[alancalverd]

Tangmere air museum is well worth a visit: a pleasant day out in the country, good cafe, well organised car park, and incidentally several amazing historic aircraft. Obviously I went for the food, fresh air, and to study the development of WAAF uniforms and hairstyles,  but I just happened  by sheer chance to wander into the English Electric Lightning simulator where I was persuaded against my mature judgement by an ex-RAF instructor to fly a 30 minute ground attack mission. Which activated the brain, several days later.

The gun pod, which only carries 7 seconds' worth of ammunition (at >1000 rounds per minute, that's still a lot of damage!), is extremely heavy, because the barrels have to contain the energy of a 30 mm cannon shell. The cartridge casing and the propellant are left behind when any gun is fired.

Now the object of the exercise is to deliver a lot of kinetic energy to the target, so I wonder if this might be done more efficiently by replacing the explosive propellant with a solid rocket charge, thus turning the barrel into an open tube with very little strength requirement, delivering more of the payload to the target, and having the characteristic of accelerating in flight rather than slowing down as a bullet does. No reason why the barrel shouldn't be rifled, or the missile grooved, so as far as the physics is concerned you end up with a lighter weapon with the same accuracy as a gun, that delivers more bang for your buck.

Military rockets are a few large devices delivering an expensive payload with a low rate of fire, from a suborbital missile down to an RPG,  but I wonder what technical issues prevent the development of a "machine rocket launcher"  to deliver a high rate of fire of small (say 30 mm?) rockets in the same space as a conventional  airborne cannon?     

[Bored chemist]

Almost all the mass of a rocket is left in the air. and never gets to the target.
A 3000 tonne rocket will deliver a 50 tonne payload.
Scaling that to deliver a few tens of grams means each rocket "round" would weigh a few kilos.
A few hundred rounds would weigh more than a heavy machine gun and its ammo.
I think the underlying physics is that a gun is much more efficient because the expanding gas has something to push against.

[Petrochemicals]

Compressed air surely would be a better way to go

[Bored chemist]

Compressed air surely would be a better way to go

Tell NASA.

[alancalverd]

Almost all the mass of a rocket is left in the air. and never gets to the target.
A 3000 tonne rocket will deliver a 50 tonne payload.

True for reaching orbital speed, but a typical tactical rocket weighing 1500 kg delivers a 500 kg warhead over about 70 km. Subsonic, admittedly, but I don't think my argument is completely demolished yet.

[Bored chemist]

Also, bullets are cheap.
It's a secondary (or less) consideration for many armies, but it may be a factor.
Rockets are hugely complicated compared to bullets.

[alancalverd]

I'm thinking of a bullet with a slow charge and a hole at the back instead of a fast charge and a percussion cap at the back.  Not much more complicated than a Guy Fawkes / 4 July rocket, surely, and no more expensive than a cannon shell. Not that cost is that important in context: the Lightning drank around 200 liters of fuel per minute just getting there!

Terminal speed of 500 mph with an active warhead would suffice for surface targets but anything above Mach 1 would be handy for air-to-air operation. Note that the muzzle velocity of a cannon is about Mach 2 but the projectile slows in flight, unlike my minirocket that accelerates.

[Bored chemist]

a bullet with a slow charge and a hole at the back

I wondered about that.
If you just drilled a hole in the back of a conventional round, fixed the bullet in  and then heated it until the charge flashed, how well would it "rocket"?

I suspect the answer is "very badly indeed".

Not much more complicated than a Guy Fawkes / 4 July rocket,

Thankfully, because these are built to be light weight, they typically do little damage on impact.

I'm sure the military have thought about  this- they probably even tried it.

[Petrochemicals]

Rate of fire could be one problem, missile launch is not fast due to speed accumulation, where as bullets  an be fired at extraordinary rates.

Compressed air via a jet engine would have the advantage of continuous high speed accelleration like hitler V3, no chemical degradation of the barrel and as the gas expands it would cool the barrel also. If you could eject some aluminium projectiles at 7000mph via compressed air you would save a lot on weight carried for no appreciable loss of fire power.

[Bored chemist]

If you could eject some aluminium projectiles at 7000mph via compressed air

You can't.

[alancalverd]

But the idea of expelling minirockets with compressed air is worth considering as it could reach  a very high rate of fire, with the rocket engines igniting thereafter. Not much point, however, in a lightweight missile: you need to deliver a hefty lump of "kinetic" steel to bash a hole in an aircraft, or a complex armor-piercing explosive charge against a surface target.   

Drilling a hole in a conventional cartridge round won't work because the propellant is necessarily fast-burn and a simple hole doesn't give optimum gas flow at any speed.  But there is no great problem in machining a laminar flow nozzle and a shaped slow-burn charge (as used in other military rockets) will give you a tailored acceleration profile. The propellant part of the system can actually be lighter than a conventional cartridge because  it doesn't have to withstand an explosive shock.

[Origin]

"Would minirockets have any military value?"
Yes, they are called RPG's.

[alancalverd]

Not quite the same. The RPG is a hefty beast that delivers a very large explosive charge. I'm thinking of something more akin to a cannon shell but with the propellant attached to the shell and not left behind in the gun - same principle as the RPG, different numbers. 

[Origin]

Not quite the same. The RPG is a hefty beast that delivers a very large explosive charge. I'm thinking of something more akin to a cannon shell but with the propellant attached to the shell and not left behind in the gun - same principle as the RPG, different numbers.

I was answering the OP.  It seems to me that an RPG is certainly a mini-rocket; Rocket Propelled Grenade...

[alancalverd]

I quote the OP:

I wonder what technical issues prevent the development of a "machine rocket launcher"  to deliver a high rate of fire of small (say 30 mm?) rockets in the same space as a conventional  airborne cannon?     

[chiralSPO]

well, a quick search showed this: https://www.arnolddefense.com/product/m26/

Not quite what you're talking about, but it looks like it can hold up to nineteen 70 mm rockets and has both "single fire" and "ripple" modes.

I don't think it would be very practical to attempt a rapid fire of rockets out of a single barrel/tube because (unlike bullets, for which their maximal speed is in in gun) rockets are very slow starting out, which will limit how many rounds can exit per unit time, and I think will also increase the potential for corrosion. (this is perhaps counter-intuitive because the gun fires the entire charge of a bullet, while most of the rocket fuel is burnt outside of the gun. But if the inside of the tube is exposed to the hot exhaust for a longer amount of time for each round fired, that could be problematic) obviously you don't want to carry too many barrels around on any type of aircraft.

That said, I feel like we are at a point technologically where we could basically dump a whole bunch of AI-controlled (target seeking or just auto-stabilizing) rocket-propelled darts out of an aircraft, and having them zoom off to bombard a target (or many).

[Origin]

I quote the OP:

I did miss that part, so an RPG would not fit the bill.

[Petrochemicals]

Not much point, however, in a lightweight missile: you need to deliver a hefty lump of "kinetic" steel to bash a hole in an aircraft, or a complex armor-piercing explosive charge against a surface target.   

Depends on the speed Alan, a good bit of titanium at twice the speed is fairly penetrating, I was surprised to discover recently the soft ball bearings in explosive shrapnel weapons actually deform into bullet shape projectiles.

[Bored chemist]

Under the forces involved, everything (steel, titanium, rocks, tanks etc)  is soft and you can model the behaviour as a liquid.

But Newton worked out that denser is better.
https://en.wikipedia.org/wiki/Impact_depth
If he had known about it, he would have advocated things like depleted uranium.
He was probably familiar with the use of lead bullets.

Steel is also a lot cheaper than titanium.

[CliffordK]

Our modern ammunition was largely developed about 150 years ago, around the time of the US Civil War. 

The advantages of cartridge and clip/magazine load became very obvious when a soldier could shoot a half a dozen shots, reload in moments, and fire another half dozen shots, all the while the enemy could fire a single poorly targeted musket shot.

I think there were some paper cartridges developed from time to time.  But, the modern brass cartridges are dependable, moderately water and humidity resistant, and can be handled fairly roughly as long as you are careful with the primer.

Most hand held "modern" guns are also recoil loaded, although there are some high speed automatic guns that are power operated (similar to the vintage Gatling Guns).

I could imagine building a reverse shell leaving only the primer behind, but that would mean a larger, and probably less aerodynamic bullet.

And, of course, with explosive acceleration, a bullet can achieve greater than the speed of sound before leaving the barrel of the gun.

Now, that doesn't mean there isn't a niche for some kind of a rocket propelled bullet.  There is a growing market for "Smart Ammunition".  In particular with person to person combat.  The ability to shoot around obstacles, or at something that isn't within the line of sight.

And, of course, sniper technology is mighty extraordinary.  A long shot of say 2 miles might take nearly 10 seconds of flight time, with the bullet at the mercy of wind direction, wind speed, etc.  Using the general gravity acceleration, , the bullet is falling quite far.  Having the ability to do precision targeting would be a huge benefit.