan

maybe the round would be big enough to contain a chemical payload or a super-heating core or some other fun gimmick that would be useful in a silent weapon.

In general 'silent' and 'rocket' don't go together, unless you're willing to use an extremely low performance rocket.

[Varyon]

Originally Posted by Black Leviathan

Since there's some control over velocity how about a Slow Gyroc, One that caps out just under the speed of sound with a very massy projectile? chances are you wouldn't get much range out of it due to the speed but with a round that accelerates rather than decelerating you could pack a whallup without breaking the sound barrier, maybe the round would be big enough to contain a chemical payload or a super-heating core or some other fun gimmick that would be useful in a silent weapon.

Slowing it down to 350 yards per second would put it subsonic. It would also take twice as long to burn through its fuel, traveling around 2800 yards in that time, so it would actually have longer range than a normal gyroc. Damage would drop to around 4d pi++. Sadly, at TL9 a 15mm projectile can't manage that great of a warhead. At least, it can't by UT - however, 18.5mm HEAT rounds are doable now (HT has one on page 103, although its stats do leave something to be desired), and 10mm HEAT is doable at TL10, so 15mm at TL9 seems feasible. Assuming TL9 is at -1/die relative to TL10, 15mm HEAT would be around 7d(5) imp inc. Alternatively, pack it with something that doesn't do direct damage, like nerve gas (a 15mm warhead can hold enough to fill a 1 yard radius) or an expendable jammer (15mm can jam a 15 yard radius).

This might not make it appreciably quieter, however - I'm pretty certain the exhaust would still be supersonic. Slow gyrocs would be used more to get extra range, so would probably be micromissiles. I'm not certain what the stall speed of a gyroc would be, but if you can get it down to 100 yards per second, that'll let it fly for around 10,000 yards (over 5.5 miles) but will drop the damage all the way down to 1d+1 pi++, so you'll very much want to make certain you pack the warhead with something that'll do the job for you. In theory a slow gyroc would let you get away with less robust engines, but I'm going to ignore that (I'd expect normal gyrocs to have normal engines that they push to the limits, and anything cheaper is going to be less reliable, which you don't want). Note you could combine this with Flight - a 0.2 lb snail flight micromissile would cost $20 plus the cost of the guidance system (probably $150 for Viper, for a total of $170) and warhead, and fly for 30,000 yards. It would have Range 30,600/33,600*, but would only fly at 100 yards per second, meaning it would take up to 5.6 minutes to reach a target at maximum range.

*As GURPS typically modifies range with the power of the round - specifically, scaling it linearly with damage for a given caliber - so adhering to +600/+3600 may not be appropriate. In this case, it would be +120/+720, for Range 30,120/30,720, which could probably just be rounded down to Range 30,000.

[Rupert]

Originally Posted by Varyon

I had some thoughts with the 25mm Payload Rifle discussion that are best put in another thread - namely, how to make gyrocs useful. Let's start with how things stand.

Pros:
Lightweight, low ST weapon
Exceptional damage and payload capability for their small versions
Extreme 1/2D range
Incredible velocity for micromissiles

Interestingly, I can't get anything like that velocity using any GURPS design system for rockets (VE2's main rules, it's rocket projectile weapons, etc.). Also, the KE damage in UT (6d pi++) is low for that velocity, and if it is correct the actual warhead must be absolutely tiny.

Cons:
Heavy, expensive ammo
Poor damage for their larger versions
Cannot benefit from ETC
Small Max range
Low Acc compared to conventional weapons
Micromissiles cost more than comparable "smart" bullets

The ammo weight is low compared to that for conventional guns and grenade launchers. The cost (for ungiunded rockets) is comparable to similar ammo for grenade launchers (payload rifle ammo is much more expensive, and very heavy). Interestingly the 25mm gauss grenades weight very little less (for the same amount of bang), but cost next to nothing. UT's gyroc ammo would seem to have a very small amount of very expensive and high-performance rocket propellant (so much so it doesn't really make sense).

Next, APDS and APEP don't really make sense for gyrocs, because discarding sabots aren't going to help them.

APEP doesn't have to have a discarding sabot - the description merely notes what the core is, not that it's a sub-calibre penetrator. I agree on the APDS thjough (and APHD at TL11).

Next up, we need to resolve some of the oddities of gyrocs. This is where we're getting solidly into houserule territory. First off is the ridiculous velocity of micromissiles (1900 yards/sec, over Mach 35). If we assume a similar mass fraction of fuel to payload as Spaceships conventional weapons (which also follow the same trend as missiles), we'd expect a 1.5cm missile to be 1/10000 tons and deal around 2.5d dDamage at 1 mile per second (1760 yards/sec). Gyrocs are about half that weight and deal around 1/4 the damage. If we assume the reduced weight is due more to having less propellant than having a smaller payload, that implies a velocity of around 440 yards/sec, on the low end of rifles. Which, honestly, doesn't seem too bad. I'd probably bump it up to 500 (on par with the IML and similar).

That's about what I managed to screw out of Vehicles, I think (though it took more, rather than less propellant).

Finally, there's the oddity of lacking a 1/2D range - apparently, the bullet just stops once it runs out of fuel, rather than continuing on a natural ballistic trajectory.

1/2D equalling Max could mean that there's a sustainer motor, and when it runs out the imprecise burnout and change in centre of gravity, etc. destabilises the bullet, so there's no effective reach past that point - this is why tracers go off-target when they burn out, and why rocket-assisted artillery rounds are less accurate than conventional ones, so it's not impossible.

Reactionless (TL^): Replaces fuel with an energy bank and rocket with reactionless thruster. Standard is x5 to flat range and x2 CPS; can be combined with Hot and Flight.

GURPS' reactionless thrusters tend not to have very high accelerations compared to chemical rockets, and I don't think even a hypothetical version that exchanges better a thrust/weight ratio for a (very) short life would do well. It'd disallow 'hot' versions, but they'd have greater range (as you say), etc.

The overall problems in UT come from 1) ETC weapons being just that good (and perhaps unfortunately RL research doesn't rule that out as a reasonable assumption), and 2) all warheads of the same calibre having the same payloads. A gyroc round intended for delivering a payload, rather than a KE-based round, could have a relatively low velocity (and long burn time to keep the range up) and thus a large payload compared to a rifle. That would make them a more accurate, but direct fire only, equivalent of a grenade launcher. Unlike a grenade launcher, you would have the option of high velocity, high-KE damage, rockets with shorter range.

Another thing is happily allowing homing rounds from rifles at the same calibre as for gyrocs, despite the harsher launching environment.

[Rupert]

Originally Posted by Rockwolf66

5,700 feet per second is very high velocity for a projectile weapon. It is faster than the highest muzzle velocity of current firearms.

Current small arms, sure. However, it's in the same ballpark as modern tank guns.

[Rupert]

Originally Posted by Varyon

500 yards per second deals well with the second note, and at least helps with the first. What sort of acceleration do you feel is more likely for a gyroc? I currently have it going from 0 to 500 in 10 yards (because UT has it reach full velocity after 10 yards), which requires an incredible 1250G acceleration, if my calculations aren't off.

If one assumes that a conventional gun's 1/2D range is the point at which damage drops to 50%, and thus velocity as well, and work that in reverse, 10 yards is where the gyroc reaches 50% of its final velocity. That implies 20 yards is where it hits its peak velocity (assuming constant acceleration), which drops acceleration to 'only' 580G.

[Hide]

Originally Posted by Varyon

I had some thoughts with the 25mm Payload Rifle discussion that are best put in another thread - namely, how to make gyrocs useful. Let's start with how things stand (...)

My idea of what a gyroc is, isn't very clear.

Is it something like this?

[lwcamp]

Originally Posted by Hide

My idea of what a gyroc is, isn't very clear.

Is it something like this?

It's probably largely inspired by the real-life gyrojet
https://en.wikipedia.org/wiki/Gyrojet
and maybe the Buck Rogers rocket guns. As a rocket-launching platform, it probably has a much lighter construction than the image you linked to.

Luke

[Daigoro]

You can see some gyrojet test shots in this video, although they're using vintage ammo.

https://youtu.be/cJAXpyt8-oQ