Launching rockets while in hover

[Distiler]

So I've read in some places (can't remember where, probably here) that many types of rockets should not be launched while in hover but need some some forward movement. The causes I've read are:

 

- because many rockets are stabilized by fixed fins and they don't get spinning movement and loose accuracy when launched with the helo in hover. Some other rockets have stabilization engines nozzles and can be launched without forward movement.

 

- because smoke from the rocket is, in some way, bad for the heli or the pilot when launched in hover.

 

So, is any of those true?

[OutOnTheOP]

As to needing forward movement to impart spin, that's just silly. There are several ways to stabilize a projectile, and both spin and fin stabilization are used in rockets, sometimes in concert. Neither has anything to do with the aircraft moving forward, but are rather imparted by aerodynamic forces from the fins or rocket motors on the aircraft itself. That's like saying a rifle won't put a spin on a bullet unless you're running when you fire it.

 

I really can't speak for rocket exhaust ingestion, though I suppose in a hover, the rotor wash might lead to some. I suspect the real reason firing rockets from a KA-50 in the hover is disadvised are twofold: 1), the KA-50 isn't as stable in a hover as it is in forward flight (particularly in yaw), and therefore won't deliver rockets as accurately, and 2) as a single-seat aircraft, holding a hover occupies most of the pilot's attention, and it's difficult or even dangerous to divide your attention between hovering the aircraft and acquiring/ slewing/ engaging targets simultaneously.

 

That said, you see AH-64s engaging from the hover with rockets all the time. Pretty standard fare for them, and they also use folding fin rockets (the Hydra-70 series). But with two pilots to share the workload and a conventional tail rotor (which theoretically grants more precise yaw control in a hover), it might be easier for the AH-64. I can't recall if AH-1, Mi-28, or any other modern attack helicopters regularly engages with rockets from the hover.

[isoul]

What I know is that having some forward speed gives some additional speed to the rocket resulting in slightly bigger range. Forward speed may reduce the possible(?) impact of recoil when a salvo of rockets are launched.

 

Still I 've seen helicopters launching rockets both while hovering and while moving forward so I believe there is no absolute rule about it.

[sobek]

- because smoke from the rocket is, in some way, bad for the heli or the pilot when launched in hover.

 

So, is any of those true?

 

Supposedly, the engines are prone to ingesting rocket fumes when fired from a hover, which can lead to flame outs.

[Distiler]

Thanks for the answers! Just because the fumes ingestion, I'll change my rocket delivery tactics ingame :) (dunno really if it's modelled or not, but anyway...)

 

About the spin movement in apache's hydra, this is a link:

 

http://www.globalsecurity.org/military/systems/munitions/hydra-70.htm

 

To provide some stability the four rocket nozzles are scarfed at an angle to impart a slight spin to the rocket during flight. The modified motor provides increased stand-off range and reduced ballistic dispersion. The MK 66 rocket motor was designed to provide a common 2.75-inch rocket for helicopters and high-performance aircraft. Compared to the MK 40 motor, it has a longer tube, an improved double base solid propellant, and a different nozzle and fin assembly. Increased velocity and spin provide improved trajectory stability for better accuracy.

 

Now I don't know about the S-8 in Ka-50, but being in hover might induce more scattering than in forward movement, not only because you're reducing the speed thus the range but because spining might be slower. Again, I'm not sure!

Edited March 13, 2011 by Distiler

[flanker0ne]

An other reason is the propeller wash.

When in hover the prop wash cause a destabilizing force on the fins resulting in a pich up for the rocket, instead if enough forward speed is gained the prop wash move backward and the rockets are launched through undisturbed air flow.

Next time try to aim just a little bit over/beyond the target, then lowering the collective and leave the pipper to fall on the target, then Fire!

With less vertical flow came from the prop the rocket are more stable.

Edited March 13, 2011 by flanker0ne

[Dash]

At least one crash of an Mi-28N was attributed to rocket gas ingestion.

[OutOnTheOP]

Huh... I guess Russian rockets burn real dirty, then? Apaches fire large volleys (20+ rockets) from the hover pretty regularly.

 

Good point about the rotor wash effects on FFAR fins. Knew I was forgetting something

 

Again, forward speed has nothing to do with rocket spin; that is imparted by the fins of the rocket and the thrust from the rocket's engines, not from any force imparted by the helicopter itself (well... I guess the additional forward speed would mean more velocity to the rocket and therefore more airflow across the fins, but I suspect the 70-100-ish knots of additional speed are pretty insignificant in comparison to the rocket's peak speed)

[Stretch]

Huh... I guess Russian rockets burn real dirty, then? Apaches fire large volleys (20+ rockets) from the hover pretty regularly.

 

Correct me if I'm wrong, but the risk of exhaust ingestion while in a hover isn't about FOD (dirty vs clean burning), but more about the fact that the engine is ingesting something other than oxygen.

 

The double-base propellant used in the S-8 and S-13 rockets produces a gaseous byproduct that displaces air that could be used to maintain fuel combustion in the Ka-50's jet engine.

 

But I'm just guessing here -- I could be wrong!

[Stretch]

EDIT: Duplicate

[OutOnTheOP]

No, I know that... when I say "dirty", I mean with a high volume of combustion byproduct. Kind of like how petrochemical fuels theoretically SHOULD be able to burn down to nothing but CO2 and H2O... but that never really happens. IE, it burns dirty. The point is that the propellant mix used in the Russian rockets seem like they produce more hazardous byproduct than the equivalent NATO rockets.

[isoul]

... The point is that the propellant mix used in the Russian rockets seem like they produce more hazardous byproduct than the equivalent NATO rockets.

 

Where did you find such info? I am not doubting your argument but it seems that someone searched in detail... extreme detail if you ask me. In such detailed sources we can find many interesting things.

Edited March 16, 2011 by isoul

[AlphaOneSix]

A little image stolen from FM 1-140 Aerial Gunnery:

 

 

What is not said but goes without saying is that firing rockets at speeds above ETL is the most accurate, as there will be no downwash effect at all on the rockets.